Temperature records going back to the late 19th Century show that the average temperature of the Earth's surface has increased by about 0.8C (1.4F) in the last 100 years. About 0.6C (1.0F) of this warming occurred in the last three decades.
Satellite data shows an average increase in global sea levels of some 3mm per year in recent decades. A large proportion of the change in sea level is accounted for by the thermal expansion of seawater. As seawater warms up, the molecules become less densely packed, causing an increase in the volume of the ocean.
But the melting of mountain glaciers and the retreat of polar ice sheets are also important contributors. Most glaciers in temperate regions of the world and along the Antarctic Peninsula are in retreat.
Since 1979, satellite records show a dramatic decline in Arctic sea-ice extent, at an annual rate of 4% per decade. In 2012, the ice extent reached a record minimum that was 50% lower than the 1979-2000 average.
The Greenland Ice Sheet has experienced record melting in recent years; if the entire 2.8 million cu km sheet were to melt, it would raise sea levels by 6m. Satellite data shows the West Antarctic Ice Sheet is also losing mass, and a recent study indicated that East Antarctica, which had displayed no clear warming or cooling trend, may also have started to lose mass in the last few years.
But scientists are not expecting dramatic changes. In some places, mass may actually increase as warming temperatures drive the production of more snows.
The effects of a changing climate can also be seen in vegetation and land animals. These include earlier flowering and fruiting times for plants and changes in the territories (or ranges) occupied by animals.
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