Very cool: trees stalling effects of global
heating in eastern US, study finds
Vast reforestation a major reason for ‘warming hole’
across parts of US where temperatures have flatlined or cooled
Oliver
Milman
@olliemilman
Sat 17 Feb
2024 10.00 GMT
https://www.theguardian.com/environment/2024/feb/17/us-east-trees-warming-hole-study-climate-crisis
Trees
provide innumerable benefits to the world, from food to shelter to oxygen, but
researchers have now found their dramatic rebound in the eastern US has
delivered a further, stunning feat – the curtailing of the soaring temperatures
caused by the climate crisis.
While the
US, like the rest of the world, has heated up since industrial times due to the
burning of fossil fuels, scientists have long been puzzled by a so-called
“warming hole” over parts of the US south-east where temperatures have
flatlined, or even cooled, despite the unmistakable broader warming trend.
A major
reason for this anomaly, the new study finds, is the vast reforestation of much
of the eastern US following the initial loss of large numbers of trees in the
wake of European settlement in America. Such large expanses have been
reforested in the past century – with enough trees sprouting back to cover an
area larger than England – that it has helped stall the affect of global
heating.
“The
reforestation has been remarkable and we have shown this has translated into
the surrounding air temperature,” said Mallory Barnes, an environmental
scientist at Indiana University who led the research. “The ‘warming hole’ has
been a real mystery and while this doesn’t explain all of it, this research
shows there is a really important link to the trees coming back.”
There was a
surge in deforestation from the start of the US’s early colonial history, as
woodland was razed for agriculture and housing, but this began to reverse from
around the 1920s as more people began to move into cities, leaving marginal
land to become populated again with trees. The US government, meanwhile,
embarked upon an aggressive tree-planting program, with these factors leading
to about 15m hectares of reforested area in the past century in the eastern US.
The
recovery of the US’s eastern forests has blunted global heating mainly through
the trees’ transpiration, in which water is drawn up through the roots to the
leaves and then released into the air as vapor, slightly cooling the
surrounding area.
By poring
over data from satellites and weather stations located across the eastern US
from 1900 to 2000, Barnes and her colleagues found reforested areas have
provided this cooling impact on a grand scale, with most of this effect
occurring within 400 meters of the trees.
In all, the
replenished forests today cool the eastern US by 1C to 2C (1.8F to 3.6F) each
year. The cooling effect is strongest on the hottest days in summer, when trees
lower temperatures by 2C to 5C (3.6F to 9F), the researchers found.
The
researchers cautioned that bringing back trees hasn’t been the sole cause of
the stalled warming, with factors such as airborne pollutants, which block
incoming sunlight, and agricultural irrigation also potential causes. But
Barnes said that the findings should further bolster efforts to provide
thoughtful reforestation, particularly near urban communities that suffer
particularly scorching temperatures due to a lack of shady trees.
“Trees have
a really beneficial impact upon surface temperatures through transpiration,
which is similar to human sweating, and they have really cooled things off a
lot,” said Barnes.
Civilian
Conservation Corps workers plant 15,000,000 trees across the wastelands of
southern Mississippi on 11 April 1940. They are part of the United Forest
Service which will re-establish forests destroyed by logging and lumbering
operations decades ago. Photograph: AP
“Moving
forward, we need to think about tree planting not just as a way to absorb
carbon dioxide but also the cooling effects in adapting for climate change, to
help cities be resilient against these very hot temperatures.”
Patrick
Gonzalez, a University of California, Berkeley, climate change scientist and
forest ecologist who wasn’t involved in the new study, said the work provides
“strong support” to the theory of cooling trees.
“Cutting
carbon pollution from cars, power plants and other human sources that burn
coal, oil and methane remains the essential solution to halt climate change,”
he said. “Natural regeneration of trees and reforestation, where ecologically
appropriate, can contribute substantially.”
Barnes,
too, stressed that reforestation was no substitute for the need to drastically
cut planet-heating emissions, which hit a new global high last year.
“Nature-based
climate solutions like tree planting won’t get us out of this climate change
problem,” she said. “If anyone thinks we can just plant a few trees and be OK,
they are wrong – we need a massive reduction in fossil fuel emissions to hit
our targets. Reforestation is something that needs to happen in addition to,
not instead of, cutting emissions.”
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