Understanding climate change

Breaking down the problem

How do we go about understanding and estimating climate change? This is done in a number of stages, shown in this figure. The first thing we need to know is what the emissions will be of greenhouse gases and other gases which affect climate change. These projections are deduced from separate models which take into account population growth, energy use, economics, technological developments, and so forth. Having obtained projections of how emissions will change, we then calculate how much remains in the atmosphere, i.e. what future concentrations will be. For the gas carbon dioxide (CO2), this is done using a model of the carbon cycle, which simulates the transfer of carbon between sources (emissions) and sinks in the atmosphere, ocean and land (vegetation). For gases such as methane and ozone, which are also important, we use models which simulate chemical reactions in the atmosphere.

Next we have to calculate the heating effect of the increased concentrations of greenhouse gases; this is often called climate forcing. This is done within the climate model which generates spatial patterns of changes in temperature, rainfall and sea level etc, across the surface of the earth and through the depth of the atmosphere and oceans. Following on from the climate change prediction, the impacts of climate change, on socio-economic sectors such as as water resources, food supply, flooding, are calculated by other research groups.

Feedback mechanisms

The prediction of climate change is complicated by the fact that, once climate change starts, there will be consequences (feedback) in the climate system which can act to either enhance or reduce the warming. For example, as the atmosphere warms it will be able to "hold" more water vapour. Water vapour itself is a very powerful greenhouse gas, so this will act as a positive feedback and enhance the warming. Similarly, when sea ice begins to melt, some of the solar radiation which would otherwise be reflected from the sea ice is absorbed by the ocean, and heats it further; another positive feedback. On the other hand when carbon dioxide concentrations increase in the atmosphere then it acts to speed up the growth of plants and trees (the fertilisation effect) which in turn absorb more of the carbon dioxide; this acts as a negative feedback. There are many of these feedbacks, both positive and negative, many of which we do not fully understand. This lack of understanding is the main cause of the uncertainty in climate predictions; this applies in particular to changes in clouds.

The climate system

In order to estimate climate change, we have to build a model of the complete climate system. Firstly the atmosphere; the way it circulates, the processes that go on in it, such as the formation of clouds, and the passage of terrestrial and solar radiation through it. Secondly the ocean, because there is a constant exchange of heat, momentum and water vapour between the ocean and atmosphere and because in the ocean there are very large currents which act to transport heat and salt. In fact the ocean does about half the work of the climate system in transporting heat between the equator and the poles. Thirdly the land, because it affects the flow of air over it, it is important in the hydrological (water) cycle. In addition we model the cryosphere, ice on land and sea. All of these components of the climate system interact to produce the feedbacks which determine how climate will change in the future. (Text courtesy of the Hadley Centre for Climate Prediction and Research, UK).

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