SciDev.Net

Summary

Critics of those demanding touch political action to curb global warming frequently point to the continued uncertainty over the scientific evidence. They argue that our current understanding of the mechanisms of climate change is insufficiently developed to justify the stringent measures that many scientists, environmentalists and governments say are needed.

This position is in conflict with the widely held view among climate scientists that, whatever gaps and uncertainties remain in our understanding of climate change, these are not sufficient to undermine the argument that urgent action is needed.

Nevertheless, such gaps and uncertainties do still exist. And some of them represent, at least in principle, a potential challenge to the argument that an unacceptable level of global warming is likely to occur without such action. It is therefore important for those engaged in the debate over climate change to be aware of these gaps — and how they can be at least partly answered.

Setting the scene
When US President George W Bush announced in March 2001 that the US administration was no longer prepared to support the Kyoto Protocol, one reason he gave was continuing uncertainty over the scientific reasoning on which its targets for reductions in greenhouse gas emissions have been based.

Keen to justify the decision, his officials, repeating words often heard from sections of the fossil-fuel industry and other opponents of the protocol, emphasized that the president was — like everyone else — eager that efforts to combat global warming are based on “sound science”. But, unlike the dominant opinion in Europe and the climate research community elsewhere, Bush’s claimed that our current understanding of the mechanisms of climate change is insufficiently developed to justify the stringent measures that many scientists, environmentalists and governments say are now needed.

Arguments about the adequacy of the science were quickly challenged by supporters of the protocol. They pointed to the conclusions of the Intergovernmental Panel on Climate Change, and in particular its Working Group No 1, the panel set up in 1988 and given the task of formulating a consensus on the current state of scientific knowledge. This states in its most recent report, published in January 2001, that “there is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities”.

The IPCC report reflects the widely held view among climate scientists that, whatever gaps and uncertainties remain in our understanding of climate change, these are not sufficient to undermine the argument that urgent action is needed.

Nevertheless, such gaps and uncertainties do still exist. And few deny that some of them represent, at least in principle, a potential challenge to the argument that an unacceptable level of global warming is likely to occur without such action. It is therefore important for those engaged in the debate over climate change to be aware of these gaps — and how they can be answered, at least in part.

Broad-based criticisms
Some of the arguments against the damage induced by man-made global warming are relatively broad-based. One, for example, is that we still lack sufficiently accurate knowledge of the carbon cycle, and in particular of the fate of carbon dioxide in the atmosphere, to give us any confidence in predictions of the future behaviour of the climate.

Critics argue that continuous debate in the scientific community over, for example, the rate at which carbon dioxide is taken up into the oceans or by different forms of vegetation on the grounds that we can have little confidence in validity of the models that are used to simulate the rate of uptake.

They point out, for example, that the most recent IPCC report provides a relatively broad range of predictions of likely temperature rise over the next century, from 1.4 to 6 degrees centigrade. Indeed this is even larger than a range of temperature rises of 1 to 3.5 degrees quoted in its earlier report. Critics argue that such a level of uncertainty means that we should hold back from making policy decisions based on modelling until we can be more confident in the accuracy of the models.

A related argument is that the current climate models do not have a particularly good track record either of explaining the behaviour of the climate in the historical past — examples commonly given are the temperature rise that took place in middle of the first half of the twentieth century, or the cooling in the mid-1970s.

Patrick Michaels, for example, a meteorologist at the University of Virginia, for example, and one of the most persistent opponents of the general consensus of climate change scientists, has stated simply that “climate models that were heavily cited as providing a scientific basis for the Framework Convention were wrong“ (Testimony to US House of Representatives, March 6 1996).

The scientists’ response
The general response to such arguments from the scientific community is that, however imperfect their current models may be, they are based on aspects of the chemical and physical behaviour of the atmosphere that are well understood. The models have already proved to be of significant value in understanding certain phenomena, and, as they are refined, are becoming increasingly accurate.

Many accept that there is a strong need to improve the quality of climate data, for example by improving the effectiveness of the Global Climate Observing System (GCOS), particular in the data gathered from Africa, South America and Asia. But most scientists argue that this does not mean that the current computer-based climate models they should not be used for attempting to predict future climate change. Merely that they should be used with caution, and with a full awareness of their actual and potential weaknesses.

Criticism of sea-level predictions
The same is true about predictions of rises in sea level. It is generally believed in the scientific community, as reflected in the IPCC report, that global warming will lead to a general rise in sea-levels around the world, significantly increasing the threat of flooding in low-level islands and coastal regions, even though there is currently little consensus on how big this rise is likely to be.

At least in theory, there are factors that could reduce — or perhaps even eliminate — the impact of the melting of the icecaps, such as increased evaporation from the ocean due to higher water temperatures. Both factors have led some critics to argue that the issue of sea-level has been overdramatized. Fred Singer, for example, of the University of Virginia, claims it is “likely” that increased evaporation from the ocean “may lead to a more rapid accumulation of polar ice, and thus a lowering of sea level”.

Most climate researchers accept that they need a better understanding of the full spectrum of factors at work before they can have confidence in their predictions about the impact of climate change on sea-level rise. They also acknowledge that what actually happens will depend on a number of social and political factors (not least whether emissions of greenhouse gases are in fact reduced).

But apart from critics such as Singer, few mainstream scientists who accept the limitations of the current data are prepared to accept that the rise is likely to be insignificant.

surface v upper air temperatures
A similar situation concerns what some have claimed to be another flaw in current climate models: the apparent discrepancy between surface temperatures and upper air temperatures which had led to controversy over whether global warming was actually taking place.

This discrepancy is based on the observation that, it apparent conflict with measurements indicating a rise in temperature in the earth’s surface in the last century, satellite and balloon data collected since 1979 indicate little to any warming of the atmosphere up to 5 miles from the surface — the low- to mid-troposphere.

This anomaly has been widely quoted by critics to throw doubt on the whole global warming hypothesis. But a panel of the National Academy of Sciences reported in January 2000 that this discrepancy — which now appears to originate in the different time periods over which the respective data was collected — did not undermine a general conclusion that that the warming of the Earth’s surface is “undoubtedly real”.

The impact of sulphate particles
Where even supporters of the main IPCC consensus agree that critics have a stronger case in pinpointing weaknesses in current climate models are on two issues that have recently attracted growing attention from researchers. One is on the impact of sulphate and other particles in the atmosphere on climate fluctuation. The second is on the role played by high-level clouds, particularly over the tropical oceans.

The particle issue was highlighted by an increasing awareness in the early 1990s of the cooling role of sulphate aerosols, based partly on observations of the impact of the eruption of Mount Pinatubo in 1991 that appeared to decrease the temperature of the upper atmosphere for several years. The issue that faced climate modellers at the time was that their earlier models did not take aerosols into account.

But rather than being a fatal flaw, these modellers say that the aerosol factor may help explain why some of the earlier predictions were less accurate than they had expected. For there was is some evidence that the ‘masking’ effect of aerosols may have been responsible for earlier overestimates of the rate of global warming. In its second full assessment of scientific knowledge of climate change, for example, published in 1995, the IPCC stated that “there is growing evidence that increases in sulphate aerosols are partially counteracting the [warming] due to increases in greenhouse gases“.

Even supporters of the IPCC consensus admit that major gaps in our scientific understanding of the behaviour of particles in the atmosphere, and the implications for this behaviour for climate models, still exist. “Everyone would agree that aerosols are the big question mark,“ says one climate scientist.

This uncertainty has been pointed out by a number of authors. A paper published in Nature in early 2001 by Mark Jacobson from Stanford University, for example, showed that soot — which is closely associated with sulphate aerosols — plays an important but opposite effect: soot, due to its intrinsic black colour, reduces the reflective properties of aerosol particles as a whole, possibly to the extent that any cooling from the sulphate component could be offset by warming from the soot component.

Critics have pointed to Jacobson’s work to argue that we lack a sufficient understanding of the effects of aerosols to give us any confidence in models of the behaviour of the atmosphere. Certainly there is widespread agreement in the scientific community that Jacobson’s work appears to suggest that some of the IPCC’s conclusions will have to be reconsidered.

But the dominant view at present is that our current knowledge — however inadequate — tends to reinforce, not undermine, current concerns. Furthermore, many point out that the climate computer models that do include the impact of aerosols are the most accurate that have been produced so far in simulating observed climate patterns, indicating that even if knowledge is lacking, it is moving in the right direction. The main need, they argue, is for more research into this issue.

Uncertainties over water vapour
A similar need for more research is widely acknowledged on the question of the feedback generated by the increasing amount of water vapour — the most important greenhouse gas from a radiative point of view — that evaporates into the atmosphere as the temperature of the globe increases, and in particular what happens at high levels.

This is an issue that has been highlighted by Richard Lindzen, professor of meteorology at the Massachusetts Institute of Technology, who has long been an active critic of some of the more strident claims being made by critics of global warming.

Lindzen’s main argument is that beyond a certain level of warming, the high atmosphere about the tropical oceans — which is normally occupied by very high clouds — literally ‘opens up’ and allows heat to be dissipated into space.

This argument has been reinforced by recent satellite observations above the Indian Ocean that such upper level clouds can indeed evaporate under warming conditions — a process claimed by critics to contradict the assumptions of climate models that warming would lead to greater cloud cover, and thus more heat trapped, in these regions (See Bulletin of the American Meteorological Society, March 2001).

Even those who disagree with the main thrust of Lindzen’s political arguments — that the problems associated with global warming have been deliberately exaggerated by those seeking to boost funding for the personal research projects — admit that the issue he has identified is a significant one.

The 1995 assessment of the IPCC, for example, admits that feedback from water vapour in response to the build-up of carbon dioxide and other greenhouse gases “remains a substantial uncertainty in climate models”. Similarly its most recent report admits that climate models “cannot yet simulate all aspects of climate“ and that there “particular uncertainties associated with clouds and their interaction with radiation and aerosols“.

The ambiguities of ‘sound science’
Virtually all of those engaged in the debate over global warming are committed to the idea that policy must be based on ‘sound science’. The difference lies in what this phrase means. To some — including those currently resisting a commitment to the Kyoto targets — ‘sound science’ implies a degree of scientific certainty that others claim to be both unrealistic and unnecessary.

The essential argument is not over whether there are still gaps in our scientific understanding of climate change; most climate scientists will admit that such gaps will always exist. Rather it is over whether these gaps are sufficiently large to undermine the conclusion that significant action is now needed to mitigate the problems that lie ahead.

Michaels and others argue (including, it would now seem, President Bush) that uncertainties in our current level of scientific understanding suggest that we should leave the situation to market forces and their impact on enhanced technological innovation; “doing very little about this is the best way to solve the problem,“ he says (See Nature, 400, 5; 2001, 'US ‘wasted vital time’ as climate-change minority sows confusion').

But relatively few of those scientists professionally engaged in the studying the behaviour of the climate accept that argument.

The author is the director of SciDev.Net