Dr Vicky Pope, Head of Climate Science Advice
Like any 'expert' trying to project into the future, scientists cannot give precise predictions of what will happen in 100 years, or even in the next 10 years. However, we are able to give a range of possible outcomes of the world's actions and to assign probabilities to these, and climate projections should be treated as an assessment of risk.
You wouldn't drive a car if you knew you had a 10% chance of causing an accident. Yet we continue to increase emissions, despite the fact that even if we could stabilise greenhouse gases at, or close to, today's levels there would still be only a 80% chance of keeping global temperature rises somewhere between 2 and 3 °C above pre-industrial levels. If we carry on increasing emissions at present rates then global average temperature rise will be 2 °C by the middle of this century.
The difficulty is that the projections do not agree on how big the changes will be. Over the next few years, better models and the use of multiple model techniques should help us to understand and reduce the uncertainties in our predictions.
Climate scientists tend to gravitate towards two approaches when it comes to describing their projections of climate change. All scientists are trained to consider all the possible problems with their results and will always point out the caveats and areas that could be improved. This can give rise to a very confused view of what results mean and leads the public dismissing to some results as too uncertain. Some scientists react against this by taking the most pessimistic view of what the future holds, to try to make it clear how serious the problem is. These results tend to be dismissed as unrealistic. By quantifying the risks of climate change, the Met Office Hadley Centre hopes to give a balanced view of the future and provide useful information that can be used to make important decisions about all our futures.
There is already a very large body of evidence that can be used to make decisions about climate change. The strengths and weaknesses of this evidence have been tested and are understood.
Climate scientists use evidence from observations of the past climate, and from computer models of the climate, to produce projections of the future. Projections are made using climate models derived from the laws of physics. A well-established set of equations result from the physical laws governing the climate and these are solved for a three-dimensional grid that spans the globe. The observations are then used to make an independent check on whether the models are good enough.
The accuracy of the results depends on a detailed understanding of the physical processes and the fineness of the grid that can be represented on the computer being used. Climate models currently have a grid spacing of 150 km, and this is likely to improve to 90 km with the next generation of models. A finer grid means more detail and more accuracy. The grid spacing and the science are the two aspects that are being continually improved, and the reason why scientists will always place caveats on their projections.
In order to generate the full range of possible futures, scientists generate model projections that take account of uncertainties in the following:
Emissions — dependent on various factors, including actions to reduce greenhouse gases
Natural variations — such as El Niño and the impact of particles (aerosols) produced by volcanic eruptions
Science — our ability to understand and model the important processes that affect climate.
If we plan ahead, we should be able to adapt to some aspects of climate change, provided that change is not too great or occurs too quickly.
If emissions continue to grow at present rates, the changes will be so large and so rapid that the adverse effects will quickly outweigh any local benefits and come to dominate in all regions of the world. Adaptation will also become increasingly difficult, and in some cases impossible.
We also need to consider that the UK does not exist in isolation, but is affected by what happens elsewhere. For example, world food prices and migration are already significant issues today and will be adversely affected by global warming.
In northern Europe (including UK) average summer rainfall for 2070-2100 is likely to decrease by between 5 and 20%. Average winter rainfall is very likely to increase. The number and intensity of extreme rainfall events will increase in winter.
For the UK as a whole, 55,000 homes and 6,000 businesses were flooded and related insurance claims were approaching £3bn by the end of 2007. In line with Europe as a whole, UK summer rainfall is very likely to decrease on average. However, it is likely that summer showers will become heavier. This is because warmer air can hold more moisture.
Global warming has already at least doubled the risk of heatwaves like that in 2003, which caused 35,000 extra deaths across Europe. By 2040 such summers are likely to be commonplace and by the 2060s they may well be unusually cool.
Incidence of drought has already increased. Severe drought has increased from 1 to 3% of the globe in the last 50 years. By 2050 it is likely to increase to 12% (if global temperatures rise by 2 °C above pre-industrial values).
There is risk of significant loss of the Amazon rainforest when global temperature rises reach 3 °C and few ecosystems will be able to adapt (IPCC).
The Greenland ice sheet will start to melt more quickly than snow accumulates once regional temperatures are high enough. It is not yet known what this exact threshold is, but it could be a global temperature rise of somewhere between 1.9 and 4.6 °C above pre-industrial values.