Climate Projections

A climate projection is usually a statement about the likelihood that something will happen several decades to centuries in the future if certain influential conditions develop. In contrast to a prediction, a projection specifically allows for significant changes in the set of boundary conditions, such as an increase in greenhouse gases, which might influence the future climate. As a result, what emerge are conditional expectations (if this happens, then that is what is expected). For projections extending well out into the future, scenarios are

developed of what could happen given various assumptions and judgments.

The IPCC published its 4th Assessment report in 2007. The report summarized information on climate projections based on the IPCC emission scenarios.

Projected temperature rise Image: IPCC

Global Temperature

The basic projection is that the global surface air temperature is going to increase with all scenarios. Even if the concentrations of all greenhouse gases and aerosols were kept constant at year 2000 levels, a further warming of about 0.1°C per decade would be expected. If emissions are kept within the range of the IPCC scenarios about twice as much warming (0.2°C per decade) can be expected.

Global Precipitation

For a future warmer climate, the current generation of models indicate that precipitation will generally increase in tropical regions (such as the monsoon regimes) and over the tropical Pacific in particular. There is projected to be general decreases in the subtropics, and increases at high latitudes. Globally averaged mean water vapour, evaporation and precipitation are projected to increase.

Precipitation Extremes and Droughts

The Intensity of precipitation events is projected to increase, particularly in tropical and high latitude areas that are due to experience increases in mean precipitation. Even in areas where mean precipitation decreases (most subtropical and mid-latitude regions), precipitation intensity is projected to increase meaning that  there would be longer periods between rainfall events. There is a tendency for drying of the mid-continental areas during summer, indicating a greater risk of droughts in those regions.

Snow and Ice

As the climate warms, snow cover and sea ice extent will decrease; glaciers and ice caps lose mass owing to a dominance of summer melting over winter precipitation increases.  There is a projected reduction of sea ice in the 21st century in both the Arctic and Antarctic (see image below). Some models project summer sea ice cover to disappear entirely in the Arctic in the latter part of the 21st century. Widespread increases in thaw depth over much of the permafrost regions are projected to occur in response to warming over the next century.

Contraction of the Greenland ice sheet is projected to continue to contribute to sea-level rise after 2100. It would take nearly 1000 years for the Greenland ice sheet to melt completely, however if temperatures increase in excess of 1.9-4.6°C this is a real possibility. The complete melting of the Greenland ice sheet would result in a sea level rise in excess of 7 meters.

Current global model studies project that the Antarctic ice sheet will remain too cold for widespread surface melting.

Tropical Cyclones (Hurricanes and Typhoons)

Results from some global models, project a likely increase in the intensity of tropical cyclones, but not in the number of them each year. Projections involving tropical cyclones  are still rather uncertain.

Mid-latitude Storms

Model projections show fewer mid-latitude storms averaged over each hemisphere, associated with the pole-ward shift of the storm tracks that is particularly notable in the Southern Hemisphere. The increased wind speeds could result in more extreme wave heights in those regions.

Ocean Circulation

The meridional overturning circulation of the Atlantic Ocean will very likely slow down during the 21st century but is very unlikely to undergo a large abrupt transition during the 21st century. Partial deglaciation of polar ice sheets would imply major changes in coastlines and inundation of low-lying areas, with greatest effects in river deltas and low-lying islands.

Particularly Vulnerable Areas

The Arctic: because of the impacts of high rates of projected warming on natural systems

Africa:  especially the sub-Saharan region, because of projected climate-change impacts and low adaptive capacity

Small islands: due to high exposure of population and infrastructure to sea-level rise and increased storm surges

Asian mega deltas: due to large populations and high exposure to sea-level rise, storm surge and river flooding.

[Further information] can also be found on the Elements of Climate Change webpage.

[Further information] on the Scenarios used to make the above projections can be found on the Emission Scenario webpage.

[More in depth information] on climate projections and impacts can be found in the IPCC Fourth Assessment report, working group I, chapters 10 and 11, and all of the working group II report