NASA - Alan Buis
It’s an intriguing premise: what if we could reduce the severity of
global climate change by planting hundreds of billions of trees to
remove excess carbon from our atmosphere? A recent
study
published in the journal Science sought to provide answers by
estimating the global potential of restoring forested lands as a
possible strategy for mitigating climate change.
The international research team, led by Jean-Francois Bastin of
ETH-Zurich in Switzerland, used direct measurements of forest cover
around the world to create a model for estimating Earth’s forest
restoration potential. They found Earth’s ecosystems could support
another 900 million hectares (2.2 billion acres) of forests, 25 percent
more forested area than we have now. By planting more than a half
trillion trees, the authors say, we could capture about 205 gigatons of
carbon (a gigaton is 1 billion metric tons), reducing atmospheric carbon
by about 25 percent. That’s enough to negate about 20 years of
human-produced carbon emissions at the current rate, or about half of
all carbon emitted by humans since 1960. The study attracted worldwide
attention, as well as some criticism within the science community.
Is the concept of planting trees to help combat climate change really
going out on a limb, so to speak, or might it take root? Sassan
Saatchi, a senior scientist at NASA’s Jet Propulsion Laboratory in
Pasadena, California, believes it has some merit. But while he says
there’s potential for using reforestation as a climate mitigation tool,
he cautions there are many factors to consider and that planting trees
will never be a substitute for decreasing fossil fuel emissions.
“I feel there’s a strong possibility a significant portion of these
lands can be reforested to their original forest cover,” said Saatchi,
an expert in global forest carbon stocks and dynamics. “It’s definitely
not a solution by itself to addressing current climate change. To do
that, we need to reduce human emissions of greenhouse gases. But it
could still have some partial impact on our ability to reduce climate
change.”
 |
| A multi-country-led effort called the African Forest Landscape
Restoration Initiative (AFR100) is working to reforest 100 million
hectares of land in Africa by 2030. Credit: Andrea Borgarello for
TerrAfrica/World Bank |
Saatchi says the study establishes a reasonable estimate of global
forest restoration potential and addresses the issue more directly than
previous work. The researchers used new satellite-based land cover and
land use maps, along with other climate and soil data and advanced
techniques to arrive at their results. He says their conclusions on tree
restoration aren’t that different from the recommendations made by the
Intergovernmental Panel on Climate Change in 2018, which suggested that
950 million hectares (2.3 billion acres) of new forests could help limit
the increase in global average temperature to 1.5-degrees Celsius (2.7
degrees Fahrenheit) above pre-industrial levels by 2050. However, he
says, “the devil is in details.”
Many Unanswered Questions
Before a global forest restoration effort is undertaken, Saatchi
says, numerous questions must first be addressed to assess the concept’s
feasibility, scientific soundness, cost-efficiency, risks and other
considerations. “We need to understand not only whether it’s possible to
do such a thing, but whether we
should do it,” he says.
“The paper has sparked a healthy debate in the science community,
which has now come forward to begin to address issues that the paper did
not,” he said. “The science community has been looking at these
questions to some extent for a long time, but there’s more urgency to
address them now, since we no longer have the same climate conditions we
had 50 or 100 years ago, when humans began massive deforestation for
agriculture and human settlements. Since then, Earth’s population and
land use have increased drastically. In some parts of the Northern
Hemisphere, countries have been able to save more forests, but other
areas, such as the tropics, have seen massive deforestations because of
the need to feed larger populations.”
 |
| Areas of degraded rainforest in the Democratic Republic of Congo. Credit: NASA/JPL-Caltech/Sassan Saatchi |
Saatchi outlined a few of the many questions scientists and others will
want to investigate. For example, how realistic are the study’s
estimates of how much carbon can be sequestered through reforestation?
How long will this approach take to make a dent in atmospheric carbon
concentrations? Can grasslands and savanna ecosystems sustain increased
tree cover? How might converting non-forest land to forests compete with
food production? How much time, money and resources will it take to
implement a global forest restoration of this magnitude? How do the
costs of adopting such a climate mitigation strategy stack up against
its potential benefits? How much carbon would be released to the
atmosphere by restoring forests? How will global climate models respond
to a massive forest restoration? Will an Earth with a billion hectares
of new forests actually be cooler?
 |
| Fire suppression tactics have allowed this forest at the edge of a
savanna in Gabon, Central Africa, to regenerate naturally. Credit:
NASA/JPL-Caltech/Sassan Saatchi |
“Planting a billion hectares of trees won’t be easy,” he said. “It
would require a massive undertaking. If we follow the paper’s
recommendations, reforesting an area the size of the United States and
Canada combined (1 to 2 billion hectares) could take between one and two
thousand years, assuming we plant a million hectares a year and that
each hectare contains at least 50 to 100 trees to create an appropriate
treetop canopy cover.”
Even once the trees are planted, says Saatchi, it will take them
about a century to reach maturity. Most forests in the United States are
less than 100 years old because they are recycled constantly. Trees in
tropical regions take a little bit longer to reach maturity, but
sequester carbon much faster. We know it will take time for new forests
to absorb atmospheric carbon.”
Saatchi says scientists will want to do a comprehensive evaluation of
all potential effects a mass reforestation may have on Earth’s climate
and the global carbon cycle.
Currently, Earth’s forests and soil absorb about 30 percent of
atmospheric carbon emissions, partially through forest productivity and
restoration. While deforestation has occurred throughout human history,
the practice has increased dramatically in the past 50 years. According
to the United Nations’ Food and Agriculture Organization, about 7.3
million hectares (18 million acres) of forest are lost every year, and
roughly half of Earth’s tropical forests have already been cleared. In
the continental United States, an estimate from the University of
Michigan found that 90 percent of indigenous forests have been removed
since 1600.
 |
| Over time, the ocean and land have continued to absorb about half of all
carbon dioxide emissions, even as those emissions have risen
dramatically in recent decades. It remains unclear if carbon absorption
will continue at this rate. Credit: NASA/JPL-Caltech |
 |
| Degraded landscapes in Colombia’s Choco region. Credit: NASA/JPL-Caltech/Sassan Saatchi |
As deforestation has ramped up, Earth’s climate has changed
significantly. Warmer, more adverse climate conditions are creating more
difficult growing conditions for forest ecosystems.
Key questions scientists will need to address are how global
reforestation might affect Earth’s surface albedo (reflectivity) and
evapotranspiration. In the near term and locally, says Saatchi, forest
restoration may actually have a warming effect. As the trees mature, the
new forest canopy cover would presumably make Earth’s surface albedo
darker, particularly in the Northern Hemisphere during periods of snow
cover, causing it to absorb more heat. Increasing forest cover,
particularly in the tropics, will increase evapotranspiration, causing a
cooling effect. With Earth already warming significantly due to
greenhouse gas emissions, will forest reforestation on a global scale
have a net warming or cooling effect on our planet, and will the
benefits of reforested areas absorbing more carbon outweigh their
increased heat absorption? These effects may vary geographically from
tropical to boreal regions and may depend largely on water and light
availability. In addition, how might these changes impact climate change
patterns?
“Recent Landsat satellite-based analyses show that close to 400
million hectares (988 million acres) of forests have been disturbed in
this century alone (2000-2017), either by human activities or through
droughts and fires – that’s almost 50 percent of the area recommended
for reforestation by the authors of the new study,” he said. Some of
these areas have gone back to being forests, but a large amount of these
degraded forests located in tropical and subtropical regions are
suitable targets for restoration.
 |
| Map of global tree loss/tree gain since the early 1980s derived from
NASA Landsat and NOAA AVHRR optical imagery, revised by Sassan Saatchi
from Song et al., 2016. Credit: NASA/JPL-Caltech/Sassan Saatchi |
Another science question concerns biodiversity. Will ecosystems in
reforested areas revert to their previous conditions and maintain their
ability to sequester carbon? While ecosystems that existed before areas
were deforested may have been highly diverse, reforesting them with only
a single type of species (known as monocultures), might result in
ecosystems that won’t function as efficiently as they did before – in
other words, they may not grow the same or stay as healthy over time.
Saatchi says each region of the world will need to address this question
for itself. But restoring a region’s original biodiversity or its
natural forests may not be easy. For example, the region’s soil health
may have changed.
Yet another concern is something Saatchi calls climate connectivity.
When ecosystems become too fragmented, they begin losing their natural
functions. “In Earth’s tropical regions, a combination of deforestation
and climate conditions may have actually changed the system so much that
climate connectivity is reduced,” he says. “Once this connectivity is
lost, it becomes much more difficult for a reforested area to have its
species range and diversity, and the same efficiency to absorb
atmospheric carbon.”
Saatchi says scientists are already studying some of these questions.
He believes that by the end of the next decade, better results from
satellite observations and modeling will likely enable us to determine
whether a global forest reforestation will produce the carbon and
climate benefits suggested by the new study, and whether it should be
undertaken. In the meantime, stopping further deforestation and
restoring these areas to their original forest cover of 50 years ago may
be the most effective mitigation strategy.
Looking to Space for Answers
Saatchi says a number of current and planned satellite missions from
NASA and other space agencies can make valuable contributions to these
research efforts:
- Instruments on NASA satellites, such as the Clouds and the Earth’s
Radiant Energy System (CERES) instrument on NASA’s Terra satellite,
continuously monitor the energy balance of Earth’s land surfaces,
measuring their albedo, a key climate parameter that would be impacted
by reforestation.
 |
| Map created from data from the CERES instrument on NASA’s Terra
satellite, showing how the reflectivity of Earth—the amount of sunlight
reflected back into space—changed between March 1, 2000, and December
31, 2011. This global picture of reflectivity (also called albedo)
appears to be a muddle, with different areas reflecting more or less
sunlight over the 12-year record. Shades of blue mark areas that
reflected more sunlight over time (increasing albedo), and orange areas
denote less reflection (lower albedo). Credit: NASA's Earth Observatory |
- NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS)
instruments on the agency’s Aqua and Terra satellites provide a suite of
measurements on global forest cover change, fire and forest carbon
cycling function.
- NASA’s ECOsystem Spaceborne Thermal Radiometer Experiment
(ECOSTRESS) aboard the International Space Station, launched last year,
measures evapotranspiration and stress on ecosystems, providing valuable
information on how Earth’s energy, water and carbon cycles interact in
ecosystems in a warming climate.
 |
| NASA's ECOsystem Spaceborne Thermal Radiometer Experiment on Space
Station (ECOSTRESS) imaged the stress on Costa Rican vegetation caused
by a massive regional drought that led the Central American nation's
government to declare a state of emergency. The image was acquired on
February 15, 2019, then processed to generate the evaporative stress
image. Credit: NASA/JPL-Caltech |
- NASA scientists conduct research to map the functional traits of ecosystems. Models, combined with satellite observations, can examine whether ecosystems will absorb more carbon if we plant new trees.
- A new NASA mission in development for launch in the next decade called Surface Biology and Geology (SBG) would give scientists a global view of the functional traits and diversity of ecosystems and their efficiency in absorbing carbon, water and energy. Other space agencies also plan to make similar measurements.
- NASA’s recently launched Global Ecosystem Dynamics Investigation (GEDI) aboard the space station is conducting high-resolution laser ranging of Earth’s forests and topography to study how deforestation has contributed to atmospheric carbon dioxide concentrations, how much carbon forests will absorb in the future, and how degradation of habitats will affect global biodiversity.
 |
| NASA’s Global Ecosystem Dynamics Investigation (GEDI) mission created
this image of a South Carolina woodland. Darker green colors show where
the leaves and branches are denser, while the lighter areas show where
the canopy is less dense. Credit: Joshua Stevens / NASA Earth
Observatory, Bryan Blair / NASA Goddard Space Flight Center, Michelle
Hofton and Ralph Dubayah / University of Maryland |
- The NASA-ISRO Synthetic Aperture Radar (NISAR) mission, a
dedicated U.S./Indian interferometric synthetic aperture radar (InSAR)
mission scheduled to launch in 2022, will be able to measure the woody
plants and forests that make up 80 percent of Earth’s living terrestrial
biomass. NISAR’s global, detailed maps of above-ground woody biomass
density are expected to cut in half current uncertainties in estimates
of carbon emissions resulting from land use changes.
- The European Space Agency’s BIOMASS mission, launching in the early
2020s, will map the global distribution of above-ground biomass in
forests to reduce uncertainties in estimates of carbon stocks and fluxes
in the terrestrial biosphere, such as those linked to changes in land
use, forest degradation and forest regrowth.
“With these new missions, we should be able to monitor how every
patch of forest around the world is absorbing carbon, and how carbon
absorption is changing, on a monthly and annual basis,” said Saatchi.
Seeing the Forests for the Trees: The Big Picture
Saatchi says the study’s results can help address policy-relevant
questions. In accordance with the Paris Agreement, after 2020, the
global community has agreed to major emission reduction programs.
Reforestation can complement these emission reduction strategies.
“With the Paris Agreement, governments around the world committed to
reduce emissions by adopting low-carbon pathways in accordance with
nationally determined contributions,” he said. “As a result, it’s become
more urgent than ever to have realistic estimates of each country’s
capacity to increase its forest cover and health. While it’s likely the
burden of restoring forests will fall primarily on the shoulders of the
world’s large and economically-developed countries, the developing world
can also contribute by reducing land use change and deforestation.” He
adds governments will need to decide which land areas to target first
and which will have the least negative economic impacts to both society
and individual communities, such as indigenous populations.
 |
| A Baka woman in central Gabon makes products from non-timber forest
materials. Without forest conservation and restoration, indigenous
forest people will be forced to re-establish themselves outside of
forest areas. Credit: NASA/JPL-Caltech/Sassan Saatchi |
If it’s determined that a global reforestation effort can be
successful, will the world’s governments have the will to do it? Saatchi
pointed to some recent examples that show what might be possible.
Over the past 15 years or so, China has planted millions of trees and
created millions of hectares of new forest cover, much of it in areas
with marginal agricultural potential. “China’s land use policy increased
forest cover in southern China between 10 and 20 percent, turning these
areas into intense managed forests,” he said. “As a result, they
created close to a carbon sink (an area that stores carbon) in their
forests, almost doubling their carbon uptake. The effort has offset 20
percent of China’s annual fossil fuel emissions, and since 2012 that
percentage has increased to 33 percent. So that’s a success story.”
Managed activities to increase the carbon sequestration of forests
have also taken place in other parts of the Northern Hemisphere,
including the United States, Canada, Europe and Russia, he says. He
believes it’s possible to increase them even further and to extend the
area or the capacity of these forests to sequester more carbon. In fact,
he says, some foresters have been doing so for decades.
“U.S. forests have actually been a net sink for carbon for many
decades,” he says. “A paper published a couple of years ago showed that
reforestation could reduce U.S. annual carbon emissions from all sources
by 10 to 15 percent. Imagine if we do that? It’s possible. We just need
to study the cost-to-benefit ratio – is it economically feasible to
plant those trees compared to how much carbon they would offset?”
 |
| The U.S. Forest Service is restoring this longleaf pine forest in Alabama. Credit: NASA/JPL-Caltech/Sassan Saatchi |
Another region Saatchi says is low-hanging fruit in terms of its
potential to extend global tree cover is the Amazon, where large
wildfires have made headlines recently. Between the 1970’s and 2010, 20
percent of the Amazon basin was deforested for land use activities —
more than 100 million hectares of trees. But prior to last year, Brazil
had significantly reduced deforestation for nearly a decade. “Restoring
these Amazonian forests, if possible, would certainly absorb more carbon
from the atmosphere,” he said.
 |
| The Amazon rainforest near Manaus, Brazil. Fragmented landscapes in Earth’s humid tropics are suitable locations for restoration of native forests. Credit: Neil Palmer, Flickr Creative Commons / CC BY-NC-SA 4.0 |
Ultimately, should a global reforestation effort be deemed feasible,
the biggest question may be whether it will be in time to make a
difference for climate change. Saatchi is hopeful.
“We know business as usual will be disastrous,” he said. “We’ve
already identified some solutions for reducing carbon emissions in parts
of our society, such as in transportation and agriculture, and we’re
working on ways to transform our energy consumption. So why not restore
our ecosystem as well? Half of what comes out of car tailpipes stays in
the atmosphere; the rest gets absorbed by the ecosystem. That’s a huge
absorptive capability that must be saved.
“Maybe we’ll find we don’t need to plant a billion hectares of
trees,” he continued. “Perhaps we can restore existing, degraded
ecosystems to their natural state, especially in the tropics, and invest
in maintaining their diversity and services. But I believe a global
reforestation effort can have a gradual climate mitigation impact. What
happens to Earth 100 years from now depends on the choices we make
today.”
Links
