The evidence continues to accumulate, strengthening the link between man's activity and a wide range of indicators of a changing climate, both globally and regionally.
Changes have now been observed in many different climate variables, in addition to temperature: the amount of moisture in the atmosphere; continuing sea-level rise; and a decreasing Arctic sea-ice extent. All are consistent with a long-term warming trend.
The period 2000-2009 was warmer than the 1990s that, in turn, were warmer than 1980s. In fact, the average temperature over the first decade of the 21st century was significantly warmer than any preceding decade in the instrumental record, stretching back 160 years.
Despite variability from year to year - which sees some years warmer and others cooler - we have identified a clear underlying trend of increasing global temperatures from the late 1970s of about 0.16 °C per decade.
Although we normally use temperature at the surface of the Earth as the primary indicator of climate change, there are other key observations that add to the evidence of a warming world.
We have compiled evidence from more than 20 institutions across the world as well as diverse sources from high in the atmosphere to the depths of the ocean. We've found changes in a number of indicators that are consistent with a warming world:
Short-term climate variations are also seen in observations, which temporarily increase or decrease the rate of change away from the long-term trends.
Most people are already familiar with the concept of natural variability, seeing the weather vary from hour to hour and day to day. In the same way, the climate varies naturally from year to year and decade to decade.
Natural variability within the climate system could explain the recent slowdown, but other factors could have contributed.
A natural downturn in radiation from the Sun occurred during some of the last decade as part of the well-known 11-year solar cycle, possibly cooling the Earth's surface.
A small reduction in stratospheric water vapour - also a greenhouse gas - has occurred in the last decade. The reasons for this reduction are not known.
A possible increase in aerosol emissions from Asia in the last decade may have contributed substantially to the recent slowdown. Aerosols cool the climate by reflecting sunlight.
The rate of warming in the last decade has been underestimated because of changes in ocean measurements and poor data coverage in the Arctic.
Changes in the way sea-surface temperatures were measured over the last decade have introduced a small artificial cooling of up to 0.03 °C over the last decade. This is now being corrected.
Satellite measurements and other evidence indicate that temperatures in the Arctic have increased at a faster rate in the last ten years. At the same time, some data suggest a slowdown in warming. This is because the region is poorly represented in datasets as there are very few observing stations. These issues are now being addressed.
There are two other factors of note, in addition to these key influences:
Since the late 1970s, when systematic monitoring of Arctic sea-ice began, there has been a marked decline in the extent of the ice, but with significant variations from year to year. There was a dramatic loss in 2007, followed by a partial recovery. 2008, 2010 and 2009 rank second, third and fourth lowest, respectively.
Highly variable atmospheric circulation in the Arctic summer plays an important part in sudden changes to sea-ice and can explain the dramatic drop which led to a minimum in sea-ice extent in summer 2007 and the low sea-ice in subsequent years. But climate models can only explain the decrease in ice extent if they take account of man-made factors as well as natural variations, strongly suggesting that human activity has contributed to the decline.