The Effect of Clouds on Climate Change

Earth's albedo in Decline - NASA

The Albedo Effect Differs According to Cloud Type

On the face of things, logic would suggest that an increase in the average atmospheric temperature will lead to greater levels of water evaporating and therefore increased cloud formation. 

This could, in turn, lead to greater reflection of solar energy from the sun (known as ‘albedo’ – see figure 1) and a reduction in global temperatures. In a way, then, amplified cloud formation could in fact negate the warming effects of increased atmospheric carbon dioxide levels, leading to an effect known as ‘negative feedback’.

Different types of clouds, however, have different effects on greenhouse warming. Lower, stratiform clouds at around 2200 m, for instance, consist of thick layers of water vapour that reflect incoming solar radiation. This helps to reduce the effect of global warming. Higher cirrus clouds (5300-14000m) that are composed mainly of ice, however, tend to trap outgoing long wave radiation while allowing solar radiation through to earth because of their low density. This subsequently results in an increase in atmospheric temperatures

The net result of these conflicting effects is around 13 watts per square metre of heat loss from the atmosphere. In other words, the negative feedback of lower to middle clouds wins out over the positive feedback produced by the higher clouds. Indeed, according to the Intergovernmental Panel on Climate Change, if all clouds were removed from the atmosphere, this cooling effect would be lost and the Earth’s climate would begin to warm up.

How Will Global Warming Affect the Behaviour of Clouds?

In the 1990s it was believed that an increase in global temperatures would simply create more of the same cloud types and therefore increase the atmosphere’s net albedo effect. Global warming would therefore be countered by greater reflection of incoming short wave radiation. Recent studies, however, have suggested that this may be too simplistic a scenario and increased atmospheric temperatures may in fact cause a reduction in the abundance of lower to middle level clouds.

According to studies carried out by Amy Clement of the University of Miami in 2009, warmer temperatures in fact resulted in decreased atmospheric circulation and reduced lower cloud formation. These studies also agreed with climate models produced at the Hadley Centre for Climate Change, and suggest that the resulting decrease in albedo will create a net warming effect on the atmosphere.

Moreover, because the atmosphere will become less stable with a rise in temperature, some scientists believe that water vapour may well be drawn up to greater altitudes, resulting in more high level clouds. As a consequence, a net warming effect (positive feedback) may result due to a reduction in albedo and an increase in the absorption of outgoing radiation by these higher, icier clouds.

What Can We Do to Reduce Global Warming?

The above study reveals that clouds are therefore not necessarily going to save us from overall global warming and may in fact act to increase its effects. With further warming it is also to be expected that polar ice, another major component of Earth’s albedo effect, will be reduced. With less short wave solar radiation being reflected back into space, a positive feedback scenario of sizeable proportions may well occur.

A general consensus among scientists and world leaders is that most global warming since the Industrial Revolution has been caused by the burning of fossil fuels and land clearing. Many countries have therefore committed to reduce their greenhouse emissions in agreements such as the Kyoto Protocol in 1997, the Copenhagen Accord in 2009 and the Cancun agreements in 2010.

Unlike the Kyoto Protocol, the Copenhagen and Cancun gatherings did not succeed in producing legally binding commitments from any countries. Nonetheless, agreements were made in each to reduce greenhouse emissions and to limit average temperature rises by less than 2 degrees above pre Industrial Revolution temperatures. This mitigation is expected to be achieved by measures such as the development of alternative energy sources, reduction of land clearing and the use of cleaner fuel burning technologies.

References

Harmon, K., 2009, ‘A Less Shady Future: Could Climate Change Mean Fewer Clouds? , scientificamerican.com

IPCC, 1990, 'Climate Change - The IPCC Scientific Assessment', ipcc.ch

NASA, 2000, ‘Clouds and Climate Change: The Thick and Thin of it’, Goddard Institute for Space Studies, nasa.gov

Walsh, B., 2009, ‘In a Warming world, Cloudy Days Are a Boon’, time.com