Future climate change depends on how global greenhouse gas emissions develop. The possible scenarios are predicted using simulations in huge climate models. These are the findings of these models on which the IPCC bases its recommendations to politicians. The models can be seen as a kind of further development of weather forecast models, which are validated by comparison with observation of the weather they predict. However, we also know that the weather is chaotic and therefore has a limited time for predictability. The climate models predict average weather and are therefore able to provide predictions that extend further into the future.  

The models are good at describing the climate change that occurs proportionally (linearly) with greenhouse gas concentration, but the sudden changes that can happen if parts of the climate system cross a so-called "tipping point" they are not good at describing. We know that the climate of the past has undergone such dramatic changes, but we do not know whether they are predictable or if we can only calculate the increased risk of meeting tipping points.