A new paper has been published in PNAS entitled “Reconciling anthropogenic climate change with observed temperature 1998-2008.”
Reconciling anthropogenic climate change with observed temperature 1998-2008
Robert K. Kaufmann, Heikki Kauppi, Michael L. Mann, and James H. Stock
Abstract. Given the widely noted increase in the warming effects of rising greenhouse gas concentrations, it has been unclear why global surface temperatures did not rise between 1998 and 2008. We find that this hiatus in warming coincides with a period of little increase in the sum of anthropogenic and natural forcings. Declining solar insolation as part of a normal eleven-year cycle, and a cyclical change from an El Nino to a La Nina dominate our measure of anthropogenic effects because rapid growth in short-lived sulfur emissions partially offsets rising greenhouse gas concentrations. As such, we find that recent global temperature records are consistent with the existing understanding of the relationship among global surface temperature, internal variability, and radiative forcing, which includes anthropogenic factors with well known warming and cooling effects.
Link to complete paper [here].
The key argument in their paper is that an increase in coal burning (primarily in China) has increased atmospheric sulfate concentration with a resulting global cooling effect. The data that they use for coal emissions can be found in this spreadsheet from EIA. The paper concludes:
The finding that the recent hiatus in warming is driven largely by natural factors does not contradict the hypothesis: “most of the observed increase in global average temperature since the mid 20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.” As indicated in Fig. 1, anthropogenic activities that warm and cool the planet largely cancel after 1998, which allows natural variables to play a more significant role. The 1998-2008 hiatus is not the first period in the instrumental temperature record when the effects of anthropogenic changes in greenhouse gases and sulfur emissions on radiative forcing largely cancel. In-sample simulations indicate that temperature does not rise between the 1940’s and 1970’s because the cooling effects of sulfur emissions rise slightly faster than the warming effect of greenhouse gases. The post 1970 period of warming, which constitutes a significant portion of the increase in global surface temperature since the mid 20th century, is driven by efforts to reduce air pollution in general and acid deposition in particular, which cause sulfur emissions to decline while the concentration of greenhouse gases continues to rise.
The results of this analysis indicate that observed temperature after 1998 is consistent with the current understanding of the relationship among global surface temperature, internal variability, and radiative forcing, which includes anthropogenic factors that have well known warming and cooling effects. Both of these effects, along with changes in natural variables must be examined explicitly by efforts to understand climate change and devise policy that complies with the objective of Article 2 of the 1992 United Nations Framework Convention on Climate Change to stabilize “greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference in the climate system.”
Science News has is writing an article on this paper (haven’t spotted it yet). Here is the complete comments I provided to the reporter via email:
This paper points out that global coal consumption (primarily from China) has increased significantly, although the dataset referred to shows an increase only since 2004-2007 (the period 1985-2003 was pretty stable). The authors argue that the sulfates associated with this coal consumption have been sufficient to counter the greenhouse gas warming during the period 1998-2008, which is similar to the mechanism that has been invoked to explain the cooling during the period 1940-1970.
I don’t find this explanation to be convincing because the increase in sulfates occurs only since 2004 (the solar signal is too small to make much difference). Further, translating regional sulfate emission into global forcing isnt really appropriate, since atmospheric sulfate has too short of an atmospheric lifetime (owing to cloud and rain processes) to influence the global radiation balance.
The alternative explanation is natural internal variability associated with the ocean oscillations. Since 1999, the Pacific Decadal Oscillation has been shifting from the warm phase (warm phase since 1976) to the cool phase, and has been mostly in the cool phase since 2007. A cool PDO is associated with more frequent La Nina events, which are associated with globally cooler temperatures. The climate model studies cited by the authors do not do a convincing job of ruling out natural internal variability as an explanation, either for the cool period since 1998, and the earlier cool period during 1940-1970.
In summary, the authors have put forward one possible explanation for the lack of warming, but an explanation associated with natural internal variability associated with the ocean oscillations is at least as plausible as the explanation put forward by the authors.
Sulfate from coal burning is but one source of atmospheric aerosol. AGW Observer provides a recent list of papers on aerosol forcing observations. See especially the Remer et al. paper, Fig 5, which shows no trend in global aerosol optical depth during the period 2000-2006. A plot for East Asia also shows no trend regional aerosol optical depth.
I also checked to see what CMIP5 is using for aerosol forcing, see here, but there doesn’t seem to be any simple way to visualize whatever is being used. I found a paper by Jones et al., see Fig. 10, that appears to be the CMIP5 aerosol forcing. Looking at the black curves (historical, to 2005), it is seen that sulfur dioxide emissions peaked during 1960-1980, and then have steadily decreased (a tiny uptick after 2005 is seen in some of the future scenarios). Fossil fuel black carbon has shown a stead increase since 1950, as has fossil fuel organic carbon. I don’t seen any signal in the total aerosol emissions that resembles the coal emissions with a flat trajectory since 1985 and an uptick after 2004 (although the historical data ends in 2005).
There is also a big international project called AEROCOM, that is coordinating the preparation, evaluation and application of global aerosol data sets. However I can’t find anything on their site that provides easy visualization of the global aerosol emissions or forcing.
JC comments: Their argument is totally unconvincing to me. However, the link between flat/cooling global temperature and increased coal burning in China is certainly an interesting argument from a political perspective. The scientific motivation for this article seems to be that that scientists understand the evolution of global temperature forcing and that the answer is forced variability (not natural internal variability), and this explanation of the recent lack of warming supports a similar argument for the cooling between 1940 and 1970. The political consequence of this article seems to be that the simplest solution to global warming is for the Chinese to burn more coal, which they intend to do anyways.
And finally, with the civil heretic discussion fresh in my mind, I checked the personal web pages of each of the co-authors: Robert K. Kaufmann, Heikki Kauppi, Michael L. Mann (not Michael E. Mann, of hockeystick fame), and James H. Stock. These authors (individually and collectively) apparently know a heck of a lot less about atmospheric aerosols (i.e. pretty much nothing) than Freeman Dyson knows about climate change. The authors don’t seem to know much about attribution, either.
This article is listed as a PNAS direct submission, which means that it gets the more rigorous review treatment by the PNAS editors. I would certainly be interested in knowing who reviewed this paper. I suspect that this paper will be criticized from both sides of the AGW debate.