We’ve used the term “consensus” here a bit recently (see our earlier post on the subject), without ever really defining what we mean by it. In normal practice, there is no great need to define it – no science depends on it. But it’s useful to record the core that most scientists agree on, for public presentation. The consensus that exists is that of the IPCC reports, in particular the working group I report (there are three WG’s. By “IPCC”, people tend to mean WG I). Fortunately that report is available online for all to read at http://www.grida.no/climate/ipcc_tar/wg1/. It’s a good idea to realise that though the IPCC report contains the consensus, it didn’t form it. The IPCC process was supposed to be – and is – a summary of the science (as available at the time). Because they did their job well, it really is a good review/summary/synthesis.
The main points that most would agree on as “the consensus” are:
- The earth is getting warmer (0.6 +/- 0.2 oC in the past century;
0.10.17 oC/decade over the last 30 years (see update)) [ch 2]
- People are causing this [ch 12] (see update)
- If GHG emissions continue, the warming will continue and indeed accelerate [ch 9]
- (This will be a problem and we ought to do something about it)
I’ve put those four points in rough order of certainty. The last one is in brackets because whilst many would agree, many others (who agree with 1-3) would not, at least without qualification. It’s probably not a part of the core consensus in the way 1-3 are. Most (all?) of us here on RealClimate are physical scientists – we can talk sensibly about past, present and future changes in climate, but potential impacts on ecosystems or human society are out of our field. If you want to see the IPCCs own summary, it’s here.
Other things we have mentioned in other posts come in as supporting evidence. That the increase in atmospheric CO2 is anthropogenic is so obvious that few people question it and in consequence few people rebut skepticism of it (though Eric has done so recently here; and the IPCC mention it). That the recent increase in temperature is unprecedented in the last 1000 years (see e.g. posts 64 or 7 by Mike) is one (but by no means the only) line of evidence indicating that recent change is likely to be unnatural (see update).
The skeptic attitude to consensus usually starts with “there is no consensus”. That’s wrong, and they usually retreat from it to “but consensus science is meaningless”, and/or “consensus has nothing to do with science”. The latter is largely true but irrelevant. The existence of the consensus doesn’t do a lot to determine what science is done; it doesn’t prevent contrary lines being explored. But the consensus view does come into the tricky interface between science and policy, and science and the media.
The existence of the consensus shouldn’t be used to hide the fact that there are areas of doubt. Climate models clearly aren’t perfect. There are questions about the differences between surface and tropospheric temperature trends. Conversely the existence of some areas of doubt shouldn’t be used to try to hide the many areas of understanding and agreement.
Update 2004/12/23: this post needs one correction and also clarification in a couple of places. Firstly, for point 1 I wrote “0.1 oC/decade over the last 30 years”. This is actually a bit of an understatement. 0.15 would be better, and the years since 2001 have been warm, pushing it up further. I’ve now replaced my first figure with the figure of 0.17 oC/decade since 1976, from the IPCC report.
For point 2, I wrote “People are causing this”. That was far too brief to cover the complexities concerned. Comments 5 and 6, and my responses, address this.
Lastly, I wrote “That the recent increase in temperature is unprecedented in the last 1000 years … is one (but by no means the only) line of evidence indicating that recent change is likely to be unnatural”. This is true, but incomplete and possibly misleading, in that it may appear to overstate the importance of the proxy record. If you follow the link to the TAR chapter 12, you’ll have found that the IPCC based its conclusion of human influence on climate on:
A longer and more closely scrutinised observational record
New model estimates of internal variability
New estimates of responses to natural forcing
Improved representation of anthropogenic forcing
Sensitivity to estimates of climate change signals
Qualitative consistencies between observed and modelled climate changes
A wider range of detection techniques
…and all of this lead them to write: The increase in the number of studies, the breadth of techniques, increased rigour in the assessment of the role of anthropogenic forcing in climate, the robustness of results to the assumptions made using those techniques, and consistency of results lead to increased confidence in these results. Detection and attribution have rapidly developed into an entire discipline and deserves its own post, sometime.