IPCC POLICYMAKER'S SUMMARY

EXECUTIVE SUMMARY

We are certain of the following:

• there is a natural greenhouse effect which already keeps the Earth warmer than it would otherwise be.
• emissions resulting from human activities are substantially increasing the atmospheric concen- trations of the greenhouse gases: carbon dioxide, methane, chlorofluorocarbons (CFCS) and nitrous oxide. These increases will enhance the greenhouse effect, resulting on average in an additional warming of the Earth's surface. The main greenhouse gas, water vapour, will increase in response to global warming and further enhance it.

We calculate with confidence that:

• some gases are potentially more effective than others at changing climate, and their relative effectiveness can be estimated. Carbon dioxide has been res- ponsible for over half the enhanced greenhouse effect in the past, and is likely to remain so in the future.
• atmospheric concentrations of the long-lived gases (carbon dioxide, nitrous oxide and the CFCS) adjust only slowly to changes in emissions. Continued emissions of these gases at present rates would commit us to increased concentrations for centuries ahead. The longer emissions continue to increase at present day rates, the greater reductions would have to be for concentrations to stabilise at a given level.
• the long-lived gases would require immediate reductions in emissions from human activities of over 60% to stabilise their concentrations at today's levels; methane would require a 15-20% reduction.

Based on current model results, we predict:

• under the IPCC Business-as-Usual (Scenario A) emissions of greenhouse gases, a rate of increase of global mean temperature during the next century of about 0.3'C per decade (with an uncertainty range of 0.2'C to 0.5'C per decade); this is greater than that seen over the past 10,000 years. This will result in a likely increase in global mean temperature of about I'C above the present value by 2025 and 3'C before the end of the next century. The rise will not be steady because of the influence of other factors.
• under the other IPCC emission scenarios which assume progressively increasing levels of controls, rates of increase in global mean temperature of about 0.2'C per decade (Scenario B), just above 0. I'C per decade (Scenario C) and about O.I'C per decade (Scenario D).
• that land surfaces warm more rapidly than the ocean, and high northern latitudes warm more than the global mean in winter.
• regional climate changes different from the global mean, although our confidence in the prediction of the detail of regional changes is low. For example, temperature increases in Southern Europe and central North America are predicted to be higher than the global mean, accompanied on average by reduced summer precipitation and soil moisture. There are less consistent predictions for the tropics and the Southern Hemisphere.
• under the IPCC Business as Usual emissions scenario, an average rate of global mean sea level rise of about 6cm per decade over the next century (with an uncertainty range of 3 - 10cm per decade), mainly due to thermal expansion of the oceans and the melting of some land ice. The predicted rise is about 20cm in global mean sea level by 2030, and 65cm by the end of the next century. There will be significant regional variations.

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.

Our judgement is that:

• Global - mean surface air temperature has increased by 0.3'C to 0.6'C over the last 100 years, with the five global-average warmest years being in the 1980s. Over the same period global sea level has increased by 10-20cm. These increases have not been smooth with time, nor uniform over the globe.
• The size of this warming is broadly consistent with predictions of climate models, but it is also of the same magnitude as natural climate variability. Thus the observed increase could be largely due to this natural variability; alternatively this variability and other human factors could have offset a still larger human-induced greenhouse warming. The unequivocal detection of the enhanced greenhouse effect from observations is not likely for a decade or more.
• There is no firm evidence that climate has become more variable over the last few decades. However, with an increase in the mean temperature, episodes of high temperatures will most likely become more frequent in the future, and cold episodes less frequent.
• Ecosystems affect climate, and will be affected by a changing climate and by increasing carbon dioxide concentrations. Rapid changes in climate will change the composition of ecosystems; some species will benefit while others will be unable to migrate or adapt fast enough and may become extinct. Enhanced levels of carbon dioxide may increase productivity and efficiency of water use of vegetation. The effect of wan-ning on biological processes, although poorly understood, may increase the atmospheric concentrations of natural greenhouse gases.

To improve our predictive capability, we need.

• to understand better the various climate-related processes, particularly those associated with clouds, oceans and the carbon cycle.
• to improve the systematic observation of climate- related variables on a global basis, and further investigate changes which took place in the past.
• to develop improved models of the Earth's climate system.
• to increase support for national and international climate research activities, especially in developing countries.
• to facilitate intemational exchange of climate data.