January 2008

From Pat Michaels, World Climate Report

Remember the good old days when “fingerprinting” was in vogue as the way to demonstrate a human impact on global climate? The idea was to show that observed temperature changes throughout the atmosphere match well the temperature changes predicted by climate models to occur there. One of the most prominent, and ultimately disproven, attempts was made by Ben Santer and colleagues, back in 1996. Santer et al. published an article in Nature magazine titled “A search for the human influences on the thermal structure of the atmosphere” in which they concluded that “Our results suggest that the similarities between observed and model-predicted changes in the zonal-mean vertical patterns of temperature change over 1963-1987 are unlikely to have resulted from natural internally generated variability of the climate system.” In other words, there must be a human influence on the observed changes. However, we (Michaels and Knappenberger, 1996) published a subsequent Comment in Nature, titled “Human effect on global climate?” describing how the correspondence between the observed patterns of vertical temperature change in the atmosphere and those projected by climate models broke down if a longer time period were considered. In other words, if the comparison was extended from 1958 to 1995 (instead of Santer et al.’s 1963 to 1987) the correspondence between model and observations became much less obvious. We concluded “Such a result… cannot be considered to be a ‘fingerprint’ of greenhouse-gas-induced climate change.” (See here for more details)

Now, 12 years later, another study appears in Nature magazine that suggests that there is a poor correspondence between the observed patterns of vertical temperature change and those predicted to occur by climate models over the high latitudes of the Northern Hemisphere. This time, Rune Graversen and colleagues from the Department of Meteorology at Sweden’s Stockholm University, conclude in their article “Vertical structure of recent Arctic warming” that variations in atmospheric heat transport from the lower latitudes into the northern high latitudes (via atmospheric circulation patterns) are largely responsible for the enhanced warming of the Arctic atmosphere. This leaves less temperature change there ascribable to our current understanding of anthropogenic global warming.

In fact, the climate model-predicted human ‘fingerprint’ doesn’t match very well at all the observed patterns of temperature change that have taken place in the Arctic atmosphere over the past several decades.

Figure 1 shows how climate models predict that the vertical temperatures in the atmosphere will evolve as more and more CO2 is added to the air. Notice that in the northern high latitudes (to the right in Figure 1), warming takes place at a greater rate at the surface than aloft—this pattern of temperature change is fundamentally different than that expected to occur elsewhere, most notably in the Tropics where more warming is predicted to occur in the middle atmosphere than occurs at the surface (not that things are working out very well there either–see here for our coverage of the latest on the model failings in the Tropics). In the Arctic, the warming is supposed to be enhanced at the surface as a result of a positive feedback loop in which a little initial warming melts some snow and sea ice, which reduces the reflectivity of the surface, allowing it to absorb more incoming solar radiation, which warms it further, leading to more snow and ice melting, and so on and so forth. Much of this feedback involves near surface processes which do not greatly effect conditions higher up in the atmosphere due to the lack of convection in the Arctic (as opposed to the Tropics where convection mixes surface changes up into the atmosphere).

Figure 1. Climate model projections of the zonal averages of the changes in vertical temperatures expected under Intergovernmental Panel on Climate Change (IPCC) scenario A1B for the years listed above each figure compared with the average from 1980-1999 (source: IPCC, AR4, Figure 10.7)

However, when Graversen et al. computed the observed vertical temperature changes which took place from 1979-2001, they found a pattern that was completely different from the one projected by climate models. Figure 2 (top) shows that instead of more warming occurring at the surface in the high latitudes of the Northern Hemisphere, more warming has been occurring aloft. This is completely opposite to how most climate models run with increasing CO2 concentrations predict conditions to evolve (and for that matter, the observed patterns in the lower latitudes were opposite the model projections as well, again, see here for more on this mismatch).

This suggests that something other than CO2 and CO2-related feedbacks (at least as we currently understand them) are playing a large role in the region’s recent temperature trends. Graversen et al. propose that the culprit is the variability of the amount of mid-level heat exchange that takes place in the atmosphere between the lower latitudes and the Arctic. They support this idea by showing how variations in heat exchange are closely related to subsequent patterns of mid-tropospheric temperature variations—the more heat exchange across 60ºN, the greater the temperature anomalies in the mid-atmosphere in the Arctic and vice versa. Furthermore, Graversen et al. report that the amount of heat exchange has been generally trending upwards over the past 20 years or so.

Using the observed relationships between heat exchange and temperature patterns, coupled with the time series of heat exchange, the authors can construct vertical temperature changes that are expected to have occurred in response to the variation in heat exchange. What they find is that the observed pattern of temperature change and the ones they calculate to result from heat exchange variations closely match (Figure 2 bottom). This is an indication that their explanation holds water. However, they freely admit that other processes could be involved as well, including changes in cloud cover and increases in moisture (which may accompany the increased heat exchange). Together, in some combination, Graversen et al. believe that these processes are largely responsible for the observed changes in the temperature patterns in the Arctic since 1979. Note that these variations must be 1) largely natural, and/or 2) poorly captured by climate models, because otherwise the observed changes and modeled changes would be in better agreement.

Figure 2. (top) Observed temperature trends in the northern extratropics during the warm season (April – October) over the years 1979-2001. (bottom) The warming trends expected from the variability in the heat exchange between the low latitude and the high latitudes during the same period. Note that north is to the left in this Figure (From Graversen et al., 2008).

Graverson and colleagues are quick to point out that just because the temperature changes in the Arctic observed over the past 20 some odd years do not well match climate model projections doesn’t meant that they always won’t. Perhaps the near surface CO2-induced processes will eventually begin to dominate the processes of natural variability, or perhaps the climate models may one day be better able to handle heat exchange-related processes. But until that ever happens, pointing to ongoing climate change in the Arctic and yelling ‘fire!’ or, in this case ‘humans!’ seems scientifically a bit premature.

Note (added Jan. 4, 2008): The folks over at RealClimate make the interesting observation that the modeled behavior of the vertical temperature trends over the Arctic during the warm season bears a different character than with the trends over the whole year (as is depicted in our Figure 1). Using the output from the Goddard Institute for Space Studies (GISS) model run for the period 1979-2001 and for the Northern Hemisphere warm season (Figure 3), there appears to be a much better match with the observations than is implied by Graversen et al.’s write-up in Nature (although clearly there remains something seriously amiss in the lower latitudes).

Figure 3. The vertical temperature trends during the Northern Hemipshere warm season (May-October) for the period 1979-2001 as produced by the NASA GISS climate model run with all forcings. Note that north is to the right (source: http://data.giss.nasa.gov/modelE/transient/Rc_pj.1.11.html).

Reference:

Graversen, R.G., et al., 2008. Vertical structure of recent Arctic warming. Nature, 541, 53-57.

Intergovernmental Panel on Climate Change, 2007. Fourth Assessment Report, http://www.ipcc.ch/ipccreports/ar4-wg1.htm

Michaels, P.J., and P.C. Knappenberger, 1996. Human effect on global climate? Nature, 384, 522–523.

Santer, B.D. et al., 1996. A Search for Human Influences on the Thermal Structure of the Atmosphere. Nature , 382, 39–45.

Santer et al., 1996. Reply to: Human effect on climate? Nature, 384, 524.