How is climate changing and how has it changed in the past?
What changes have been observed so far in climate? What is causing the present-day changes in climate?
Observed changes in climate
The warming of global climate is now unequivocal. There are many observations of increasing air and ocean temperatures, widespread melting of snow and ice, and rising sea levels.
More specifically, 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), global temperature has increased by 0.74°C. Global sea level has risen by 17 cm during the 20th century, in part because of the melting of snow and ice from many mountains and in the polar regions. More regional changes have also been observed, including changes in Arctic temperatures and ice, ocean salinity, wind patterns, droughts, precipitations, frequency of heat waves and intensity of tropical cyclones.
The temperatures of the last half century are unusual in comparison with those of at least the previous 1300 years. The last time that the polar regions remained significantly warmer than now for a very extended period (125 000 years ago), the sea level rose by 4 to 6 meters.
Causes of present-day climate change
Most of the increase in global temperature observed over the past fifty years is very likely due to human emissions of greenhouse gases.