Temperature changes

SURFACE TEMPERATURE CHANGE

Recent Global temperatures have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900, according to the instrumental temperature record. The urban heat island effect is unlikely to have significantly influenced this value and is estimated to account for about 0.02 °C of warming since 1900.[51] Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[52] Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.[citation needed] Ocean temperatures increase more slowly than land temperatures because of the larger effective heat capacity of the oceans and because the ocean loses more heat by evaporation.[53] The Northern Hemisphere has more land than the Southern Hemisphere so it warms faster. The Northern Hemisphere also has extensive areas of seasonal snow and sea-ice cover subject to the ice-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the difference in warming because the major greenhouse gases persist long enough to mix between hemispheres.[54] Based on estimates by NASA's Goddard Institute for Space Studies 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.[55] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[56][57] Temperatures in 1998 were unusually warm because the strongest El Niño-Southern Oscillation in the past century occurred during that year.[58] Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation.[59][60] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.[61]