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.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.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