Amazonian tropical rainforest near Manaus, Brazil. (Image credit: William Anderegg)

In a surprising new study, scientists say they’ve pinned down the climate factor most strongly tied to variations in terrestrial carbon storage — that is, the ability of plants and other features of the Earth’s surface to take up carbon, thus preventing it from going into the atmosphere. They’re arguing that the biggest driver is tropical nighttime temperatures, which are expected to warm at a faster rate than average temperatures otherwise will.

As these temperatures rise, the researchers say, they could begin to seriously interfere with forests’ ability to store carbon, even helping tip the scales one day in the future so that our global forests turn into a net source of greenhouse gas emissions — leaking carbon into the atmosphere rather than sucking it out, and thus exacerbating global climate change.

The amount of carbon taken up by land ecosystems “bounces around from year to year, largely due to the climate bouncing around,” said the paper’s lead author, William Anderegg, an assistant professor of biology at the University of Utah and an assistant research scholar at Princeton University. It can be affected by annual fluctuations in environmental factors, such as temperature and precipitation, which can cause plants or soil to take up more or less carbon in a given year.

Anderegg said he and his colleagues were interested in the “big unknowns” when it comes to carbon uptake — that is, what factors influence carbon uptake versus carbon release in land systems, such as forests. So they designed a study, which was published Monday in Proceedings of the National Academy of Sciences, to test four hypotheses on the factors that might be most important.

The first three factors — tropical mean temperature, tropical precipitation and both temperature and precipitation in semi-arid regions of the Earth — had already been well-established in the scientific community as important drivers of the global carbon cycle. These regions of the world — and tropical regions in particular — have been shown to be disproportionately important when it comes to the total amount of carbon poured into or sucked out of the atmosphere.

The fourth hypothesis — tropical nighttime temperatures — was a much newer idea in the scientific community, Anderegg said. The idea rose out of the some local studies in Costa Rica that had implicated night temperatures as an important influence on how much carbon is taken up by land ecosystems in that region.

The researchers used datasets on climate and carbon uptake by vegetation — largely compiled from satellite data — to test the four hypotheses and see which one seemed to have the biggest influence on variations in carbon uptake worldwide. And while all the factors played a role, they found that tropical nighttime temperatures, out of the four, was the most important.

That hypothesis “hadn’t been tested at the global scale,” said Anderegg. “So it was surprising to us that that was the one that always rose to the top in our statistical analysis.”

The reason for its importance is likely tied to a metabolic process called respiration, in which plants convert sugar into energy and release carbon dioxide in the process. It’s basically the opposite of photosynthesis, which takes up carbon dioxide and uses it to make sugar. In very simple terms, higher levels of photosynthesis can drive stronger carbon uptake, and higher levels of respiration can drive higher levels of carbon release in plants.

Since photosynthesis requires sunlight and mainly occurs during the day, respiration is the dominant process happening at night. And in higher temperatures, the process speeds up in many plants, Anderegg said.

“The idea here is that on warm nights, plants — and other parts of the ecosystem as well, potentially — are consuming and using up more of their sugars and losing that carbon to the atmosphere more than they do on cooler nights,” Anderegg said.

The fact that respiration is so sensitive to nighttime temperatures “isn’t really surprising,” said David Schimel, a senior research scientist at NASA’s jet propulsion laboratory, who has studied ecosystems and the carbon cycle. Schimel was not involved with this particular study, although he was familiar with the research before its publication.

However, he said, he’s not sure that this is actually the most important driver of carbon uptake variability. “The support for it that they’ve put together is clear and reasonable, but the reality is that the observations that we have in order to rank [these factors] with aren’t very good yet,” he said.

While satellite data can tell us a lot already about carbon uptake in ecosystems, scientists are still improving the technology, Schimel pointed out. More and more, scientists are starting to use a process called solar-induced fluorescence — which allows satellites to remotely detect the occurrence of photosynthesis — to examine questions about plant productivity and carbon uptake. As our technology and improves and we’re able to put more years of data collection behind us, Schimel said, it will be interesting to go back and reevaluate the conclusions in studies like this one.

Even so, he added, “it’s an interesting idea, and now that it’s out there as potentially a global phenomenon, it’s one that needs to be very carefully assessed. Even if it’s not the most important effect, it’s an important effect and needs to be looked at.”

Ben Bond-Lamberty, a research scientist with the Pacific Northwest National Laboratory (who was also not involved with this study), added that there are “a lot of moving parts” when it comes to researching carbon uptake.

“This [study] provides a really nice testable hypothesis basically for other people to look at hard,” he said, echoing Schimel’s idea that revisiting the paper’s conclusions will be valuable down the line. But he also agreed that tropical nighttime temperatures are likely to play an important role in the carbon cycle either way.

“Daytime temperatures, we’re going to expect, will affect both sides of the carbon uptake and loss  equation — the photosynthesis uptake side and then the respiration loss side — whereas changes in nighttime temperature are just going to affect the respiration side,” he said. “I would say, sort of a priori — and there have been a couple of previous papers on this subject — you would expect that the nighttime temperatures would exert an unbalanced effect.”

The support for this idea offered in the new paper could be used to strengthen the Earth-system models used to make predictions about carbon balancing, and to see if the models can duplicate the effects observed in the paper, Bond-Lamberty noted.

Making accurate predictions will likely be more important moving forward as the Earth continues to warm — especially if scientists are correct in predicting that nighttime temperatures will warm at a faster rate than daytime temperatures. This could suggest that the influence of night temperatures could become even more important in the coming years.

The authors suggest in the paper that, in a worst-case scenario, future climate pressures could turn forests into a carbon source, rather than a carbon sink. In this situation, the massive additional amount of carbon released into the atmosphere would likely have a major effect at exacerbating global warming.

How long it would take for this turnover to occur, if warming were to continue unabated, remains unclear — it could be anywhere from decades to hundreds of years. And there’s also the hope that some plants might be able to adapt to warming over time, Anderegg said, noting, “In terms of when this could be a problem, it’s still really hard to know.”

So the scenario is by no means an immediate given. It’s rather a concern to think about when considering forest conservation in a warming world — and this paper serves as yet another warning sign.

“Tropical regrowth and forest growth right now is probably responsible for absorbing a significant amount of fossil fuel carbon, and that’s an amazing service that these ecosystems are providing,” Schimel said. “To me the concern is that these systems are — they’re valuable, but they’re vulnerable, so my takehome message is that this should increase the motivation to manage them carefully and protect them even more than people think now.”

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