A major new study in Nature, “Climate extremes and the carbon cycle” (subs. req’d), points to yet another significant carbon cycle feedback ignored by climate models. The news release sums up the key finding of this 18-author paper: Researchers “have discovered that terrestrial ecosystems absorb approximately 11 billion tons less carbon dioxide every year as the result of the extreme climate events than they could if the events did not occur. That is equivalent to approximately a third of global CO2 emissions per year.”
Measurements indicate, for instance, that the brutal 2003 European heat wave “had a much greater impact on the carbon balance than had previously been assumed.” We’re already seeing a rise in extreme weather in North America. Last year, Munich Re, a top reinsurer, found a “climate-change footprint” in the rapid rise of North American extreme weather catastrophes: “Climate-driven changes are already evident over the last few decades for severe thunderstorms, for heavy precipitation and flash flooding, for hurricane activity, and for heatwave, drought and wild-fire dynamics in parts of North America.”
The new Nature study found that one type of extreme weather event is worse than the others:
Periods of extreme drought in particular reduce the amount of carbon absorbed by forests, meadows and agricultural land significantly. “We have found that it is not extremes of heat that cause the most problems for the carbon balance, but drought,” explains [lead author] Markus Reichstein…. Drought can not only cause immediate damage to trees; it can also make them less resistant to pests and fire. It is also the case that a forest recovers much more slowly from fire or storm damage than other ecosystems do.
And this is worrisome because as a major 2013 review of obsevartions and climate models pointed out, “historical records of precipitation, streamflow and drought indices all show increased aridity since 1950 over many land areas.” That study, by Aiguo Dai, “Increasing drought under global warming in observations and models,” found:
… the observed global aridity changes up to 2010 are consistent with model predictions, which suggest severe and widespread droughts in the next 30–90 years over many land areas resulting from either decreased precipitation and/or increased evaporation.
According to Climate Central, “Reichstein singled out the ongoing drought in the Southwest as a particularly damaging extreme weather event that could affect ecosystems’ carbon dioxide absorption in the U.S.” And that is worrisome because, as Dai has explained, thanks to climate change, “The U.S. may never again return to the relatively wet conditions experienced from 1977 to 1999.”
The new study finds “a few major events dominate the global overall effect, while the more frequent smaller events occurring throughout the world play a much less significant part.” And that is worrisome because global warming — by shifting the bell curve and causing step-function changes in the climate — dramatically increases the odds of the most extreme events, like, say, Superstorm Sandy.
And so we may be headed to what Reichstein calls “a self-reinforcing effect” whereby increased CO2 in the air increases the frequency of the most extreme kinds of weather, which in turn puts more CO2 in the air. We can add this to the amplifying feedback I reported on Monday, whereby ocean acidification may amplify global warming this century up to 0.9°F. And there’s a third major carbon feedback unmodeled by the new IPCC report — the thawing permafrost — which is projected to add up to 1.5°F to total global warming by 2100.
Climatologist Michael Mann wrote in an email, “This study is another sober reminder that uncertainties in the science of climate change are a reason for concern rather than complacency. In this case, this new finding implies that the terrestrial biosphere is likely to become a less efficient ‘sink’ of carbon than previously acknowledged. That, in term, means that the airborne fraction of CO2, and, hence, the human-caused greenhouse warming, may well be greater than most previous assessments have suggested.”
Reichstein issued a stark warning about this latest feedback:
“I think counting on the biosphere’s ability to absorb carbon is a risky thing because you don’t know how long it will continue to take up carbon dioxide that we emit.”
Is anyone listening? The time to act to slash manmade carbon emissions is NOW — before we devastate the biosphere’s ability to store carbon.