April 8, 2009

Has the climate recently shifted?

“Has the climate recently shifted?” is the title of a just-published paper in Geophysical Research Letters by researchers Kyle Swanson and Anastasios Tsonis from the University of Wisconsin-Milwaukee. Their examination of this topic was undoubtedly prompted by the recent behavior of global temperature which shows that the rate of warming has dramatically slowed during the past 7-12 years.

Updating a methodology that they had previously developed and used to identify several changes in the climate state that occurred during the 20th century, Swanson and Tsonis examined the temperature data from recent years to see if another state change had taken place:

Here, a new and improved means to quantify the coupling between climate modes confirms that another synchronization of these modes, followed by an increase in coupling occurred in 2001/02. This suggests that a break in the global mean temperature trend from the consistent warming over the 1976/77–2001/02 period may have occurred.

In other words, the authors think that they have identified another in a string of break points that signal a change in the general state of the earth’s climate.

The prevailing climate state generally lasts for a couple of decades. Swanson and Tsonis identified previous break points occurring around 1910 (which ended a period of steady to cooling temperatures and began a period of warming), the early 1940s (which ended a period of warming and began a period of steady to cooling temperatures), the mid 1970s (which ended the period of slight cooling and began a period of warming), and now seemingly in the early 2000s (which possibly ended a period of warming and began a period of steady to slightly cooling temperatures). The authors find that the temperature change during the past century is episodic in nature, rather than a smooth increase over time as atmospheric greenhouse gases have built up and that such a behavior is “difficult to reconcile with the presumed smooth evolution of anthropogenic greenhouse gas and aerosol radiative forcing with respect to time” and “suggests that an internal reorganization of the climate system may underlie such shifts.”


Figure 1. Global temperature history since 1900 with the approximate breaks in the prevailing temperature trends indicated (from Swanson and Tsonis, 2009).

Swanson and Tsonis carefully reviewed other possible causes of the recent temperature behavior and eliminated either volcanic eruptions or strong La Niñas as a possible mechanisms for the lack of warming in recent years and concluded that:

This [recent] cooling, which appears unprecedented over the instrumental period, is suggestive of an internal shift of climate dynamical processes that as yet remain poorly understood. [emphasis in original]

There are several interesting and potentially important implications of this work.

The first is that if a change of climate state did take place in 2001/2002, it could mark the beginning of a relatively extended period during which the rise in global temperatures proceeds at a rate far less than forecast by today’s climate models. Swanson and Tsonis describe this possibility:

If as suggested here, a dynamically driven climate shift has occurred, the duration of similar shifts during the 20th century suggests the new global mean temperature trend may persist for several decades. Of course, it is purely speculative to presume that the global mean temperature will remain near current levels for such an extended period of time. Moreover, we caution that the shifts described here are presumably superimposed upon a long term warming trend due to anthropogenic forcing. However, the nature of these past shifts in climate state suggests the possibility of near constant temperature lasting a decade or more into the future must at least be entertained.

Of course, if this did occur, it would challenge how well climate models understand the inner workings of the earth’s climates, because generally, they do not predict such an occurrence to take place in the 21st century when run under emissions scenarios approximating reality. You can hardly blame the models however, because apparently our understanding of the climate system is not good enough to produce accurate models of it. Swanson and Tsonis write:

The apparent lack of a proximate cause behind the halt in warming post 2001/02 challenges our understanding of the climate system, specifically the physical reasoning and causal links between longer time-scale modes of internal climate variability and the impact of such modes upon global temperature. Fortunately, climate science is rapidly developing the tools to meet this challenge, as in the near future it will be possible to attribute cause and effect in decadal-scale climate variability within the context of a seamless climate forecast system. Doing so is vital, as the future evolution of the global mean temperature may hold surprises on both the warm and cold ends of the spectrum due entirely to internal variability that lie well outside the envelope of a steadily increasing global mean temperature.

Swanson and Tsonis go on to add this word of warning:

Finally, it is vital to note that there is no comfort to be gained by having a climate with a significant degree of internal variability, even if it results in a near-term cessation of global warming. It is straightforward to argue that a climate with significant internal variability is a climate that is very sensitive to applied anthropogenic radiative anomalies. If the role of internal variability in the climate system is as large as this analysis would seem to suggest, warming over the 21st century may well be larger than that predicted by the current generation of models, given the propensity of those models to underestimate climate internal variability.

However, it is quite possible that this concern is misplaced.

It is not that the real climate system has more internal variability and thus is more sensitive to applied anthropogenic forcings—it simply is as it is, and we have observed how it has behaved during the past 100 years under increasing anthropogenic influences. But it is that the real climate has more internal variability than the climate models expect and thus the climate models may have the wrong sensitivity (how much the climate will warm when atmospheric carbon dioxide is doubled).

But do the models have the the climate sensitivity too low (as Swanson and Tsonis suggest) or too high? Or is the climate model climate sensitivity even coupled to the size of the model internal variability (as it is in the real world)?

There are hints that the latter two may be the case—that is, the model climate sensitivity is (artificially) disconnected from the model-produced internal variability, and that the model climate sensitivity it too large.

Support from the first comes from the results of Keihl (2007) (among other sources) which shows that despite the range of climate sensitivities found across climate models, the different models each simulate the temperature evolution of the 20th century correctly. In other words, the models have about the same amount of temperature variability despite having much different climate sensitivities—something which should be impossible.

So, how do the models pull it off? Well, it happens because each climate model is tuned such that the total amount of forcing that is applied is just the right amount to produce a match to 20th century temperatures. Therefore, since models are so constrained, it does not immediately follow that if the models underestimate the size of the real world internal climate variability that they necessarily will project greater warming in the future—they haven’t in the past, so why should they be expected to do so in the future?

The biggest tuning knob for getting models to match reality is the one labeled “forcing from aerosols.” This knob is especially useful because we really don’t know where exactly it should be set—thus different modeling groups can set it just about anywhere they would like to get their models to produce the right answer. The knob was installed in the first place because without it the models produce too much warming from greenhouse gas increases alone. So, add a knob that can be used to counteract some of that warming (taken together aerosols in the models produce a cooling), and you get the right answer without having to make any other major changes to the model!

Unfortunately (for the modelers), is that the results from Swanson and Tsonis do not support a large real-world role of aerosols in determining the shape of the global temperature history. Instead of aerosols being responsible for the cooling from the 1940s through the mid-1970s and then the warming (accompanying a reduced aerosol load) from the late 1970s to the present (as in the models), Swanson and Tsonis explain these episodes by internal climate processes—no aerosols needed. In earlier work (Tsonis et al., 2007), the authors concluded that:

The standard explanation for the post 1970s warming is that the radiative effect of greenhouse gases overcame shortwave reflection effects due to aerosols. However, [our result] suggests an alternative hypothesis, namely that the climate shifted after the 1970s event to a different state of a warmer climate, which may be superimposed on an anthropogenic warming trend.

Their new work further supports this conclusion as do plain and simple observations—after all, there is no way that declining influence of aerosols which was invoked to help explain the warming of the 1980s and 1990s can be used to explain the lack of warming thus far during the 21st century.

So if aerosols don’t play a large role in the 20th century temperature behavior, then the models get things right for the wrong reasons and, when fed the right reasons, they would get things wrong (i.e. produce too much warming—an indication that their climate sensitivity is too large).

The most important lesson of all that should be drawn from the work of Swanson and Tsonis is that the modeling community still has some important work to do, not only for projecting the future, but also for properly replicating the past. And until they can do the latter, they have no business even attempting the former.

References:

Kiehl, J.T., 2007. Twentieth century climate model response and climate sensitivity. Geophysical Research Letters, 34, L22710, doi:10.1029/2007GL031383.

Swanson, K.L., and A.A. Tsonis, 2009. Has the climate shifted? Geophysical Research Letters, 36, L06711, doi:10.1029/2008GL037022.

Tsonis, A. A., K. Swanson, and S. Kravtsov, 2007. A new dynamical mechanism for major climate shifts. Geophysical Research Letters, 34, L13705, doi:10.1029/2007GL030288.




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