Chapter 4. The Earth System feedbacks that matter for contemporary climate
Pierre Friedlingstein, Angela V. Gallego-Sala, Eleanor M. Blyth, Fiona E. Hewer, Sonia
I. Seneviratne, Allan Spessa, Parvadha Suntharalingam, Marko Scholze
4.1 Introduction
Chapter 2 dealt with physical feedbacks that contribute to climate sensitivity, and with
the general principles of terrestrial ecosystem function, fire regimes and the carbon cycle.
Chapter 3 discussed feedbacks of many kinds, including some involving ice sheet
dynamics and operating on time scales of thousands of years. This chapter focuses more
closely on feedbacks due to biogeophysical and biogeochemical processes involving the
land and ocean, operating on time scales that are relevant to climate change in the 21st
century; with a brief glance at additional, relatively rapid feedbacks involving human
behaviour. These feedbacks are not included in the climate sensitivity, yet they will
contribute to determining the effect of any change in greenhouse gas emissions on
climate. Quantifying these feedbacks in an active research area and it is not yet possible
to provide a complete picture of their magnitudes. But recent research has narrowed some
of the largest uncertainties and ruled out some of the more extreme possibilities; while
drawing attention to the general tendency of feedbacks that involve microbial processes
to be positive, i.e. to amplify climate change.
4.1.1 Background
The importance of feedbacks in the Earth system has been recognised for more than a
century. In a seminal paper published in 1896, Svante Arrhenius calculated the ‘variation
of temperature that would ensue in consequence of a given variation of the carbonic acid
in the air’. He already considered in his calculations the amplification of the CO2-induced
warming due to the resulting changes in the atmospheric water vapour content, and in the
surface albedo due to changes in snow cover (Arrhenius, 1896). Without explicitly
naming them, Arrhenius thereby identified two major physical feedbacks in the climate
system. These feedbacks, along with a few others (clouds, the atmospheric lapse rate),
were further quantified more than 50 years later by modelling studies (e.g. Manabe and
Wetherald, 1967, Sellers, 1969, Cess, 1975).
Climate sensitivity is defined as the equilibrium temperature change DT2´CO2 for a change
in radiative forcing DF2´CO2 equivalent to a doubling of the CO2 concentration:
DT2´CO2 =
DF2´CO2
l
In this equation, l is called the feedback parameter. Without physical feedbacks, l is
approximately equal to 4 W m-2 K-1: this is BB, the ‘black body’ value of . BB is simply
the derivative of the Stefan-Boltzmann law, assuming the Earth to be an idealised black
body radiating energy to space. With this value of , doubling atmospheric CO2 would
translate into a warming of only ~ 1 K. Results from both observations and climate
models indicate that the actual value of l is much smaller, leading to a larger warming.
One of the earliest attempts to quantify the climate sensitivity was in the report of the US
National Academy of Sciences, chaired by Jules Charney (NRC, 1979). Based on the
GFDL and GISS general circulation models available at that time, they estimated a range
of 1.5 to 4 K, with the most likely value near 3 K. The most recent estimates are not
much different. The Fourth Assessment Report of the IPCC gave a likely range of 2 to
4.5 K with a best estimate of 3 K (IPCC, 2007). New observational constraints from
palaeoclimate data yield a similar range (1.4-3.5 K likely range, with a median estimate
of 2.4 K). Chapter 3 provides further background on the history and methods of
estimating climate sensitivity, explaining the use of palaeoclimate constraints.
To understand how the climate sensitivity arises as a consequence of different physical
feedbacks, it is necessary to analyse results of climate models that include these
feedbacks. The strength of a given physical feedback can be estimated by comparison of
the modelled climate response obtained with and without accounting for that feedback.
Using a three-dimensional general circulation model of the atmosphere, Hansen et al.
(1984) proposed a framework for the estimation and comparison of different feedbacks.
They borrowed a terminology from electronics and defined the net feedback factor f by
DTeq = f DT0
where DTeq is the equilibrium change in global mean surface temperature and DT0 is the
change in temperature in the absence of feedback (for example, if atmospheric water
vapour content is held constant). The gain of this feedback, g, is the relative change in
temperature:
DT - DT0
g = eq
DTeq
which can be rearranged in terms of the equilibrium change in temperature to give:
DT
DTeq = 0
1- g
It follows that feedback and gain are related, with f =1 (1- g) .
A positive value of g means a positive feedback. In such case the equilibrium
temperature, DTeq , will be higher than the one in the absence of feedback, DT0 . Negative
feedbacks have negative values of g, resulting in an lower temperature change relative to
the no-feedback situation. It can be shown that the gain for each physical feedback is the
ratio of its individual climate feedback parameter li to lBB :
DT0
DTeq =
l
1+ å i
lBB
This framework makes it possible to compare the different physical feedbacks and to
assess their respective contribution to the climate sensitivity and its uncertainty. This gain
theory continues to be applied in present day studies, e.g. in Bony et al., (2006), an
analysis that highlights the particularly large uncertainty arising from cloud feedbacks
(see chapter 2).
During recent decades, additional feedbacks involving biological and chemical
components of the Earth system have been identified and quantified, using a variety of
models ranging from “back of the envelope” calculations to full Earth System models
simulations as discussed in Chapter 5. The climate-carbon cycle feedback is of principal
importance. Climate change alters the efficiency of land and ocean in taking up the CO2
that is added to the atmosphere. Evidence from both observations and modelling attests
that overall this is a positive feedback: CO2 uptake is reduced by warming, leading to a
faster growth rate in atmospheric CO2 concentration and amplifying the warming.
Feedbacks of this type are by definition not included in the climate sensitivity, which is
framed in terms of a fixed relationship between temperature change and radiative forcing
change. The climate-carbon cycle feedback was quantified by Friedlingstein et al. (2003,
2006) in terms of a number of sensitivity coefficients. The gain of the climate-carbon
cycle feedback is not directly comparable to the gain of physical feedbacks, because the
climate-carbon cycle feedback directly affects the radiative forcing (F) and not just the
response (T) to a given F. Gregory et al. (2009) reconciled the two concepts in a
unified framework, allowing the comparison of physical and non-physical feedbacks
strengths and highlighting the importance of the various contributions to the carbon cycle
feedback (Figure 4.1).
The Gregory et al. (2009) framework has recently been applied to other biogeochemical
feedbacks (Arneth et al., 2010a). Both nitrous oxide (N2O) and methane (CH4) are potent
greenhouse gases with large terrestrial emissions, driven by biogenic processes, and there
is evidence that these emissions increase with temperature. N2O is released by soils as a
by-product of microbial re-mineralization of organic matter under suboxic conditions.
CH4 is produced by methanogenic decomposers under anoxic conditions. A climate
feedback due to increased N2O emissions from natural soils has been described based on
observations and modelling (Xu-Ri et al., submitted). A growing number of studies have
investigated the potential climate feedback through increased emissions of CH4 from
natural environments, mainly wetlands (Eliseev et al., 2008, Ringeval et al., 2011). Other
potential biogeochemical feedbacks have been identified, but to date these have not been
systematically quantified using comprehensive models. We consider some of these
poorly quantified biogeochemical feedbacks that are nonetheless candidates for being
significant in the context of climate change, including those involving marine N2O
emissions, CH4 emissions linked to clathrate destabilization, and pyrogenic emissions of
CO2, trace gases, black carbon and other aerosol precursors.
4.1.2 Is there observational evidence for Earth system feedbacks?
It is impossible in principle to measure the gain of any Earth system feedback directly.
The strength of a feedback is the ratio between the change of a quantity (e.g. global
temperature) accounting for the feedback and the change in the same quantity in the
theoretical case where the feedback is inactive. Only the first term can be measured in the
real world. What we can observe, however, is the strong correlation between observations
of different components of the Earth System, giving hindsight at least on the sign of the
feedbacks.
The ice-core record shows that polar temperatures (inferred from isotopic measurements
in the ice) covary with the atmospheric concentrations of the long-lived greenhouse gases
(CO2, CH4 and N2O), which are directly measured in the air bubbles entrapped in the ice.
Glacial periods have cold climates and low concentrations of these gases, interglacial
periods warm climates and higher concentrations. But this information does not allow us
directly to estimate the strength of the feedback. The recorded change in temperature is
not just due to the change in greenhouse gases, but also to changes in orbital parameters,
planetary albedo, etc. To quantify the feedbacks, therefore, we have to invoke our
knowledge of the radiative properties of greenhouse gases. This has been done for CO2,
using the observed changes in atmospheric CO2 both during the most recent glacialinterglacial cycle and the last millennium including the Little Ice Age.
One additional, highly promising approach to estimating the strength of Earth system
feedbacks involves examining the sensitivity to climate of a given process at a time scale
over which observations are available, and extrapolating this to longer time scales using
an ensemble of models. Hall and Qu (2006) were the first to apply this method. Using
both historical and future climate simulations from a series of models, they showed that
in the “model world” there is a close linear relationship between the present-day response
of snow cover to the seasonal cycle of temperature (i.e. snow melting that take place
during the warming in spring), and the longer-term response of snow cover to global
warming during the 21st century. Models with a large snow feedback tend to simulate
both a rapid spring snowmelt and a strong response of snow cover to warming.
Conversely, models with a small snow feedback tend to simulate both a slow spring
snowmelt and a weak response of snow cover to warming. With the help of this
relationship, Hall and Qu (2006) were able to use the observed seasonal cycle of snow
cover to constrain the future snow feedback. A similar approach was proposed by Cox
and Jones (2008) to estimate the climate-carbon feedback, using the observed interannual
variability of the carbon cycle.
4.2 Land-atmosphere biogeophysical feedbacks
The land surface affects the atmosphere through biogeophysical feedbacks involving
evapotranspiration and energy partitioning between latent and sensible heat fluxes;
surface albedo; and surface roughness length. We present these three pathways and their
consequences, and then consider their implications for the quantification of
biogeochemical feedbacks.
4.2.1 Impacts on evapotranspiration
Evapotranspiration from the land is a major component of the global water and energy
cycles (Seneviratne et al. 2010). Evapotranspiration returns as much as 60% of total land
precipitation to the atmosphere (Oki and Kanae 2006), and the associated latent heat flux
uses up more than half of the net radiation incoming on land surfaces (Trenberth et al.,
2009). Plant evapotranspiration and CO2 uptake are closely coupled through
photosynthesis, implying that changes in evapotranspiration have an impact on the carbon
cycle while changes in the carbon cycle have an impact on the water and energy cycles
(Section 4.2.4 below).
Figure 4.2 displays the (idealized) dependency of the evaporative fraction (latent heat
flux normalized by net radiation) on soil moisture, following the classical framework of
Budyko (1956). Under wet soil conditions (right side of Figure 4.2), actual
evapotranspiration is close to potential evaporation (evaporation from a wet surface) and
thus determined primarily by the available energy. Under those conditions, land surface
states and processes have limited control on land-atmosphere water exchanges, except
through impacts on albedo and roughness length (discussed in Sections 4.2.2 and 4.2.3.
below). Under dry soil conditions (left side of Figure 4.2), evapotranspiration is sensitive
to soil moisture but too small to affect atmospheric conditions and regional climate. Land
surface conditions therefore mainly influence evapotranspiration in transitional zones
between dry and wet climates (Koster et al. 2004, Guo et al. 2006, Seneviratne et al.
2010) where evaporation is substantial but limited by soil moisture. Within this regime,
small spatial or temporal variations in soil moisture content, land cover or vegetation
state lead to variations in surface energy fluxes.
Several studies have highlighted the importance of soil moisture control on
evapotranspiration in transitional climate regions. These include contributions to soil
moisture-precipitation feedbacks, whereby enhanced soil moisture content leads to either
enhanced or diminished precipitation depending on boundary-layer conditions (e.g.
Findell and Eltahir 2003, Ek and Holtslag 2004, Betts 2004; see Seneviratne et al. 2010
for an overview). Changes in evapotranspiration can also affect sensible heat flux and
thus air temperature (reduced or enhanced evaporative cooling), with effects of up to
several degrees, relevant to the occurrence of heat waves (e.g. Seneviratne et al. 2006a,
Fischer et al. 2007, Vautard et al. 2007, Jaeger and Seneviratne 2011). In the context of
climate change, it has been shown that modified land-atmosphere coupling characteristics
linked with shifts in the location of transitional climate regions can explain projected
changes in temperature variability and extremes (Seneviratne et al. 2006a). Large
differences between climate models are linked to differences in the representation of soil
moisture-evapotranspiration coupling in the models (Boé and Terray 2008).
Evidence for the role of soil moisture-evapotranspiration feedbacks in climate and
atmospheric processes is mostly based on modelling, starting with the landmark study by
Shukla and Mintz (1982) and expanded to the recent multi-model analyses within the
international project Global Land-Atmosphere Coupling Experiment (GLACE) and those
based on IPCC AR4 simulations (Koster et al. 2004; Guo et al. 2006; Seneviratne et al.
2006a,b; Dirmeyer et al. 2006b; Koster et al. 2010). Evidence directly based on
observations is still scarce, but correspondences have been shown between modellingidentified ‘hot spots’ of land-atmosphere coupling and related observational diagnostics
(Koster et al. 2003, Teuling et al. 2009, Seneviratne et al. 2010) and observed impacts
during heat waves have been documented using satellite data (Zaitchik et al. 2006).
Figure 4.3 displays diagnostics related to land-atmosphere coupling for the boreal
summer season (JJA) based on both modelling and observational investigations
(Seneviratne et al. 2010). The top two panels (adapted from Teuling et al. 2009) are
analyses of evapotranspiration regimes based on correlation measures applied to data
from the second phase of the Global Soil Wetness Project (GSWP-2, Figure 4.3a: yearly
correlation of evapotranspiration with radiation and precipitation) and to FLUXNET
measurements (Fig. 4.3b: daily correlations with radiation). The GSWP-2 data are a
multi-model product driven with observation-based meteorological forcing (Dirmeyer et
al. 2006a). The FLUXNET data are eddy-covariance flux measurements of landatmosphere exchanges made at more than 500 sites worldwide (Baldocchi et al. 2001).
The network sites are unevenly distributed with the greatest concentrations of sites in
North America and Europe. The middle panels of the figure (redrawn from Koster et al.
2004, 2006) display a measure of soil moisture-atmosphere coupling (∆Ω, as defined in
Koster et al. 2006) applied to simulations from the GLACE experiment and diagnosing
the strength of coupling between soil moisture and temperature or precipitation (Figure
4.3c and 4.3d respectively). The bottom two panels (Figure 4.3e and 4.3f) diagnose soil
moisture-evapotranspiration/temperature coupling in IPCC AR4 simulations (using the
GFDL, HadGEM1 and ECHAM5 models) for two time periods (1970-1989 and 20802099) from the correlation between evapotranspiration and temperature. Negative values
(red shadings) indicate strong coupling, positive values (blue shadings) indicate weak
coupling (Seneviratne et al. 2006a).
Coherent patterns can be identified across datasets in Figure 4.3. The GLACE hot spot of
soil moisture-temperature and soil moisture-precipitation coupling in the Great Plains is
also identified in the IPCC AR4 simulations, and corresponds to an area with a soil
moisture-limited evapotranspiration regime. The IPCC AR4 20th-century simulations and
the GSWP-2/FLUXNET analysis also all suggest a gradient of evapotranspiration
regimes across Europe, with the Mediterranean region identified as a hot spot of soil
moisture-atmosphere coupling (projected to expand to Central Europe in the 21st century,
Figure 4.3f). This regional hot spot in the Mediterranean region is not captured in the
GLACE simulations, possibly due to the use of prescribed sea surface temperature (SST)
conditions from a single year in the experiment (Seneviratne et al. 2006a). On the other
hand, the GSWP-2, GLACE and IPCC AR4 analyses agree on a soil moisture-limited
evapotranspiration regime on the Indian subcontinent.
These preliminary comparisons suggest some level of qualitative agreement between
observations and models regarding soil moisture-evapotranspiration interactions. But
quantitative assessments remain difficult both due to lack of observations and
methodological issues (Seneviratne et al. 2010). Climate models still vary greatly in the
way they represent land-atmosphere coupling (Koster et al. 2004, Dirmeyer et al. 2006,
Pitman et al. 2009). Future research will need to focus on a more systematic evaluation of
soil moisture-evapotranspiration coupling in current climate models, in particular with
the help of newly developed global evapotranspiration datasets (Jimenez et al. 2011,
Mueller et al. 2011).
4.2.2 Impacts on surface albedo
Land surface conditions affect the climate system through impacts on surface albedo and
resulting modifications of the radiation balance. Lower albedo leads to a warming of the
near-surface atmosphere due to enhanced shortwave radiation absorption, and the
converse also holds – higher albedo enhances reflection and results in a cooling. Albedo
can be altered by changes in snow cover, soil moisture content, vegetation cover, and
dust levels of dry regions.
The literature on snow-albedo feedback mechanisms is extensive (e.g. Groisman et al.
1994, Bony et al. 2006, Qu and Hall 2006, Brown and Mote 2009). Snow cover leads to a
cooling of the surface due to enhanced albedo, which in turn tends to maintain the snow
cover, resulting in a positive feedback between low surface temperatures and snow cover.
Forest cover in high-latitude regions significantly affects this positive feedback by
increasing surface albedo sharply during winter through snow masking of dark leaves
(e.g. Bonan et al. 1992, Betts and Ball 1997). Quantifying these effects using Earth
system models that include better representations of vegetation types is of relevance for
assessing the net radiative forcing of boreal forests on climate. Snow aging and
metamorphosis also influence snow albedo, with old (compacted, dirty) snowpacks being
less reflective. Snow also affects climate through other pathways, in particular associated
with snowmelt impacts on soil moisture content and land surface insulation by the
snowpack (e.g. Lawrence and Slater 2009).
Soil moisture has a three-fold impact on land surface albedo (Seneviratne et al. 2010)
acting through changes in bare soil albedo with soil moisture content (soil has a lower
albedo under wetter conditions), changes in vegetation albedo with soil moisture content
(yellowing of leaves occurring with soil drying), and changes in the exposed bare soil
fraction due to drought impacts on vegetation (which affect leaf orientation and leaf area
index). However, the effects of drought on surface albedo are probably smaller than
previously assumed, because of compensating changes in the visible and near-infrared
spectral bands (Teuling and Seneviratne, 2008).
4.2.3 Impacts on roughness length
Climate models include measures of surface roughness because frictional processes
influence the atmospheric circulation. Several modelling studies have shown how an
increase in surface roughness increases the convergence of the water vapour in the
overlying atmosphere, thereby promoting rainfall. The impact on the Amazon rainforest
has been studied extensively (see e.g. Pitman et al. 1993, Lean et al. 1996). The impact of
this feedback between the land and the atmosphere is local (Pitman et al. 2009) but it
strongly affects the viability of large forest systems such as the Amazon (Cowling et al.
2005).
Observational evidence of this biophysical feedback is hard to obtain. But a unique study
of the Les Landes forest in France by Blyth et al. (1994) demonstrated that the observed
patterns of cloud over the region were most likely caused by the presence of the forest,
with its high roughness. Another example is from Los et al. (2006), who analysed
satellite data of vegetation cover and rainfall and demonstrated that there is a likely
causal link between the presence of vegetation and rainfall, in addition to the more
obvious causal link between rainfall and vegetation growth.
4.2.4 Implications of biogeophysical feedbacks for biogeochemical feedbacks
The implications of biogeophysical feedbacks for biogeochemical feedbacks can be
considered from two main perspectives: first, the biosphere, where soil moisture impacts
on plant function and CO2 uptake; and secondly, the atmosphere, where feedbacks affect
radiative forcing, which needs to be evaluated against those changes produced by
biogeochemical feedbacks.
Soil moisture-evapotranspiration interactions have implications for carbon cycling, given
the tight coupling between water loss and CO2 uptake in plants. As an example, during
the extreme drought and heat wave summer of 2003 in Europe, it was found that plants
became less productive; the European continent became a source of CO2 in summer
(Ciais et al., 2005; Reichstein et al., 2007). The 2005 drought in the Amazon rainforest
also had major impacts on carbon uptake (Phillips et al., 2009). Uncertainties associated
with the simulation of soil moisture-plant relationships in current climate models are
likely to affect the representation of biogeochemical feedbacks in carbon-climate models.
Assessments of carbon-cycle feedbacks in these models suggest large variations in land
feedback characteristics (Friedlingstein et al. 2006).
Quantification of the radiative forcing associated with biogeophysical versus
biogeochemical feedbacks feedbacks has a bearing on assessments such as the total
climate impacts of afforestation (Claussen et al. 2001, Betts et al. 2000, Bonan 2008).
Regional modifications of surface temperature associated with land use and land cover
changes need to be taken into account beside the first-order radiative forcing that they
induce (Davin et al. 2007).
4.3. Carbon cycle feedbacks
The atmospheric content of carbon dioxide represents the balance between anthropogenic
emissions and natural sources and sinks. (See chapter 2 for a general description of the
carbon cycle, and the mechanisms of CO2 uptake.) From observations, we know that only
about 55 % of anthropogenic CO2 emissions is taken up by land and ocean processes,
,and that the remaining ‘airborne fraction’ is responsible for the atmospheric CO2
increase. One effect of this increase in atmospheric concentration is to set up a
concentration gradient across the air-sea interface, which causes a net uptake of CO2 into
the surface ocean. The CO2 is progressively exported to deeper layers through a
combination of biological and physical processes. Increasing atmospheric CO2 also
progressively stimulates land plant photosynthesis. Combined with the 20-30 year
residence time of land carbon pools, this effect provides a quantitative explanation for the
continuing net uptake of CO2 into vegetation and soils. The land and ocean together thus
exert a major negative feedback on the carbon cycle, absorbing together more than half of
the human-induced CO2 currently being emitted. But there are good reasons to expect
that this negative feedback is offset to some (increasing) extent by positive feedbacks due
to the effect of climate on CO2 uptake.
4.3.1 Climate-carbon cycle interaction
Many Earth System modelling studies have concluded that future climate change will
reduce the efficiency of land and ocean in removing CO2 from the atmosphere, leading to
more warming. But the size of this positive feedback has proved hard to pin down. The
two first studies, by Cox et al. (2000) and by Friedlingstein et al. (2001) and Dufresne et
al. (2003), agreed on the sign (positive) of this feedback, but Cox et al. (2000) estimated a
feedback three times larger than that of Friedlingstein et al. (2001) and Dufresne et al.
(2003). The Coupled Climate-Carbon Cycle Model Intercomparison Project (C4MIP),
engaging 11 Earth System models, confirmed the original finding of a positive feedback,
with both land and ocean showing a reduced uptake under a warming climate; but did
nothing to reduce the uncertainty in its magnitude (Friedlingstein et al., 2006).
The main processes leading to reduced uptake, at least, are common to the models.
Increased stratification of the surface ocean due to warming at the surface reduces the
export of carbon from the surface to the deep ocean, and hence limits the air-sea
exchange of CO2. Declining productivity in tropical forests and a general increase of the
rate of soil carbon decomposition (heterotrophic respiration) partially offset the CO2
fertilization effect. But the responses of temperate ecosystems, especially, are highly
variable among models.
The C4MIP analysis provided a first estimate of the magnitude of the uncertainty in both
the CO2 concentration-carbon cycle (negative) feedback and the climate-carbon cycle
(positive) feedback: revealing that the current uncertainty in these processes is larger than
the uncertainty in many physical feedbacks (Figure 4.1, two bottom bars) (Gregory et al.,
2009).
4.3.2 Carbon-nitrogen cycle interaction
Nitrogen (N) is a key limiting nutrient for most terrestrial ecosystems, in the sense that
addition of reactive forms of nitrogen produces an increase in vegetation growth
(Vitousek et al., 1991). Terrestrial models that include C-N cycle interactions (with fixed
vegetation distributions) have existed since the early 1990s (e.g. Melillo et al., 1992), but
none of the C4MIP models accounted for N. Several modelling groups have recently
enhanced dynamic global vegetation models by including an explicit, prognostic N cycle
coupled to the C cycle and vegetation dynamics (e.g. Thornton et al. 2007, Xu-Ri and
Prentice, 2008, Zaehle et al., 2010). According to these modelling studies, N limitation
on land has two main implications for the global-scale analysis of feedbacks. First, in
regions where N is limiting to plant growth, the CO2 fertilization effect is restricted
(compared to a reference case with no active N cycle), reducing the magnitude of the
negative feedback due to CO2 fertilization. Second, the effect of warming on soil organic
matter decomposition becomes more complex: CO2 is released from soil more quickly (a
positive feedback) but this process also accelerates soil nitrogen mineralization (the
conversion of N from organic to inorganic forms in the soil solution), hence increasing in
the amount of N available for plant growth (a negative feedback).
The introduction of C-N interactions into coupled models therefore alters the modelling
of the climate-carbon cycle feedback. All published C-N modelling studies agree that the
strength of the climate-carbon cycle feedback is reduced when accounting for N.
However, the amplitude of that reduction varies considerably. The most extreme model
(Thornton et al. 2007, 2009) produces an increase in growth larger than the increase in
decomposition, leading to an unrealistic net negative feedback. Other models, such as
Zaehle et al. (2010), have concluded that N dynamics merely reduces the strength of the
climate-carbon cycle feedback. These studies agreed that N constraints reduce CO2
fertilization, leaving more CO2 in the atmosphere. Hence an apparent contradiction:
although accounting for N in Earth system models reduces the strength of the climatecarbon cycle feedback, the overall effect of accounting for N is to enhance the simulated
atmospheric CO2 growth rate, and hence to accelerate global warming.
4.3.3 Observational constraints on climate-carbon cycle feedback
An emerging approach for assessing the realism of modelled climate-carbon cycle
feedbacks involves the examination of independent global observations of CO2 and
climate. The CO2 response to climate anomalies can provide constraints on the magnitude
of the global carbon cycle sensitivity to climate on several timescales (Friedlingstein and
Prentice 2010). The year-to-year variation in atmospheric CO2 growth rate is strongly
correlated with climate variability. In particular, El Niño years are associated with higher
than average CO2 growth rates. Many studies have analysed CO2 variability on El NiñoSouthern Oscillation (ENSO) time scales either using carbon cycle models or top-down
methods such as deconvolution or inversion of atmospheric CO2 and 13CO2 data. All
evidence points to terrestrial carbon release during El Niño events, with the global ocean
showing slightly enhanced uptake, but the terrestrial effect being dominant.
Interannual variations in the CO2 growth rate have been used to constrain global climatecarbon cycle models and to estimate the carbon cycle sensitivity to climate (Cox and
Jones 2008). Tropical lands are anomalously warm, and also anomalous sources of CO2,
during El Niño events, implying that the climate-land carbon cycle feedback is positive
on the ENSO time scale. In the “models world”, there is a strong positive correlation
between modelled atmospheric CO2 interannual variability and modelled future tropical
land sensitivity to climate change. Recent analysis using the observed atmospheric CO2
and tropical surface land temperature variability leads to an estimate of about 40PgC loss
per degree warming for tropical ecosystems, excluding negative feedbacks as suggested
by some recent modelling, but also ruling out extremely large positive feedbacks.
On longer time scales, ice-core records allow us to investigate coupled CO2 and climate
variability and potentially to infer some properties of the coupled climate-carbon cycle
system. The last millennium is of special interest as the CO2 record is at a high resolution,
and time series data for reconstructions of temperature are available. It includes periods
of change over the same time scales of concern (decadal to centennial) as the ones in play
over the coming century. For example, the low CO2 and temperature excursion
associated with the Little Ice Age (roughly 1600-1850 AD) should be a relevant
constraint. The larger CO2 and climate oscillations over glacial-interglacial cycles may be
less useful in the context of the 21st century climate-carbon cycle feedback, as the
processes and time-scales involved are different.
The main problems with using these data are the multiple sources (different methods for
temperature reconstruction and different sets of ice-core measurements for atmospheric
CO2), and the necessary choices in the treatment of the data, including the analysis
period, smoothing techniques, age calibration, consideration of time lags, and handling of
uncertainties. Several studies have nevertheless used the last millennium record to derive
the carbon cycle sensitivity to climate (Joos and Prentice, 2004, Scheffer et al., 2006, Cox
and Jones, 2008, Frank et al., 2010). Frank et al. (2010), the most comprehensive study so
far, concluded that the carbon cycle sensitivity to climate is positive with a median
amplitude of 7.7 ppm CO2 K-1 and a likely (broad) range of 1.7 to 21.4 ppm CO2 K-1.
Again, this analysis excludes both negative and very large positive feedback.
4.4. Nitrous oxide feedbacks
4.4.1 N2O production pathways
Nitrous oxide (N2O) is a potent greenhouse gas produced by nitrifying and denitrifying
bacteria during the degradation of organic matter in soils and water. The specific
formation pathway is determined by local environmental conditions, including oxygen
level, substrate availability, pH and temperature (Ritchie and Nicholas, 1972, Baggs and
Philippot, 2009 and references therein). In oxic environments nitrifying bacteria produce
N2O as a byproduct during the oxidation of ammonium to nitrate. Under anoxic and
suboxic conditions more complex pathways dominate, often with a much higher yield of
N2O. These include N2O production by denitrifiers (during the reduction of nitrate to
nitrite and N2); denitrification by nitrifiers (consisting of the oxidation of ammonium to
nitrite and subsequent reduction to N2O); and nitrate ammonification (reduction of nitrate
to nitrite and ammonium, during which some ammonifying bacteria can produce N2O).
Kelso et al. (1997), Codispoti and Christensen (1985) and Naqvi et al. (2010) have
reviewed the microbial pathways that produce N2O.
The largest sources of N2O are terrestrial (both natural and fertilized soils). N2O
production pathways in soils are highly sensitive to local environmental conditions, and
can alternate between low-yield nitrification pathways and higher-yield pathways,
primarily driven by changes in soil oxygen levels. The largest terrestrial sources of N2O
are in the wet and seasonal tropics. In aquatic environments N2O is produced during the
biologically mediated remineralisation of organic matter. Current understanding is that
the dominant source pathway in the oxygenated open ocean is via nitrification while
combinations of higher-yield pathways dominate in suboxic marine environments,
including coastal and estuarine zones and sediments (Bange, 2008).
4.4.2 Impact of climate change on N2O production
Feedbacks between terrestrial N2O fluxes and climate can arise due to climate-driven
variations in soil temperature and soil water content, both of which are important controls
on the global pattern of soil N2O emission. Both nitrification and denitrification respond
positively to temperature over most of the physiological range. There is experimental
evidence for the enhancement of soil N2O emissions in response to temperature, soil
moisture and CO2 (e.g. Barnard et al., 2005, Ketunnen et al., 2006, Kamman et al., 2008,
Cantarel et al., 2011, van Groenigen et al., 2011) although some experiments have
reported a decline in N2O emission with increased CO2 due to increased plant demand for
N, which can deplete the soil pools of inorganic N from which N2O is derived.
Experiments on permafrost soils indicate a potential positive feedback mechanism due to
increased N2O emissions leaking from the thawing soil (Repo et al., 2009). There are
possible counteracting effects, such as greater denitrification to N2 instead of N2O if soil
moisture increases (e.g. Kesik et al. 2006). A global modelling study of N2O terrestrial
emissions during the 20th century has suggested a dominant control by temperature, and a
positive sensitivity of ~ 1 Tg N yr-1 K-1 (Xu-Ri et al. submitted).
Climatic influences on marine N2O formation include changes in the extent of suboxic
zones, and changes in local temperature. The potential expansion of oceanic suboxic
regions under climate warming (Stramma et al. 2008) could result in large increases in
N2O yield (Bange et al. 2005; Codispoti 2010). In situ temperature is also reported as a
possible influence on marine N2O production (Butler et al. 1989); this would imply
potentially higher N2O yields. The magnitude of the oceanic N2O source is also
determined by the rate of organic matter production and sub-surface remineralization.
Factors influencing these flows include changes in vertical mixing and upper ocean
stratification (Bopp et al. 2002, Behrenfeld et al. 2006), and changes in nutrient supply
via surface deposition and river outflow (e.g., iron and reactive inorganic nitrogen,
Jickells et al. 2005; Duce et al. 2008). Bopp et al. (2002) note possible reductions in
marine export production under scenarios of future climate warming and increased ocean
stratification; this could imply reduction in marine N2O production. But such warming
scenarios would be accompanied by increased ocean deoxygenation, and increased N2O
formation in suboxic waters. These separate effects could result in feedbacks of opposite
sign. The overall impact of warming on ocean N2O fluxes remains to be determined.
4.4.3 Observational constraints on climate-N2O feedback
The recent atmospheric measurements and the ice-core records of N2O concentration are
underexploited resources for the analysis of climate feedbacks involving N2O. Xu-Ri et
al. (submitted) showed a relationship between global mean land temperature variations
and the reconstructed growth rates of N2O concentration in the atmosphere during the
20th century, of similar magnitude to variations in modelled N2O emissions from soils.
They also pointed out that the so-called anthropogenic source in fertilized soils is likely
to respond to climate in a similar way to natural soils, amplifying the feedback: a
possibility that has so far been neglected in global N2O budgets.
4.5. Methane feedbacks
4.5.1 CH4 production pathways
Wetlands are environments where high water levels inhibit the diffusion of oxygen into
the soil and produce anoxia. Anoxia slows down soil organic matter decomposition, and
allows a reduced compound such as CH4 to be the terminal electron acceptor in the
decomposition chain. CH4 production (methanogenesis) is carried out mainly by archaea
which use either plant-derived acetate, or CO2 and H2, as substrates for growth. Natural
wetlands cover about 7% of the global land surface area (Lehner and Döll, 2004) and are
the largest source of CH4 in the atmosphere (Hein et al. 1997). The emission of methane
from wetlands represents a balance between production by methanogens, and
consumption by methane-oxidizing (methanotrophic) bacteria in the drier top layers
(Frenzel 2000).
4.5.2 The carbon balance and CH4 emissions of wetlands
The CH4 concentration in the atmosphere and the climate are linked in a feedback loop
(Figure 4.4): climate is the main control on wetland CH4 emissions while a change in
CH4 concentration derived from those emissions must affect the climate, because CH4 is
a potent greenhouse gas. The climate-carbon cycle feedback loop is connected to the CH4
feedback loop, via the effect of CO2 on climate, and the direct CO2 fertilisation effect on
primary productivity in wetlands which increases the substrate available for CH4
production (Ringeval et al. 2011). Some so-called anthropogenic sources are also affected
by climate and atmospheric CO2 concentration: for example, rice paddies react to climate
and CO2 concentration changes in a similar way to natural wetlands.
Northern wetlands have been extensively studied in terms of their involvement in the
carbon cycle. Because of their low decomposition rates, roughly 20% of the world’s soil
carbon is stored in boreal and subarctic peatlands where it has been accumulating
gradually under wet and cool climatic conditions since the end of the last ice age (Maltby
and Immirzi 1993). Northern peatlands are a small sink for CO2 but an important source
of CH4 and dissolved and particulate organic carbon which will eventually be oxidized to
CO2 (Billett et al. 2004; Blodau et al. 2004). Global warming, drainage and peat fires
may be causing a decline of carbon accumulation in peatlands, and converting these
ecosystems into a net source of carbon (Gorham 1991; Billett et al. 2004). It has been
predicted that increased summer droughts in mid-continental regions and increases in
precipitation in oceanic regions will further reduce carbon uptake by peatlands (Belyea
and Malmer 2004).
The carbon budget of tropical wetlands is not as well understood as that of northern
wetlands. Tropical wetlands are notable because their high productivity has led to the
accumulation of deep peat deposits in certain tropical areas of Amazonia and Southeast
Asia (Page et al. 2002; Lähteenoja et al. 2009). Amazonian peatlands have been
estimated to cover 150,000 km2. This area, combined with the great depth of peat
deposits and high accumulation rates found by Lähteenoja et al. (2009), make these
peatlands a potentially significant component of the global carbon budget.
4.5.3 Impacts of climate on CH4 emissions
CH4 emissions from wetlands depend on temperature, water table, pH, plant cover and
type, time of year (i.e. the supply of metabolites), nutrient status, depth of the oxic zone
and the presence of competing microorganisms. The rate of methanogenic activity
increases with temperature. Some models predict an increase of methane emissions for a
small temperature rise due to changes in net ecosystem production (NEP) and/or
increased methanogenesis (e.g. Cao et al. 1998). On the other hand, a decrease of this
flux is predicted for an increase of temperature of more than 4C because of soil moisture
depletion and a subsequent decrease in net ecosystem production (Cao et al. 1998). It has
been suggested that CH4 and CO2 emissions are only controlled directly by temperature
in wetter sites, while in drier sites water table position is more important (Christensen
1993; Bubier et al. 2003). In general, lowering the water table increases the rate of CO2
emission and decreases the emission rate of CH4 in the long term (Crill et al. 1991;
Priemé 1994; Daulat & Clymo 1998; Blodau et al. 2004; Strack & Waddington 2007).
4.5.4 Climatic controls on wetland distribution
Climate affects not just the rates of carbon mineralization and methane fluxes in
wetlands, but also the extent and distribution of these ecosystems. Wetland ecosystems
exist within well-defined climatic boundaries (Wieder & Vitt 2006) and are highly
susceptible to changes in climate because they rely on the maintenance of high water
tables. Models of peatland distribution for Canada suggest a retreat northwards of the
southern boundary of peatlands under future warming (Gignac et al. 1998). The climate
envelope of British blanket bogs has been predicted to retreat towards the north and west
when subjected to warming scenarios (Clark et al. 2010; Gallego-Sala et al. 2010).
Globally, blanket bogs may be under threat from a changing climate (Gallego-Sala et al.
2009). Palsa mires, which are marginal permafrost features and highly vulnerable to
changes in the climate, are predicted to be degrading due to warming (Luoto et al. 2004).
Temperature changes are the most important driver in all of these model projections. The
importance of temperature as a factor controlling the destabilization of peatlands has been
pointed out in many studies (Freeman et al. 2001). Paludification, the positive feedback
between accumulation of soil organic carbon and water table rise due to the high water
holding capacity of peat and its low hydraulic conductivity, has been suggested to
increase the sensitivity of peat decomposition to temperature when the climate is
unfavourable to peat accumulation by accelerating carbon loss (Ise et al. 2008).
The large area of peatlands currently under permafrost is especially vulnerable to climate
change (Denman et al. 2007). Thawing of the permafrost in arctic and boreal peatlands
may stimulate net primary productivity and hence the high-latitude carbon sink.
However, permafrost thawing may also lead to an increase in soil organic matter
decomposition. Increased CH4 production may counteract any potential increase in the
carbon sink, in terms of radiative forcing (Turetsky et al. 2007). A recent study, based on
a terrestrial ecosystem model accounting for both permafrost and wetlands dynamics,
predicts that, by the end of the 21st century, the land north of 60° latitude could shift from
a CO2 sink to a source, once high latitude processes such as soil insulation, cryoturbation
and microbial heat release are accounted for. In addition, CH4 emissions from these
regions are predicted to experience up to a two-fold increase, but this rise will be partly
compensated by a reduction in wetland extent (Koven et al. 2011).
Several dynamic vegetation and/or land surface models have added a module for wetland
extent and/or a methane emission (Gedney et al. 2004; Shindell et al. 2004; Wania et al.
2009a; Wania et al. 2009b; Ringeval et al. 2010; Ringeval et al. 2011). These models
differ in the way they calculate wetland extent (dynamic or fixed), whether they consider
human-managed rice paddies, and whether permafrost dynamics are modelled (Table
4.1). However, all models agree in predicting an increase in the methane fluxes from
wetlands by the end of the 21st century. The only one of these models currently able to
calculate global fluxes of CO2 due to losses of carbon from peatlands is LPJ WHy-Me
(Wania et al. 2009b).
Table 4.1 A brief summary of methane models and their main features.
Model name
HadleyMOSESLSH
ORCHIDEE
ORCHIDEE
GISS
LPJWHyMe
IAP RAS CM
Dynamic
Wetland
Rice
paddies












Permafrost






Methane
increase
x2
X1.5
x2
x1.8
x2.5
x1.5
Carbon
accum/loss






Climate
Scenario
IS92a
SRES A2
SRES A2
2xCO2
SRES A2
All IPCC
Reference
(Gedney et al. 2004)
(Ringeval et al. 2011)
(Koven et al. 2011)
(Shindell et al. 2004)
(Wania et al. 2009b)
(Eliseev et al. 2008)
4.5.6 Climate change and methane release from clathrates
Contemporary warming is greatest in the northern high latitudes and this high rate of
warming is expected to continue, so these regions are liable to produce the strongest
changes in natural emissions of CH4. Shakhova et al. (2008) estimated that there is a vast
store of carbon in the form of methane and methane bound in clathrates lattices on land
and at the bottom of the oceans. Much of this is locked up in frozen states. The warming
of the Arctic region potentially threatens this carbon store. The question of how much of
this store can or will be released in a future warmer arctic region in the next 100 years is
key to any feedback estimates. Sokolov et al. (2009) predicted potential increases of
naturally produced CH4 in the case of a very large (12 K) temperature rise. The increased
methane release in that case was only 40 Tg CH4 yr-1; a small fraction of the
anthropogenic source (currently 400 Tg CH4 yr-1), and contributing an additional
temperature increase of only 0.5 K. Archer and Buffett (2005) estimated the total store of
deep-ocean methane clathrates to be ~ 5000 Pg C and used a model to indicate that a
large total anthropogenic CO2 release could produce a large (thousands of Pg C)
additional release from these ocean clathrates, but only on a time scale of millennia or
longer. Archer (2007) concluded that ‘on the timescale of the coming century … most of
the hydrate reservoir will be insulated from anthropogenic climate change. The
exceptions are hydrate in permafrost soils, especially those coastal areas, and in shallow
ocean sediments where methane gas is focused by subsurface migration. The most likely
response of these deposits to anthropogenic climate change is an increased background
rate of chronic methane release, rather than an abrupt release’.
4.5.7 Impact of biogenic volatile organic compound emissions on methane life
time and atmospheric chemistry
A number of biogenic volatile organic compounds (BVOC) are emitted from terrestrial
sources and, although not direct greenhouse gases, are capable of impacting the lifetime
of methane in the atmosphere through indirect mechanisms, mainly via reaction with the
hydroxyl radical, i.e. consuming the main methane sink and prolonging its life time
(Folberth et al. 2006). BVOCs can also impact atmospheric chemistry in other ways,
producing or destroying tropospheric ozone (depending on the NOx concentration) and
promoting the production of secondary aerosols. Altogether, BVOCs represent a large
emission source of carbon annually, around a 1000 Tg C yr-1. Plants emit BVOCs as
defense against high temperature, toxic compounds or signal molecules. Amongst all
BVOCs, isoprene is the most important in terms of annual flux (400 to 600 Tg C).
Models of isoprene and other BVOCs (mainly monoterpenes) are used to upscale
observed measurements of leaf fluxes to whole ecosystems (Arneth et al. 2010b) but the
process of production and release of isoprene is complex and our knowledge is still
limited, and so there remains a large uncertainty in the effect of a change in climate on
the emissions of isoprenes and other BVOCs (Arneth et al. 2008; Peñuelas & Staudt
2010).
Different plant species produce and emit isoprene at greatly differing rates. In addition,
isoprene emissions have been observed to increase hyperbolically with radiation influx
and exponentially with temperature. There is also a CO2-isoprene interaction, with
emissions increasing at low CO2 concentrations and decreases at high CO2 concentration,
but this may be offset by the fertilization effect of CO2 on plant productivity (Arneth et
al. 2008). Most models of isoprene predict an increase of emissions in the future due to
the temperature effect, the CO2 fertilization on photosynthetic activity and land use
changes (Guenther 2007; Andersson & Engardt 2010; Arneth et al. 2010b) but others
predict either no significant change (Arneth et al. 2007) or a small reduction in emissions
(Heald et al. 2009) due to the impact of higher CO2 on the production of isoprene.
4.6. Fire feedbacks
Fire is a major influence on vegetation dynamics and community structure, an element of
the global carbon cycle, and a mechanism by which CO, CH4, NOx and other trace gases
and aerosol precursors are emitted to the atmosphere (Carslaw et al. 2010, van der Werf
et al. 2010, Spessa et al. 2010a, Bowman et al. 2009). Fires modify surface characteristics
such as surface roughness, albedo or soil integrity via changes in vegetation cover, with
consequences for the land-atmosphere exchanges already discussed in Section 4.2.
Smoke-borne aerosols from fires can disrupt radiative processes in the troposphere, as
well as normal hydrological processes, potentially reducing rainfall (Andreae 2007,
Carslaw et al. 2010). Fire is thus a cross-cutting process in the Earth system with the
ability to perturb the Earth’s vegetation, its carbon, hydrological and energy cycles, and
to generate land-atmosphere feedbacks (Figure 4.6). General principles and recent
developments in observing and understanding fire regimes are summarized in Chapter 2.
4.6.1 Climate change and land use impacts on fire
Fire is extremely sensitive to climate. Analysis of remotely sensed burnt area data in
relation to climate has shown that globally, burnt area increases with temperature across
the full range of temperatures, whereas the relationship between burnt area and
precipitation is unimodal due to limitation by fuel in dry climates and fuel moisture in
wet climates (Prentice et al., 2011; Daniau et al., submitted). The dependence of fire on
temperature and fuel availability has been confirmed in several regional studies. For
example, Westerling et al. (2006) showed that fire activity in the western USA increased
markedly in the mid-1980s, along with increased spring and summer temperatures, and
earlier spring snowmelt which produces an earlier, prolonged dry season, more
opportunities for ignition and reduced fuel moisture. Spessa et al. (2005) and Harris et at
(2008) showed a strong positive relationship between antecedent rainfall and area burnt
in the tropical savannas of northern Australia, due to the positive effect of rainfall on
biomass.
Many studies have linked fire activity to specific atmospheric circulation patterns,
including an increase in fire activity during La Niña events in the southern United States
and Central America (Kitzberger et al. 2001, van der Werf et al. 2010; Le Page et al.
2008), and a marked increase in fire activity in tropical rainforests during El Niño events
(van der Werf et al. 2008, 2010; Le Page et al. 2008). Anomalous drought and severe fire
episodes in the Iberian Peninsula have been linked to the negative phase of the North
Atlantic Oscillation (NAO) (Trigo et al. 2006), while Balzter et al. (2005) reported a
close correlation between the Arctic Oscillation (closely related to the NAO) and fire in
the Russian boreal forests. However, all of these linkages are indirect; they come about
through the effects of local climate on fuel supply and burning conditions.
In moisture-limited (high biomass) fire regimes, land-use practices can exacerbate the
impacts of drought on fire. For example, active fire suppression is thought to have led to
fewer but much more intense fires during la Niña periods in the USA (Pyne et al. 1996;
Rodgers et al. 2011) and El Niño periods in southeastern Australia (Gill & Catling 2002).
Logging in closed forests produces more residual biomass, and can lead to more severe
dry conditions because the canopy is fragmented and the surface exposed to increased
radiation. The increased dry fuel increases the risk of fire, which can then further open up
the canopy. Furthermore, logging and fires in areas not usually subject to fires can create
conditions that encourage future fires because of incursions by pyrophytic grass and
woody shrubs which, in turn, lead to higher fine fuel loads and drier fuels surrounding the
forest (Siegert et al. 2001; Cochrane 2003; Page et al. 2002, 2010; Spessa et al. 2010a).
Peat drainage for agricultural production can worsen impact of El Niño drought on peat
moisture, thereby increasing the vulnerability of peats to fire (Page et al. 2002, 2010,
Ballhorn et al. 2009).
Studies of future climate impacts on future wildfires have traditionally focused on using
outputs from GCMs (and some cases regional climate models as well), forced with one or
more future plausible emission scenarios (Flannigan et al. 2009). These climate model
outputs are typically then used to examine changes in fire weather related climate
variables such as temperature, rainfall and relative humidity; or various climate fields
combined to form a fire danger index (FDI), such as those typically used by forest
operations managers (e.g. the Canadian Fire drive fire danger index: Stocks et al. 1989).
Flannigan et al. (2009) lists over 20 such studies in the last decade that have shown a
general increase in fire risk under a future climate regime. Despite the consensus, studies
relying on fire danger index alone inform us only about the climatic risk of fire, and
disregard the effects of vegetation properties and land use.
4.6.2 Modelling fire-vegetation interaction
Modelling fire, burnt area and emissions of trace gases under climate change requires
process-based modelling approach (Denman et al. 2007; Spessa et al. 2010a). Prognostic
fire models, embedded in process-based vegetation models, have been developed to
simulate the effects of changes in climate and vegetation dynamics on fire activity and
emissions (Lenihan and Neilson 1998; Thonicke, et al. 2001; Venevsky, et al. 2002,
Lehsten et al. 2009; Arora and Boer 2006, Thonicke et al. 2010, Kloster et al. 2010,
Spessa & Fisher 2010, Prentice et al. 2011). Only a handful of studies have used GCM
outputs to drive coupled vegetation-fire models to investigate fire activity under future
climate change. All have reported a general increase in burnt area (i.e. not just fire risk)
under various 21st century scenarios (Scholze et al. 2006, Arora & Boer et al. 2006,
Lenihan et al. 2008, Rodgers et al. 2011, Thonicke and Cramer 2006). The caveat
remains that future fire regimes are not entirely predictable from climate; changes in fire
regimes will depend on land-use decisions as well as on climate change (Archibald et al.
2009, Aldersley et al. 2011).
4.6.3 Fire effects on the emissions on CO2 and other trace gases
The most recent global assessment reports that wildfires (excluding peat fires) emit about
2 to 4 Pg C yr-1 to the atmosphere, with about half of this amount due to savanna fires in
Africa (van der Werf et al. 2010). Mediterranean forests and shrublands, tropical forests
and boreal forests, where fuel loads (and therefore emissions) per unit area are much
greater than in grasslands, are also globally significant for the carbon cycle even though
the annual areas burnt are much smaller (Figure 4.7).
CO2 emissions from these fires are expected to be balanced over decadal time scales by
the additional CO2 uptake due to regenerating vegetation. But many other trace
atmospheric constituents are released by fires (Andreae & Merlet 2001) including
greenhouse gases and other reactive gases that influence the greenhouse effect through
atmospheric chemistry. The emissions of these substances from fire amount to additional
climate forcing agents, which are not balanced by uptake in the same way as for CO2.
CO2 released from deforestation and peatland fires, especially in South America and
Southeast Asia, also counts as an additional, unbalanced emission. These fires are
thought to emit about 0.6 Pg C yr-1 (Schultz et al. 2008; van der Werf et al. 2010) albeit
with large uncertainty.
Fire is estimated to have been responsible for ~ 1 Pg C yr-1 of the interannual variability
of total land-atmosphere CO2 fluxes during the past decade (van der Werf et al. 2010).
Short-term increases of 10% or more in the atmospheric growth rates of CO, NOx and
CH4 are associated with El Niño-induced droughts in the tropics (van der A et al. 2008,
Yurganov et al. 2008, van der Werf et al. 2010, Patra et al. 2011). The influence of both
wetlands and peatland fires on large global CH4 pulses in 1997-1998, 2002-2003 and
2006 (all three coinciding with El Niño periods) has been confirmed by inverse
modelling based on atmospheric CH4 measurements (Bousquet et al. 2011).
Peat has a high carbon content, and can burn when dry. Once ignited by the presence of a
heat source (e.g. a wildfire penetrating the subsurface), it smoulders. Smouldering peat
fires can burn undetected for weeks to months (Page et al. 2010). Over the past 15 years
or so, the burning of Indonesian peatlands and forests has contributed to detectable
increases in global CO2, CO, CH4 and aerosol concentrations in the atmosphere (Page et
al. 2002, 2010; van der Werf et al. 2008, 2010; Spessa et al. 2010b). During the El Niñoinduced drought of 1997-1998, the most severe in recorded history, peat and forest fires
in Indonesia released at least 1 Pg C (Page et al. 2002). High-resolution analysis of fires
in Borneo during 1997-2007 (Spessa et al. 2010b) has shown positive correlations
between fire frequency and negative rainfall anomalies. The strength of these correlations
increases with declining forest cover, implying a nexus between fire, climate and
deforestation or forest degradation. Equatorial Southeast Asia is projected to generally
experience reduced rainfall in the next decades implying that the risks of drought-related
fire will increase (Li et al. 2007; Dai 2011). At the same time, economic demands for
establishing pulp paper and palm oil plantations to replace native rainforests, especially
on peatlands where tenure conflicts among land-owners tend to be minimal, are expected
to increase as well (Hooijer et al. 2006; Page et al. 2010).
Climate change projections show increased melting of permafrost and drought frequency
in the northern high latitudes (Solomon et al. 2007). This suggests that peat fires in the
boreal zone will also become more common, and the emissions from such fires will
increase (Flannigan & de Groot 2009).
4.6.4 Fire effects on aerosols and radiative forcing
Aerosols emitted from biomass burning are particularly important for land-climate
interactions because of their light-absorbing properties, which can affect absorption of
radiation in the atmosphere and at the Earth’s surface, especially when the particles are
deposited on snow and ice (principally in the form of black carbon); and because of their
solar scattering properties (principally due to organic carbon), which can lead to cooling
at the surface (Andreae 2007, Carslaw et al. 2010). However, estimates of aerosol
production from fires vary widely (Schulz et al. 2008; van der Werf et al. 2010). And in
contrast to trace gas emissions, there is no agreement on the sign of the direct radiative
forcing due to aerosols. Equivocal outcomes arise from several factors. Aerosols from
wildfires contribute to either warming or cooling of the climate depending on the season
or region or from which ecosystems they are emitted. Also, model structure and
complexity vary between different studies (Carslaw et al. 2010). For example, Tosca et
al. (2010), showed that biomass burning aerosols from regional peat and forest fires can
increase the intensity of future droughts in SE Asia during El Niño events via an increase
in tropospheric heating from black carbon, which leads to a reduction in short-wave
radiation forcing at the surface, which in turn, leads to a further reduction in rainfall via
increased surface cooling and hence reduced convection. On the other hand, positive
forcing has been predicted to occur in areas with a very high surface reflectance such as
desert regions in North Africa, and the snow fields of the Himalayas, mainly due to
increased absorption of radiation by black carbon (Forster et al. 2007).
Aerosols have additional indirect effects on climate and ecosystems. Aerosol loading can
increase photosynthesis (up to a point) by increasing the diffuse solar radiation fraction
(Chapter 3, Mercado et al. 2007). On the other hand, aerosols can disrupt droplet
formation in clouds and thereby reduce rainfall (Andreae 2007).
4.6.5 Fire effects on surface albedo
Surface albedo varies between ecosystems and interacts with climate and vegetation
(Bonan 2008). Fire regimes can influence climate through changes in surface albedo
because of the interplay between how much vegetation cover is removed, how much
black ash is produced, sensible and latent heat fluxes from fire scars, and available
atmospheric water vapour.
In the Australian tropics, for example, the concentration of water vapour abruptly
increases at the end of the dry season until increasing cloud cover results in the
predominance of diffuse solar radiation. At this time recently burnt areas (relatively dark
in colour with low albedo) can induce precipitation because of intense heating and deep
moist convection. The development of such convective storms does not occur in the early
dry season, because of lower levels of humidity and solar radiation (Bowman et al. 2007).
Throughout the North American boreal region, forest stands are being continuously
renewed by stand-replacing fires. Fires of such severity not only kill the above-ground
vegetation but also blacken the surface, influencing the albedo and the partitioning of
sensible versus latent heat flux. An analysis of a single boreal forest fire by Randerson et
al. (2006) showed a net effect of decreased radiative forcing, mainly from sustained
multi-decadal increases in the surface albedo. Amiro et al. (2006) and Goetz et al. (2007)
indicated that fire-induced lowering of albedo in these fire-prone regions has a net
cooling effect. Randerson et al. (2006) concluded that this albedo effect exceeded the
warming effects from fire-emitted greenhouse gases. The generality of this conclusion is
unclear.
Another albedo-related effect on climate arises when black carbon particles are deposited
on regions with summer snow cover at high latitudes or elevations (Carslaw et al. 2010).
The global radiative forcing for black carbon is certainly positive, but its magnitude
remains an open question.
4.7 Human feedbacks
Most of the physical and biogeochemical feedbacks discussed above are likely to be
modified by human activities, especially land use, even though these interactions have
not been studied much yet in a systematic way. While observations of the global
environment certainly enable the intricate interactions of humans and the natural
environment to be detected, monitored and even mapped (Ellis and Ramankutty, 2008)
existing modelling efforts still struggle with representations of the complexity of human
feedbacks.
To start with an example that has already been explored in detail from a biogeophysical
angle, the incidence and distribution of fire is as sensitive to human decisions about land
use as it is to climatic controls (Marlon et al., 2008). Land use has the potential either to
amplify or reduce feedbacks due to changes in the land-atmosphere fluxes of CO2, N2O
and CH4. A richer understanding of the natural processes involved in these feedbacks
potentially enables human society to target its efforts and activities in ways that optimise
the climate benefits. Possibilities include modifying the location of fire and of fire
suppression, and the balance between intense and smouldering events, and the
(re-)sequestration of CO2 into geological formations.
We also need to consider another class of feedbacks whereby as individuals, communities
and a global population, people’s reactions to changes in the climate can have
consequences for greenhouse gas emissions. We confine attention to processes identified
as feedbacks; the wider societal context of climate change mitigation and adaptation is
considered in Chapter 1, and some policy implications in Chapter 8. Table 4.2 below
summarises some examples of potential positive and negative human feedbacks on the
climate system, and we discuss them in the following.
Deliberate, planned measures to reduce energy use in response to society’s perception of
climate change as a key political and economic issue could result in a small negative
feedback to climate change. However, Lenton and Watson (2011) have argued that a
high-energy, high-recycling society is not incompatible with a stable climate. Shifts to
sustainable energy production and enhanced recycling could have greater effects than
absolute energy use itself. Another trigger of a negative feedback is that milder winters
in cold climates could reduce use of space heating, currently a major use of fossil-fuel
energy in developed countries.
Table 4.2. Examples of positive and negative human feedbacks; how changes in climate
can cause people to increase or decrease greenhouse gas emissions
Meteorological
measure of
climate change
Higher
temperatures
Impact on human
wellbeing
Human response
hermal discomfort in
buildings
Drier climate in
populated coastal
regions
Increased water
shortage
Increased use of
air-conditioning
for cooling
buildings
Desalination
Milder winter
climates
Improved thermal
comfort in buildings
Reduced use of
heating and
firewood
Milder spring,
autumn.
Extended period of
thermal comfort in
peripheral domestic
areas (e.g.,
conservatory
extensions, especially
with double glazing)
Add space
heating to
conservatories for
all year use
Greenhouse
gas emissions
implication
Higher energy
consumption
Higher energy
consumption
Reduced
energy
consumption
and
deforestation
Higher energy
consumption
Feedback
Raised
emissions;
positive
feedback
Raised
emissions;
positive
feedback
Reduced
emissions:
negative
feedback
Raised
emissions;
positive
feedback
Of more concern is the risk of ‘maladaptation’ (Barnett and O’Neill, 2010), where
rational responses to an immediate climate-related pressure aggravates the climate
problem. In some coastal regions where the climate is becoming drier, energy-intensive
desalination technology has been adopted to partially make up for freshwater shortage.
The net effect on climate of this response to a pressing societal need depends on the
energy source: if increased greenhouse gas emissions result from the energy generated to
desalinate seawater, then a positive feedback ensues. The increased use of airconditioning in hot years, using greater amounts of greenhouse-gas producing fossil
fuels, is another example of a positive feedback. These two examples are direct
feedbacks, but human behaviour changes also have unintended consequences. For
example, a policy decision to encouraging householders to use double-glazing for
windows in cold climates should in principle reduce household energy consumption and
hence greenhouse gas emissions. However, using double glazing in conservatory
extensions has encouraged space heating in conservatories, which if single-glazed would
be very difficult to heat to a comfortable temperature on a cold winter’s day.
The examples shown in Table 4.2 are simple and involve local actions. However, the
linkages between humans and the Earth system occur at multiple temporal, spatial and
socio-political scales (Turner et al., 2003; Adger et al., 2009; van der Leeuw et al., 2011)
through trade, migration, international policy and information exchange.
It is virtually impossible to assess the current sign and magnitude of human feedbacks
because of the overwhelming impacts of non-climatic factors, such as poverty and access
to natural resources. The IPCC Fourth Assessment Report considered non-climatic
factors from the perspective of human vulnerability to climate change. It concluded that
decision-makers considering actions for economic development have available to them
emissions-increasing or emissions-reducing options, and that it is difficult to determine
the impact of development generally on emissions (Yohe et al. 2007). It also concludes
that climate change will make it harder to achieve the Millenium Development Goals
which
embody
basic
human
rights
for
all
people
on
Earth
(www.un.org/millenniumgoals/bkgd.shtml). In the face of the known current economic
uncertainties and development challenges, it is hard to imagine that our lack of
quantitative understanding of human feedbacks constitutes a significant obstacle either to
sustainable development in general, or to climate change mitigation in particular.
However, a better grasp on human feedbacks is important. Human feedbacks can act
against climate mitigation policies. Acknowledging the existence of human feedbacks
may inform individual decisions, raise awareness of climate change mitigation, and
improve larger-scale decision-making by taking account of unplanned changes in
behaviour.
Figures and table captions
Table 4.1. Examples of positive and negative human feedbacks; how meteorological
changes can causes humans to increase or decrease greenhouse gas emissions.
Figure 4.1. Comparison of physical feedbacks (black body response, cloud, surface
albedo, water vapour and lapse rate, and ocean heat uptake) with carbon cycle feedbacks
(the carbon cycle response to change in atmospheric CO2 and in change in climate).
From Gregory et al. (2009).
Figure 4.2. Soil moisture and evapotranspiration regimes following the classical
framework of Budyko (1956). EF refers to the evaporative fraction (latent heat flu
xdivided by available net radiation). EFmax is its maxium value, qCRIT the critical soil
moisture content under which evapotranspiration becomes soil-moisture limited, and
qWILT the plant wilting point. From Seneviratne et al. (2010).
Figure 4.3. Diagnostics of land-atmosphere coupling, applied to observations and model
results for the boreal summer: from Seneviratne et al. (2010), adapted from Teuling et al.
2009 (a,b), Koster et al. 2004, 2006 (c,d) and Seneviratne et al. 2006 (e,f). Top row:
Estimation of evapotranspiration drivers of moisture and radiation (after Teuling et al.,
2009) based on simulations from the GSWP project (left, yearly correlations with global
radiation (Rg) and precipitation (P), redrawn for whole globe) and FLUXNET
observations (right, daily correlations with global radiation). Middle row: Estimates of
the soil moisture–temperature (left) and soil moisture–precipitation (right) coupling,
based on ΔΩ-coupling diagnostic and output of 12 GCMs from the GLACE project: after
Koster et al. (2004, 2006). Bottom row: Estimaes of soil moisture–temperature coupling
for 1970–1989 (left panel) and 2080–2099 (right panel) climates, based on three general
circulation models and diagnosed with the correlation between evapotranspiration and
2-m temperature: after Seneviratne et al. (2006a).
Figure 4.4 Methane and carbon dioxide feedback mechanisms. Red arrows represent the
climate-carbon dioxide feedback and purple arrows the climate-methane feedback. The
arrows labelled with letters a-d are discussed in the text: a) the effect of methane on the
climate as a greenhouse gas; b) the effect of climate on methane emissions by increasing
rates of decomposition and influencing wetland extent; c) the CO2 fertilization effect on
wetlands; and d) the melting of permafrost, increasing the inundated area in northern
areas. Figure modified with permission from Ringeval et al. (2011).
Figure 4.5. Methane emissions as simulated by the ORCHIDEE model. a) control
emissions (for pre-industrial conditions), b) additional emissions by 2100 due to
atmospheric CO2 increase, c) additional emissions by 2100 due to climate change, and d)
additional emissions by 2100 due to the combined effect of atmospheric CO2 increase
and climate change. From Ringeval et al., (2011)
Figure 4.6. Fire functioning and feedbacks in the Earth System, illustrating the three
requisites for fire to occur: i) a sufficient amount of fuel; ii) sufficiently dry fuel; iii) an
ignition source.
Figure 4.7. Average carbon releases from above- and below-ground wildfires, 19972008 (g C m-2 yr-1). From van der Werf et al. (2010)
(http://www.falw.vu/~gwerf/GFED/index.html).
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