Recently, the world celebrated an International Day of Climate Action, called “350”, which is based on lifting public awareness on the need for an international climate treaty to reach a 350 parts per million CO2 level as a target threshold. I didn’t really join in on the fun or follow it in any detail, but from what I understand it was a pretty big deal, and I hope that they had some success in raising awareness.
In any case, I just wanted to say a word about the communication of climate science. It is very easy to justify, scientifically, that the globe is warming and that anthropogenic (human-induced) activities are responsible for at least a large part of that warming trend. It is easy to justify that continued use of fossil fuels under “business-as-usual” scenarios will eventually lead to consequences which can be dangerous to socio-economic and ecological welfare. The first two Working Groups of the IPCC 2007 report have an extensive background into these issues, as does many other reports and papers. It is therefore not necessary to over-inflate the consequences of climate change or to assume “worst-case” scenarios in analysis, and I urge people to consider this, since the consequences of this for a public attention, policy, and scientific viewpoint can be just as bad as those who downplay or misrepresent the urge to take climate action.
This brings me to a report online entitled The Economics of 350: The Benefits and Costs of Climate Stabilization (PDF file). The report is written by a group of economists (Ackerman et al.) who focus on the subject of emission targets, which urge quick action, and while I agree with much of what they write (and their concern about climate change and the urgency to act), much of the science in the paper is flawed or incomplete to emphasize their point that “climate change is occurring faster, and its consequences could be more severe, than previously expected” (Pg. 3). Such broad statements should be much more detailed (e.g., sea ice loss might be going up faster than expected, but temperatures are not) and the Copenhagen Synthesis report is a suitable citation for post-AR4 findings on this topic. I would just like to touch upon some points:
— Ackerman et al. describe climate sensitivity values cited in the IPCC AR4 (2 to 4.5 C per doubling of CO2) and sensitivity values in Hansen et al (2008) (~ 6 C per doubling of CO2) in “Target Atmospheric CO2: Where Should Humanity Aim?” in The Open Atmospheric Science Journal 2: 217-231. Ackerman et al. believe these these values are inconsistent, and specifically “That is, [Hansen et al] argue that the global warming likely to result from any given atmospheric concentration of CO2 is approximately twice as great as AR4 projected.”
This is not correct and is an apples-to-oranges comparison. IPCC 2007 uses the so-called Charney sensitivity, which refers to an equilibrium state in which CO2 is doubled. This accounts only for so-called “fast feedbacks” which respond relatively rapidly to climate change, such as sea ice, or water vapor content. The Hansen et al. value refers to a much longer term sensitivity value which takes into account slower feedbacks such as ice sheet and long-term vegetation changes. Hansen et al does not argue for a sensitivity outside the IPCC range when you compare the two appropriately.
— The assumption of a 6 degrees per 2x CO2 sensitivity is too high and unwarranted. The authors do explore alternative scenarios, however.
–Ackerman et al. continue to assert that scientific findings are becoming too troublesome. For instance,
The “climate sensitivity,” that is the amount of warming that will result from a doubling of the atmospheric concentration of greenhouse gases, may be inherently uncertain — because in a system such as the earth’s climate with strong positive feedbacks, small errors in estimating the size of the feedbacks inevitably cause large errors in the outcome (Roe and Baker 2007).
However, Ackerman et al ignore Hannart et al (2009) which suggests the Roe and Baker analysis may be an artifact of a strange use of the word ‘uncertainty.’
— Ackerman et al state:
Low-level clouds, one of the least understood aspects of the climate system until recently, may be a source of additional positive feedback to the warming process; of the major climate models, the one that simulates clouds most accurately is also the one that predicts the most rapid warming (Clement et al. 2009).
Although it may very well be true that clouds act as a positive feedback, Clement et al. can only be taken as a very small part of the literature on this topic. In particular, they only focus on a very small region of the globe (roughly 115° to 145°W, 15° to 25°N) and focus only on lower-level clouds that are important for albedo, and not higher ones which also can have a very large impact on sensitivity due to their impact on outgoing terrestrial radiation. A more comprehensive discussion of the literature, or at least a reference to more comprehensive discussions (e.g. Bony et al 2006) should be included.
–Ackerman et al. state,
There are also some lesser known, serious threats (Lenton et al. 2008; Weitzman 2009). For example, rising temperatures could trigger abrupt, massive releases of methane from undersea geological formations (clathrates) or from permafrost; this could lead to a runaway greenhouse effect (Hall and Behl 2006).
A runaway greenhouse effect is not something that is taken seriously as a possibility right now, and it has a very precise meaning in the study of climate physics and in the evolution of planetary atmospheres. It does not simply refer to a “tipping point” or dangerous amount of warming. Certainly, I can find no support for such a proposal in the reference by Hall and Behl (I cannot even find the word ‘runaway’ in that document).
–Ackerman et al state
Hansen’s assertion that 300–500 ppm CO2 corresponds to a 25 percent chance of harm is consistent with Harvey’s results under the assumptions that non-CO2 gases either remain at their current levels or decline, and that aerosols have no effect on temperature
There’s some sketchiness about what exactly “harm” means here, but Hansen et al only quote this CO2 range once, and they simply do it in the context of referencing Harvey’s results for background on the topic. It should not be taken to be “consistent” with Harvey’s reference. It came from Harvey’s reference!
— Ackerman et al. summarize the strategy by Hansen et al (2008) to stabilize emissions, although I personally don’t feel they do a good job of error bars (there are essentially none in their descriptions or assumptions). A lot of numbers are given in this document which should have some brackets next to them. Another example of overselling certainty is the claim that “The combined radiative forcings from aerosols is -1.18 W/m2”. It is not until a later graph that one can inspect a see a range of about 0.3 to 2 W/m2.
These are just some of the quibbles I have with this document. I also would recommend that documents which go into even some detail about the science (and not just the economics) have better correspondence with scientists in that field. I do agree with their focus on how to best stabilize emissions, although I find it difficult to get quantitative information from this piece.