The Intergovernmental Panel on Climate Change released its summary for policymakers on the "current scientific understanding" of climate change in Brussels, Belgium, on April 6. Its assessment of the possible climate-change threat is predictably bleak. The Weekly asked Travis Huxman, a University of Arizona physiological ecologist specializing in North American deserts and climate change, to help Southern Arizonans understand what the summary could mean to them.

What are the implications from this report for people living in Southern Arizona?

There are the human implications and the ecological implications. They're related. From the perspective of how climate might change and how it will affect the human endeavor, we know that there's going to be a decrease in winter precipitation; there's going to be sort of a general increase in temperature in both the summer and the winter. That really puts a stress on our local water resources, and our distant water resources, i.e., the Colorado River and the water that supports our growth. So you imagine there is the potential for an increased demand on those resources regionally. Along those lines, the increases in temperature mean that maintaining our current lifestyle--i.e., the air conditioning in the summer, those sorts of things--is going to become more expensive. It's going to hit people in the pocketbooks.

And ecological impacts?

In terms of ecological impacts, they're also related to water. They have to do with landscape water balance, and how that affects the way vegetation behaves. The decrease in winter precipitation means that many of our long-lived, woody, perennial-type species will experience greater periods of drought, and that will put their populations at risk. The relative increase in summer precipitation, as compared to winter, suggests that some of these non-native, summer-active grasses may have a competitive advantage.

Like buffelgrass?

Like buffelgrass. And we know that means that the landscapes are more predisposed to fire, and that's kind of like the double-whammy for our long-lived, charismatic megaflora--you know, saguaros, mesquites, the creosote bush. If you went west, the Joshua tree. Those kinds of things.

So the saguaro could be in danger?

Yeah. We already know that non-native-grass-spread fires are burning saguaros. So you can imagine an acceleration of that process under this climate-change scenario.

I read in the summary that there might be "poleward and upward shifts in ranges in plant and animal species." Tell me about that.

Cold temperatures limit the northern and high-elevation distributions of plants and animals, and so if we see a general warming, we should see a relaxation of that constraint--and movement north and up in elevation. But the thing that will trump that is how precipitation changes. If we don't see the same bands of precipitation with latitude that we currently see, that just may not happen. For example, if the North American jet stream continues to push water into the northern part of North America, rather than moving significantly south in the wintertime, then it doesn't matter for these low-latitude species. There's just no water for them to grow any higher up on the continent.

I see.

The other thing that trumps that--and which is absolutely one thing we've already seen--is the disturbances in land use, (which) are so profound on our landscape that they would either alter how species might migrate or they would prevent migration altogether.

The summary said that 20 to 30 percent of plants and animals are at "increased risk of extinction if increases in global temperature" surpass about 2.5 degrees Celsius. I want to play devil's advocate; why is that necessarily a bad thing? Why is it bad that plants and animals die out, if some are dying out all the time?

The idea here is that extinction rates are greater than speciation rates. Species are constantly being produced and constantly being lost from the Earth. If one of those rates exceeds the other, then we have this profound change in biodiversity. If global climate change is accelerating extinction rates, then we have a general reduction in species likely happening on the planet. People look at biodiversity as this kind of reservoir of potential. ... For us, it's important for maintaining healthy landscapes, and ensuring the goods and services that come from those landscapes, like water delivery, low sediment transport, low erosion rates--those kinds of things. Tucson and the Southwest are famous for their diversity. I mean, that's why people come here in the wintertime to see our landscape. This is why we have a tourist industry; the biodiversity means something in terms of dollars. You can imagine losing that diversity would be sort of (the same as) losing our unique characteristics on the continent.

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