C. Mitchell and B. Geerts (1)
The greenhouse effect…global warming…climate change…phrases that have been at the centre of public debate around the world since the 1980's. An exploration of the uncertainties of the science relating to greenhouse needs to start with those elements of the subject that are the most broadly agreed before moving to the more contentious issues. This shorticle is based on Mitchell (1).
About 100 years ago (1896) a Swedish scientist made the first calculation of global warming that could be expected due to a doubling of carbon dioxide in the atmosphere.
As our understanding of climate expanded through the decades of the 20th century, it became clear that a number of other gases in a planet's atmosphere act to trap net radiation. These gases, known as greenhouse gases, are more transparent to solar radiation than to the infrared radiation emitted by the Earth's surface. Since a planet's temperature is determined by the balance between incoming solar and out-going planetary radiation, the overall effect of gases that trap net radiation near the surface of a planet will be to elevate the surface temperature to a level above that which would otherwise be the case (Section 2.8).
By careful observation on planet Earth, it also became clear that the comfortable temperature of the planet - averaging about 15°C - is largely due to the effect of these greenhouse gases. In the case of the Earth the major naturally-occurring greenhouse gases are: water vapour, carbon dioxide, methane, nitrous oxide, and ozone where it occurs near the Earth's surface.
In1957, the International Geophysical Year, high on a volcano in Hawaii, Dr Dave Keeling started continuous measurement of carbon dioxide at the Mauna Loa Observatory. The idea behind the observatory was to situate it far away from sources of city pollution, so that scientists could determine the composition of 'unpolluted' or baseline air. Data from Mauna Loa and elsewhere have shown, without doubt, that major greenhouse gases (carbon dioxide, nitrous oxide and methane) have been, and are, continuing to increase in the global atmosphere.
In parallel with these developments, scientists realised that it is possible to recover and analyse air from bubbles trapped in polar ice. Ice core measurements have enabled us to place recent changes in the atmosphere in an historic context. Very precise records are available for the last thousand years, with other records reaching back 200,000 years. The most surprising discovery was that during the last 200,000 years there is an excellent correlation between the atmospheric carbon dioxide concentration and the average temperature. During cold periods there was less CO2. The data do not, of course, explain this correlation.
During this century a number of artificial compounds: chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs), which are also powerful greenhouse gases, have been introduced into the atmosphere.
Furthermore, scientific analysis clearly shows that the combustion of fossil fuels and large-scale deforestation have driven the increase in carbon dioxide. Increases in methane and nitrous oxide are also linked to human activity.
Given this solid basis in scientific dogma, it surprises some that 'climate change' has become such an intense topic of discussion. However, the uncertainties really begin to intrude when we look to the future. In order to determine whether the changing composition of the atmosphere is cause for concern we need to answer a series of ever more difficult questions:
Each of these questions has been subjected to intensive investigation. For brevity, the remainder of this account will focus largely on the role of carbon dioxide.
Future emissions of greenhouse gases are unknown. However, by predicting population growth, economic activity, rates of technological change, and the energy mix thought likely to be used by different nations, it is possible to develop scenarios of future greenhouse gas emissions. Taking the many different views of the future into account, it is clear that the range of possibilities for future greenhouse gas emissions is very wide.
In fact, uncertainties surrounding the future growth of greenhouse gas emissions is a major contributor to the uncertainty surrounding estimate of future levels of greenhouse gases in the atmosphere, and hence future warming.
From a climate perspective, the emissions of greenhouse gases are not as important as the concentrations of the greenhouse gases in the atmosphere. Future concentrations are even less certain than future emissions.
When carbon dioxide is released into the atmosphere three things can happen to it:
The mass of planetary carbon remains constant. Therefore as we release carbon dioxide to the atmosphere the rate at which it will accumulate there will depend on the rate at which it can be incorporated into plants and the soil (the biosphere) and on the rate at which the oceans can remove it. Huge amounts of carbon are exchanged between the atmosphere, biosphere and oceans each year. This global carbon cycle has operated in one form or another for millions of years.
The annual natural exchange between biosphere and atmosphere is about 60 billion tonnes of carbon in each direction. By comparison, the yearly exchange between the oceans and atmosphere is about 90 billion tonnes. Against this massive exchange of carbon the human-caused changes are very small: we are adding about 7 billion tonnes of carbon to the atmosphere each year (2). Processes such as weathering of rocks, volcanism and coral reef-building also result in exchanges of carbon between the atmosphere and geosphere, but only over very long time periods. Nevertheless, the human additions are enough to change the fine balance of the global carbon cycle.
Scientists cannot be completely sure that as climate change proceeds the rate at which the oceans to absorb carbon will stay the same. Furthermore, levels of carbon dioxide in the atmosphere also influence plant growth. As carbon dioxide rises it is expected that elevated plant growth may increase the rate of removal of carbon from the atmosphere. Of course, this 'CO2 fertilisation' will only make a difference provided the rate of deforestation slows.
The interplay of these factors means that scientists can predict the amount of carbon remaining in the atmosphere for a given rate of future emission to within 15%.
It is interesting that the most stringent proposal put forward by any nation at the Kyoto, Japan, 1997 Climate Summit (a 2% per annum cut in emissions for the developed world) would, by the end of next century, reduce the projected increase in carbon dioxide concentrations in the atmosphere by about 100 part per million. The concentration of carbon dioxide in the atmosphere would still be more than double pre-industrial levels (3).
The discussion thus far begs the question: how important is global warming? Since greenhouse gases are responsible for keeping the planet warmer than it would be without them, it is reasonable to expect that any addition to the level of greenhouse gases in the atmosphere would cause the planet to warm. In fact, in an idealised case the effect of doubling the amount of carbon dioxide on temperature can be calculated quite accurately. All things being equal, a doubling of CO2 in the atmosphere would elevate the temperature of the Earth by just over 1°C (4).
The problem is that the Earth is not an idealised case. The climate of the Earth is the result of a series of complex interactions between the atmosphere, the oceans, the great polar icecaps and snow and ice covered regions. Each component of the climate system operates on many time-scales. For example the lower atmosphere mixes within a year, whereas the oceans mix over decades to centuries or even millenia (depending on whether we are considering the upper layers of the ocean or the full ocean depths).
Any change, or 'forcing' of one part of the climate system will affect other parts. In some cases the initial effect of perturbing the climate system will lead to other changes that will amplify that effect; in other cases the climate system will act to negate the perturbation. There are many potential feedbacks in climate (Note 7.A).
The complexity inherent in the climate system means that calculating the amount of warming resulting from changes of the composition in the atmosphere is not simple. Only two approaches are viable:
The latter approach is climate modelling. Although past and present observations are carefully analysed, it is the climate modelling approach that is mostly used for climate change studies.
One reason for the emphasis on climate modelling is that data from past climate changes are scarce and comparable circumstances from which we can learn are even rarer. After all, the level of carbon dioxide in the atmosphere today is higher than it has been for 400,000 years(5), and the influence of humans on the climate system is unprecedented.
In 1990, the international scientific community reviewed the science of climate change and attempted to agree on an estimate of global warming due to a doubling of carbon dioxide. At that stage it noted, but did not take into account any other influences of human activities on climate (6).
Thus in answer to the question: If we double the amount of carbon dioxide in the atmosphere (from pre-industrial levels) by how much will the Earth warm? The answer was, and still is: "somewhere between 1.5 and 4.5°C". This answer is further complicated by the fact that this warming will not eventuate immediately, but will flow through the system for decades, if not centuries.
The global warming resulting from a doubling of carbon dioxide in the atmosphere is known as the 'climate sensitivity'. The estimate of the climate sensitivity has remained unchanged for over a decade.
Unfortunately, even if we knew the climate sensitivity of the Earth more accurately, scientists would still be called on to 'please explain'.
First, humans are influencing the climate system in ways other than through changes in greenhouse gases. The increase in sulphur dioxide aerosols, due mainly to industrial emissions, has reduced global mean solar radiation at sea level by about 1 W/m2 during the twentieth century (1). This reduction is augmented by an increase, within clouds, of tiny droplets, nucleated by the aerosols. The combined (direct and indirect) aerosol effect is in some places comparable in magnitude to the greenhouse gas forcing, especially over and downstream of industrial areas in the northern hemisphere. Sulphur dioxide aerosols are fairly short-lived, they are usually washed out of the atmosphere within a few days (leading to acid rain). Greenhouse gases are long-lived and therefore have on global effect, which varies slightly with latitude (7). The most recent estimates of future climates, taking both the gases and the aerosols into account, show a larger warming in the mid-latitude belt of the southern hemisphere (which is largely ocean), but cooling in parts of the northern hemisphere (8).
Second, accompanying global warming, there is every reason to expect associated climate change. Even a very superficial understanding of physics would lead to an expectation that global warming may effect rates of evaporation, hence cloudiness and therefore precipitation. For reasons similar to those already outlined, computer models of the global climate system are the only viable method for investigating these possibilities.
As the underlying science of climate change has become more widely accepted and understood this decade, global climate models have come under ever-increasing scrutiny.
Much of the debate relating to the science of climate change relates not to whether or not there is a greenhouse effect (there is), nor whether humans will continue to release greenhouse gases (almost inevitable, even though the rate is unclear); nor whether concentrations will increase (they will as long as emissions continue); nor even whether global warming is expected (it is, provided the previous conditions are met).
The key issue at present is whether global climate models provide a sufficiently reliable picture of future climate for policymakers to determine what action should be taken to reduce greenhouse gas emissions. Economists face similar problems when they use complex economic models to determine the costs of reducing emissions.
Unfortunately, there is not a simple answer to the question: how good are climate models?
Much of the science associated with future climate change is engaged with assessing the performance of climate models. A full account of this the status of this aspect of the science is given in the Intergovernmental Panel on Climate Change Second Assessment Report (Chapter 6, Volume 1).
State-of-the-art global climate models (or general circulation models,GCMs) contain detailed representations of the atmosphere (its dynamics, radiative transfer, cloud and precipitation processes, and chemistry), oceans, ice, the land surface and vegetation. But GCMs also have well-recognised limitations:
On the other hand climate models:
In any event, scientists and policymakers do not have a real choice. Climate models offer an unmatched ability to explore the potential ramifications of atmospheric change.
As humanity's experiment with greenhouse gases proceeds, we will find out what is really happening and we can compare this to what climate models suggest. Differences between the two will be used to improve the models, and therefore reliability of model projections. Of course we cannot afford to wait and see too long.
Given the complexity of the science and the uncertainties that have been described, it is not surprising that climatologists have described the confidence in regional estimates of climate change as 'low'. This is a major difficulty, since climate affects people where they live. A simple measure of climate, such as global average temperature, because it is not regionally specific, is singularly uninformative to those who wish to assess the impacts of climate change. Thus, there is a need for more detailed regional assessments of potential climatic changes. Scientists who are asked to provide guidance to policymakers on possible changes in climate over the next century, refer to these assessments scenarios rather than predictions.
Over the past decade or so there have been many studies into the possible impacts of climate change. Some of these studies have involved multi-disciplinary teams investigating the multiple effects of climate change, social changes and the economies of particular regions. Other studies focus on the potential impacts of climate change in very specific terms. One example is work that shows that in the Australian Alps snow amounts are likely to decline as the planet warms.
Impacts research is as complex an area of investigation as any in the climate change area. The IPCC has recently completed a major region-by-region review (9). This review points out that human health, ecosystems and economic activities are sensitive to changes in climate. As a result of this sensitivity, many regions are likely to be adversely affected by climate change, in some cases irreversibly. On the other hand, some impacts of climate change are likely to be beneficial.
Even though climate change cannot be predicted for a specific region, our experience with natural climatic variability leads many scientists to believe that even relatively small changes to climate may be socially and economically significant. The effects of climate on our agriculture, natural resources and even our cities are all pervasive, but are often subtle or influence our systems through a chain of interactions. For instance, electricity companies plan supplies around demand curves. The influence of climate on demand may only become apparent when an unexpected summer heatwave creates a demand for electricity that cannot be met as millions of city-dwellers turn on their air conditioners. Major surges in demand like this can lead to power blackouts.
The current scientific knowledge of climate change presents policymakers with a number of conundrums: