December 3, 2008

Rethinking Observed Warming

The United Nations Climate Change Conference is underway this week in Poznan, Poland, and literally thousands of folks have convened and reinforced the notion that the buildup of greenhouse gases has caused substantial warming in recent decades and that left unchecked, the continued buildup will undoubtedly cause significant warming in the decades to come. Believe it or not, it is possible that aspects of the traditional greenhouse gas explanation could be largely wrong, and if you think we are crazy, let’s visit an article just published in the prestigious journal Climate Dynamics.

The interesting (to say the least) work was conducted Gilbert Compo and Prashant Sardeshmukh of the Climate Diagnostics Center, Cooperative Institute for Research in Environmental Sciences and Physical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration (these guys must have oversized business cards) and the work was supported financially by the NOAA Climate Program Office. The first sentence of the abstract reads “Evidence is presented that the recent worldwide land warming has occurred largely in response to a worldwide warming of the oceans rather than as a direct response to increasing greenhouse gases (GHGs) over land.” This sentence certainly captured our interest at World Climate Report – anyone suggesting that some warming may have been caused by something other than the buildup of greenhouse gases will always get a second look.

The approach used by Compo and Sardeshmukh is actually quite simple and clever. The pair collected sea surface temperature data from 1961 to 2006 and they determined the difference between the 1991-2006 and 1961-1990 sub-periods. As seen in the Figure 1 below, the oceans of the world generally warmed between the two sub-periods. Next, they took only the change in SSTs to force global climate responses in a suite of climate models. Basically, instead of increasing greenhouse gas (GHG) concentration to examine the global climate response, they wondered how the climate would be impacted by the spatial pattern of sea surface temperature changes over the global oceans. In addition, they conducted a set of numerical modeling experiments that did include the set of known forcings “included time-varying solar irradiance and volcanic aerosols, anthropogenic sulfate aerosols, tropospheric and stratospheric ozone, well-mixed GHGs (CO2, CH4, N2O), halocarbons, and black carbon aerosols.”


Figure 1. Surface temperature change, 1991-2006 minus 1961-1990 (from Compo and Sardeshmukh, 2008)

In the second sentence of their abstract, Compo and Sardeshmukh tell us “Atmospheric model simulations of the last half-century with prescribed observed ocean temperature changes, but without prescribed GHG changes, account for most of the land warming.” Are they kidding? Are they really suggesting that the warming of the land areas of the Earth may not have been caused directly by the increased concentration of greenhouse gases? The answer is … yes!

Their figure (Figure 2 below) shows the results and to the amazement of the greenhouse advocates, the model runs forced by SSTs only did as good a job replicating the observed temperature rise as the model runs with elevated greenhouse gases or the many other forcings explored in their research. In explaining their results, the scientists write “In summary, our results emphasize the significant role of remote oceanic influences, rather than the direct local effect of anthropogenic radiative forcings, in the recent continental warming. They suggest that the recent oceanic warming has caused the continents to warm through a different set of mechanisms than usually identified with the global impacts of SST changes. It has increased the humidity of the atmosphere, altered the atmospheric vertical motion and associated cloud fields, and perturbed the longwave and shortwave radiative fluxes at the continental surface.”


Figure 2. Simulated mean change in surface temperature, comparing model runs with prescribed SSTs to those with additional prescribed natural and anthropogenic forcings. Panels a and c (lefthand column) as using (a) the 14 NASA/NSIPP low-resolution climate model simulations with prescribed observed SSTs and (c) the 8 simulations forced additionally with time-varying CO2. Panels b and d (righthand column) Mean change in the 1991–2000 average minus the 1961–1990 average teusing 10 NCAR/CAM3 simulations with prescribed (b) observed SSTs and sea ice, and (d) additional anthropogenic and natural forcings. Coloring and smoothing are the same as the figure above (from Compo and Sardeshmukh, 2008)

It only gets better as they state “Although not a focus of this study, the degree to which the oceans themselves have recently warmed due to increased GHG, other anthropogenic, natural solar and volcanic forcings, or internal multi-decadal climate variations is a matter of active investigation.” That’s interesting – it seems there is some lingering debate about why the oceans have warmed, and they note “a role for natural causes of at least some of the recent oceanic warming should not be ruled out.” Here at World Climate Report, we never rule out the role of natural variability of the climate system.

Their final paragraph has some priceless phrases as they report “Regardless of whether or not the rapid recent oceanic warming has occurred largely from anthropogenic or natural influences, our study highlights its importance in accounting for the recent observed continental warming. Perhaps the most important conclusion to be drawn from our analysis is that the recent acceleration [sic—see our last couple of WCRs demonstrating a slowdown of recent warming trends] of global warming may not be occurring in quite the manner one might have imagined. The indirect and substantial role of the oceans in causing the recent continental warming emphasizes the need to generate reliable projections of ocean temperature changes over the next century, in order to generate more reliable projections of not just the global mean temperature and precipitation changes, but also regional climate changes.”

Stating “global warming may not be occurring in quite the manner one might have imagined” is an interesting way to report that the entire global warming – greenhouse gas buildup link (largely unchallenged) may be a quite a bit off. Time will tell, but don’t look for a lot of press coverage coming from the Poland meeting of this interesting research challenging the gospel of global warming.

Reference:

Compo, G.P. and P.D. Sardeshmukh. 2008. Oceanic influences on recent continental warming. Climate Dynamics, DOI 10.1007/s00382-008-0448-9.




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