Global ChangeIntersection of Nature and Culture

Climate warming worse than previously thought because plants become nutrient starved?

Tuesday, November 24th, 2009

The biosphere—the sum of all living organisms on earth—has tremendous influence on our atmosphere and climate.   One active area of research is how the biosphere might respond to rising atmospheric CO₂ and temperatures and how this change, in turn, can influence (“feed back on”) climate change.

There are several reasons to believe that the biosphere will generate positive feedbacks on climate (i.e., things that cause warming to accelerate) as a result of warming.  For instance, rising temperatures can

• increase soil organic matter decomposition, causing the release of CO₂ to the atmosphere, which causes more warming;
• increase permafrost thaw, thereby releasing old carbon and methane bubbles previously locked in a deep freeze;
• cause treeline to advance northwards.  As conifer trees move north, the formerly tundra landscapes darken as evergreen trees replace open expanses of snow in the winter.  This causes regional climate to warm.  In fact, one study¹ showed that without its boreal forest tree cover, Canada would be about 5-10 degrees C colder in the winter and spring!

Positive feedbacks are a nightmare scenario because they mean that climate can warm faster than we currently expect, indicating that we have even less time to deal with it.  This is part of the reason why scientists are beginning to sound the alarm that CO₂ is rising faster than expected and the impacts warming are happening faster than predicted.

There are also reasons to believe that the biosphere will generate negative feedbacks on climate (i.e., things that cause warming to slow) as a result of rising CO₂.

• For example, because CO₂ is limiting to photosynthesis, rising CO₂ is causing plants to grow better.  This removes CO₂ from the atmosphere and can slow warming.

In a forthcoming article² in Geophysical Research Letters, Wang and Houlton analyze this negative feedback, and offer some sobering news about it’s ability to slow warming.

Specifically, they argue that most models used in the IPCC assessments do not take into account the fact that nitrogen becomes limiting in soils as plants grow more.  Nitrogen is one of the most limiting nutrients to plant growth, which is why it’s often the #1 ingredient in fertilizer used on crops and lawns.  Many field studies (example) have shown that trees grown in elevated CO₂ experiments eventually stop growing any better than those in ambient air after a few years because soil nitrogen runs out.

Without nitrogen limitation, the IPCC models allow plants to grow more and remove more CO₂ from the atmosphere via photosynthesis than they do in reality.   This means that forests may not be as strong a carbon sink as we suspect, and we are therefore underestimating the rate of CO₂ rise and magnitude of warming—maybe by as much as 0.5 degree by 2050 and 1 degree by the year 2100.

¹Bonan, G. et al. (1992) Effects of boreal forest vegetation on global climate. Nature 359:716-718.

²Wang, Y. and B. Houlton (in press). Nitrogen constraints on terrestrial carbon uptake: Implications for the global carbon-climate feedback. Geophysical Research Letters
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