On election day, the U.S. was busy voting for a president who openly scoffs at the reality of human-caused climate change and has expressed the goal of withdrawing the U.S. from the international process for dealing with it. So if climate change is something you worry about, you’re probably extra worried right now.
But on the same day, a paper published in Nature Communications Tuesday actually contained some of the better news about climate change that we’ve heard in a while. Granted, even that doesn’t make it very sunny — just a modest bit of evidence suggesting a slight, temporary reprieve in the rate at which we’re altering the planet.
Trevor Keenan of Lawrence Berkeley National Laboratory and his colleagues examined what is called the “terrestrial carbon sink”: How much trees, plants, and other aspects of global land surfaces are managing to pull carbon dioxide out of the air. Every year, you see, humans pump many billion tons of the stuff into the atmosphere, but not all of it stays there to warm the planet. A very significant fraction ends up getting absorbed by the ocean. Another large fraction gets pulled in by land-based plants which use it for photosynthesis. And so on.
What the new study shows is that from 2002 to 2014, plants appear to have gone into overdrive, and started pulling more carbon dioxide out of the air than they had before. The result was that the rate at which carbon dioxide accumulates in the atmosphere did not increase during this time period, although previously, it had grown considerably in concert with growing greenhouse gas emissions.
Granted, this does not mean that plants revved up nearly enough to prevent steady accumulation of the long-lived gas in the atmosphere each year. There is still more added every year. But they did increase the fraction of it that they were gobbling up, in a way that scientists could measure.
“That portion [of carbon dioxide] that stays in the atmosphere, that’s called the airborne fraction, and that has reduced by about 20 percent over the last 15 years,” said Keenan in an interview. “So around the year 2000 it was about 0.5, and currently it’s about 0.4.” In other words, in recent years the land and ocean started absorbing 60 percent of all emitted carbon dioxide, whereas before they had been absorbing more like 50 percent. Carbon dioxide in the atmosphere still went up steadily each year, but maybe not at quite as fast a rate as before.
Keenan completed the work with scientists from U.S., U.K., Australian and Chinese institutions — including two researchers with the Global Carbon Project, which every year tracks how much CO2 the world emits, as well as how much of it ends up in the atmosphere as opposed to being trapped by the ocean and land “sinks.”
One reason for the change is the so-called global warming “pause” that persisted during much of the 2000s (and now appears to have ended), explained Keenan. You see, plants engage in photosynthesis which pulls in carbon dioxide, but they also engage in respiration, which releases it again. The same is true of microorganisms living on land – in its soil, for instance. And respiration rates increase at higher temperatures.
But during the pause, Keenan continued, carbon dioxide levels kept rising but temperature didn’t rise as much over land surfaces. That seems to have triggered a disproportionate response from living things on land.
“We have more and more CO2 going into photosynthesis, but we don’t have more and more coming out from respiration,” he explained.
One related trend may be what has been termed “Arctic greening.” Scientists have been finding, of late, that as northern portions of the planet warm up even as total atmospheric carbon dioxide increases, there’s been an increase in plant growth in these regions:
If all of this were to continue — if more carbon dioxide in the air continually meant an increased response from photosynthesis, and if respiration stayed in check too — then you might think the growing land carbon “sink” would be able to really help save us. And indeed, climate change “skeptics” have often pointed to plant growth as a possible factor that would mitigate climate change.
However, that is most emphatically not the conclusion of the new paper. Rather, what it has detected seems to have been a temporary effect. It’s even possible the situation could fully reverse itself in the future.
“What we do know is that the effect of CO2 fertilization diminishes as CO2 goes up, so it’s a diminishing return,” said Keenan. “Whereas the effect of temperature on respiration is thought to increase.”
Especially in a context in which Trump’s election could significantly hobble global climate progress, the new research underscores how effective it could be to curtail another major climate driver – deforestation, especially in tropical countries.
If we not only stopped cutting down tropical forests, but also reclaimed lost forested areas and allowed them to grow back on a mass scale, research like this does serve as a reminder that there is a way to further fire up the land sink, and reduce the rate of global warming still more. And that could buy the planet some much needed time.
In the meantime, it remains to be seen whether the 2015-2016 El Nino event has already shifted the trends reported in the new paper, making the land “sink” weaker once again. There are, after all, good reasons to think that El Nino events, by causing droughts and large scale wildfires (such as what we saw in Indonesia a year ago), can actually hurt the land’s ability to pull carbon dioxide out of the air. Which just goes to show you how fragile any shift in the land “sink” could be.
“It’s good news for now,” said Keenan of the study. “We can’t expect it to continue.”
Correction: This article had previously stated that the land and ocean carbon sinks had gone from absorbing 40 percent of airborne carbon dioxide to 50 percent. It should have been from 50 percent to 60 percent.
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