Global Warming

The scientificuncertainty 217

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 also 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 loom largest 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 regionalscale, there are
bound also to be uncertainties in our assessment of the impacts of cli-
mate change. As Chapter 7 shows, however, some important general
statements can be made with reasonable confidence. Under nearly all
scenarios of increasing carbon dioxide emissions next century, the rate
of climate change is likely to be large, probably greater than the Earth
has seen for many millennia. Many ecosystems (includinghuman be-
ings) will not be able to adapt easily to such a rate of change. The most

The reasons for scientific uncertainty

The Intergovernmental Panel on Climate Change^1 has described the sci-

entific 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 incompleteunderstanding of:


sourcesand sinks of greenhouse gases, which affect predic-

tions 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 confi-

dent that the uncertainties can be reduced by further research. How-

ever, the complexity of the system means that we cannot rule out

surprises.