already are only a small fraction of the three tril-
lion trees and 10 billion acres of forest on this
planet. Climate change still poses less of a threat
to forests than logging and land clearing, but the
threat is growing fast. “How big does that frac-
tion get over time, and when does it overwhelm
the other?” asks Matt Hansen of the University of
Maryland, who monitors forests using satellites.
The problem is, we can’t yet quantify the
planetwide scope of climate impacts. Satellite
data show that Earth’s tree-covered area actu-
ally expanded from 1982 to 2016 by 7 percent,
an area larger than Mexico. But that doesn’t
mean forests are doing fine: The data don’t dis-
tinguish between natural forests and industrial
tree farms, such as the millions of palm, euca-
lyptus, and pine trees planted as crops while
rainforest is cleared. The data also don’t show
which forests were lost to chain saws and which
were killed by climate-related events.
No computer model can yet project how cli-
mate will change forests globally—or how their
carbon stores will feed back on climate. “Earth
system models historically haven’t done a good
job of capturing this,” says Charlie Koven, a
climate scientist with the Lawrence Berkeley
National Laboratory, who worked with the UN’ssignificant stands are instead transitioning to
something new. Some will never be the same.
Others may not come back at all.
IT’S A TOUGH TIME TO BE A TREE. Earth has lost a
third of its forests over the past 10,000 years—half
of that just since 1900. We logged them for timber.
We cut them to make way for farms and cattle. We
cleared land to build homes and roads. Globally,
deforestation has decreased from its peak in the
1980s, but trends vary by region. In Indonesia,
which had been mowing down forests for oil palm
plantations, primary forest loss has declined
since 2016. From August 2020 to July 2021, the
Brazilian Amazon lost 5,000 square miles of rain-
forest, a 22 percent increase over the previous
year. Since 1990, we’ve cut down more forest
globally than there is forest in the United States.
Now fossil fuel emissions spewing from coal
plants and tailpipes are rearranging forests in
other consequential ways. As carbon dioxide
and other greenhouse gases warm the planet,
some of its estimated 73,000 tree species are
pushing poleward and higher up slopes, drag-
ging other life with them. Alders, willows, and
dwarf birches are expanding across the Arctic,
from Scandinavia to Canada, providing cover and
food for snowshoe hares and moose. Trees are
growing faster as they soak up excess CO 2 —a key
ingredient for photosynthesis. That “greening” of
the planet has so far helped slow climate change,
protecting us from ourselves.
But climate change also is killing trees. And
what has forest scientists increasingly uneasy
is the quickening pulse of extreme events—fire,
more powerful storms, insect infestations, and,
most notably, severe heat and drought, which
can worsen the effects of all the rest. These sin-
gular, frequently unprecedented episodes can
swiftly inflict mass tree mortality, shifting for-
ests that have been around since the last ice age
to entirely new states.
“We have a whole set of mechanisms that are
pushing Earth’s forests to grow more and suck
up more CO 2 ,” says University of Utah biologist
William Anderegg. But those mechanisms “are
fundamentally in tension with mechanisms that
are pulling Earth’s forests toward a cliff—with
more tree death and more loss of carbon.”
The forests that have plunged over that cliff
SINCE 1990, WE’VE
CUT DOWN MORE
FOREST GLOBALLY THAN
THERE IS FOREST IN THE
UNITED STATES.Intergovernmental Panel on Climate Change
(IPCC). Only two of its 11 models include both
fire and geographic shifts in plants.
The global number of trees isn’t the only thing
that matters. Climate change is reshaping forests
locally almost overnight, transforming them even
where there are policies to protect them. It’s hap-
pening so fast we can’t discern the consequences.
While we’re losing trees of all types and sizes, the
biggest and oldest harbor the most carbon, are
important for biodiversity, and will be the hard-
est to get back. “Big trees are disproportionately50 NATIONAL GEOGRAPHIC