On the Nature paper, BBC online reported that “temperatures around the world could rise by as much as 11ºC“; on the latter report it headlined: “Climate crisis near ‘in 10 years’”. Does this mean there is new evidence that climate change is more serious than previously thought? We think not. [...]
Is there a “point of no return” or “critical threshold” that will be crossed when the forcings exceed this level, as reported in some media? We don’t believe there is scientific evidence for this. However, as was pointed out at an international symposium on this topic last year in Beijing by Carlo Jaeger: setting a limit is a sensible way to collectively deal with a risk. A speed limit is a prime example. When we set a speed limit at 60 mph, there is no “critical threshold” there – nothing terrible happens if you go to 65 or 70 mph, say. But perhaps at 90 mph the fatalities would exceed acceptable levels. Setting a limit to global warming at 2ºC above pre-industrial temperature is the official policy target of the European Union, and is probably a sensible limit in this sense. But, just like speed limits, it may be difficult to adhere to.
Uncertainty in climate sensitivity is not going to disappear any time soon, and should therefore be built into assessments of future climate. However, it is not a completely free variable, and the extremely high end values that have been discussed in media reports over the last couple of weeks are not scientifically credible.
I left out the juicy bits from the selection above—the background and discussion on climate sensitivity and validation of models. It’s very much worth a read. The science surrounding global warming is complex and continually developing. Navigating through the claims and counterclaims requires some basic education—a mission that RealClimate is tackling very well.
To be clear: the team at RealClimate is not disparaging the project or the effort. As noted in a reponse to just that comment, they respond:
As a project, this is very useful because it’s the kind of thing that could not have been done before and that is always worthy in modelling. There will be further work done on these results, and there will be more interesting experiments performed. Eventually, we will get a better idea of how wide the spread of climate model parameterisations can be while still passing the stringent valdiation tests that state-of-the-art models must pass. My crticism [sic] mainly concerns how these preliminary results have been presented in the media.