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.

Long-term climate change

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.

Other indicators of a warming world

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:

• A discernable increase in air temperature observed above both the land and sea.
• Increases in water temperature at the sea-surface down to hundreds of metres below the surface.
• An increase in humidity as a warmer atmosphere holds more moisture.
• Increases in sea-level as warmer waters expand and land-based ice melts.
• Shrinking of Arctic sea-ice, glaciers and Northern Hemisphere spring snow cover.

Short-term trends in climate

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.

Changes in solar activity

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.

Changes in stratospheric water vapour

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.

Increased aerosol emissions from Asia

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 to sea-surface temperature measurements

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.

Strong warming in the Arctic is poorly represented

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:

• Taking the temperature of the deep ocean - it is possible that more heat is being transported to the deep ocean, leading to less warming at the surface. The deep ocean temperature remains a major uncertainty, as we have only been able to monitor the ocean, to about 2000 m, since 2002.
• Natural factors important in previous decades - El Niño / La Niña variability has contributed a net warming effect over the last decade. There have also been very few climatically significant volcanic eruptions, which tend to cool the climate, in the same period.

Arctic summer sea-ice

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.

The climate in 2011