Later this month the IPCC will release the first bit of its Fifth Assessment Report (AR5), the Summary for Policy Makers (SPM). This is the scary story part, for the full reports of the Working Parties with the real evidence and argument won’t appear until next year. There is a certain breathless anticipation around the blogosphere about how the IPCC will deal with the pause in global warming, which is 12, 15, 16 or 17 years long, depending on what data you choose and which year you begin with.
An added interest in the SPM flows from the cut-off for acceptance of papers to be considered, which was back in March, but the flow of interesting papers since, one of which last week (in Nature, no less) proposed that much of the pause is due to ocean movements in the Pacific. If that is true, then it must also be true that some of the warming in the past will have been related to ocean movements. And I did hear about it on the ABC, though the news item seemed to suggest that warming would continue once the oceans had settled down. Actually, the authors said that too.
Judith Curry devoted one of her essays to what might or might not be in the SPM, and I thought it so crisply summarised the IPCC’s dilemma that I’ve adapted it slightly for this post. She begins by pointing out that the take-home message from the last IPCC Assessment Report, AR4, went like this:
There are two qualifiers in that statement. How much is ‘most’, and how likely is ‘very likely’? The orthodox tend to ignore the qualifiers, and speak as though most means ‘all’ and very likely means ‘certainly’. But it ain’t so. The data are too rubbery, and the argument too contingent to overcome the uncertainty. Indeed, Professor Curry has made her reputation in this area by focussing on uncertainty, and coined the phrase ‘The Uncertainty Monster’ to draw attention to how little we know, not how much.
In her essay she went on to ask:
So what is the evidence for, and against, a dominant role in the climate since the mid-20th century of increasing human-induced greenhouse gas concentrations, and what are the major uncertainties? Below is my summary interpretation of the available evidence.
- Long-term trend of increasing surface temperatures, for more than a century.
- Theoretical support for warming as greenhouse gas concentration increases.
- Long-term trend of increasing ocean heat content, although the trend for the past 10 years has been small in the upper 700 m of the ocean.
- Decline in Arctic sea ice since 1979, with record autumn minimum in 2012.
- Sea level rise since 1961, although multi-decadal variability and confounding factors such as coastal land use and geologic process hamper interpretation of these data.
- Results from climate model simulations.
- No significant increase in globally averaged temperature for the past 15 years.
- Lack of a consistent and convincing attribution argument for the warming from 1910-1940 and the plateau from the 1940s to the 1970s.
- Growing realization that multidecadal natural internal variability is of higher amplitude than previously accounted for in IPCC attribution analyses.
There are major uncertainties in many of the key observational data sets, particularly prior to 1980. There are also major uncertainties in climate models, particularly with regards to the treatments of clouds, solar indirect effects and the coupled multidecadal oscillations between the ocean and atmosphere. Further, there are meta-uncertainties regarding the methods used to make arguments about attribution of climate change and determine sensitivity to increasing greenhouse gases. And finally, climate models are apparently incapable of simulating emergent phenomena such as abrupt climate change.
In light of these uncertainties, what can we say about the future climate of the 21st century? Most scientists anticipate a decrease in solar forcing in the coming decades, but noting the absence of understanding the solar indirect effects on climate, this is not expected to dominate climate change in the 21stcentury. If the climate shifts hypothesis is correct, then the current flat trend in global surface temperatures may continue for another decade or two, with a resumption of warming at some point during mid-century. The amount of warming from greenhouse gases depends both on the amount of greenhouse gases that are emitted as well as the climate sensitivity to the greenhouse gases, both of which are associated with substantial uncertainties.
Leaked documents and general scuttlebutt seem to indicate that these matters are concerning governments around the world, which have to sign off on the SPM, which is the political, rather than the scientific, document that emerges at this time. While governments seem to me to be backing off from the AGW scare — what has happened in Australia over the past year is not peculiar to us — there are still a lot of civil servants whose careers are presently built on global warming, and they need a credible, indeed convincing, story to tell.
I’m not at all sure that they are going to have one.