Increase in mass loss by the West Antarctic ice sheet. The mass loss has been steadily increasing since the 1970s as a result of accelerations in glacier flow; snowfall has not changed significantly in Antarctica over the past 50 years.

Initially, meltwater was assumed to be the prime cause of glacier acceleration, making its way to the ground beneath ice sheets, lubricating it and causing the glaciers to flow more quickly to the sea.

Regional variability in sea level change relative to the global average by the end of this century.

Arctic sea ice and atmospheric circulation.

Mixing ratio of methane in the air above the water surface measured along a ship’s route in September 2005. The dotted line shows the Latitude-specific monthly average of 1.85 parts per million by volume established for the Barrow, Alaska, USA, monitoring station at 71° 19’ N, 156° 35’ W (http://www. cmdl.noaa.gov/ccgg/insitu.html); this is the normal level of methane in the atmosphere at this latitude.

Changes in mass of the Greenland Ice Sheet from 1958-2007.

No data

A new assessment has estimated that there are 1,650 gigatonnes of carbon stored in the northern circumpolar permafrost region4, more than twice the amount of carbon in the atmosphere.

Consistent with the rapid retreat of sea ice, the surface waters of the Arctic Ocean have been warming in recent years, because declining sea-ice cover allows the water to absorb more heat from the sun.

Atmosphere-ocean exchanges of carbon.

MOC strength in a suite of coupled climate models.

Sea-level rise is accelerating.

Reduced sea ice amplifies warming.

At the regional, ocean basin scale, the area between the insulating sea-ice cover and the open ocean (known as the ice margin) is characterized by particularly strong temperature gradients during winter, favoring the development of low pressure systems along the edge of the ice, as well as smaller, intense features known as polar lows that present hazards to shipping.

Satellite altimetry data has revealed that sea level is not rising uniformly.