Sound Science Recognizes Potential Confounding Effects

If you want to guess (or, more impressively, hypothesize) what kinds of impacts global warming might have, there are two approaches you can use. One is what we’ll call the 8th-grade earth science approach. It assumes simple, direct relationships. For example, you might hypothesize that increased precipitation from global warming will cause more flooding. Such a guess would assume that it doesn’t matter what time of year the extra precipitation arrives or what type of weather system delivers it. It’s difficult to flood a low-flow river and there is a difference in the effect of gentle spring rains, summer thunderstorms, and snow. The complex and ostensibly sophisticated climate models that serve as the basis for today’s global warming concerns use the 8th-grade earth science approach when they seek to inform us about what to expect from higher atmospheric carbon dioxide concentrations.
     The alternative is to consider a variety of possibilities, including the potential for the result to actually be opposite to that which might be assumed. This highlights the obvious advantage of the 8-grader’s approach to scientific hypothesis: it appeals to people who lost interest in the study of science at about the time puberty onset. Most environmental advocacy organizations understand this. They incorporate this approach in their fundraising appeals; they make everything seem 'oh, so “logical.”' But those who bother to follow the evolution of the climate change issue and its ebb and flow of hypothesis tested by observation know quite well that all things global warming are rarely what they seem.
     Stanford’s Erika Zavaleta and several co-authors offer a fine example of this dynamic in a recent edition of Proceedings of the National Academy of Sciences. What would an 8th-grader guess about global warming’s effect on soil moisture in traditionally dry ecosystems — say grasslands, by way of example? You can see the science fair poster in your mind’s eye: “My Hypothesis: Global warming will make dry ecosystems drier because of increased evapotranspiration (the combination of evaporation from open water surfaces and water lost through plant leaves).” That’s not just a science fair poster; it’s precisely what climate models predict.
     But vegetation responds in different ways to warming. Warming can change a plant’s phenology — the annual timing of budbreak, flowering, seed production, and other things linked to seasonal climate change. It can alter the number, size, and orientation of leaves. It might change the depth of the plant roots.
     What Zavaleta and her colleagues did was examine potential warming impacts on California grassland vegetation in control plots exposed to higher temperatures and/or higher CO2 levels over two growing seasons. They warmed the soil surface by about 1°C and increased the ambient carbon dioxide levels by 300 parts per million (ppm). That amount would almost double the current concentration.
     When averaged over two seasons, they find that spring soil moisture increased by 1.1% with warmer soil and 2.7% under elevated CO2 (see Figure 1). That’s right —warming and higher CO2 raised soil moisture levels during the critical transition from the wet season to summer drought. In all cases, the soil moistening was statistically significant.
     The researchers propose that a change in the timing of the plants’ annual cycle resulted in reduced transpiration losses because the plants began their life cycle earlier in the year. We’ll note, too, the higher temperatures did not impact total plant production. What might surprise a budding 8th-grade scientist (or a climate modeler) is not so surprising to these researchers. “Our findings illustrate the potential for organism-environment interactions to strongly modify global change effects on ecosystem function,” they conclude. “We suggest that in at least some ecosystems, declines in plant transpiration mediated by changes in phenology can offset direct increases in evaporative water losses under future warming.”
     They go on to hypothesize that such results very well could be applicable to most of the world’s Mediterranean-type ecosystems. Such ecosystems are plant life in unique regions where plants have adapted themselves to winter rainfall and extreme summer dryness. As a consequence, the importance of this research has potential to extend beyond a simple study of Mediterranean annual grasses. It might demonstrate how plants in general, are capable of adapting to changing environmental conditions in an effort to increase the likelihood of their survival. But that’s not something an 8-grader naturally contemplates, nor an environmental copywriter, either.

Reference:
Zavaleta, E.S., Thomas, B.D., Chiariello, N.R., Asner, G.P., Shaw M.R., and C.B. Field, 2003. Plants reverse warming effect on ecosystem water balance. Proceedings of the National Academy of Science, 100, 9892–9893.

Figure 1. The effects of warming and elevated CO2 on spring soil moisture. Elevated CO2 and temperatures lead to elevated soil moisture. (Source: Zavaleta et al., 2003)

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