A river snakes its way through the Amazon rain forest in Peru. Credits: USDA Forest Service
The finding comes out of an effort to map where vegetation is emitting and
soaking up carbon dioxide from the atmosphere.
Earth’s trees and plants pull vast amounts of carbon dioxide out of the
atmosphere during photosynthesis, incorporating some of that carbon into
structures like wood. Areas that absorb more carbon than they emit are called
carbon sinks. But plants can also emit the greenhouse gas during processes like
respiration, when dead plants decay, or during combustion in the case of fires.
Researchers are particularly interested in whether – and how – plants at the
scale of an ecosystem like a forest act as sources or sinks in an increasingly
warming world.
A recent study led by scientists at NASA’s Jet Propulsion Laboratory in
Southern California identified whether vegetated areas like forests and
savannas around the world were carbon sources or sinks every year from 2000 to
2019. The research found that over the course of those two decades, living
woody plants were responsible for more than 80% of the sources and sinks on land,
with soil, leaf litter, and decaying organic matter making up the rest. But
they also saw that vegetation retained a far smaller fraction of the carbon
than the scientists originally thought.
In addition, the researchers found that the total amount of carbon emitted
and absorbed in the tropics was four times larger than in temperate regions and
boreal areas (the northernmost forests) combined, but that the ability of
tropical forests to absorb massive amounts of carbon has waned in recent years.
The decline in this ability is because of large-scale deforestation, habitat
degradation, and climate change effects, like more frequent droughts and fires.
In fact, the study, published in Science Advances, showed that 90% of the
carbon that forests around the world absorb from the atmosphere is offset by
the amount of carbon released by such disturbances as deforestation and
droughts.
The scientists created maps of carbon sources and sinks from land-use
changes like deforestation, habitat degradation, and forest planting, as well
as forest growth. They did so by analyzing data on global vegetation collected
from space using instruments such as NASA’s Geoscience Laser Altimeter System (GLAS) on board ICESat and
the agency’s Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Terra and
Aqua satellites, respectively. The analysis used a machine-learning algorithm
that the researchers first trained using vegetation data gathered on the ground
and in the air using laser-scanning instruments.
Taking Stock
“A lot of research that has come before hasn’t been spatially explicit – we
haven’t had a map of where carbon fluxes were occurring,” said Nancy Harris,
research director of the forest program at the World Resources Institute in
Washington and one of the study authors.
Other ways of estimating how much carbon is exchanged between vegetated
areas and the atmosphere include looking at how many plants or forests are in a
particular region and studying land-use changes, combining that information
with carbon emission estimates. But those methods have spatial or temporal
limitations that the study authors tried to address with their machine-learning
method.
Knowing where plants are taking up carbon and where they’re emitting it is
important for monitoring how forests and other vegetated regions respond to a
changing climate. “The Amazon was considered a substantial carbon sink because
of large tracts of pristine forest that soak up carbon dioxide,” said Sassan
Saatchi, principal scientist at JPL and the study lead investigator. “However,
our results show that overall, the Amazon Basin is becoming almost neutral in
terms of carbon balance because deforestation, degradation, and the impacts of
warming, frequent droughts, and fires over the past two decades release carbon
dioxide to the atmosphere.”
Saatchi and his colleagues developed their analysis so that it’s easier to
track changes in vegetated areas based on data collected on the ground as well
as remotely. “Our approach is designed to make sure we can systematically
balance the global carbon budget every year, and that countries can use the results
and methodology for carbon management and their own reporting needs,” he said.
This map shows the change in how much carbon a vegetated area stored or emitted between the years 2000 and 2019. Greener areas absorbed more carbon than they emitted, while more pink and purple regions released more carbon than they stored. One megagram of carbon (MgC) is one metric ton. Credits: NASA/JPL-Caltech
This budgeting analysis helped the researchers better understand the
dynamics of how forests and other vegetated areas around the world were storing
the carbon that they’re absorbing from the atmosphere. “Many previous studies
found that vegetation around the world absorbs a lot of atmospheric carbon
dioxide,” said study lead author Alan Xu, a carbon researcher at JPL and UCLA.
“It gives the impression that global forests are growing and getting bigger
everywhere, but that’s not the case.”
Missing Pieces
This study helps to fill in the picture of where and how trees and plants
are absorbing or emitting carbon, but there’s more work to be done. The
satellite-based carbon maps in this study covered about 39 square miles (100
square kilometers) at a time, but they couldn’t necessarily pick up changes
happening on smaller scales. And there was some information about how forests
stored and emitted carbon within those maps that wasn’t necessarily accounted
for in the researchers’ source-sink calculations. Some of these information
gaps should be remedied by higher-resolution carbon maps provided by newer
satellites already in orbit, as well as upcoming missions like the NASA-Indian
Space Research Organization’s NISAR.
It’s important to understand how regions around the world absorb and emit
carbon dioxide, said Harris. “If we’re not getting these patterns right, we may
be missing some of these ecosystems and how they’re affecting the carbon
cycle.” But she is encouraged by the sheer amount of data becoming available to
climate scientists on how the greenhouse gas moves between the atmosphere and
Earth’s forests, grasslands, and other vegetated areas.
Saatchi is hopeful that having a more systematic and consistent approach to
keeping track of which parts of the world are acting as carbon sources or sinks
will enable better monitoring across regions and countries. “It could allow
countries around the world to use the data as guidance for meeting their
national commitments to the Paris Climate Agreement.”
Learn more about climate change and carbon dioxide here: https://climate.nasa.gov/vital-signs/carbon-dioxide/
No comments:
Post a Comment