by Gabriel Popkin
avoid overlapping crowns as they grow,
which creates a jigsaw-puzzle pattern
when viewed from below.
Trees are supposed
to slow global warming,
but growing evidence suggests
they might not always be
With nations making little progress controlling their carbon emissions, many governments and advocates have advanced plans to plant vast numbers of trees to absorb carbon dioxide from the atmosphere in an attempt to slow climate change.
But emerging research
suggests that trees might not always help as much as some hope.
China aims to plant trees over an area up to four times the size of the United Kingdom.
California is allowing forest owners to sell credits to CO2-emitting companies, and other US states are considering similar programs, which could motivate projects that establish new forests and protect existing ones.
The European Union is
moving towards allowing countries to include forest planting in
their plans to fight climate change; some nations in the bloc have
also pledged billions of dollars to tropical forest programs.
They argue that forests have many more-complex and uncertain climate impacts than policymakers, environmentalists and even some scientists acknowledge. Although trees cool the globe by taking up carbon through photosynthesis, they also emit a complex potpourri of chemicals, some of which warm the planet.
The dark leaves of trees
can also raise temperatures by absorbing sunlight. Several analyses
in the past few years suggest that these warming effects from
forests could partially or fully offset their cooling ability.
Nobody denies that trees are good for the environment; after all, forests provide a host of benefits, and harbor much of the world's terrestrial biodiversity.
And no researchers are suggesting cutting down existing forests or curtailing efforts to combat deforestation.
But as governments, corporations and non-profit organizations advance ever-more ambitious programs to slow climate change, some scientists warn against relying on forests as a solution to 'global warming' until a better understanding emerges.
Researchers are involved
in major campaigns to collect data using airplanes, satellites and
towers in forests to sample the full suite of chemicals that trees
emit, which can affect both climate and air pollution.
At the same time, increasingly 'dire' warnings about climate change - and the potential for huge amounts of money to go towards planting forests - have made working out how trees affect climate a matter of urgency.
When it comes to forests and their ability to cool the climate, he says,
One widely cited 2017
study 1 estimated that forests and other ecosystems could
provide more than one-third of the total
required to keep 'global warming' below 2°C through to 2030.
The first inkling that plants suck CO2 from the air dates back to the 1780s, when Swiss pastor Jean Senebier grew plants under different experimental conditions.
He suggested that plants
decompose CO2 from the air and incorporate the carbon, an
idea corroborated by subsequent discoveries about the mechanisms of
The 1997 climate treaty known as the Kyoto Protocol allowed rich countries to count carbon storage in forests towards their targets for limiting greenhouse-gas emissions.
In practice, few nations did so because of the agreement's unwieldy accounting mechanisms and other factors.
Later negotiations laid out a framework for enabling wealthy countries to pay poorer tropical countries to reduce emissions from deforestation and to increase carbon in forests.
The framework was
formalized under the
2015 Paris agreement, which
required countries to commit to reduce greenhouse-gas emissions;
more than 50 nations have pledged to add tree cover or protect
existing forests (see image below 'Where are the trees?').
560, 639-643 (2018).
In the past few decades, scientists have worked to create national estimates of carbon loss and gain from vegetation by studying field plots and by combing through satellite data.
In 2011, an international
group led by researchers at the US Department of Agriculture's
Forest Service concluded that forests globally are a large carbon
sink, taking more carbon out of the air through photosynthesis and
wood production than they release through respiration and decay.
Researchers have known for decades that tree leaves absorb more sunlight than do other types of land cover, such as fields or bare ground. Forests can reduce Earth's surface albedo, meaning that the planet reflects less incoming sunlight back into space, leading to warming.
This effect is especially pronounced at higher latitudes and in mountainous or dry regions, where slower-growing coniferous trees with dark leaves cover light-colored ground or snow that would otherwise reflect sunlight.
Most scientists agree, however, that tropical forests are clear climate coolers:
More-recent studies have branched out to include other ways in which forests can influence climate.
As trees live, grow and
die, scientists have learnt, they are in constant conversation with
the air, swapping carbon, water, light and a bewildering array of
chemicals that can interact with the climate.
These include isoprene, a small hydrocarbon that can warm the globe in several ways. It can react with nitrogen oxides in the air to form ozone - a potent climate-warming gas when it resides in the lower atmosphere.
Isoprene can also
lengthen the lifetime of atmospheric methane - another greenhouse
gas. Yet isoprene can have a cooling influence, too, by helping to
produce aerosol particles that block incoming sunlight.
Clearing forests liberated carbon stored in trees, but,
As a corollary, Unger suggested that reforestation would also have uncertain climate effects.
Trees in tropical and temperate zones emit 'huge' quantities of isoprene that is not accounted for in most forestry schemes.
Higher-latitude boreal forests emit mostly terpenes, which help to cool the climate by forming aerosols that can block sunlight and promote the formation of cloud particles - although Unger didn't attempt to quantify this cloud-seeding effect.
She acknowledged that her
study was a first step, and called for increased monitoring of
forest chemicals and their atmospheric interactions.
The article, and especially the headline (which Unger did not write), triggered a tsunami of complaints from researchers, who disputed the science and said the piece threatened to undermine years of research and advocacy.
A group of 30 forest scientists wrote a response on the environmental news website Mongabay, saying,
At 304 meters high, the Zotino Tall Tower Observatory
measures gases and aerosols above taiga forest in central Siberia.
A similar tall tower in the Amazon
makes measurements above the tropical rainforest.
Credit: Michael Hielscher/MPI
Subsequent studies have both supported and contradicted Unger's 2014 analysis.
A team led by Dominick Spracklen and Catherine Scott, atmospheric chemists at the University of Leeds, UK, ran a model that included how aerosols from forests can seed clouds, which reflect sunlight.
They concluded that the
net effect of VOCs from forests is to cool the global climate.
The latest findings are piling on even more complexity.
Ecologist Sunitha Pangala at Lancaster University, UK, spent much of 2013 and 2014 in the Amazon rainforest, where she placed gas-measuring chambers around the trunks of more than 2,300 trees.
She and Vincent Gauci at the Open University in Milton Keynes, UK, and their colleagues reported in 2017 that trees account for around half of the Amazon's total methane emissions. 5
Researchers had previously assumed that methane leaked into the air directly from the soil, where it is produced by microbes.
The new work suggests
that trees could be another conduit for that microbial methane,
potentially explaining why more methane has been detected above
tropical wetlands than has been measured emanating from soil alone.
Pangala and Gauci both estimate that the cooling effect of trees taking up carbon greatly outstrips the warming from tree emissions of methane and nitrous oxide.
But Kristofer Covey, an environmental scientist at Skidmore College in Saratoga Springs, New York, has found methane leaking from non-wetland trees in temperate forests, 7 and argues that such emissions could, in some places, diminish the climate benefits of trees more than researchers and environmentalists realize.
The recent explosion of results underscores the need for a full account of the impacts of forests, says Unger.
Researchers are now turning to sophisticated computer models and using larger and more-comprehensive data sets to nail down exactly what forests in different places do to the climate.
In some cases, the results have been sobering.
Last October, a team led by ecologist Sebastiaan Luyssaert at the Free University of Amsterdam modeled a variety of European forest-management scenarios. 8
The researchers concluded
that none of the scenarios would yield a significant global climate
impact, because the effects of surface darkening and cloud-cover
changes from any added forests would roughly eliminate their
He has found in preliminary work that adding trees to the US west coast and to regions east of the Mississippi River makes sense, climatically speaking.
But albedo changes make forest planting in the Rockies and the southwestern United States a bad deal for the climate in most cases, because the conifers that thrive in those regions are dark and absorb more sunlight than do underlying soils or snow.
He hopes to turn this
research into a standardized methodology that forest managers can
use to assess a project's climate impact.
Williams has found that some resist considering albedo effects, including representatives of companies hoping to sell carbon credits for forest projects.
More data about the climate impacts of forests could come from long-term studies that track the gases and chemicals that trees emit and absorb.
Researchers are using a
325-metre tower in the Amazon to monitor carbon, water and other
chemical fluxes over a roughly 100-square-kilometre area of intact
rainforest northeast of Manaus in Brazil. A
companion tower in Siberia does the
A tower in Norway, for example, will soon be the first in that country to start taking data in a forest. But many important areas have not yet been covered.
Two NASA instruments
launched in the past year - the Global Ecosystem Dynamics
Investigation (GEDI) and the
Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) -
should soon provide a more consistent global picture of forests'
And even those who are firmly convinced that forest projects can fight climate change welcome the added rigor of more-comprehensive studies.
Peter Ellis, for one, acknowledges that the analysis he co-authored 1 considered albedo effects only crudely; the team did not consider VOCs and methane emissions from trees.