(Part of the How to Talk to a Climate Sceptic guide)
Objection:
There is no indication of how much confidence we should have in the models. How are we supposed to know if it is a serious prediction or just a wild guess?
This is a pretty sure indicator your objector hasn't ever looked at an actual climate model prediction.
Answer:
There is indeed a lot of uncertainty in what the future will be, but this is not all because of an imperfect understanding of how the climate works. A large part of it is simply not knowing how the human race will react to this danger and/or how the world economy will develope. Since these factors control what emissions of CO2 will accumulate in the atmosphere, which in turn influences the temperature, there is really no way for a climate model to predict what the future will be.
What modelers can do, however, is talk about and estimate the climate's sensitivity to CO2, usually in terms of how high the temperature will rise given a doubling of CO2. See the Real Climate glossary entry for climate sensitivity.
So how much certainty is there? This varies from model to model but typically a projection is given as a most likely temperature together with a range that encompasses all the likely values. In the IPCC report "likely" is defined as a 70% probability. If you want a specific number and range, you must chose a specific scenario of emissions over time and a specific model. The IPCC does note that:
The globally averaged surface temperature is projected to increase by 1.4 to 5.8°C over the period 1990 to 2100. These results are for the full range of 35 SRES scenarios, based on a number of climate models.
But what about the certainty of this summary of model predictions? Well, a recent paper by James Annan et al. has attempted to clarify this question by statistically combining the certainties of a wide variety of models in a variety of situations. Focusing on climate sensitivity, they conclude that in terms of the climate's response to a doubling of CO2, the model's say:
"The resulting distribution can be represented by (1.7,2.9,4.9) in the format used throughout this paper. That is to say, it has a maximum likelihood value of 2.9oC, and, using the IPCC terminology for confidence levels, we find a likely range of 2.2-3.9oC (70% confidence) and a very likely range of 1.7-4.9oC (95%). We can also state that climate sensitivity is very likely to lie below 4.5oC(95%). These results represent a substantial decrease in uncertainty over those originally presented in NAS [1979] and in subsequent research. They also imply that the sensitivity range of modern GCMs (2.1-4.4oC) is likely to include the correct value (with greater than 80% confidence)"
So, most likely value is 2.9oC with a 95% probability of falling between 1.7oC and 4.9oC.
There is a summary and discussion of this paper at Real Climate.
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