by Judith Curry
This post draws heavily from Bishop Hill’s post with (almost) the same title, related to the Cambridge Workshop discussed on the previous thread. Specifically, this addresses the attempt by Dr. Eric Wolff to find a measure of agreement between the two sides in the climate debate. I attempted something similar on a previous Climate Etc. thread titled What we know with confidence.
Dr. Wolff’s summary points (as per Bishop Hill) are listed below:
1. Everyone in the room agrees that CO2 does absorb infrared radiation, as observed in the lab
JC comment: I think we can declare this one 99.99% accepted
2. I think everyone in the room agrees that the greenhouse effect (however badly named) does occur in practice: our planet and the others with an atmosphere are warmer than they would be because of the presence of water vapour and CO2.
JC comment: I tried valiantly to lay this one to rest with the threads on Slaying a Greenhouse Dragon, but the skydragon group isn’t buying it. I have yet to see an interesting (let alone valid) argument from this group. I assume they will continue to generate noise (but no light).
3. The greenhouse effect does not saturate with increasing CO2.
JC comment: a slightly complicated issue, but one that has been more than adequately dealt with in the published literature and the technical climate blogosphere (e.g. here).
4. It seemed that everyone in the room agreed that the CO2 concentration in the atmosphere has risen significantly over the last 200 years
JC comment: should be 100% agreement on this.
5. Almost everyone in the room agrees that this is because of anthropogenic emissions (fossil fuels, cement production, forest clearance). We did hear Ian Plimer arguing that volcanic emissions of CO2 are more important than most scientists claim.
JC comment: the anthropogenic contribution is (should be) undisputed. However, uncertainties in the global carbon budget do leave room for arguing about the relative contribution from fossil fuels.
6. I then suggested that if we agree all these statements above, we must expect at least some warming.
JC comment: as it stands, this is an incomplete argument.
7. I think everyone in the room agrees that the climate has warmed over the last 50 years, for whatever reason: we saw plots of land atmospheric temperature, marine atmospheric temperature, sea surface temperature, and (from Prof Svensmark) ocean heat content, all with a rising trend.
JC comment: there should be 100% agreement on the sign of the temperature change, although there is some room to debate the actual magnitude of the increase.
8. We probably don’t agree on what has caused the warming up to now, but it seemed that Prof Lockwood and Svensmark actually agreed it was not due to solar changes, because although they disagreed on how much of the variability in the climate records is solar, they both showed solar records without a rising trend in the late 20th century.
JC comment: the role of solar variability on 20th century warming is not settled; the IPCC AR4 gave “low confidence” to our understanding of solar variability. There has been a spate of recent solar analyses, but the issues of direct vs indirect solar effects (e.g. Svensmark’s ideas) is far from settled.
9. On sea level, I said that I had a problem in the context of the day, because this was the first time I had ever been in a room where someone had claimed (as Prof Morner did) that sea level has not been rising in recent decades at all. I therefore can’t claim we agreed, only that this was a very unusual room. However, I suggested that we can agree that, IF it warms, sea level will rise, since ice definitely melts on warming, and the density of seawater definitely drops as you warm it.
JC comment: if it warms and there are no confounding factors like coastal subsidence and isostatic causes and sedimentation in deltas, then yes sea level will rise. However in many locations geological factors swamp eustatic sea level rise, and sea level is actually decreasing. The scientific and socioeconomic impacts of sea level rise are fundamentally local, and emphasizing the global rather than local sea rise issues isn’t very useful IMO.
10. Finally we come to where the real uncertainties between scientists lie, about the strength of the feedbacks on warming induced by CO2, with clouds a particularly prominent issue because they have competing effects that are hard to quantify. I suggested to the audience that we could probably agree that the likely range of warming from a doubling of CO2 was 2-4.5 degrees C (which is actually the IPCC range). This was the first time I really got any dissent, so I then asked whether we could all agree on at least 1 degree (implying no positive feedbacks at all, even from increased water vapour and sea ice loss). I got one dissenting voice for that, but there wasn’t a chance to find which of the preceding statements he had disagreed with (it would be necessary to disagree somewhere up the line to be consistent with dissenting on this one).
JC comment: indeed, this is where the biggest uncertainties lie. I don’t buy the IPCC’s likely/very likely ranges for climate sensitivity (see this previous post). Re the 1 degree no feedback response, see this previous thread.
Summary
This was certainly an interesting exercise by Wolff, see also the Bishop Hill thread for Montford’s comments. The bottom line is that there are a few basics that everyone agrees upon, and plenty of topics where there is legitimate disagreement (which signals significant ignorance about the topic).