Many complex processes shape our climate
Based just on the physics of the amount of energy that CO
2
absorbs and emits, a
doubling of atmospheric CO
2
concentration from pre-industrial levels (up to about 560
ppm) would, by itself, cause a global average temperature increase of about 1 °C (1.8 °F).
In the overall climate system, however, things are more complex; warming leads to further
effects (feedbacks) that either amplify or diminish the initial warming.
The most important feedbacks involve various forms of water. A warmer atmosphere
generally contains more water vapour. Water vapour is a potent greenhouse gas, thus
causing more warming; its short lifetime in the atmosphere keeps its increase largely in
step with warming. Thus, water vapour is treated as an amplifier, and not a driver, of climate
change. Higher temperatures in the polar regions melt sea ice and reduce seasonal snow
cover, exposing a darker ocean and land surface that can absorb more heat, causing further
warming. Another important but uncertain feedback concerns changes in clouds. Warming
and increases in water vapour together may cause cloud cover to increase or decrease
which can either amplify or dampen temperature change depending on the changes in the
horizontal extent, altitude, and properties of clouds. The latest assessment of the science
indicates that the overall net global effect of cloud changes is likely to be to amplify warming.
The ocean moderates climate change. The ocean is a huge heat reservoir, but it is difficult
to heat its full depth because warm water tends to stay near the surface. The rate at which
heat is transferred to the deep ocean is therefore slow; it varies from year to year and from
decade to decade, and helps to determine the pace of warming at the surface. Observations
of the sub-surface ocean are limited prior to about 1970, but since then, warming of the
upper 700 m (2,300 feet) is readily apparent. There is also evidence of deeper warming.
Surface temperatures and rainfall in most regions vary greatly from the global average
because of geographical location, in particular latitude and continental position. Both
the average values of temperature, rainfall, and their extremes (which generally have the
largest impacts on natural systems and human infrastructure), are also strongly affected
by local patterns of winds.
Estimating the effects of feedback processes, the pace of the warming, and regional
climate change requires the use of mathematical models of the atmosphere, ocean,
land, and ice (the cryosphere) built upon established laws of physics and the latest
understanding of the physical, chemical and biological processes affecting climate, and
run on powerful computers. Models vary in their projections of how much additional
warming to expect (depending on the type of model and on assumptions used in
simulating certain climate processes, particularly cloud formation and ocean mixing), but
all such models agree that the overall net effect of feedbacks is to amplify warming.