Nobody can tell what the weather will be like in a few days, so how can we predict what the weather will be like in ten years or a hundred? Will it be hot, very hot? Probably. Scientists most definitely don not want taken for astrologers or fortune-tellers.
The IPCC’s models are mathematical models that use the planet’s most powerful computers. The atmosphere and oceans are divided into a huge number of cubes (hundreds of millions). Their evolution is then calculated gradually. On a yearly scale, local meteorological variations seem insignificant.
The models are verified in several ways. By examining, if, taken backwards, they can help to trace past evolution. Or by seeing if they can take the most recent data into account. Or by comparing them to each other because there are many models with technical names like GISS-ER or HadCM3, each attached to different research centres. But none of these results are perfect: for example, researchers freely admit that they have difficulty imitating the role of clouds – this is obviously an essential parameter. This is why they provide “ranges” of values: for example, the precipitation decrease in Southern Europe could be between -4 % and -27 % by 2099.
But, particularly, these models establish scenarios for the next century, not predictions. They are possible stories – “narrative frameworks” – not future reality. Researchers know that the future will be different from all the scenarios they have suggested. But by giving us several benchmarks, they show us how our planet could evolve in one way or another. They thus give us the tools we need to reflect and make decisions.