Analysis of CO2, climate and land use effects in twentieth century
terrestrial carbon balance simulated using Met Office Surface
Exchange
Scheme (MOSES)
Introduction:
Over the past several centuries human intervention has markedly impacted land surface
characteristics and atmospheric composition, in particular through large-scale land conversion
for cultivation and burning of fossil fuels.
Land cover changes lead to large direct emissions of CO2 into atmosphere and impact
atmospheric composition and climate by biogeochemical and biogeophysical mechanisms.
McGuire et. al., 2000 estimated that a cumulative emissions of between 56.2 and 90.8 Pg C
over the period 1920-1992 using four process based land carbon cycle models. Houghton et.
al., 2003 estimated 156 Pg C for the whole industrial period 1850-2000 using a simple
bookkeeping approach.
The major components of the atmospheric carbon budge on the timescale of human lifetimes
are fossil-fuel CO2 emissions, exchange of CO2 between the ocean and atmosphere and
exchange of CO2 between the terrestrial biosphere and atmosphere ( Mcguire et al, 2000), the
net carbon exchange ( NCE) between the terrestrial biosphere and the atmosphere can be
described by the equation
NCE = NEP – RH
Where NEP is net ecosystem productivity, RH is heterotrophic respiration (i.e.,
decomposition). The fluxes of NEP and RH represent production and decomposition of
organic matter in terrestrial ecosystem. A positive NCE indicates a terrestrial source of
atmospheric CO2, where as a negative NCE indicates a terrestrial sink.
Here, we present a study on effect of CO2, climate and land use changes in twentieth century
terrestrial carbon balance simulated using Met Office Surface Exchange Scheme (MOSES)
Data and Methods:
Monthly forcing data:
The observation data set of 0.5° resolution for period between 1901 and 2002 has taken from
1
Climate Research Unit. The data set contains the climate variables of monthly mean
temperature, diurnal range, humidity, cloud cover, amount of precipitation and wet day
frequency. Climatological wind speed and surface pressure fields ware taken form a hadley
centre global climate model simulation of HadCM3 (Gordon et. al, 2000). The empirical
formulations of Albrecht (1989) are used to derive surface downward shortwave and
longwave radiation from Climate Research Unit data set. These radiation components are
compared to the monthly mean radiation data from the second Global Soil wetness Project
(Zhao and Dirmeyer, 2003) which is a combination of reanalysis and observations, The
average difference between the two is less than 5 Wm-2 for both longwave and shortwave
radiation, implying that these formulations are sufficient for this purpose. As we are using
cloud-cover observation in the reconstruction of surface downward shortwave radiation, a
component of the second indirect aerosol effect (cloud lifetime) is already incorporated.
Historical atmospheric CO2
The historical CO2 forcing is provided as annual mean concentration taken from a spline fit to
the ice core record from Law Dome Antarctica (Etheridge et al. 1996) and a combined
atmospheric record from the South Pole (“spo”) and Mauna Loa (“mlo”) records of the
Scripps Institute of Oceanography (Keeling et al. 1995). The atmospheric record is
calculated as a weighted sum (0.75<mlo + 0.25*spo) which is a good fit to the global mean
marine boundary layer value from the Global view compilation when both records overlap.
The Law Dome values have been adjusted to bring both records onto the same calibration
scale. The spline uses different weightings for different periods to account for varying data
density. Throughout it is arranged to preserve decadal-scale variability but suppress
shorter-term variations. Before 1960 (in the period covered by the ice core measurements) it
produces a 50% attenuation at frequencies less than about 20 years, during the period of the
atmospheric record this frequency cut-off drops to about 12 years (figure 3.1)
Figure 3.1 The historical CO2 forcing as annual mean concentration
2
Historical Land Use:
A spatially-explicit, global-scale dataset of crop and pasture area for each year from
1700-1900 was constructed by merging, interpolating and reconciling two independent source
datasets (Klein Goldewijk, 2001 – KG; Ramankutty and Foley 1999 - RF). These data may
be used as input to Terrestrrial Ecosystem Models (TEMs) which require anthropogenic
disturbance as a forcing.
The construction of annual pasture fraction datasets based on KG required interpolation, and a
number of assumptions regarding fractional coverage and the exact nature of grazing land.
With respect to the latter it is important to know its history as this determines to a large extent
the (soil) carbon pool, e.g. is it intensively grazed land converted from either natural grassland
or forest, or is it natural grassland.
In the absence of more precise information, datasets of the fractional coverage of pasture at
years other than 1700, 1750, 1800, 1850, 1900, 1950, 1970 and 1990 were obtained by simple
linear interpolation between those dates.
In many grid squares it was necessary to deal with the dual presence of both pasture from KG
and crops from RF, as a result of which a grid square could contain a pasture fraction of 1.0
and also a non-zero crop fraction, giving a total “disturbed fraction” greater than 1.0. The crop
and/or pasture fractions are therefore adjusted in order to avoid total disturbed fractions of
more than 1.0.
Figure 3.2 Distributions of total anthropogenically-disturbed fraction of land for 1750
and1990.
The final datasets provide fractional coverage of crops and pastures at 0.5˚ resolution, each
year from 1700 to 1990. The distributions of total anthropogenically-disturbed fraction of
land (i.e. crop + pasture) for 1750 and 1990 are shown in Figure 3.2.
3
Model Simulations Set-up
The vegetation growth is ‘spun up’ by repeatedly forcing the model with monthly data
averaged over the period 1901-1910 and setting CO2 and land cover to 1901 values. After
getting vegetation at equilibrium values, we setup following three transient runs of model.
S1. Model runs for period between 1901 and 2002 using varying CO2, mean climatology
averaged over a period between 1901 and 1910. 1901 land cover is used for simulations.
S2. Model runs for period between 1901 and 2002 using varying CO2, climate and 1901
land cover is used for simulations.
S3. Model runs for period between 1901 and 2002 using varying CO2, climate and land
cover.
The cumulative changes in Net carbon storage associated with increasing atmospheric CO2 is
estimated from a simulation (S1) that considered only increasing CO2. The cumulative change
in net carbon storage associated with climate variability was estimated by subtracting the
cumulative change of a simulation (S1) considered only increasing atmospheric CO2 from that
of simulations (S2) that considered both increasing CO2 and climate variability. The
cumulative change in net carbon storage associated with land use changes was estimated by
subtracting the cumulative change of a simulation (S2) that considered rise of CO2 and climate
variability from that of a simulation (S3) that considered rise of CO2, Climate variability and
land use change. Positive values indicate net release to the atmosphere and negative values
indicate net storage in terrestrial ecosystems.
Results:
Changes in Carbon Storage:
The land surface model, MOSES (Met Office Surface Exchange Scheme) captures
land-atmosphere exchanges of CO2 by simulating photosynthetic activity and plant and soil
respiration responses to imposed climate. Figure 3 (top panel) shows total gain of carbon for
each year by the land surface is presented (red line) for the period 1960-2002. Also plotted are
the changes in atmospheric carbon content for each year (derived from the Mauna Loa CO2
record). The key result that this figure 3 (top panel) shows a general "anti-correlation" so that
when the forcing climate is such that in any particular year the land surface "fixes" large
carbon contents, the rise in atmospheric carbon content is less. This provides quantitative
evidence that the land surface model is capturing the main features, at the large-scale, of
land-atmosphere exchanges of carbon dioxide.
4
Figure 3. (Top Panel)Annual Carbon uptake by the terrestrial biosphere simulated by the
MOSES model (red line) and the annual rise in atmospheric CO2 at Manuna Loa (green line).
(Middle Panel) Year-to-year fluctuations in increments of emissions and land uptake. (Bottom
panel) Cumulative emissions and uptake since 1960.
Figure 3 (middle) shows the year-to-year fluctuations in the increments of simulated global
land carbon uptake (red line) which show very similar patterns to the year-to-year fluctuations
in the rate of rise of atmospheric CO2 ( green line). There is larger fluctuation in the land
carbon uptake than the fluctuations in the rate of CO2 rise. The cumulative carbon uptake and
emission as shown in figure 3 (bottom panel) indicate there is gradual increase in land uptake
and CO2 rise since 1960. There is slight divergence in carbon uptake from the cumulated
emissions after 1980 shows that the carbon uptake is not matching with emission this is due to
the time lag associated with the carbon uptake by the terrestrial biosphere.
5
Simulations indicate Terrestrial ecosystems have lost small amounts of carbon because net
releases associates with land use are greater than storage associated with the effects of CO2
fertilization.
The global releases associated with landuse generally track the releases simulated by the
bookkeeping models of Haughton [2003] and where there is variation in the magnitude of
increase. Figure 4 shows the net carbon flux associated with land use over the period between
1920 and 2002. It is compared with the fluxes calculated by book keeping models. The
simulation calculated the upward trend while the magnitude of increase varies from the book
keeping models. It also shows that the magnitude of interannual variability in net fluxes
simulated by MOSES more than the bookkeeping models.
Figure 4 Global annual means of net carbon fluxes associated with land use changes
compared with the fluxes calculated using book keeping method [Houghton, 2003]
The effect of climate on carbon storage is small in comparison with the effects of CO 2
fertilization and effect of land use changes. (figure.5). Figure 5 shows the spatial distribution
of mean net carbon exchange with the atmosphere from 1990 through 2002. Positive values
are net releases while negative values are net storage in terrestrial ecosystem.
These results showed that the terrestrial carbon has decreased till 1960 since industrial
revolution and there has been gradual build up of the carbon in the terrestrial biosphere since
then. Figure 6 shows the net effect of CO2, Climate and Land use changes on Carbon
exchange. The effect of climate on carbon storage is small in comparison with CO2 and land
use changes.
At global scale the effect of CO2 fertilization are stronger than the release associated with
landuse as seen in top panel of figure 6. They show that effects of CO2 fertilization are slightly
stronger in the tropics than in north of tropics.
6
Figure 5 Spatial distribution of mean net carbon exchange with the atmosphere from 1990
through 2002. (Top panel) Net carbon exchange simulated using CO2 only. (Middle panel)
Net carbon exchange simulated using CO2 and Climate Variability (Bottom Panel) Net
carbon exchange using CO2, Climate Variability and land use changes.
7
Figure 6 Spatial distribution of Net effect of mean net carbon exchange with the atmosphere
from 1990 through 2002. (Top panel) Net carbon exchange simulated using CO2 only.
(Middle panel) Net carbon exchange simulated climate variability (difference between S 2 and
S1 fluxes) (Bottom Panel) Net carbon exchange simulated by land use changes. (difference
between S3 and S2 fluxes)
8
Conclusions
The study presents analysis of carbon cycle over last century. Anthropogenic land cover
changes during the last millennium played an important and complex role in climate change.
The MOSES model (Met Office Surface Exchange Scheme) was used to simulate the sinks
and sources of carbon over the global land surfaces. Taking observed and reconstructed
changes in climate, CO2 concentrations and land use as inputs, MOSES simulated biological
processes in the vegetation across the different continents and produced estimates of local
uptakes and release of carbon in response to the imposed environmental changes. The
temporal pattern of net carbon exchange simulated by the models is primarily associated with
the relative effects of rising of CO2 and land use changes. The global releases associated with
landuse generally track the releases simulated by the bookkeeping models of Haughton [2003]
and where there is variation in the magnitude of increase. In Summary, the results presented
here are consistent with the atmospheric data and provide a partitioning of total terrestrial
carbon exchange. It has shown that the carbon exchange at the tropical regions compensated
by carbon storage at high latitudes.
References:
Gordon, C. et al. (2000). The simulation of SST, sea ice extents and ocean heat transports in a
version of the Hadley Centre coupled model without flux adjustments. Clim.Dyn. 16,
147-168.
Henning, D. (1989). Atlas of the surface Heat Balance of the Continents 6-9 (Gebruder
Borntraeger), Berlin.
Zhao, M and Dirmeyer, P (2003) Production and analyses of GSWP2 near-surface
meteorology data sets. COLA Technic. Rep. 159, 1-36.
McGuire, AD., S.Sitch, J.S.Clein, R.Dargaville, G..Esser, J.Foley, M.Heimann, F.Joos, J.
Kaplan, D.W.Kicklighter, R.A.Meier, J.M.Melillo,B. Moore III, I.C.Prentice, N.RamanKutty,
T.Reichenau, A.Schloss, H.Tian, L.J.Williams, and U.Wittenberg (2001). Carbon Balance of
the terrestrial biosphere in the twentieth century: Analysis of CO2, climate and land use
effects with four process based ecosystem models, Global Biogeochem.Cycles, 15(1),
183-206.
Houghton, R.A. (2003). Revised estimate of annual net flux of carbon to the atmosphere from
changes in land use and land management 1850-200, Tellus, 55B, 378-390.
Etheridge, D.M., L.P. Steele, R.L. Langenfelds, R.J. Francey, J.-M. Barnola, and V.I. Morgan.
(1996). Natural and anthropogenic changes in atmospheric CO2 over the last 1000 years from
air in Antarctic ice and firn. Journal of Geophysical Research 101:4115-4128.
Keeling, C. D., T. P. Whorf, M. Wahlen and J. van der Plicht, 1995: Interannual extremes in
9
the rate of rise of atmospheric carbon dioxide since 1980. Nature, 375, 666-670.
Klein Goldewijk, K. (2001). Estimating global land use change over the past 300 years: the
HYDE database. Global Biogeochemical Cycles, Vol 15(2): 417-434.
Ramankutty N. and Foley J. A. (1999) Estimating historical changes in global land cover:
Croplands from 1700 to 1992. Global Biogeochemical Cycles 13(4), 997-1027.
10