The scientific uncertainty

Before considering the 'weighing' process and the cost of action, we begin by explaining the nature of the scientific uncertainty and how it has been addressed by the scientific community.

In earlier chapters I explained in some detail the science underlying the problem of global warming and the scientific methods that are employed for the prediction of climate change due to the increases in greenhouse gases. The basic physics of the greenhouse effect is well understood. If atmospheric carbon dioxide concentration doubles and nothing else changes apart from atmospheric temperature, then the average global temperature near the surface will increase by about 1.2 °C. That figure is not disputed among scientists.

However, the situation is complicated by feedbacks and regional variations. Numerical models run on computers are the best tools available for addressing these complications because they are able effectively to add together all the non-linear interactions. Although they are highly complex, climate models are capable of giving useful information of a predictive kind. As was explained in Chapter 5, confidence in the models comes from the considerable skill with which they simulate present climate and its variations (including perturbations such as the Pinatubo volcanic eruption) and from their success in simulating past climates; these latter are limited as much by the lack of data as by inadequacies in the models.

However, model limitations remain, which give rise to uncertainty (see box below). The predictions presented in Chapter 6 reflected these uncertainties, the largest of which are due to the models' failure to deal adequately with clouds and with the effects of the ocean circulation. These uncertainties become of greatest importance when changes on the regional scale, for instance in regional patterns of rainfall, are being considered.

With uncertainty in the basic science of climate change and in the predictions of future climate, especially on the regional scale, there are bound also to be uncertainties in our assessment of the impacts of climate change. As Chapter 7 shows, however, some important statements can be made with reasonable confidence. Under nearly all scenarios of increasing carbon dioxide emissions this century, the rate of climate change is likely to be large, probably greater than the Earth has seen for many millennia. Many ecosystems (including human beings) will not be able to adapt easily to such a rate of change. The most noticeable impacts are likely to be on the availability of water (especially on the intensity of heat waves, the frequency and severity of droughts and floods), on the distribution (though possibly not on the overall size) of global food production and on sea level in low-lying areas of the world. Further, although most of our predictions have been limited in range

The reasons for scientific uncertainty

The Intergovernmental Panel on Climate Change1 has described the scientific uncertainty as follows.

There are many uncertainties in our predictions particularly with regard to the timing, magnitude and regional patterns of climate change, due to our incomplete understanding of:

• sources and sinks of greenhouse gases, which affect predictions of future concentrations,

• clouds, which strongly influence the magnitude of climate change,

• oceans, which influence the timing and patterns of climate change,

• polar ice-sheets, which affect predictions of sea level rise.

These processes are already partially understood, and we are confident that the uncertainties can be reduced by further research. However, the complexity of the system means that we cannot rule out surprises.

to the end of the twenty-first century, it is clear that by the century beyond 2100 the magnitude of the change in climate and the impacts resulting from that change are likely to be very large indeed.

The statement in the box regarding scientific uncertainty was formulated for the IPCC 1990 Report. Eighteen years later it remains a good statement of the main factors that underlie scientific uncertainty. That this is the case does not imply little progress since 1990. On the contrary, as the subsequent IPCC Reports show, a great deal of progress has taken place in both scientific understanding and the development of models. There is now much more confidence that the signal of anthropogenic climate change is apparent in the observed climate record. Models now include much more sophistication in their scientific formulations and possess increased skill in simulating the important climate parameters. For regional scale simulation and prediction, regional climate models (RCMs) with higher resolution have been developed that are nested within global models (see Chapters 5 and 6). These RCMs are beginning to bring more confidence to regional projections of climate change. Further, over the last decade, a lot of progress has been made with studies in various regions of the sensitivity to different climates of these regions' resources, such as water and food. Coupling such studies with regional scenarios of climate change produced by climate models enables more meaningful impact assessments to be carried out2 and also enables appropriate measures to be assessed. Particularly in some regions large uncertainties remain; it will be seen for instance from Figure 6.7 that current models perform better for some regions than for others.

Socio-economic assumptions

Emissions scenarios

Concentration projections (Greenhouse gases and aerosols)

Radiative forcing projections

Sea level projections

Climate scenarios

Global change scenarios

Global change scenarios

Summarised in Figure 9.1 are the various components that are included in the development of projections of climate change or its impacts. All of these possess uncertainties that need to be aggregated appropriately in arriving at estimates of uncertainties in different impacts.

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