Climate Change is happening

== Overview ==

Over the last hundred years or so, the instrumental temperature record has shown a trend in climate of increased global mean temperature, i.e., global warming. Other observed changes include Arctic shrinkage, Arctic methane release, releases of terrestrial carbon from permafrost regions and Arctic methane release in coastal sediments, and sea level rise.

Global average temperature is predicted to increase over this century, with a probable increase in frequency of some extreme weather events, and changes in rainfall patterns. Moving from global to regional scales, there is increased uncertainty over how climate will change. The probability of warming having unforeseen consequences increases with the rate, magnitude, and duration of climate change. Some of the physical impacts of climate change are irreversible at continental and global scales. Sea level is expected to rise 18 to 59 cm (7.1 to 23.2 inches) by the end of the 21st century. Due to a lack of scientific understanding, this sea level rise estimate does not include all of the possible contributions of ice sheets. Slowing of the Meridional Overturning Circulation is very likely to occur this century, but temperatures in the Atlantic and Europe will probably still be higher due to global warming. For a global warming of 1-4°C (relative to 1990-2000), there is a moderate chance that partial deglaciation of the Greenland ice sheet would occur over a period of centuries to millennia. Including the possible contribution of partial deglaciation of the West Antarctic Ice Sheet, sea level would rise by 4?6 m or more.

The impacts on human systems of climate change will probably be distributed unevenly. Some regions and sectors are expected to experience benefits while others will experience costs. With greater levels of warming (greater than 2-3°C, relative to 1990 levels), it is likely that benefits will decline and costs increase. Low-latitude and less-developed areas are probably at the greatest risk from climate change. With human systems, adaptation potential for climate change impacts is considerable, although the costs of adaptation are largely unknown and potentially large. Climate change will likely result in reduced diversity of ecosystems and the extinction of many species. Adaptation potential for biological and geophysical systems is estimated to be lower than that for human systems.