The notion that nature itself will act as a check on the atmospheric excesses of humanity has long held a fair amount of appeal, not least because it draws on a nugget of high-school science that most people can quickly comprehend. Plants inhale carbon dioxide, after all -- they need it to grow. Add more CO2 to the air, as human civilization has been doing in copious amounts since the dawn of the Industrial Revolution, and the result will surely be thicker, more expansive biomass.
More trees, plants and crops, the thinking goes, means that more and more carbon dioxide will be naturally absorbed from the atmosphere, and ... voila! The climate problem is elegantly solved!
It's a conviction readily embraced by climate skeptics, and one enthusiastically peddled in some scientific cul-de-sacs like the Center for the Study of Carbon Dioxide and Global Change, a pet project of Sherwood B. Idso, a former research physicist with the Department of Agriculture, and his two sons, Craig and Keith.
From the center's Web site:
For years environmentalists have warned us about how fragile earth's biosphere is; and in many cases dealing with specific species or ecosystems, they have been correct. In its totality, however, the biosphere is much more resilient than most people give it credit for being. As atmospheric CO2 -- the lifeblood of the planet -- has gradually risen over the course of the Industrial Revolution, for example, the biosphere has begun to reveal its true strength, with the plants of the planet growing ever more robustly and profusely, as they expand their ranges over the face of the earth and extract ever greater quantities of CO2 from the air and sequester its carbon in their tissues and the soil into which they sink their roots.
Nothing inherently nonsensical here -- but the Idsos take it to the extreme, essentially arguing that the planet can't have too much of a good thing.
"Science tells us that putting more CO2 in the air would actually be good for the planet," the Idsos assert.
The fact is, most climate models do suggest that higher concentrations of carbon dioxide will spur increased plant growth, and that this will have some mitigating effect on what would otherwise be unchecked global warming. Just how significant this or myriad other planetary feedback effects might be in tweaking the Earth's thermostat over time is a matter of some speculation -- and the subject of fervent activity among designers of evermore accurate climate models.
But scientists generally agree that the influence of increased biomass will be modest, essentially acting like a brake on a runaway freight train. It might be able to slow steadily rising temperatures, but it will hardly be enough to stop global warming in its tracks.
What's more, most studies have ignored the influence of other potent global warming compounds like nitrous oxide and methane. Both are known to be released from soil in varying amounts, depending on the landscape, as atmospheric carbon dioxide increases.
A new study from Northern Arizona University, published Wednesday in the journal Nature, throws N2O and methane into the mix, and the results suggest that nature's defenses against global warming are even less effective than previously thought.
"This is one more reason why we should not expect nature to help clean up our climate change problem," said Bruce Hungate, a co-author of the study and a professor of biological sciences at Northern Arizona University.
The study -- a meta-analysis of dozens of separate studies of soil emissions in variety of ecosystems, including forests, farmland, rice paddies, grasslands and wetlands in North America, Europe and Asia -- found that increased atmospheric carbon dioxide causes soil to release methane and nitrous oxide in amounts significant and sustained enough to reduce the overall cooling effect of increased biomass by nearly 20 percent.
What's going on? Organisms in soil, it seems, thrive on both nitrate and carbon dioxide. These microbes also produce methane and nitrous oxide, which are, respectively, about 25 times and 300 times more effective at trapping heat than even CO2. As humans pump more carbon dioxide into the atmosphere, these organisms pump out more N2O and methane.
The body of literature on this effect suggested that it varied in degree from one ecosystem to another. This variation made it difficult to determine whether, on the whole, its impact on the global climate was significant or merely a wash. Yet the new study provides a clear answer: the effect is significant.
Hungate said he was surprised by the results, which he obtained no matter how he and his co-authors weighted the various studies. "We're not just looking at a statistical fluke," he said, adding that the reduction in cooling that the study ultimately attributed to soil emissions was probably an underestimate, given the lack of available data from tropical ecosystems, which have yet to be substantively studied in this regard.
Still, the allure of the notion that nature will heal itself is a powerful one.
A study from NASA in December, for example, was widely heralded as suggesting that global warming was no longer a worry.
"A new NASA computer modeling effort," stated a press release issued at the time, "has found that additional growth of plants and trees in a world with doubled atmospheric carbon dioxide levels would create a new negative feedback - a cooling effect - in the Earth's climate system that could work to reduce future global warming."
That the study clearly asserted that this cooling effect would be no real match for the overall trend of rising temperatures was missing from some reports of the news.
"A group of top NASA boffins says that current climate models predicting global warming are far too gloomy," began an article at The Register, a British-based technology news site, "and have failed to properly account for an important cooling factor which will come into play as CO2 levels rise."
Still, Ken Caldeira, a prominent climate scientist at the Carnegie Institution's department of global ecology, suggested in an email that the new research out of Northern Arizona University made the situation facing humanity quite plain.
"To solve the carbon-climate problem, we need to transform our energy system into one that does not dump its waste into the sky," Caldeira said. "Land plants help. It looks like they won't help quite as much as we thought they would. Clearly, we can't expect nature to solve our problems for us."
In other words, while the ability of the planet to heal itself is a factor, it's ultimately up to humans to sort out the climate mess. Nature won't save the day.
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