CLIMATE

Ecosystem Overload

More Carbon Dioxide Isn’t Necessarily a Plus for Plants

An argument I have often heard from global warming skeptics is that we should not rush to create greenhouse gas emissions caps because doing so might hurt—or, at least, hinder—our climate change mitigation efforts.

Come again?

Their logic goes something like this: Because plants require carbon dioxide to do photosynthesis, wouldn’t it make more sense to allow atmospheric levels to further increase beyond their long-term average—rather than reduce them—so as to make more available for the planet’s flora, thus ensuring a lush supply of greenery? In other words, pump more carbon dioxide into the atmosphere and, ipso facto, you’ll have more plants doing more photosynthesis—naturally taking care of any pesky (supposed) warming problem that might arise. Or so skeptics and their allies in the energy industry would have you believe.

Ecosystems are already struggling to keep up with the meteoric growth in emissions over the past few decades.

I have to admit that, to a certain audience, that logic must have a distinct appeal. It effectively removes much of the burden of emission regulation from our shoulders and places it squarely on those of the planet’s ecosystems. While it does not necessarily give us carte blanche to continue emitting at an unabated rate, it seemingly provides some measure of relief: the knowledge that Mother Earth will take matters into her own hands should we fail to do so ourselves. If only that was the case. Thanks to evidence from several recent studies, scientists now know that, far from helping ecosystems, higher emission levels could actually harm their short- and long-term ability to take up and store carbon dioxide—producing, in effect, a so-called climate “double whammy.” That is, as emissions rise due to human activity, some plants will naturally absorb less of that CO2, which will in turn further accelerate atmospheric concentrates because those very same plants will no longer be able to move the greenhouse gas from the air to the soil.

Because plants and soils act as major carbon sinks, any reduction in their ability to draw down and store CO2 could have dramatic consequences for the climate. As things stand, ecosystems are already struggling to keep up with the meteoric growth in emissions over the past few decades; placing any further undue stress would only make matters worse—and, at the same time, make our efforts to fight climate change that much harder. A warmer world in which plants, one of our first-line defenses against rising CO2 levels, become impotent would force us to drastically revise our current emission scenarios and make it much more difficult for us to mount an effective response in time. A study published in a recent issue of Nature suggests that we may be approaching this dangerous tipping point.

A large team of researchers, led by Jay Arnone of the Desert Research Institute, organized a four-year study to track the response in CO2 uptake and loss in ecosystems during abnormally warm years.^[1] To do so, they sealed large plots of native Oklahoma tall grasses inside simulated environment chambers in which they were able to replicate daily and seasonal temperature and rainfall changes. After letting the plots condition for the first year, the scientists exposed half of them to a range of temperatures typical of a regular year and exposed the other half to temperatures on average 4°C higher (in line with the predictions made by the United Nations’ Intergovernmental Panel on Climate Change). These anomalously high temperatures were turned down during the third year to match those in the control plots.

Arnone and his colleagues found that the plots exposed to the higher temperatures experienced a net reduction in CO2 uptake for at least two years; furthermore, they only captured and stored about one third as much carbon during those two years as did the control plots. They attributed this to two main causes: a suppression of net primary productivity, which refers to the amount of CO2 absorbed by a plant during photosynthesis minus the CO2 it emits during respiration (the process by which plants break down sugars into useable energy) induced by the drought-like conditions and higher respiration rates by the soil’s microorganisms during the second year. The authors conclude that, “more frequent anomalously warm years, a possible consequence of increasing anthropogenic carbon dioxide levels, may lead to a sustained decrease in carbon dioxide uptake by terrestrial ecosystems.”

These grim findings echoed the results of an earlier study done this year in which a team of scientists investigated the ability of northern ecosystems to store carbon in response to autumnal warming.^[2] Using data from past atmospheric records to examine year-to-year variations in atmospheric CO2 concentrations and ecosystem CO2 fluxes, they found a trend over the last two decades that pointed to an earlier autumn-to-winter build-up, suggesting a much shorter net carbon uptake period. They then combined this data with observations gathered from a terrestrial biosphere model and satellite imaging to further study the ecosystems’ response to autumnal warming, which revealed that both photosynthesis and respiration increased during that period—though respiration did so at a faster rate. In other words, the plants were emitting more CO2 than they were storing—resulting in a net release of CO2 to the atmosphere.

Shilong Piao, the lead author, and his colleagues conclude that the loss in carbon uptake during this period could offset as much as 90 percent of the increased carbon uptake witnessed during spring warming. This means that if future autumnal warming continues to outpace spring warming, the ability of northern ecosystems to take in and store CO2 could be sharply curtailed much sooner than previously expected.

Now, granted, we may still be a ways off before global temperatures rise another 4°C (though we’ve already seen some isolated incidents). And, admittedly, much more work needs to be done in more locations before we can truly accept these findings as fact. Indeed, a study done in 2001 suggested that grasslands could act as potent carbon sinks under conditions of elevated atmospheric CO2 levels because higher levels would inhibit microbial respiration—at least for the short term.^[3]

Yet, as with much environmental science, what really matters here are the general trends—and these mostly point to a warmer climate in which ecosystem carbon uptake will be significantly reduced. For an indication of what may be to come, it helps to look at a recent real-life example: the European heat wave of 2003. That year, temperatures during the summer surged above their long-term means—July temperatures alone were up to 6°C higher—resulting in tens of thousands of deaths. Scientists who studied the impact of the continent-wide drought on primary productivity discovered that it caused a significant drop in CO2 uptake and reversed the effect of four years of net carbon sequestration.^[4] Of particular note was their conclusion that future drought events could turn Europe’s temperate ecosystems into carbon sources—not sinks—thus contributing to the onset of intense climate change. Could the same happen in southwestern states such as California and Arizona, which have been battered by a wave of severe droughts in recent years? Only time will tell of, course—there are many factors at play—though I can’t say the European example fills me with much confidence.

In the end, what all of this tells us is that we can’t take it for granted that plants will always be there to bail us out. Though they will remain a major carbon sink for the foreseeable future, we may not have the luxury of relying on them as much if we don’t start taking responsibility for our actions soon.

Jeremy Jacquot is a graduate student in marine environmental biology at the University of Southern California and is a contributing writer for VentureBeat, DeSmogBlog, and TreeHugger.

Notes

[1] Arnone, J.A. et al. Prolonged suppression of ecosystem carbon uptake after an anomalously warm year. Nature, 455 (2008): 383—385.

[2] Piao, S. et al. Net carbon dioxide losses of northern ecosystems in response to autumn warming. Nature, 451 (2008): 49—54.

[3] Hu, S. et al. Nitrogen limitation of microbial decomposition in a grassland under elevated CO2. Nature, 409 (2001): 188—191.

[4] Ciais, Ph. et al. Europe-wide reduction in primary productivity caused by the heat and drought in 2003. Nature, 437 (2008): 529—533.

Tags: Climate Change, environment

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