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Date Published: 22/10/01
Author: Peter Bunyard Almost all scientists are now agreed that human-induced climate change is under way. But new evidence shows that it will be even worse than we thought. Peter Bunyard reports. |
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When Britain was awash last winter, in the back of most people’s minds was the notion that those seemingly endless torrents of rain had something to do with global warming. Whenever an extreme climatic event happens anywhere in the world, we now often question whether it was entirely ‘natural’. Few doubt that the climate is changing, but how certain can we be that humans are to blame with their emissions of greenhouse gases? Are we implicated in what could be the biggest swing in climate for many millions of years?
The Intergovernmental Panel on Climate Change, a body of scientists, economists and policy makers, which the UN first brought together in 1988 to inform governments of the likely causes and consequences of climate change, and solutions to mitigate it, have been deliberating for the past 12 years over the evidence for global warming. In what can only be described as a landmark development, the IPCC confirms in its recently published 'Third Assessment Report' not just that global warming is occurring, but that it is largely man-made, has begun to accelerate sharply and will increase far faster than previously thought.(1)
Global warming is here
Six years ago, in its 'Second Assessment Report', the IPCC predicted that the overall warming of the planet during the 20th century would be some 0.45ºC. Now, in the summary of its latest report, published in March 2001, the IPCC points out that the Earth’s surface has warmed by an additional 0.15ºC. The 1990s, a run of the warmest years since instrumental records began in 1861, has forced the IPCC to up its assessment of 20th-century temperature rise to 0.6ºC. In the northern hemisphere, the IPCC adds, the increase during the 20th century is ‘likely to have been the largest of any century during the past 1,000 years’.
Not that global warming just means hotter days in the sun, rather one of the manifestations is an increase in night-time temperatures. ‘On average’, states the IPCC, ‘between 1950 and 1993, night-time daily minimum air temperatures over land increased by about 0.2ºC per decade. This is about twice the rate of increase in daytime daily maximum air temperatures. This has lengthened the freeze-free season in many mid- and high-latitude regions.’
The IPCC also reports that the planet has lost about 10 per cent of its snow cover since the 1960s and that lakes and rivers in the high latitudes of the Northern Hemisphere remain frozen over for two weeks less than they did a century ago. Glaciers in non-polar regions are also retreating, while Arctic sea ice has not only thinned by some 40 per cent since the 1950s, the surface area that it covers during the spring and summer is also down by 10–15 per cent. All in all these figures are extremely significant, revealing that rapid changes are afoot.
We are responsible
After taking a range of different factors into account, such as sunspot activity, erupting volcanoes and wind-blown dust, the IPCC concludes that: ‘There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.’
The present atmospheric level of carbon dioxide (CO2) – the main heat-trapping or ‘greenhouse’ gas, generated from the burning of fossil fuels – is now up by 31 per cent from its level in 1750. As the IPCC points out, the present level ‘has not been exceeded during the past 420,000 years and likely not during the past 20 million years. The current rate of increase,’ continues the IPCC, ‘is unprecedented during at least the past 20,000 years.’
On average approximately 240 watts per square metre (W/m2) of the sun’s energy reaches the Earth’s surface. The- -IPCC has found that increases in greenhouse gases since 1750 have increased the heat absorbed at the surface by 2.43W/m2 – with CO2 emissions responsible for 60 per cent of that increase; methane 20 per cent; nitrous oxide 6 per cent; and halocarbons 14 per cent. Changes to the amount of ozone in the atmosphere have resulted in an additional net warming of 0.2W/m2.
The IPCC also carefully assessed whether other factors have had an impact. It looked at solar activity and found that during the first half of the 20th century the sun became more radiant, and when the 11-year solar cycle is taken into account, overall the sun has added an extra 0.3 W/m2 to the earth’s budget since 1750. However, when the cooling impact of volcanoes is added, the combined effect for the past two and possibly the past four decades, is negative.
For those, like astrophysicist Professor Sally Baliunus of Harvard(2) who insist that variations in solar activity are enough to explain all the warming of the past century, all the models used by the IPCC not only factor in such variations, they also show unequivocally that the enhanced warming from human activities swamps the ups and downs in surface temperature caused by solar activity.
Could natural variability, internal to the climate system, be of a sufficient magnitude to have resulted in the global warming that we have experienced over the past few decades? The IPCC realises the vital importance of detecting a real signal of climate change against chance fluctuations and, on the basis of models that factor in the dynamic interactions of the different components of the climate system, it concludes that ‘the warming over the past 100 years is very unlikely to be due to internal variability alone’. Again, in the same cautious language, the IPCC points out from reconstructions of climate over the past 1,000 years, that ‘this warming is unusual and is unlikely to be entirely natural in origin’.
Taking all the evidence into account, the IPCC concludes, ‘most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations,’ which is ‘attributable to human activities’.
Turning up the heat
But what about the future? What does the IPCC predict? Just as it had to revise upwards the actual warming of the latter part of the 20th century, compared to what it had predicted a few years previously, is it also predicting greater increases in temperature and therefore greater impacts than it did in 1995?
In its latest report, to predict future trends, the IPCC has used a range of 35 different scenarios reflecting different estimates of rates of economic growth, types of growth, the energy technology used to fuel that growth, population numbers and whether global or local development initiatives are applied. The greenhouse gas emission consequences of those different scenarios are then fed into climate models that have been calibrated to give equivalent responses in temperature and sea-level rise (following the melting of ice cover and the thermal expansion of sea water). The range of predictions therefore reflects projections for different modes of development. While some of those modes lead to a flattening out of greenhouse gas concentrations in the atmosphere and hence of surface temperatures, others show a steep rise in CO2 emissions and hence in atmospheric levels of the gas.
A scenario in which fossil fuels remain the prime source of energy gives rise to nearly 1,000 parts per million by volume (ppmv) of CO2 in the atmosphere by 2100, even with some energy conservation (compared with the 280ppmv of the pre-industrial era and the 370ppmv of today). A scenario in which energy conservation and recourse to alternative energy systems is aggressively followed will lead to a state in which the CO2 content of the atmosphere is stabilised at 540ppmv, double that of pre-industrial times.
The high-emission scenario indicates an increase in temperature of 5.8ºC over 1992 levels by 2100. The scenario in which we use less energy shows a rise of 1.4ºC by 2100. We also see a similar range in sea level rise, with the upper range scenarios leading to a projected 0.09m rise by 2100, and the lower range, about 0.01m.
As the situation stands, we are currently embarked on the more extreme scenario, with all its potential for catastrophic climate change. That makes it all the more critical that we take action now to curb emissions – without question we would gain immeasurably just from stabilising climate at a point close to current values.
The new emissions scenarios and their consequences reveal a substantially greater warming compared with that of the IPCC’s 1995 Second Assessment Report, which projected a temperature increase in the range of 1–3.5ºC.
Positive feedback – negative impact
More worrying still, the scale and rate of change could be even greater than the IPCC predicts because of so-called ‘feedbacks’ – reactions in the biosphere triggered by a changing climate. These feedbacks could go both ways. As temperatures rise and precipitation changes, living organisms could respond by releasing carbon from soils and biomass – thus adding to the warming effect. On the other hand, in some instances, organisms could take up carbon – through the spread of boreal forests to higher latitudes, for example.
The IPCC’s scenarios include feedbacks between the land and the ocean, but still neglect potential feedbacks between terrestrial vegetation and soil micro-organisms and climate. Even so, the models suggest that even after several centuries, approximately one quarter of the increase in CO2 concentration caused by our emissions will still be present. But how much CO2 gets drawn down out of the atmosphere or released depends crucially on the biosphere – which the IPCC pays little attention to. Forest die-back and permafrost melting – resulting from higher temperatures – have the potential to release at least as much carbon into the atmosphere, in the form of CO2 and methane, as is currently there, doubling the atmospheric concentration in one stroke.
And those same feedbacks, by causing additional warming, are likely to cause less carbon uptake overall compared with today. Therefore, as our emissions increase, proportionately more CO2 will accumulate in the atmosphere and take longer to leave. A warmer ocean holds less CO2 compared with a cooler one, and warmer soils will have more respiratory activity, so ‘burning up’ carbon (instead of storing it) as soon as it accumulates.
The IPCC has tried to encompass that which it cannot yet measure with any certainty by assuming that at best the biosphere will reduce atmospheric levels of CO2 by 10 per cent, and at worst cause levels to rise by 30 per cent. Consequently, atmospheric CO2 could vary from 490ppmv for curbed emissions at the bottom range to 1,260ppmv for unrestrained emissions at the upper range (four times pre-industrial levels).
According to Jerry Mahlman, director of the Geophysical Fluid Dynamics Laboratory, the consequences of the unrestrained emissions scenario could be a temperature rise of 10–14ºC compared with today. Such an increase in greenhouse gases would generate untold changes to global climate. It would lead to droughts on an unprecedented scale, damaging torrential rain, longer lasting heat waves, the wide-scale spread of pests and disease to over-stressed systems, and devastating storm surges and cyclones in low-lying delta regions such as in Bangladesh. Against that background of destabilised climate, we would somehow be seeking to feed a population at least double today’s.
As The Ecologist stated in its special issue on climate change of 1999 (Vol 29/2), the response of the biosphere to global warming is clearly critical to any analysis. Any models need to reflect the interactions between all the factors that impinge on climate. To some degree, the IPCC admits to its current ignorance. It therefore sees the projected climate changes during the 21st century as having: ‘the potential to lead to future large-scale and possibly irreversible changes in Earth systems resulting in impacts at continental and global scales.’ Among these changes it cites ‘significant slowing of the ocean circulation that transports warm water to the North Atlantic, large reductions in the Greenland and West Antarctic ice sheets, accelerated global warming due to carbon cycle feedbacks in the terrestrial biosphere, and releases of terrestrial carbon from permafrost regions and methane from hydrates in coastal sediments’.
However, the IPCC then states, ‘these possibilities are very climate scenario-dependent and a full range of plausible scenarios has not been evaluated,’ and concludes, ‘the likelihood of many of these changes in Earth systems is not well-known, but is probably very low’.
Hadley Centre forecasts a more troubled future
Since life is so heavily implicated in what goes on in the atmosphere, (both in generating it in its current form and, through the hydrological cycle, playing a powerful role in the transfer of energy from the equator to the poles), it would seem that the IPCC is flying in the face of reason in its downplaying of the feedbacks involving the biosphere.
However, recent climate models developed at the Hadley Centre of the UK Met Office, one of the world’s leading centres on climate research, indicate that feedbacks involving the biosphere play a truly significant role in creating climate and could lead to temperature rises by 2100 that are significantly higher than those projected by the IPCC.(3)
Peter Cox and his colleagues at the Hadley Centre find that feedbacks between terrestrial vegetation and climate are likely to accelerate global warming. As they pointed out in 'Nature' (9 November, 2000): ‘General circulation models have generally excluded the feedback between climate and biosphere, using static vegetation distributions and CO2 concentrations from simple carbon-cycle models that do not include climate change.’ In contrast, the Hadley Centre’s ‘fully coupled, three-dimensional carbon-climate model... indicates that under a ‘business-as-usual’ scenario, the terrestrial biosphere acts as an overall carbon sink until about 2050, but turns into a source thereafter’.(4)
The Hadley Centre’s coupled carbon-climate model indicates an atmospheric concentration of CO2 for the present that differs little from the IPCC’s standard upper-range projection. But as feedbacks kick in, the two models begin to diverge. Twenty years from now, even without any cutting down of the rainforest, the Amazon forest begins to die-back as Amazonia begins to warm and dry out. The net result is that South America as a whole begins to lose carbon to the atmosphere. Then, after 2050, the land biosphere as a whole switches from being a weak sink for CO2 to becoming a strong source. Even though increased concentrations of CO2 in the atmosphere stimulate photosynthesis, particularly in temperate regions, any such gains are more than offset by increased respiration from soil organisms and from vegetation owing to the maintenance costs of resisting higher temperatures.
Even though land as a whole may have accumulated some 75 billion tonnes of carbon (GtC) between 1860 and 2000, during this century, as much as 170 GtC may be vented back into the atmosphere. As a result, the modelled CO2 concentration approaches 1,000ppmv by 2100 which is 250ppmv higher than that predicted in the ‘business-as-usual’ scenario of the IPCC. The carbon cycle model also results in higher global average temperatures, over 8ºC up on 1860 by 2100, compared to the 5.8ºC of the IPCC.(5)
Climate change could be worse still
This latest work is a salutary reminder of the limitations of climate models that do not include life in the dynamics of energy transfers across the planet. Yet, for all that Cox and Betts and their colleagues have now begun to include life, and are obtaining such important results, it must be appreciated that they can include only those factors that can be quantified with the current state of knowledge.
Uncertainties, such as the venting of methane from permafrost as it melts or from oceanic deposits of methane hydrates as they warm are not yet incorporated into the models. These are significant omissions as they comprise large potential sources of carbon, more than enough, in fact, to double or triple atmospheric concentrations.
Other omissions include the impact on climate of human-induced land-use changes, such as deforestation and the displacement of people onto marginal lands through the machinations of agribusiness. Brazil’s aid-assisted plan, Avança Brazil, for example, aims to convert half of its share of the Amazon into industrialised agriculture, taking no account of the dynamics of the Amazon Basin in which the forest is a self-sustaining system surviving through recycled rain from evapo-transpiration. As of now, we do not know the critical point of collapse. Even were Brazil to stick absolutely to its plan, that degree of forest destruction may be more than enough to bring about the forest’s total collapse. Such a possibility, however, is not included in any climate model.
Nevertheless, the Hadley Centre’s findings are of crucial importance: they tell us that a climate model that does not integrate the impact of life on the processes of energy exchange between the atmosphere and the earth’s surface is inadequate. Over and above that, their findings provide us with a stark warning about the consequences that will befall us if our behaviour remains unchanged, leaving us with only one sensible strategy: to act now to minimise those runaway impacts that could make our planet uninhabitable.
Peter Bunyard is science editor of The Ecologist.
1. IPCC: Summary for Policymakers. Third Assessment Report, March 2001.
2. Donald A Yerxa. ‘Harvard Astrophysicist Throws Cold Water on Global Warming’, Research News, April 2001, Templeton Foundation Press.
3. Richard A Betts, 1999. ‘Self-beneficial effects of vegetation on climate in an Ocean-Atmosphere General Circulation Model’, Geophysical Research Letters, Vol 26, No 10, 15 May.
4. Peter M Cox, Richard A Betts, Chris D Jones, Steven A Spall & Ian J Totterdell, 2000: ‘Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model.’ Nature, Vol 408, 9 November.
5. Richard A Betts: ‘Offset of the potential carbon sink from boreal forestation by decreases in surface albedo.’ Nature, Vol 408, 9 November, and Richard A Betts: ‘Biogeophysical impacts of land use on present-day climate: near-surface temperature change and radiative forcing.’ Atmospheric Science Letters, Vol 1, No 10.
6. Science (24 March, 2000 and April 2001), Levitus, Antonov, Boyer and Stephens, the US National Oceanographic Data Center and National Oceanic and Atmospheric Association.
7. Richard Wetherald and colleagues at the Geophysical Fluid Dynamics Laboratory. Geophysical Research Letters, 15 April, 2001.
8. Equinox, Channel Four, 17 June, 2001
9. Fred Pearce: ‘Something nasty brewing in the bog’, New Scientist, 2001, 25 August p 8.
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