2. KNOWLEDGE AND RISK: WHAT WE KNOW
ABOUT WHAT WE DO AND DON’T KNOW
Note. It is proposed that the final chapter 2 will comprise a short section on oil and fossil fuel
reserves and the risk characteristics associated with oil depletion; a substantive section on climate
science; and a section on the analytics of cost-benefit and risk under uncertainty.
Current material on oil reserves has been included in Chapter 4. The draft chapter here presents the
material on climate science. Economists and risk scientists may also wish to look at the draft
“Chapter 2 material input on cost-benefit and risk under uncertainty”, on the specialist review
material part of the book website; key elements of this will be presented in the first section of the
review meeting at Imperial College.
1. What We Do Know.
One thing we certainly do know is that the Earth is warming. To call it ‘global warming’ wouldn’t be
inappropriate. What’s more, the science behind it is really quite simple. The greenhouse effect has
been known about for nearly 250 years and has been linked to ‘greenhouse gases’ for well over a
century. These gases are warming our planet. The large number of ways in which this warming
impacts our intricate climate system, however, is a far more complicated story. This range of impacts
we call climate change. Although there are still uncertainties, change is happening and is gaining
momentum. The most obvious and easily measured changes are temperature increases, but changes
in weather patterns and other natural systems are also occurring and look set to continue to do so.
But let’s go back to the basics.
The burning of fossil fuels is changing the atmosphere. The inevitable increase of carbon in the Earth
system is the most direct impact of human activity on our planet, and evidence shows rapid rises in
atmospheric greenhouse gases in recent years. Records from air bubbles trapped in ice show that
atmospheric CO2 levels, which have been steadily fluctuating between 180 and 280 ppm for the last
800,000 years (Dieter Lüthi, 2008), have shot to a current 387 ppm since the industrial revolution
began, 150 years ago (Aresta, 2010). This current level, and the rate of this increase, is backed up by
direct air measurements (Keeling, 2009), and it unparalleled in the entire ice core record (Weitzman,
2009).
What else do we know? Well, greenhouse gases keep the Earth warm. Like the glass in a
greenhouse, the layer of gases in the upper atmosphere lets sunlight pass through, but traps other
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forms of heat – such as the heat re-radiated from the surface of the Earth after it’s been warmed by
the sun’s radiation (figure 2.1). This greenhouse effect is responsible for keeping our planet at a
habitable temperature; without carbon dioxide (CO2) amongst other gases in the atmosphere, the
sun’s heat would be reflected straight back into space, and the surface temperatures of Earth would
be on average 33°C colder (Le Treut, 2007). The planet’s water bodies would be permanently frozen,
and life probably wouldn’t have evolved. Hence, it is the huge amount of CO2 released by volcanic
activity in Earth’s early history that enabled the planet to make the shift from bare, rocky and
uninhabitable to one where life could exist.
Figure 2.1. The greenhouse effect. Source: http://www.co2crc.com.au
The greenhouse effect was first noted by H.B. de Saussure in the 1760s, who was surprised that the
natural phenomena of the “hot room or carriage”, where glass lets in heat in the form of sunlight
but traps it in other forms, hadn’t yet been formally studied. Citing his work in 1824, Joseph Fourier
related the effect to the Earth’s atmosphere, and the subsequent warming of the Earth’s surface
(Fourier, 1824), and from this the idea of the greenhouse effect was developed.
Of the so-called ‘greenhouse gases’ (GHGs), carbon dioxide (CO2) is most often mentioned, but
methane (CH4), nitrous oxide (NO), and water vapour (H20) also play a large part. They are all
released during human activities – primarily through the burning of fossil fuels, but also large scale
land-use change, deforestation and certain industrial processes – and add to the gas layer already
present in our atmosphere. Human activities have been thickening this blanket of gases since the
industrial revolution, and It’s been predicted for a long time that this could lead to the warming of
the Earth. But is there evidence for this?
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Well, yes; and it’s not easy to ignore. A general trend of increases in surface, oceanic and
atmospheric temperatures are among the most clear and obvious examples, each adding to the
picture of a planet heating up slowly but surely. Most strikingly, the ten hottest years since records
began have all occurred since 1997 (based on land and marine surface temperatures from the
Meteorological Office (MetOffice, 2009)). The last time the annual global mean temperatures were
below the 1901-2000 mean was in 1976; since then, they’ve consistently been warmer than that
average (NOAA, 2008); see fig. 2.2). Additionally, the decadal scales strengthen the pattern. The
2000s decade (2000–2009) was warmer than the 1990s, which in turn were warmer than the 1980s
and all earlier decades (MetOffice, 2009). Although there are recent reports of a recent slowdown in
the observed warming between 2000-2009 (Easterling., 2009), these are apparently not inconsistent
with expected internal variability, and are therefore consistent with the 0.2°C/decade observed
trend (Knight J, 2009). has been It’s clear; surface temperatures are increasing.
Fig. 2. 2. Surface temperatures are increasing.
Global mean temperatures compared to the 1901-2000 average. Source: NOAA, 2008;
http://www.epa.gov/climatechange/science/recenttc_triad.html.
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Worryingly, so are the temperatures of the oceans themselves – by far the planet’s largest heat
reservoir, estimated to accumulate over 80% of the climate system’s excess heat (Le Treut, 2007)
(Levitus, 2005a). There has been a general trend of increases in the heat content of the entire
oceans over the last half century (see Figure 2.3), despite a significant amount of spatial and
temporal variability (S. Levitus, 2009). Although almost all the warming occurrs in the upper 700m of
the oceans, the measurable warming of such a vast amount of water indicates a significant increase
in the climate system’s external radiative forcings.
Fig. 2. 3. The Oceans are Warming.
Yearly ocean heat content (1022J) for the 0–700 m layer from Levitus et al. [2009] (solid) and Levitus et al. [2005a]
(dashed). Each yearly estimate is plotted at the midpoint of the year. Reference period is 1957–1990. Source: Levitus et
al. 2009 (S. Levitus, 2009).
Finally, certain parts of the Earth’s atmosphere are warming while other parts are cooling, as is
expected from anthropogenic greenhouse gas-driven changes. The troposphere, the part of the
atmosphere closest to the Earth’s surface and where all greenhouse gases are found, has warmed,
whilst the part directly above, the stratosphere, has cooled. Evidence comes from radiosconde
observations with near-global coverage date from 1958, and satellite-based temperature
measurements which began in 1979. All data sets show that the troposphere has warmed, at
between 0.12 to 20°C per decade since 1979 (Qiang Fu, 2004) (Karl, 2006) (Le Treut, 2007).
Distribution of tropospheric warming can be seen in Figure 2.4. Meanwhile the same datasets show
that the stratosphere, the part of the atmosphere directly above the troposphere, has undergone
considerable cooling over the same period (Ramaswamy, 2001) (Karl, 2006) a conspicuous
fingerprint of the greenhouse effect – but more on that later.
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Figure 2.4. The Troposphere is Warming.
Patterns of linear global temperature trends from 1979 to 2005 estimated for the troposphere from the surface to about
10 km altitude, from satellite records. Grey areas indicate incomplete data. Source: Modified from IPCC AR4 WG1 CH3
(2007).
So the direct evidence that the Earth is warming comprises three distinct types of temperature
record, each from different sources and largely long-term datasets:



Surface temperatures
Ocean heat content
Troposphere temperatures
Just as predicted from pumping out vast quantities of greenhouse gases, the concentrations of these
gases in the atmosphere have increased rapidly, enhancing the greenhouse effect, and warming the
Earth. Furthermore, these changes in our climate system have been heavily buffered (Miller, 2008),
thanks to carbon sinks such as the oceans and forests absorbing much of the carbon (Sabine et al.,
2004) and resulting heat (Le Treut, 2007). This has two worrying implications: if we were to stop
emitting gases completely, the inertia of the system might mean that changes continue to occur for
many years afterwards; and, we’re in the risky position of being at the mercy of these carbon sinks –
should the capacity or function of any of them start to reduce, the impacts of emissions would likely
rapidly accelerate.
2. What else is changing?
The knowledge that the world is warming, in ways consistent with expectations, is reinforced by
numerous other observed changes, which also start to give us a sense of the areas in which
accumulating heat might impact the world around us.
In geological terms, the presence of sizeable ice sheets at each of Earth’s poles mean that we can be
described as being in an icehouse world. The Earth has fluctuated between icehouse states, where
permanent ice exists, and greenhouse states with no permanent ice, throughout the last few
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hundred million years (e.g. (Kenneth G. Miller, 1991)). These changes are driven by a number of
interacting factors, including variation in the Earth’s orbit around the sun, ocean circulation, albedo
effects, etc., which combine to set about changes to the Earth’s state. These changes between
greenhouse and icehouse states and involving the melting of icecaps happen over timescales of tensor hundreds of millions of years, although smaller fluctuations can happen over periods of tens of
thousands of years (e.g. (Thomas, 2008)). Global change over shorter timescales than this is virtually
unheard of in the geological record, the exceptions involving tipping points leading to localised rapid
change.
Yet currently sea ice on both poles is declining, and at an appreciable rate. Arctic sea-ice has been
decreasing since records began in the late 1970s, losing an area of ice roughly equal to the size of
France every decade since then (MetOffice, 2009). Recent years have witnessed huge declines, with
the years 2007, 2008 and 2009 being respectively the first, second and third lowest recorded extents
of sea ice since records began, although these still fall within the expected long-term trend of
decline (MetOffice, 2009).
In addition to sea-ice, evidence of the retreat of glaciers and ice caps from both direct and satellite
observations provide among the most visible and definite indications of changes in our climate
system. The most recent report by The World Glacier Monitoring Service (WGMS) which reviews
changes in over 700 glaciers from 27 different countries and regions worldwide, describes a ‘strong
acceleration of glacier melting’. They state that rates of mass losses have more than doubled
compared to 1980-2000 rates in their 30 ‘reference’ glaciers , monitored almost continuously since
1976 (Haeberli, 2008).
Even the most persistent glaciers, having survived large climate fluctuations in the past, are currently
experiencing rapid declines. The northern ice fields of the glacier on Mount Kilimanjaro, dating back
at least 11,700 years, survived a widespread drought 4,200 years ago that lasted around 300 years.
Yet it has lost 85% of the ice cover ice cover present in 1912, and 26% of the cover present in 2000 is
now gone (L. G. Thompson et al., 2009). This hints at the severity of current conditions.
The melt water has to go somewhere and the inevitable consequence of sea-ice and glacier melt is
an associated sea-level rise. The Antarctic and Greenland ice sheets are each vast, between them
holding enough freshwater to cause sea-levels to rise by 73 metres if they were to melt completely
(MetOffice, 2009). This is unlikely to happen in a hurry, but significant increases in global mean sea
level have been documented (see Figure 2.5).
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Figure 2.5.Sea levels are rising.
Variation in sea level from the 1980-1999 mean. Before 1870, global measurements are not available and the grey
shading shows the uncertainty in the estimated long-term rate of sea level change. The red line shows the global mean
sea level from tide gauges. The green line shows global mean sea level observed from satellite altimetry. Source:
adapted from IPCC 2007 AR4 WG1 CH5.
It appears that the sea-level has risen by increasingly greater amounts since the early 1900s, and ice
melt has contributed as much as 0.8 ± 0.2 mm per year to sea level rise for the period 1993-2003
(Dyurgerov, 2005) (Hegerl, 2007) (Domingues, 2008). To put these figures into some sort of
perspective, this would require the melting around 290 giga-tonnes of ice each year (Wouters,
2008).This sea level rise due to melting has been attributed to the heat from extra radiative forcing
due to increased greenhouse gases. This however isn’t the only effect the rising temperatures have
on sea-level. The warming causes the oceans to undergo thermal expansion, which adds to the sealevel increases. The sea surface temperature increases from 1880 until the present time are
estimated to have produced a sea level rise of around 35 mm through thermal expansion, with as
much as 16 mm occurring between 1993 and 2003 (Marcelja, 2010), exhibiting a recent increase in
expansion rate due to temperature increases (S. Levitus, 2009) (Fasullo, 2009) (Domingues, 2008)
(Hegerl, 2007).
Additional observed changes worth noting are the changing precipitation patterns. The global trend
shows a general increase in drying areas (see Figure 2.6a), but there is a marked variation in the
pattern over different regions, as shown in Figure 2.6b. The PDSI (Palmer Drought Severity Index)
measures the cumulative deficit in land moisture compared to the historical average. Red and
orange indicates areas that are drier than average, and blue and green areas are wetter than
average. As shown, certain areas have become significantly wetter, for example northern Europe,
parts of North and South America and northern and central Asia, whilst there has been a drying
trend over many land areas, including much of Africa and especially the Sahel, the Mediterranean,
southern Eurasia and Canada and Alaska. In fact, the number of areas classified as ‘very dry’ (with a
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PDSI of less than -3.0) have more than doubled since the 1970s.
Fig. 2. 1 Precipitation patterns are changing (b), with a global drying trend (a).
The spatial pattern of the monthly Palmer Drought Severity Index (PDSI) for 1900 to 2002. The lower panel shows how
the sign and strength of this pattern has changed since 1900. The smooth black curve shows decadal variations. Note
also the wetter areas, especially in eastern North and South America and northern Eurasia. (Although the PDSI is not
precise in its treatment of all precipitation (snow accumulation isn’t fairly represented, (Karl, 1986)) and some other
processes (Alley, 1984) it does take precipitation and surface temperature as inputs, setting it apart from most other
drought indices which are based on precipitation alone (Keyantash, 2002)). Source: IPCC 2007 WG1 CH3.
The observed widespread and significant increases in the frequency of heavy precipitation events,
even in regions where total amounts have decreased, are associated with the warming of the
world’s oceans, which leads to increased water vapour in the atmosphere (Trenberth, 2007).
Concentrations of water vapour in the troposphere have indeed increased since 1976 (Trenberth,
2007), and is not only expected to alter precipitation but also, being a greenhouse gas, to increase
the greenhouse effect.
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Further observed changes that indicate the effects of global warming are decreased northern
hemisphere snow cover (see figure 2.8, (Mote, 2009)), increased water temperatures in lakes
worldwide (Livingstone and Dokulil, 2001; Ozaki et al., 2003; Arhonditsis et al., 2004; Dabrowski et
al., 2004; Hari et al., 2006), a decrease in the number of travel days of vehicles over frozen roads in
the Arctic, reduced from 220 to 130 since 1971 (ACIA, 2005), an increase in number, and
enlargement of, glacial lakes in Nepal (Agrawala et al., 2005), a landward retreat of coastlines due to
sea level rise in the UK and USA (Rosenzweig, 2007), and a decrease in number of ski areas from 58
to 17 over 1975-2002 in New Hampshire, north-eastern USA [Hamilton, 2003b].
The physical changes occurring in the climate system are insurmountable; only a few are
summarised here but the list could be extended much further. But are these physical changes
affecting the Earth’s biological systems, if so how, and are they consistent with a climate change
hypothesis?
The Biological Response
One direct effect of changes in the physical biomes on the biological systems is the disruption of krill
reproduction. Krill, a major food source and a keystone producer in the marine ecosystem,
reproduce underneath sea ice in polar regions. A reduction in sea ice due to warming has caused a
krill population decline, and the impacts of this are visible all the way up the food chain ( (Loeb VJ,
1997)– including Emperor penguin population declines (Jenouvriera et al., 2009 ) and struggling right
whale populations (Russell Leaper, 2006 ).
There are many more intricate effects of climate change on the biological systems. In a warming
planet, a species would have to move polewards or to higher altitudes in order to maintain preferred
environmental conditions, with particular regard to temperature. This is exactly what was seen in an
iconic study by Parmesan and Yohe (2003). Data for over 1700 species were analysed, and an
average migration of 6.1 km per decade towards the poles was documented. Many studies since
have shown similar results, such as a broad northwards extension of many plant species over the last
decades, as noted by the European Environmental Agency (EEA, 2004), and upward changes in the
altitudinal ranges of British lagomorphs (B. J. Anderson, 2009). The result of this is that species and
ecosystems at high altitudes, latitudes or approaching the edges of a polewards-pointing continent
(such as South Africa) are particularly at risk as an alternative habitat to migrate to doesn’t exist.
Whole ecosystems, such as the South African fynbos ecosystem, are under threat of extinction.
Ocean warming provides a huge risk to coral ecosystems, with fluctuations of 1°C over the summer
average causing mass bleaching of coral reefs (Lough, 2008), and a significant proportion of coral
species being identified as susceptible to climate change (Foden W, 2008).
The temperature changes also allow pests and invasive species to widen their ranges, as is currently
occurring in North America with several invasive plant species and insect pests (Dukes et al., 2009). A
dramatic example of this is the North American mountain pine beetle, which since 1997 has infested
and destroyed 13 million hectares of forest in areas that were historically found to be unsuitable
(Anonymous, 2003), (Range, 2007) .
It’s not just the terrestrial biomes that are affected; similar poleward trends are also evident in the
marine environment, as species adjust to warming ocean temperatures. Warm-water plankton have
9
moved northwards by 10° over 4 decades in the North-East Atlantic (Beaugrand et al., 2002), and
similar results have been seen for algae and fish species in the North-East Atlantic (Brander et al.,
2003; Beare et al., 2004; Genner et al., 2004; Perry et al., 2005), and the Bering Sea (Grebmeier et
al., 2006).
Timing shifts
Shifts in the timings of seasonal activities of animals and plants have been recognised and tracking
changes in these is one of the simplest ways to gauge species’ responses to climatic change.
Examples of observed phonological events include leaf unfolding, flowering, fruit ripening,
amphibian chorusing, bird migration and the emergence of butterflies. Changing the annual timing
of biological events can have large negative impacts on ecosystems as interspecies interactions
become decoupled. If this happens to the phonologies of two species related in a food chain,
dramatic population decline can occur for one of those species. For example, A mis-matching
between the timing of the arrival of migrant birds and first insect appearance has caused significant
reductions in bird populations (Parmesan, 2007).
A large number of studies and meta-analyses document a significant forward progression of spring
dates by 2.3 to 5.2 days/decade in the last 30 years, as a response to climate warming (e.g.
(Rosenzweig, 2007) (Parmesan, 2003)). A meta-analysis of 983 data series show strong evidence
that bird arrival dates are earlier (Lehikoinen, 2004), and there is a clear signal across Europe of
changing spring event timing with 78% of leaf unfolding and flowering records advancing, along with
75% of fruit ripening records. These findings come from a study of more than 100,000 observational
series of phases in 542 plant species in 21 European countries, between 1971 and 2000. Only 22%
and 25% of the records were delayed, respectively (Menzel et al., 2006b), (Rosenzweig, 2007).
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Again, these results are apparent in the aquatic environment also. Many plankton taxa have been
found to be moving forward in their seasonal cycles (Edwards & Richardson, 2004), but more
importantly, different responses to warming were seen between different groups, leading to a
mismatch between trophic levels.
Coherent with the biological responses to warming, the annual break-up dates of river and lake ice
across the northern hemisphere are occurring progressively earlier, and the yearly freeze-up date of
the ice is occurring later, in comparison to the last 150 years. The dates have shifted by a rate
around of 6.5 ± 1.6 days and 5.8 ± 1.2 days per century respectively, consistent with planetary
warming. An apparent response to this earlier ice break-up and warmer water temperatures in the
advancement of spring algal blooms, which now occur up to 4 weeks earlier in several large lakes
(Gerten & Adrian, 2000), (Weyhenmeyer, 2001), (Winder & Schindler, 2004b). Often, the
zooplankton communities have not followed suit and undergo population decline, missing out on
their annual food bloom (Winder & Schindler, 2004b) (Gerten & Adrian, 2000)
3. So is it human-induced climate change?
So we’ve shown that climate change is definitely happening, and that the changes are consistent
with what we would expect from a human emissions-induced scenario. But this doesn’t prove
anything – it’s simply a correlation. So how do we determine exactly why the climate is changing like
it is? Or, how do we move from detection to attribution?
Detecting climate change, or demonstrating that the climate has changed in a defined statistical
sense, is relatively easy. Detection seeks to determine whether observed data indicate that the
climate is changing, or are simply consistent with fluctuations from natural internal variability. One
way to do this is to compare observational data with a modelled expected result, considering only
natural influences, and perform a statistical test to determine the possibility that the observed trend
is coherent. Figure 2.10 shows an example of this method; clearly the observed temperature (red
lines) cannot be explained by the natural variation shown in the ‘control’ part of the model (black
lines), and a change is detected.
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Figure 2.10. Observed temperature changes are inconsistent with expected change.
Observed global mean temperature changes from 1850 to 2008 (red) with uncertainties (yellow band) as anomalies to
the 1861-1899 mean, overlain on a 1000 year control simulation from the HadGEM1 model (black line). Source: Stott et
al., 2010.
Attribution, or establishing the most likely causes of this detected change to a defined level of
confidence, is much harder. In climate change attribution we are trying to determine which external
factors have significantly affected the climate. In traditional science, attribution requires
experimentation where several factors are changed and the effects recorded, and a control model is
normally used for comparison. However, without a spare identical Earth with an atmosphere we can
manipulate and record the resulting changes, we must rely upon other methods of attribution.
Climate models provide a suitable tool to study influences on the Earth’s climate. Models are made
to replicate past variations as closely as possible, using all inputs we are aware of. By changing the
combination of inputs we change the model’s output, and by comparing different model simulations
with observational data, we can calculate the agreement between them. The closer a model
simulation to the observed effects, the more realistically it describes the actual processes involved,
and so the more likely the chosen inputs are significant drivers of the observed change. In this way,
the influence of individual factors on observed climate change can be investigated.
The big question is whether natural influences can explain the warming patterns observed. Although
some internal processes such as the El Niño Southern Oscillation, and some external ones such as
variability in solar output and volcanic activity have all been shown to influence global temperatures
to an extent (e.g. (J Lean, 1995), (A Robock, 1995 )), observed temperature change cannot be
12
explained by known natural causes alone. A huge number of models have been created, run and
analysed, and not a single simulation thus far has succeeded in reproducing the global temperature
changed over the 20th century without including anthropogenic forcing as an input (Hegerl, 2007).
Models including anthropogenic effects, however, can match observations quite closely. Figure 2.11
shows the comparison between models using both natural and anthropogenic forcings (a), and just
natural forcings (b), compared to observed temperature changes (the black line). It is clear that
natural forcing alone cannot explain the rapid warming seen over the last century, and that human
effects are a significant factor. Anthropogenic change has very likely (less than 1% error probability)
influenced surface temperatures (Hegerl, 2007), (Stott et al., 2010).
Figure 2.11. Models cannot explain observed temperature changes if they do not include human emissions.
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Global mean surface temperature anomalies relative to the 1901 – 1950 mean, as observed (black line) and as obtained
from climate model simulations with (a) both anthropogenic and natural forcings (red lines) and (b) natural forcings only
(blue lines). Grey vertical lines indicate the timings of major volcanic eruptions, predicted to reduce warming. Source:
Stott et al., 2010; originally reproduced from IPCC AR4 WG1 Figure TS.23.
As for single events, however, there is considerable debate about whether we can attribute them
directly to global warming with any level of confidence. There is no trend to compare with, and a
one-off event can be caused by a great many different factors. Events such as the 2009 Australian
forest fires and the prolonged drought over Eastern Africa make this a particularly pressing issue.
Recent research at the Meteorological Office Hadley Centre is developing a new approach to this
problem by instead determining how human influence could have changed the odds of such extreme
event occurring (MetOffice, 2009) (Jones, 2008). They claim that when applied to the European
heatwave of 2003, they can show that it is very likely that human influence has more than doubled
the probability of events like this occurring. The weather associated with the 2009 Australian forest
fires is also thought to have become more likely due to climate change. Additionally, the same
treatment can be applied to cold spells such as was seen in late 2009; events like this are expected
to continue, but become more and more rare as the probability of them occurring decreases. They
expect extreme cold events like this to happen around once in every 20 years, rather than once in
every 5 as would be expected without human effects.
This leads us to attributing the responses seen in the physical and natural systems, as summarised in
the previous section. Can we determine the causes of phonological change and melting glaciers?
Broadly speaking, the responses analysed are consistent with observed temperature changes. In
fact, of the more than 29,000 data sets recording changes in the physical and biological systems,
more than 89% are in the direction expected as a response to warming, and a statistical comparison
shows that agreement between regions of significant warming and locations of significant changes in
the systems is very unlikely to be due to natural variability in either the temperatures of the systems
(Rosenzweig, 2007).
Further, when these physical and biological responses undergo the same treatment as the
temperature changes, we find that they too cannot be explained without anthropogenic forcing. A
study by Root et al. (Root, 2005), reviewed in the IPCC Fourth Assessment Report (Rosenzweig,
2007), compared observed responses of natural systems to different modelled temperature trends.
The agreement between the observed data and the models is weakest for the model considering
only natural effects, stronger with anthropogenic effects, and strongest when both effects are
combined. Changes observed in animals and plants are therefore shown to be most likely
responding to both natural and anthropogenic influences.
Additional evidence that changes are anthropogenic comes from the specific warming pattern in the
atmosphere. As discussed earlier in this chapter, warming of the troposphere (the lowest part of the
atmosphere) has been observed by numerous satellite and radiosconde observations. If this
warming was due to natural external factors, such as increased solar output, we would expect both
the troposphere and the stratosphere to have warmed. However, models considering anthropogenic
emissions predict warming in the troposphere and cooling in the stratosphere immediately above.
The layer of greenhouse gases in the troposphere trapping heat that is re-radiated from the Earth’s
surface would cause the air immediately around that layer to warm, and the atmospheric layer
above that the radiation doesn’t reach, to cool. Observations agree with these models; the
14
stratosphere has cooled, while the troposphere has warmed (Qiang Fu, 2004) (Hegerl, 2007) (Karl,
2006).
Each of these points towards observed changes in the climate and natural systems being a result
human influence, namely greenhouse gas emission.
4. Where is this leading us?
The evidence leaves little room for doubt that anthropogenic climate change is a real threat, and
already causing serious impacts across the globe. But what does this mean for the future? How do
we predict climate change, and what can we expect in the next ten, twenty, one hundred years?
Although relatively straightforward to detect and even attribute, climate change is still difficult to
predict. We might know a lot about how and why changes are occurring, but there still remains a
large amount of uncertainty in forecasting future impacts. There are several reasons for this. Firstly,
the magnitude of future change is obviously dependent upon future emissions of CO2 and other
greenhouse gases. Although serious international mitigation strategies are being formed (see
chapter 1), the level of emissions cuts decided upon and the actual success or impact of any future
mitigation strategy will affect future climate change. And these are hard to predict with any certainty
– as December 2009’s COP15 conference clearly demonstrated.
Additionally, the climate system is essentially an extremely complex collection of interconnecting
and interdependent processes, the precise functioning of which is complicated to understand in
retrospect, let alone foresee. As such, for any process altering the climate system, there is a
multitude of possible impacts. For example, one result of warmer temperatures is an increased rate
of evaporation. This could further enhance the warming through greenhouse forcing, as water
vapour is a greenhouse gas. Yet it may also generate more clouds, which affect heat transfer in
various ways – reducing external solar radiation hitting Earth, and trapping heat re-radiated from the
Earth’s surface to name but a few. Additionally, warmer temperatures will melt ice and snow,
thereby potentially reducing the Earth’s albedo (ability to reflect light) and so increasing heat
absorption at the surface. However, a sped up hydrological cycle could increase snowfall, obscuring
this effect. For every factor affecting the climate system, a similar number of possible impacts can
probably be drawn; the levels of uncertainty remaining after decades of research are then less
surprising.
An additional factor complicating precise prediction is the chance of large-scale discontinuities, or
catastrophic events, occurring. It is difficult to predict exactly when these events might occur and at
what rate changes will happen once they do, but once set into motion, these events may trigger
positive feedback mechanisms and hence dramatic change. As highlighted in recent reports by the
IPCC (Rosenzweig, 2007) and the Met Office (MetOffice, 2009), these include the die-back of the
Amazon rainforest, sea-ice melting, ocean acidification and methane release from melting
permafrost or methane hydrants.
The Amazon rainforest, being one of the world's most biodiverse ecosystems, play a key role in
carbon storage and exchange (Philips, 2005) making it one of the world’s greatest carbon sinks. As
models consistently predict increased temperatures and reduced precipitation over Amazonia (as
15
reviewed by (Cochrane., 2009)), dramatic increases in forest fires are likely, and large scale die-back
of the forest could occur (Malhi et al., 2009) (Williams JW, 2007). Additionally, new research has
suggested that climate forcing agents (e.g. CO2, ozone, aerosols) can have very different effects on
vegetation, which could affect the functioning of terrestrial carbon sinks (Huntingford C,
(submitted)), (Collins, undergoing peer review). Not only could these factors reduce the Amazon’s
function as a carbon source, but initiate the release of huge amounts of carbon, thereby exerting a
positive feedback to exacerbate climate change (Philips et al., 2009). Predictions of when this might
occur are very imprecise, but the effects would certainly be dramatic.
Increasing acidity of the oceans is an added impact of rising CO2 and has only recently been
recognised as a possible further source of large scale problems. Carbon dioxide reacts with seawater
to produce carbonic acid, thereby increasing the ocean acidity (Zeebe, 2001) while removing CO2
from the atmosphere. Oceans are therefore a huge carbon sink, currently absorbing up to 50% of all
anthropogenically released CO2 (Sabine et al., 2004). The resulting production of carbonic acid and
associated reduction in ocean pH, however, has many negative effects on marine life. The carbonic
acid reacts with carbonate making it unavailable to marine organisms that need it to build their
shells, thereby greatly increasing environmental stress and chance of population decline in marine
organisms, including ones crucial to the marine food chain (TheRoyalSociety, 2005). The acidification
can also cause calcification problems in corals (Langdon et al., 2003), which in turn would negatively
impact ocean fish stocks (Garcia et al., 2001), having severe social impacts. However, the increasing
acidity of the oceans reduces their ability to take up further CO2, hence reducing their function as a
sink and potentially accelerating future climate change (Turley et al., 2006) (MetOffice, 2009).
The melting of the ice caps is another catastrophe waiting to happen. In addition to sea level rise,
the Earth’s albedo will decrease. White ice has an albedo of 0.8 or 0.9 (meaning it reflects 80-90% of
the solar radiation it receives), whereas dark surfaces, such as the sea, can have an albedo of less
than 0.1, meaning that as the ice caps reduce in extent, increased areas will ‘flip’ from reflecting 8090% of the sun’s energy to absorbing 90% of it. This is known as the ‘albedo flip’ and increases
regional warming, further strengthening processes that caused the ice melt, thereby inducing yet
another positive feedback mechanism that could spiral out of control and cause rapid, catastrophic
change. Recent research by the US National Center for Atmospheric Research (NCAR) found that the
associated warming could be far greater than previously predicted by models, with coastal areas
experiencing on average 5°C land temperature rise over a very short space of time (David M.
Lawrence et al., 2008). The ice melting resulting from such a change would be huge, yet currently no
climate models have predicted such dramatic regional heating, so the precise impacts are difficult to
quantify.
A further ticking time-bomb is the melting of permafrost in northern latitudes. As it sounds,
permafrost is permanently frozen ground, and is often rich in carbon. In fact, recent research has
shown that there is more than double the 750 billion tonnes of carbon already in the atmosphere
locked up in the permafrost (Edward Schuur et al., 2008). If all this permafrost were to melt this
carbon would be released in the form of carbon dioxide or methane, a greenhouse gas about 20
times more potent than CO2 (Hegerl, 2007), setting further warming into motion. Early models
predict that these additional GHGs could increase warming by several percent (Peter Good,
2010)Unfortunately, this feedback ties in with the previous one - rapid warming in the Arctic region
as described above would melt a huge proportion of the world’s permafrost (location of permafrost
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shown in Figure 2.14. (David M. Lawrence et al., 2008)). Alternatively, if permafrost melting happens
first, it could kick-start the melting of the ice-sheets and both mechanisms would act simultaneously.
Not a nice thought.
Figure 2.14 Permafrost in the Northern Hemisphere.
Source: UNEP’s Global Outlook for Ice and Snow report (2009)
Alarmingly, reports suggest that permafrost melting may already be happening (Programme, 2009).
Recent research shows that methane emissions from the Arctic have increased by 31% from 2003 2007, due to melting permafrost (Anthony Bloom, 2010). This increase represents about 1 million
extra tonnes of methane each year, or 20 million extra tonnes of CO2-equivalent. Since permafrost is
not incorporated in any global climate models, the impact on the climate system is unknown, but as
these carbon deposits have accumulated over thousands of years, a rapid release would be
effectively irreversible (MetOffice, 2009).
All of these scenarios have a real possibility of occurring, but when and just how likely they are we
still don’t know to much accuracy. In fact, a certain amount of caution is required when assessing
the risks of such rapid changes occurring, since there is considerable variation in the scientific
literature regarding the method of assessing the chance of such possibilities occurring (Kriegler E,
2009) (Peter Good, 2010), although a study by Challenor et al. (2006) suggests that risk estimates
may be much greater than generally appreciated, with a 30-40% chance of certain rapid changes
occurring.
Despite these complicating factors, highly sophisticated models have been constructed with inputs
from every known factor, enabling us to make detailed predictions of future climate risks. Just how
accurate they are, we can’t be sure, but many are able to accurately reproduce observed change
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over the last century so confidence in them is high. Worryingly, however, there is increasing
evidence that the best models available are underestimating the rate at which climate change is
occurring. Observations are already racing ahead of the predictions made in the IPCC’s 2007 report
in the short time since it was published (Hawkins, 2008). Summarises leading NASA climate scientist
Zwally, in reference to the 2008 Arctic sea ice record low, “climate warming is coming larger and
faster than the models are predicting and nobody’s really taken that into account yet.”
As illustration of this, sea-ice is melting ahead of schedule. The previous worst case estimate for
complete Arctic sea-ice summer melting was about 2070, but following the extreme melting since
2007, new estimates suggest it may actually be less than a decade (see Fig. 2.15).
Figure 2.15. Sea ice might be declining well ahead of schedule
Minimum summer sea-ice extent, observed and predicted, 1950-2100. Observed Arctic ice extent loss to 2007 (black
line) compared to IPCC modelled changes (IPCC AR4 WG2).
Source: Modified from chart courtesy of Dr. A. Sorteberg, Bjerknes Centre for Climate Research, University of Bergen,
Norway, and climatesafety, Public Interest Research Centre – www.pric.info.
Whether these accelerated impacts are signs that positive feedback mechanisms discussed above
are already taking effect is not clear. Yet, despite all these complications, unknowns and
uncertainties, we do have a detailed set of model predictions under varying future scenarios, which
we will now examine.
Firstly, the expected temperature changes for three different emissions scenarios are outlined in the
IPCC 2007 report are detailed in figure 2.16 below. The three scenarios, B1, A1B and A2, represent
the ‘low’, ‘medium’ and high’ emission scenarios respectively, as defined by the IPCC Special Report
on Emissions Scenarios (Nebojsa Nakicenovic, 2000). The scenarios are specified in Figure 2.16.
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Figure 2.16 mean of annual mean surface warming (surface air temperature change, °C) for the scenarios B1 (top), A1B
(middle) and A2 (bottom), and three time periods. Anomalies are relative to the average of the period 1980 to 1999.
Scenarios are characterised by: B1 – A declining global population after 2050, reductions in industrial intensity, and
emphasis on global solutions to economic and environmental stability, and the introduction of clean, resource efficient
technologies; A1B – As B1 but with less emphasis on new technologies; and A2 – A continuously increasing population, a
non-global approach to economic and environmental solutions, and slower, more fragmented technological changes.
Source: IPCC AR4 WG1 Ch.10.
As shown, predictions for the periods 2011-30 for the different scenarios are similar. It is suggested
that this is because most of the warming over this period has already been committed; it is
determined by what we’ve already done, and these changes would still occur even if atmospheric
concentrations were maintained at current levels (Meehl, 2007). It’s later that choice of scenario
becomes important, as future emissions take effect. By late century (2080-2099), differences are
large, and only 20% of this warming arises from climate change that is already committed (Meehl,
2007). By 2080 in Europe, there is just a 10% chance that temperatures will only rise by 1-2°C. The
worst-case scenario gives a mean global warming of 2.8°C, which equates to around 5.6°C over land.
A possible temperature rise of this magnitude is put into perspective when we consider that the
2003 European heat wave caused over 70,000 heat-related deaths in Europe alone (Robine, 2008),
and summer temperatures were only around 2°C higher than normal.
Models also predict large changes in precipitation patterns. For the mid-case scenario, mean
precipitation is expected to increase in many tropical areas, with general decreases in the
subtropics. Increases at high latitudes are attributed to a general intensification of the hydrological
cycle. Intensity of precipitation events is also projected to increase, in the same areas that will
experience mean precipitation increases. Even in areas with mean decreases, precipitation intensity
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is projected to increase, but accompanied by longer periods between rainfall events. Certain midcontinental regions are at risk of summer drying and therefore drought.
So what do these projections mean for human welfare? Unfortunately, it looks probable that
agriculture will be negatively affected and so food security threatened. Droughts and floods will
decrease yields, and water availability and quality will be reduced. Predicted productivity changes by
2050 see large decreases in rice and wheat productivities (27 and 42% decreases respectively), with
maize also reducing by 13%. With predicted increases in energy prices, food security looks certain to
be at risk. And that’s not even considering the projected increased demand due to a continually
increasing human population. Battisti & Naylor ((Battisti DS, 2009) predict that suggests that half of
the world’s population could face severe food shortages by the end of the century, and this would
hit developing countries hardest.
Water stress is certain to increase. Even without the impacts of climate change more than half the
world’s population are forecast to be water stressed by 2025 (Institute, 2000) . Climate change is
predicted to decrease water availability further (Kundzewicz, 2007), partly through the salinisation of
freshwater due to sea level rise, and partly through increased water pollution as a result of both
temperature rises and increased precipitation intensity (Fischlin, 2007).
Increased vulnerability to extreme weather events is expected, and the spread of disease is
projected to increase (Fischlin, 2007). Rising temperatures affect rates of pathogen maturation and
replication, and the range and abundance of vectors (e.g. mosquitoes) is expected to increase. This
could trigger large scale disease outbreaks (Tabor, 2008) of malaria, for example. Indeed, the impact
of climate change on malaria may be even greater than initially suspected (Hayashi et al., 2010;
Chaves and Koenraadt, 2010; Wandiga et al., 2010).
Increasing research suggests that those most affected will be poor populations in developing
countries (Nelson, 2009) (Mertz et al., 2009) (Hess, 2008), due to a greater dependence on
agriculture, a higher risk from food price increases or security, and more limited capacities and
resources to cope with change.
Predictions of large scale migration as land is lost to the sea, combined with all other factors
mentioned above, and a human population expected to reach 9 billion between 2040 and 2050
(World Population Prospects:The 2008 Revision, 2009), leave many to predict a much increased risk
of conflict, both inter- and intra-nationally (Watson, 2010).
As shown, climate change incorporates an incredible number of issues, risks and uncertainties.
Totally unprecedented are the levels of risk involved, and the range of impacts – both in type and
geographical, demographic and social range. Accordingly, climate change is a unique and global
challenge where international diplomacy, technological advancement and behavioural change will
be pivotal in addressing it.
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