All Aflutter About Butterflies

Threats to the Monarch butterfly’s winter range published in the Proceedings of the National Academy of Sciences have reporters on the “global warming” beat all aflutter. The researchers claim to demonstrate how, within 50 years, altered climate conditions likely will prove inadequate to support Monarch populations (Bye, bye butterfly). We’re aflutter too because this conclusion is based upon a chance occurrence and faulty underlying principles. This research turns out to be an unfortunate mix of science and science fiction.
     This kind of flighty analysis all too often clogs the scientific literature and stimulates the popular press to flights of seemingly science-based silliness. Once again, the conclusions prove to be dependent upon a single version of a particular climate model run under very specific scenarios of anthropogenic emissions. Tweak any of the knobs and you can produce virtually any result that you desire. In this instance the desired conclusion was the one capable of garnering lots of attention: The premise that a huge dark cloud is looming over one of the world’s most recognized and beloved butterfly species. (Cue “Theme From Jaws”).
     Monarch butterflies are among the world’s greatest migrators. They spend the summer in wide distribution all across North America. Then the eastern population (those east of the Rocky Mountains) gather in massive numbers and begin an airborne trek south of the border, down Mexico way. They winter over in a few remote locations west of Mexico City where, it is theorized, ecological and climate conditions are just to the Monarch’s liking. It’s cool, but not too cold, and it’s relatively dry.
     Two butterfly researchers, Karen Oberhauser and Townsend Peterson, decided to examine how the potential for future changes in climate might alter this preferred habitat. Could it any longer support the Monarch population?
     First, they devised a model identifying specific conditions sought by the butterflies. They used a technique called ecological niche modeling in which they combine topographical data, ecological data, and climatological data with the observed overwintering locale to produce a precise understanding of the conditions that the Monarchs seek. So far, so good. But here’s where science fiction creeps in.
     To ascertain how future climate change might impact the Monarch butterflies, Oberhauser and Peterson used the output of a somewhat dated climate model (HadCM2) originally developed at the Hadley Centre in the United Kingdom. The model’s output was produced by using two different input scenarios. One is labeled a “conservative” scenario, in which carbon dioxide concentrations are increased at a rate of 0.5% per year. It also includes the cooling effects of sulfate emissions aerosols. If you don’t keep track of this kind of thing, we’ll point out that the actual annual rate of increase in carbon dioxide has a 25-year average of about 0.43% per year. In other words, the conservative scenario incorporates an assumed increase greater than what currently is observed.
     A “less conservative” (dare we say, liberal) scenario projects a rate of carbon dioxide increase of 1.0% per year. This is totally unrealistic because it results in an atmospheric carbon dioxide concentration 2.5 times the current levels of increase over the course of a century. And, oh yes, this approach doesn’t incorporate sulfate cooling.
     Oberhauser and Peterson used results from these two model runs to conclude that the Monarchs’ overwintering ground will become slightly cooler and much wetter within 50 years. How much wetter? Six times!
     Would it surprise you to learn that this new set of conditions doesn’t match up very well with the current set of conditions the butterflies prefer? It probably didn’t surprise our intrepid researchers, either. They say the model results, “[S]uggest that current over-wintering sites will become less suitable for monarchs over the next 50 years,” and that when the current distribution of the tree species in which the Monarchs gather are folded in “[N]one of the present wintering sites was predicted to be suitable in 50 years.”
     They claim “science”; we respond “science fiction.” Here’s why: For purposes of civil discourse we’ll ignore the fact that the smallest spatial scale produced by the HadCM2 is 2.5º latitude by 3.75º longitude, something on the order of a grid 175 miles by 250 miles, while the butterflies’ over-wintering locations represent a couple of square miles on a few specific mountainsides in a narrow elevation band that has a very particular set of climate conditions. We’ll grant the authors' assumption that future conditions within the 175-by-250-mile grid cell (a grid that includes oceans, shoreline, coastal plain, mountains, and valleys among other topographical features) is representative of the butterflies’ preferred locale. (Good science fiction requires suspension of disbelief.)
     But there is a fact we simply cannot ignore: There are more than 20 different climate models. The UN’s Intergovernmental Panel on Climate Change also describes more than 35 different future emissions scenarios (though it has been suggested that few fairly represent observed trends). That combination of models and scenarios would result in more than 700 different outcomes. If you perturb models that have slightly different initial conditions, the models can produce sometimes significantly different output, especially when examined at the level of the smallest spatial scales (as this study does).
     Might a reasonable person conclude it to be possible that 700 different papers could be written and published on the specific and somewhat esoteric topic of the overwintering grounds of a single subpopulation of single species of insect? The results from the current paper are no more or nor any less representative of the Monarchs’ potential fate in Mexico’s mountains fifty years from now than would be the results generated from any of the other model/scenario combinations.
     To illustrate how different results can depend on the choice of model and emissions scenario, Figure 1 shows the change in winter precipitation produced by the HadCM2 model run under the IS92a emissions scenario. Figure 2 shows the change in winter precipitation produced by the updated Hadley Centre model, HadCM3 using the same scenario. Notice how the in the relevant region of central Mexico the HadCM2 model predicts slightly drier conditions while the HadCM3 model indicates there will be no change or that it might become slightly wetter.
     If you really want to pursue this, visit the IPCC’s climate model results visualization page (http://ipcc-ddc.cru.uea.ac.uk/cgi-bin/ddcvis/gcmcf) and examine what happens in central Mexico (just west of the Yucatan peninsula) as you choose different models and scenarios. There is a huge array of possible climates and, interestingly enough, only very few result in cooler temperatures and much wetter conditions — those described by Oberhauser and Peterson.
     Any person can draw any number of conclusions from a random sample. But doing so is without scientific merit or meaning. But it could get your paper published in the Proceedings of the National Academy of Sciences, apparently.



Reference:
Oberhauser, K., and Peterson, T., 2003. Modeling current and future potential wintering distributions of eastern North American monarch butterflies. Proceedings of National Academy of Sciences, 100, 14063-14068.

Figure 1. Winter (December, January, February) change in precipitation projected by the HadCM2 model run using the IS92a emissions scenario. Figure 2. Winter (December, January, February) change in precipitation projected by the more updated HadCM3 model run using the same IS92a emissions scenario that produced the results in Figure 1 from the older HadCM2 model. Do Oberhauser and Peterson intend to publish a new paper every time the Hadley Centre tweaks their model?

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