Read here. The IPCC and all the alarmist climate scientists/pundits base their catastrophic global warming predictions on the output of a handful of climate models. They present the models' predictions and forecasts as evidence to policymakers that action must be immediately taken to curb human CO2 emissions, or else "the world will end as we know it."
While presenting climate models' forecasts as gospel, there is ample real world evidence that the models are terrible at climate predictions, be they global temperature predictions or other climate condition forecasts (eg., sea-level, snow-pack predictions).
A new study by experts confirms what the majority of scientists are now reluctantly admitting: the IPCC climate models are worthless when used for future climate prognostications.
"“Forecasting researchers, with few exceptions, have ignored the current major forecasting controversy: global warming and the role of climate modelling in resolving this challenging topic. In this paper, we take a forecaster’s perspective in reviewing established principles for validating the atmospheric-ocean general circulation models (AOGCMs) used in most climate forecasting, and in particular by the Intergovernmental Panel on Climate Change (IPCC)...The omissions in Chapter 10 of the IPCC report and most of the subsequent research lie in the lack of evidence that the models actually produce good forecasts. There is ample testimony in the forecasting literature of the difficulties of forecasting beyond the range of data on which a model is constructed...“…the structural weaknesses in the GCM identified here suggest that a reliance on the policy implications from the general circulation models, and in particular the primary emphasis on controlling global CO2 emissions, is misguided...“The scientific community of global climate modellers has surely taken unnecessary risks in raising the stakes so high when depending on forecasts and models that have many weaknesses.”" [Robert Fildes, Nikolaos Kourentzesa 2011: International Journal of Forecasting]