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Common Questions about Climate Change
Published in 1997 by the United Nations Environment Programme - World Meteorological Organization




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What Climate Changes Are Projected?

The IPCC has projected further increases in globally averaged surface temperatures of 1 to 3.5°C (about 2 to 6°F) by the year 2100, as compared with 1990 (Figure 6.1). This projection is based on estimates of future concentrations of greenhouse gases and sulfate particles in the atmosphere.

Figure 6.1

(top)The possible range of globally average surface temperature increase is shown for the period 1990 to 2100.

(bottom) The possible range of globally averaged sea level rise is shown for the period 1990 to 2100.

The average rate of warming of the Earth's surface over the next hundred years will probably be greater than any that has occurred in the last 10,000 years, the period over which civilization developed. However, specific temperature changes will vary considerably from region to region.

As a result of the warming, global sea level is expected to rise by a further 15 to 95 cm (about 6 to 37 inches) by the year 2100, because sea water expands when headed and some glacial ice will melt.

Although globally averaged surface temperature increases and sea level rise are the most certain of the IPCC projections, other effects can be projected with some confidence. Greater warming is expected to occur over land than over the oceans. The maximum warming is expected to occur in the Arctic in winter. Nighttime temperatures are expected to increase more than daytime temperatures. In general, there will probably be an increase in the number of very hot days at mid-latitude locations in summer, such as in most of North America, Europe, and parts of South America, with a decrease of very cold days in the same locations in winter.

Extreme events such as heavy rains and droughts are the most destructive forms of weather, and the frequency and duration of these events are likely to increase as the climate continues to change. Increases in the global averages of both evaporation and precipitation are expected. In winter at mid-latitudes, higher surface temperatures are expected to cause an increased portion of the precipitation to fall in the form of rain rather than snow. This is likely to increase both wintertime soil moisture and runoff, leaving less runoff for summer. In spring, faster snow melt is likely to aggravate flooding. In the summer, increased heating will lead to increased evaporation, which could decrease the availability of soil moisture needed both for natural vegetation and agriculture in many places, and increase the probability of severe drought. Droughts and floods occur naturally around the world, for example in association with El Niño events, but are likely to become more severe, causing water management to become an even more critical problem in the future.

The most uncertain projections of future climate relate to changes in particular locales, as well as how weather events such as tropical storms, including hurricanes, typhoons, and cyclones, will be affected. This uncertainty results from the existence of large natural regional variations, as well as limitations in computer models and the understanding of the relationship between local and global climate at the present time.

The range of estimated warming of 1 to 3.5°C (about 2 to 6°F) by the year 2100 arises from uncertainties about the response of climate to the buildup of greenhouse gases and particles, as well as the total amount of future emissions of these gases. Factors such as estimates of human population growth, land use changes, life styles, and energy choices yield a range of plausible greenhouse gas emissions. For example, concerted efforts to reduce emissions of greenhouse gases would lead to a significantly lower projected temperature rise.

All of these predictions are based on the assumption that the global climate will change gradually. However, there is evidence to suggest that the Earth's climate has occasionally changed rather rapidly in the distant past. There may be similarly abrupt transitions due to human-induced climate change. These abrupt transitions raise the possibility of significant surprises as the world warms over the next century, perhaps with rapid and unexpected changes in ocean currents and regional climate. The likelihood that such rapid changes could occur increases with increasing emissions of greenhouse gases.

Go to "How Reliable Are Predictions of Future Climate?"

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