Climate Change 2007 Update

3. How is the climate going to change in the future?

• 3.1 What are the projected changes in temperature for the 21^st century?
• 3.2 What are the other projected changes for the 21^st century?
• 3.3 What changes are projected on the longer term?

3.1 What are the projected changes in temperature for the 21^st century?

Modeled temperature projection to 2100

According to a series of emission scenarios, global temperature is projected to increase by about 0.2°C per decade for the next two decades. Previous projections had suggested a warming of 0.15 to 0.3°C per decade for 1990 to 2005.

Even if the concentrations of all greenhouse gases and aerosols had been kept constant at year 2000 levels, a further warming of about 0.1°C per decade would be expected, mainly due to the long time it takes for oceans to release the heat they accumulated. Continued greenhouse gas emissions at or above current rates would cause further warming and induce many changes in the global climate system during the 21^st century that would very likely be larger than those observed during the 20^th century. In addition, warming tends to reduce land and ocean uptake of atmospheric CO₂, increasing the amount of human-induced emissions that remains in the atmosphere

Best estimates for globally average surface air warming between the 1980s and the 2090s in the selected scenarios range from 1.8°C (likely range is 1.1°C to 2.9°C) to 4.0°C (likely range is 2.4°C to 6.4°C). Uncertainties are related to model differences and to differences in energy use scenarios. More...

3.2 What are the other projected changes for the 21^st century?

Projected temperatures up to 2100

Global average sea level is projected to rise by 18 to 59 cm by the end of the 21^st century (2090-2099), depending on the scenario (Table 3). However, models used to date do not include uncertainties about certain climate mechanisms because of lack of knowledge. For instance, projections of sea level rise do not take into account the fact that the flow of ice from the ice sheets in Greenland and Antarctica could be faster in the future than they were in recent years. These changes could increase the projections by 10 to 20 cm, maybe even more, but understanding is still too limited to include them in the models with any level of certainty.

Geographical patterns in climate changes are expected to remain similar to those observed over the past several decades. Warming is expected to be greatest over land and at most high northern latitudes, and smallest over the Southern Ocean and parts of the North Atlantic Ocean.

Other projected changes include:

Relative changes in precipitation (in percent) for the period 2090–2099, relative to 1980–1999

• increased acidification of the oceans caused by increasing carbon dioxide concentrations in the atmosphere;
• shrinking snow cover and sea ice, and decreased permafrost;
• increasingly frequent hot extremes, heat waves, and heavy precipitation events  
• more intense tropical cyclones (typhoons and hurricanes);
• a moving of extra-tropical storm tracks towards the poles, with consequent changes in wind, precipitation, and temperature patterns; 
• greater amounts of precipitation in high-latitudes and less rain in most subtropical land regions; and
• a slowing of the Atlantic Ocean circulation.

More...

3.3 What changes are projected on the longer term?

From 1980 to the end of the 21^st century, temperatures are projected to increase by 1.8°C to 4.0°C.

Warming and sea level rise caused by human activities will continue for centuries, even if greenhouse gas concentrations were to be stabilized, because of the long timescales associated with climate processes and feedbacks.

Warming is expected to affect the carbon cycle, resulting in even higher carbon dioxide concentration in the atmosphere, but the magnitude of this is uncertain.

If greenhouse gas concentrations in the atmosphere were to stabilize in 2100 at levels projected in the B1 and A1B emission scenarios, a further increase in global average temperature of about 0.5°C would still be expected around 2200. Under this A1B scenario, the thermal expansion of the oceans alone would lead to an increase of 30 to 80 cm of global sea level by 2300, and this rise would continue over many centuries after that.

The melting of the Greenland ice sheet is projected to continue and to contribute to sea level rise after 2100. If it were to keep melting for millennia until Greenland ice disappeared completely, global sea level would rise by about 7m.

The vulnerability of the ice sheets to warming could be increased by dynamical processes related to ice flow (not included in current models but suggested by recent observations) thereby increasing future sea level rise. Current global model studies project that the Antarctic ice sheet will remain too cold for widespread surface melting and is expected to gain in mass due to increased snowfall.

Both past and future human emissions of carbon dioxide will continue to contribute to warming and sea level rise for more than a millennium, due to the long time it takes for this gas to disappear from the atmosphere. More...

The Emission Scenarios of the IPCC Special Report on Emission Scenarios (SRES)