Recent Changes

Over the last century, global average temperatures and sea levels have risen significantly, while snow cover in the Northern Hemisphere has declined (see Figure SA-17; National Geographic Society, 2007). The total temperature increase from 1850-1899 to 2001-2005, estimated at 0.76°C (0.57°C to 0.95°C), occurred during a warming trend that appears to be gaining momentum. The rate of warming for the last 50 years was double that during the previous half-century, and 11 of the last 12 years (1995-2006) rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850). Over the last 50 years, cold days, cold nights, and frost have become less frequent, while hot days, hot nights, and heat waves have become more frequent.

Measurements conducted since 1961 show that the average temperature of the global ocean has increased to depths of at least 3,000 meters and that oceans have absorbed more than 80 percent of the heat added to the climate system.

44 GLOBAL CLIMATE CHANGE AND EXTREME WEATHER EVENTS

44 GLOBAL CLIMATE CHANGE AND EXTREME WEATHER EVENTS

Temperature Change Recent Years
Number of years before present (quasi-log scale)

FIGURE SA-16 Variation in Earth's average surface temperature over the past 20,000 years.

SOURCE: Reprinted from WHO et al. (2003) with permission from the World Health Organization. Copyright 2003.

Such warming causes seawater to expand, contributing to sea level rise, as have widespread decreases in glaciers and ice caps. Global average sea level rose at an average rate of 1.8 mm per year between 1961 and 2003 and at a rate of about 3.1 mm per year between 1993 and 2003. Current estimates indicate that sea levels rose 0.17 m over the course of the twentieth century (see Figure SA-18; IPCC, 2007).

Present Effects and Future Projections

A warmer global climate system accelerates the hydrological cycle, increasing the likelihood of extreme weather phenomena such as droughts, heavy precipitation, heat waves, hurricanes, typhoons, and cyclones (see Figure SA-19; National Geographic Society, 2007). More intense and longer droughts, which have been observed over wider areas since the 1970s and particularly in the tropics and subtropics, have been associated with higher global temperatures, but also

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