Science - USA (2020-05-22)

SCIENCE sciencemag.org 22 MAY 2020 • VOL 368 ISSUE 6493 813

PHOTO: JAKE BRYANT

T

ropical forests have been one of

Earth’s best defenses against rising

carbon dioxide levels. The trees suck

carbon from the atmosphere as they

grow, and researchers estimate that,

despite ongoing deforestation, tropi-

cal forests hold more carbon than humanity

has emitted over the past 30 years by burn-

ing coal, oil, and natural gas. But scientists

have worried that the ability of tropical

forests to act as carbon sinks will dimin-

ish and ultimately reverse with continued

global warming, as trees stressed by heat

and drought die and release their carbon.

Now, on p. 869, researchers report that

measurements of carbon stor-

age and growing conditions

for some 500,000 trees around

the world suggest some tropi-

cal forests, particularly in Africa

and Asia, will—if left intact—

continue to sequester large

amounts of carbon even as global

temperatures rise. But only up to

a point. “There are certain levels

where forests can’t respond,” says

William Anderegg, a forest eco-

logist at the University of Utah.

If warming reaches 2°C above

preindustrial levels, the study

finds huge swaths of the world’s

tropical forests will begin to lose

more carbon than they accumu-

late. Already, the hottest forests

in South America have reached

that point.

Trees, with their long lives

and massive woody trunks, are particularly

good at storing carbon. But just how much

carbon tropical forests can capture as the

planet warms depends on the balance be-

tween tree growth spurred by higher atmo-

spheric carbon levels and tree stress and

death caused by rising temperatures and

increasing drought. “It comes down to a

tug of war,” Anderegg says, “between the

benefits of carbon dioxide and the poten-

tial impact of climate change.”

To see which side might ultimately win,

a global team of more than 200 researchers

measured more than half a million trees

in 813 forests in 24 countries. The team,

led by tropical ecologist Oliver Phillips of

Leeds University and his postdoc Martin

Sullivan, calculated how much carbon the

different forests now store based on the

height, diameter, and species of each tree.

Researchers also looked at how carbon

storage varied from place to place using

data from 590 long-term monitoring plots.

To forecast how carbon accumulation

might change in the future, the researchers

assumed that the hottest forests, which are

mostly in South America, are bellwethers

of the future. By comparing carbon storage

in forests across the range of climates, they

could use space as a proxy for time. They

analyzed how changes in temperature and

precipitation might affect carbon storage,

looking for those changes that best ex-

plained what they observed in the forests.

(The analysis takes into account differ-

ences in the forests’ mix of tree species.)

Previous studies had suggested the low-

est temperature a forest experiences at

night has the biggest impact on its long-

term carbon storage capacity, perhaps

because warm nights cause trees to boost

respiration and release more carbon. But

this study found that the maximum day-

time temperature is most important, per-

haps because on hot days trees slow their

carbon dioxide intake to reduce water loss

through pores in their leaves.

The study showed that, overall, the for-

ests now take up more carbon than they

lose. But it found that at a tipping point—

when the average daily maximum tempera-

ture during the warmest month of the year

rises to 32.2°C—long-term carbon storage

capacity declines steeply and carbon loss

increases. The decline is even greater in

drier forests, notes Sullivan, now at Man-

chester Metropolitan University, likely be-

cause the lack of water makes trees more

vulnerable to stress and death.

The team calculated that, worldwide,

each 1°C increase in maximum tempera-

ture reduces carbon storage in tropical

forests by 7 billion tons (roughly equiva-

lent to total U.S. carbon emissions over

5 years), although much of that loss is cur-

rently offset by increased growth. If global

temperatures rise 2°C above preindustrial

levels, however, 71% of tropical forests will

be pushed past the thermal

tipping point, the researchers

found. Carbon losses would be

four times greater, with South

America seeing the largest loss.

The “massive data compila-

tion ... allows us to draw con-

clusions with much higher

confidence than individual

studies would allow,” says Julia

Pongratz, a climate scientist at

Ludwig Maximillian Univer-

sity of Munich. But ecosystem

ecologist Lara Kueppers of the

University of California, Berke-

ley, worries the study might be

too optimistic in forecasting

that cooler forests, especially in

Asia and Africa, will continue

to accumulate large quantities

of carbon as they warm. It’s not

clear whether those forests will

behave like their counterparts in South

America, she notes, or that they can adapt

to the speed of human-induced climate

change. “I don’t have confidence that for-

ests are going to be able to adjust on the

time scale they will need to,” she says.

Other researchers see the findings as a

wake-up call for action, noting the world

has already warmed about 1°C above pre-

industrial levels. “Even though tropical

forest sinks will weaken, conserving them

is still better than not having them at all

and turning them into carbon sources,”

says Richard Betts, a climate modeler spe-

cializing in the global carbon cycle at the

University of Exeter. “It is not too late,” he

adds, “to avoid the most severe impacts.” j

By Elizabeth Pennisi

CLIMATE CHANGE

Tropical forests store carbon despite warming

But if global temperatures reach key threshold, dying trees will release warming gases

NEWS | IN DEPTH

Researchers collect leaves to measure carbon storage in an Andean cloud forest.

Published by AAAS