by Judith Curry
On Lucia Liljegren’s Blackboard (commonly categorized as a “lukewarmer” site), Zeke has a post titled “Agreeing.” Zeke’s motivation for this is:
My personal pet peeve in the climate debate is how much time is wasted on arguments that are largely spurious, while more substantive and interesting subjects receive short shrift. While I’m sure a number of folks will disagree with me on what is spurious vs. substantive, I think it would be useful to outline which parts of the debate I feel are relatively certain, are somewhat uncertain, and quite uncertain.
I attempted something similar on an earlier thread “What we know with confidence,” which was based on the conclusions of the IPCC FAR, which have been out there long enough to have stood the test of time (or not). I also attempted to put to rest debate about whether or not the greenhouse effect exists, on the Slaying the Sky Dragon threads.
Zeke puts forth a list that is pretty consistent with the IPCC AR4, and he doesn’t get much push back from the comments (which are well worth reading). A very different response from the earlier confidence thread here.
So lets try this again, with Zeke’s statements, and i will append my own comments, as a spring board for discussion.
What is extremely likely [>95% probability]
- The greenhouse effect is real, albeit poorly named. While reams of comments have been written on this subject (witness the whole Sky Dragon debacle over at Judy’s blog, or Science of Doom’s heroic efforts to explain every facet of the issue), I’d hope that readers here won’t argue with this one. JC: OK
- Carbon dioxide is a greenhouse gas. JC: OK
- Atmospheric carbon dioxide concentrations are increasing. Folks like Beck notwithstanding, there is no serious challenge to Keeling’s measurements, especially as they have been verified by hundreds of additional methods in the subsequent decades. JC: OK
- Anthropogenic emissions of carbon dioxide are increasing. We have reasonably good data on the consumption of carbon-heavy fossil fuels over the past few centuries. JC: OK
- The majority of the increase in carbon dioxide concentrations since pre-industrial times is due to anthropogenic emissions of carbon dioxide. This is confirmed both by the isotopic signature of the carbon and the fact that concentrations rise proportionate to emissions. JC: OK
What I think is very likely [>90% probability]
- A doubling of carbon dioxide, holding everything else equal, would lead to a global average surface temperature increase of about 1 C. This follows from a basic derivation of forcing from changes to absorption bands, though it is complicated by the inherent difficulty of defining what exactly a no-feedback system is. JC: I have a problem with the way this is formulated, but agree that more CO2 will warm the surface.
- Stocks of atmospheric carbon have a relatively long lifetime. While any individual molecule of atmospheric carbon remains in the atmosphere for only a few years on average, the growth limitations of sinks means that the stock will not decline quickly should emissions stop increasing. As a corollary, arguments that 95 percent of annual CO2 emissions are natural rather miss the point. JC comment: the lifetime of CO2 in the atmosphere is debated, so as a qualitative statement this is ok.
- Water vapor primarily acts as a feedback rather than a forcing in the climate system due to its short atmospheric residence time and the limitation to absolute humidity at a given temperature for saturated air. Science of Doomcovers this rather well. Pointing out that water vapor is Earth’s dominant greenhouse gas does not minimize the climate’s sensitivity to carbon dioxide. JC: OK
- As a corollary to 3., a warmer world will have an atmosphere with more water vapor. This will tend to enhance the greenhouse effect, though the situation is complicated by the difficulty in both projecting changes in cloud formation and determining the radiative forcing effect of clouds. JC: the coupled water vapor and cloud feedbacks are uncertain, doesnt belong in very likely IMO.
- Direct solar forcing has played a relatively minor role in the last four decades, as TSI has been flat-to-modestly-decreasing during that period. JC: this is still being debated, in terms of calibrations of the satellites, etc. I would bump this down to likely.
What I think is likely [>66% probability]
- Climate sensitivity is somewhere between 1.5 C and 4.5 C for a doubling of carbon dioxide, due to feedbacks (primarily water vapor) in the climate system. This is supported by multiple lines of evidence, including GCMs, paleoclimate evidence (including climate response to forcing during glacial periods as well as millennial proxies), the instrumental record, and the climate response to volcanic forcings among others. That said, this range is large enough that it could mean that climate change will be a moderate issue (1.5 C) or potentially quite dangerous (4.5 C). JC comment: I think we can bound this between 1 and 6C at a likely level, I don’t think we can justify narrowing this further.
- Land and ocean temperature measurements over the past century are largely accurate at a global level, though there are some regions that have limited data, especially toward the earlier part of the century. That said, factors like UHI, instrument change, siting issue, and other data quality issues could potentially change the global trend modestly. JC: I’m waiting to see the results of the Berkeley analysis (coming soon) before passing judgment. I have serious concerns about the ocean data.
- Indirect solar forcing is not particularly significant in recent decades. While the role of cosmic rays in cloud formation is interesting and deserving or more study, the lack of a trend and large uncertainty in modalities precludes it being a major player in modern warming. JC: even the IPCC AR4 did not have confidence on this one; it is at the knowledge frontier, border with ignorance.
What I think is more likely than not [>50% probability]
- Intrinsic (unforced) variability plays a relatively large role globally at an intra-decadal scale, but is relatively insignificant at multidecadal scales. In this view, the early 20th century warming was due primarily due to solar forcings and a volcanic lull. JC: I disagree with this one.
- Recent warming is unprecedented over the past millennium. While there are plenty of problems with paleoclimate reconstructions, enough corroborating work has been done to at least elevate this to more likely than not in my personal judgment. Were there reconstructions clearly showing MWP temperatures comparable with, say, the running 50-year mean of the instrumental record I would be less certain. JC: I don’t think we know; the white part of the Italian flag is very big on this one.
When an individual is assessing these, the epistemic level of the assessor is relevant. I propose the following levels:
1. Research scientist publishing papers on relevant topics
2. Individual with a graduate degree in a technical subject that has investigated the relevant topics in detail.
3. Individual spending a substantial amount of time reading popular books on the subject and hanging out in the climate blogosphere
4. Individual who gets their climate information from talk radio
Note: personally, I would rate an epistemic level of 1 on some of the topics, and level 2 on others.
Moderation note: this is a technical thread, will be moderated for relevance.