Imagining the future
We can measure things now, and infer what the past was like, but how do we predict what will happen next?
Early humans may have noticed annual cycles, spotting that spring invariably follows winter. Until very recently this was pretty much the limit of our predictive abilities. With so much depending on the weather, meteorology has long been a focus of human endeavour.
In recent years weather forecasting has moved from being reasonably dependable over 24 hours to being accurate over a five-day period, in part thanks to the application of chaos theory to forecasting. Weather forecasters now use the chaos theory insight that small differences in the initial conditions of the atmosphere can lead to big differences a few days later.
Forecasters also make use of mathematical models called algorithms. These are computational procedures that can solve mathematical problems and be used to produce computer simulations of complex systems. Programmed into the model are key parameters describing different parts of the system and how they interact with one another. Starting conditions are entered and when the ‘on’ switch is flipped, the model runs until a predetermined point – a day, a year, a decade or a millennium into the future.
The beauty of modelling is that the program can be rerun time and again with slight modifications. How accurate a model is depends on how well it describes a system. Uncertainty is inevitable – no system will ever be understood in perfect detail. Some simplification is also required, as climate systems are enormously complex.
So how can we be sure that they are reliable predictors of the future? One way is to test their ability to recreate known patterns from the past. Modellers can test whether the predictions of their model match what actually happened.
Over the past couple of decades, climate models have become increasingly refined, as more has been discovered about the environment. But they are only simulations – which is why they generally produce a range of values of greater or lesser likelihood. None can say with absolute certainty what the climate will be like in, say, 20 years’ time.
The big unknown, of course, is what will happen to greenhouse gas levels. Climate models compare the impact of different stabilised greenhouse gas levels – generally, the higher the ceiling, the worse the impact will be.