In ACPD this week, a new paper by P.K.Quinn et. al. (co-authors include Shindell), looks at the things which effect the Arctic climate in the short-term (anthropogenic forcings other than CO2), and discuss possible strategies for mitigation. Link to the abstract here, from where you can download the discussion paper.
The paper focusses on Methane, Tropospheric Ozone and Aerosols (with an emphasis on black carbon in the latter part). For those who can’t be bothered, here’s what the abstract says:
Short-lived pollutants in the Arctic: their climate impact and possible mitigation strategies
Abstract. Several short-lived pollutants known to impact Arctic climate may be contributing to the accelerated rates of warming observed in this region relative to the global annually averaged temperature increase. Here, we present a summary of the short-lived pollutants that impact Arctic climate including methane, tropospheric ozone, and tropospheric aerosols. For each pollutant, we provide a description of the major sources, the mechanism of forcing, seasonally averaged forcing values for the Arctic, and the corresponding surface temperature response. We suggest strategies for reducing the warming based on current knowledge and discuss directions for future research to address remaining uncertainties.
The most powerful forcing is still related to tropospheric aerosol indirect effects, though it is interesting to note that, whilst the biggest factor by far is in Summer, the largest net effects are felt in other seasons, notably Winter. The table at the end of the paper provides a useful summary.
There may be some questions about the forcing values of cloud longwave emissivity; I am not sure that this is an entirely convincing part of the paper, but I’ll hope that one of you can provide a reasoned discussion of this section.
Another slightly surprising result of their analysis is the (relatively) low values of forcing from Methane to date. The paper does, however, point out that this variable is particularly open to sudden changes as a result of changes in land – permafrost degradation and wetland production, as well as vegetation typology – and still contains a degree of uncertainty. My intuition is that this particular forcing has considerable potential for non-linear, rapid change in the coming decades, and may play a more significant role than is implied in the paper’s estimates of current relative forcing values.
The paper is worth reading for anyone who is interested in looking for an explanation of why, for example, 2007 might have been such an exceptional year for Summer sea-ice loss. The unusually high number of boreal forest fires in Siberia and Alaska in 2006 may well have been an exceptional factor affecting ice-melt onset, and thus the ensuing record low.
This is a rich paper with much to consider, not least in the implications for our expectations of future changes in the Arctic, and possibly for those of you who have taken a bet with William. I’d appreciate some feedback from those in the know…