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Climate Change 2007 Update

2. How is climate changing and how has it changed in the past?

  • 2.1 What changes have been observed so far in climate?
  • 2.2 How has climate changed in the past?
  • 2.3 What is causing the present-day changes in climate?

2.1 What changes have been observed so far in climate?

Since the Third Assessment Report (TAR), improvements in terms of data, geographical coverage, understanding of uncertainties, and variety of measurements have allowed for a better understanding of how climate is changing in space and time. The warming of global climate is unequivocal and is evidenced by numerous observations of increasing air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level (see Figure SPM-3).

Eleven of the last twelve years (1995 -2006) rank among the 12 warmest years ever recorded since global surface temperatures are measured (1850). Over the last 100 years, (1906–2005) there has been an increase in surface temperature of 0.74°C, which is larger than the 0.6°C increase given in the TAR for the 1901-2000 period. And the warming trend over the last 50 years (0.13°C per decade) is nearly twice that for the last 100 years. Temperatures in the higher atmosphere and in the oceans (to depths of at least 3000m) have also been rising, along with the water vapor content of the atmosphere. Mountain glaciers, snow cover and ice caps have declined on average in both hemispheres, contributing in part to the rise of global sea level. The Greenland and Antarctic ice sheets have also contributed to the observed rise of sea level, which amounted to 17cm in total over the course of the 20th century.

At continental, regional, and ocean basin scales, numerous long-term changes in climate have been observed. These include changes in Arctic temperatures and ice, widespread changes in precipitation amounts, ocean salinity, wind patterns and aspects of extreme weather including droughts, heavy precipitation, heat waves and the intensity of tropical cyclones (Table SPM-1). Some aspects of climate have not been observed to change. The difference of temperature between day and night, for example, has remained the same, since daytime and nighttime temperatures have risen by the same amount. Contrary to sea ice in the Arctic, there has been no significant decrease in sea ice in Antarctica, which fits with the lack of observed warming in Antarctica. More...

2.2 How has climate changed in the past?

Studies of past climate have allowed inferences to be made about past changes in global climate on time scales ranging from a few decades to millions of years. The uncertainties related to these conclusions on past climate generally increase with time into the past.

This information on past climate show that the overall temperatures of the last half century is unusual in at least the previous 1300 years. The last time that the climate was significantly warmer than now for an extended period (about 125,000 years ago, during the last interglacial period), reductions in polar ice volume led to 4 to 6 metres of sea level rise. More...

2.3 What is causing the present-day changes in climate?

It is very likely that a significant part of temperature variability in the Northern hemisphere during the past seven centuries prior to 1950 is due to volcanic eruptions and changes in the intensity of solar radiation. However, most of the observed increase in global temperature since then is very likely due to the observed increase in atmospheric greenhouse gas concentrations due to human activities. Human activities now clearly affect other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns.

It is likely that these increases in greenhouse gas concentrations alone would have caused more warming than observed, had volcanic and human-induced aerosols not offset some of the warming that would otherwise have taken place.

Current climate models that simulate the observed temperature evolution on each of six continents provide stronger evidence of human influence on climate than was available in the 2001 Third Assessment Report (TAR). Difficulties remain in simulating temperature changes at smaller scales, where natural climate variability is larger and makes it harder to estimate the current and future impact of greenhouse gas increases due to human activities. More...

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