Predicting future climate

 

Scientists use powerful computer-based models to predict how the future climate will change. These models are the only tools currently available for simulating the complex set of processes that determine climate at global and regional levels, and it is important to remember that they represent our current understanding of how the climate system works. The models are constantly being improved as we learn more about meteorology and the physics and chemistry of the atmosphere – this often generates headlines of the ‘Scientists now say …’ variety.

Climate models can produce scenarios that describe future climate given certain assumptions about the growth of the emissions of greenhouse gases and about other factors that may influence climate in the future. A scenario is a sophisticated ‘what if’ game; ‘if we assume this happens, then that would be the result, then that …’ and so forth. For example, greenhouse gas generation depends on a complicated interaction of global economics, policy and social interactions, and these are examined by building scenarios. When developed by a panel of experts, they are a powerful technique for analysing future eventualities and options. However if they are used as predictions for the future, they are, as can be imagined, quite uncertain.

The model used in the UK is developed by the Hadley Centre (the part of the Met Office which provides a focus in the UK for the scientific issues associated with climate change) and predicts climate at the level of 5 km squares. The latest global climate model from the Centre has been used by the United Kingdom Climate Impacts Programme (UKCIP) – the main source of advice on preparing for climate change in the UK. UKCIP has produced four different scenarios based on four different greenhouse gas emission scenarios to simulate future changes in the UK's climate. These climate change scenarios are referred to as UKCIP02 – as they were published in 2002, and they replace the UKCIP98 scenarios published in 1998.

Uncertainties

Although many scientific predictions can have a level of uncertainty attached to them (i.e. the odds of them being correct), climate modelling is so complicated that it is unrealistic to do this about for any single model. Therefore the UKCIP scenarios and the climate predictions associated with each do not have assigned probabilities. The high emissions future is not, for the scientists, any more likely to happen than the low emissions one. The scenarios provide alternative views of the future, and together show a broad range of changes that we may face.

What scientists have done to tackle uncertainty in climate modelling is to compare different models. There are a several very good models around the world in addition to the Hadley model. These predict rather different future climates from each other but a comparison of outputs using the same inputs shows clear common directions. Therefore although we have to accept with some caution the precise amount of warming predicted by any one model – we need far less caution to accept that it will get warmer.