2 May 2008

Dr Vicky Pope, Head of Climate Change for Government, Met Office Hadley Centre explains.

Climate change is perhaps the most pressing problem facing the world today and it will remain a challenge for decades, if not centuries, to come. Difficult decisions need to be made on reducing emissions and adapting to inevitable change. These involve significant economic and social costs but the right choices, made soon, might lead to significant economic benefit. Scientists and other experts are under increasing pressure to provide usable dispassionate evidence to support these decisions, without becoming involved in the inevitable political aspects of the decisions themselves.

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. If we could, we would all have made our fortunes! However, we are able to give a range of possible outcomes of the world's actions and to assign probabilities to these. 

Hence, 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. 

Any reputable scientist will tell you both the strengths and weaknesses of their predictions and point out what more could be done with more research. However, this doesn't mean that their evidence should be dismissed or set aside until 'better' evidence comes along. 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. However, a climate projection is not simply a collection of all observations of past climate and an extrapolation of these into the future. Rather, 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 typically have a grid spacing of 150 km currently, and this is likely to improve to 90 km with the next generation of models in the next year or so. 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.

Global climate models have become more complete over the years as our understanding of climate change has increased and computer power has increased. The next generation of climate models (such as the Met Office Hadley Centre's model, which will be available in 2009) will include detailed chemistry and interactions with the biosphere (plants, soil and ocean biology). This will help us to answer some of the key questions on how climate will change in the future.  It is only with the introduction of new supercomputers over the next year or so that we will have enough computing power to run these models.

How do scientists analyse the risks of climate change?

In order to generate the full range of possible futures, scientists generate model projections that take account of uncertainties in the following:

• Emissions - which depend on factors such as economic growth and actions to reduce greenhouse gases.
• Natural variations – where gradual change is overlaid with variations that occur naturally and make up the day-to-day weather and variations from one year to the next, for example El Niño. Other variations arise through changes in natural factors such as variations in the heating from the sun and the impact of particles (aerosols) produced by volcanic eruptions.
• Science – which depends on our ability to understand and model the important processes that affect climate. In particular, there are still uncertainties in the strength of feedbacks in clouds and the carbon cycle (how plants and the biosphere react to carbon dioxide).

Emissions could increase rapidly, if there is rapid economic growth and little change in technology, or more slowly, if growth is less rapid and/or technology improves. Climate models have been used to make projections which span a range of possible future emissions. 

The effects of natural variations are included by varying the starting conditions in the models to generate different 'weather'. Natural forcings are included in 'hindcasts' of past climate – hindcasts are used to test the models and compare them with real climate observations. Natural forcings are not included in projections of future climate. For example, it is not possible to include volcanic eruptions, because they are not predictable. However, their effect is only temporary, cooling the globe by up to 2 °C for at most a couple of years. 

Until recently the uncertainty in the science was not quantified in a systematic way. Rather, climate models from different modelling centres were simply averaged and the ranges of projections were compared. 

New techniques developed in 2004, using multiple computer models developed at the Met Office Hadley Centre, allowed scientists to provide a more systematic risk assessment of climate change on the global scale. The first of these multiple model projections assumed that greenhouses gases were stabilised at various levels. Using this technique the Met Office Hadley Centre has been able to quantify that there is a 80% chance of limiting global warming to between 2 and 3 °C if we stabilise greenhouse gases at 450 ppm (parts per million). There are already 380 ppm of carbon dioxide in the atmosphere and when other greenhouse gases are included this rises to a total equivalent value of 455 ppm. 

Why should we care if global mean temperatures rise by more than 2 to 3 °C?

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. For example, increasing temperatures and carbon dioxide could give increased opportunities to grow certain crops in some regions, if there is enough water available during the growing season. However, this must not generate any complacency. 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. Some examples of the global impacts are given below.

What are the risks of climate change?

This multiple model technique has been improved and extended over the last few years to provide information that can be used in risk assessment for the UK. Particular improvements include:

• projections that include the time variations in emissions and climate;
• the way that observations are used to decide how well a model represents reality, and how its results should be used;
• the use of results from other modelling centres to inform the statistical analysis;
• fine-scale regional models with a 25 km grid to give more detailed results for the UK. (This compares with the global model grid of 300 km.) 

The final results, which will include all of the improvements, will be published in November 2008 as the UK 21st Century Climate Change Scenarios for the UK Climate Impacts Programme (UKCIP08). UKCIP08 will provide information on climate change, based on Met Office Hadley Centre projections, combined with the results from other models, to help organisations plan to adapt to inevitable climate change. Results published by UKCIP in 2002 gave a useful indication of the range of possible changes in climate taking account of different possible greenhouse gas emissions. However, at the time it was not possible to quantify the uncertainties in the projections. UKCIP08 will update this information with finer grid models (25 km instead of 50 km). In addition, for the first time, it will provide probabilities of different levels of change in temperature, rainfall and other aspects of climate, using the Met Office Hadley Centre's multiple model technique.  

Ensemble results based solely on Met Office Hadley Centre global models are already revealing some interesting results. Early results for northern Europe have shown that average summer rainfall for 2070-2100 is likely to decrease by between 5 and 20% (the dark blue shaded area on the chart opposite). Average winter rainfall (not shown), on the other hand, is very likely to increase. The results from UKCIP08 will be in a similar format. However, UKCIP08 results will be different to these because, as well as being focused on the UK, they will include information from other modelling centres and from fine-scale regional models.

But are these changes really significant for the UK?

The latest climate model projections indicate that by the middle of the century it is very likely that average winter rainfall will increase and that summer rainfall will decrease. In line with this, we are confident that the number and intensity of extreme rainfall events will increase in winter. Although, it is very likely that overall summers will become drier, it is likely that summer showers will become heavier. This is because warmer air can hold more moisture. The difficulty is that the projections do not agree on how big the changes will be. Over the next few years, better models run and the use of multiple model techniques should help us to understand and reduce the uncertainties in these predictions.

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.

A final point

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 so 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 lead to some results being dismissed as too uncertain by the public. Some scientists react against this by taking the most pessimistic view of what the future holds, to try and 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 to provide useful information that can be used to make important decisions about all our futures.