A Surprise in the Greenhouse

by Bill Chameides | January 27th, 2011
posted by Erica Rowell (Editor)

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California plants facing global warming: “I won’t go up.”

If you are a human being worried about climate change, then you should also be concerned about how plants will respond to those changes. For example, if you want forests in the world and forests need to move from one location to another as a result of global warming, then you’re gonna want to make sure there is some habitat for those forests to move into in that new location.

Movin’ on Up? Not Necessarily

For the most part, scientists have assumed that the direction of plant movement in response to global warming is pretty self-evident. The reasoning goes something like this: plants today locate themselves in places that optimize the temperature to their needs. Thus, as the temperature warms in a given location, plants will tend to move to the cooler locations. Latitudinally, this usually means moving northward (in the Northern Hemisphere). Altitudinally, this means moving upward. (For more, see also this Smithsonian article.)

It ain’t necessarily so, says Shawn Crimmins from the University of Montana and co-authors in a paper published last week in the journal Science. The authors combined long-term data on 64 plant species in California to see if the average (or optimal) altitudes of plants have changed from the 1930s to today and if that change could be correlated with changes in three key climatic conditions: temperature, sunlight and water availability. It was a fairly comprehensive study of what’s been going on in California with 13,746 survey plots sampled in the 1930s and about 33,000 plots between 2000 and 2005.

Surprisingly, the authors found that even though average temperatures increased over the periods studied by about one degree Fahrenheit, 72 percent of the plants studied moved downward —  the average change in altitude was 290 feet. As a result, on average the plants are now hanging out at temperatures that are about 0.6 degrees Fahrenheit warmer than those back in the 1930s.

Water Is the Key

Why would plants choose to reside at warmer temperatures? Water, argues Crimmins et al. Over the study period, precipitation in northern California increased, making water more readily available and thus allowing plants to grow optimally at higher temperatures (and lower altitudes) without wilting. The authors found that the change in precipitation — which favors downhill movement — dominated over the increase in temperature — which favors uphill movement. And so the plants of California have tended to shift down the hill, so to speak, rather than uphill.

The authors’ bottom line: if you want to be able to predict how the biosphere is going to respond to global warming, you better consider water as well as precipitation.

Adaptive Strategies Are Key to Dealing With Climate Change but the X Factors Make Developing Them Tricky

Learning to adapt to climate change is an imperative. The climate is warming today at a remarkably rapid pace, and regardless of what we do about emissions of carbon dioxide and other greenhouse gases, it will continue to warm for at least decades. Ideally, we could suss out what’s going to happen as a result of warming and prepare for it. But the Crimmins et al study provides a cautionary reminder that we are dealing with a complex system and the response of that system may be quite different from what we expect. In other words, surprising results should not be all that surprising.

So, you want to get ready for global warming? Get ready to be surprised. You never want a simple plan in global warming land.

filed under: carbon dioxide emissions, climate change, faculty, global warming, habitat, plants, temperatures, water
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