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CGD Climate Highlights: Climate Change

Why do we believe the climate is changing?

The globe is warming. Averaged over all land and ocean surfaces, temperatures today are about 0.75°C warmer than at the beginning of the 20th Century. Rates of temperature rise are greater in recent decades: since 1979, global surface temperatures have increased more than 0.4°C. Land regions have warmed the most (0.7°C since 1979), especially over the Northern Hemisphere continents. The warmest year in the 145-year global instrumental record is 1998, since the major 1997-98 El Niño enhanced it. The year 2005 is the second warmest on record, followed by the years 2002-2004. Based on reconstructions of temperature from proxy data, like tree rings and ice cores, several studies have also concluded that NH surface temperatures are warmer now than at any time in at least the last 1,000 years.

This warming is also consistent with a large amount of other observations. For example, there has been a widespread reduction in the number of frost days in middle latitude regions, principally due to an earlier last day of frost in spring. There has been an increase in the number of warm extremes and a reduction in the number of daily cold extremes, especially at night. Ocean temperatures have warmed not only at the surface, but at depth as well. As oceans warm, seawater expands and sea level rises. Over the 20th Century, sea levels have risen 15-20 centimeters.

There has been a nearly worldwide reduction in mountain glacier mass and extent. Snow cover has decreased in many regions of the Northern Hemisphere, and sea-ice extents have decreased in the Arctic, where average annual temperature has increased significantly since the 1960s.