Using 29 years of data from Landsat satellites, researchers at NASA have found extensive greening in the vegetation across Alaska and Canada. Rapidly increasing temperatures in the Arctic have led to longer growing seasons and changing soil for plants. (Cindy Starr/NASA’s Goddard Space Flight Center)
this month, NASA scientists provided a visualization of
a startling climate change trend — the Earth is getting
greener, as viewed from space, especially in its rapidly warming
northern regions. And this is presumably occurring as more carbon
dioxide in the air, along with warmer temperatures and longer growing
seasons, makes plants very, very happy.
roughly three-decade greening trend itself is apparent, the study
notes, in satellite images of “leaf area index” — defined as
“the amount of leaf area per ground area,” as Robert Buitenwerf
of Aaarhus University in Denmark explains in a commentary
accompanying the study — across most of the northern hemisphere
outside of the tropics, a region sometimes defined as the
“extratropics.” Granted, there are a few patches in Alaska,
Canada and Eurasia where greening has not been seen.
from this set of observations, the researchers, led by Jiafu Mao
of Oak Ridge National Laboratory but including 18 others from
multiple institutions in the United States, France, and China,
conducted what scientists call a “detection
and attribution” study.
This is an experiment in which differing sets of climate model
runs are used to determine whether a particular event or change —
ranging from an extreme heat wave, to a coral bleaching event, to a
major trend like Arctic greening — is more likely to happen in
simulations that include human greenhouse gas emissions, than it is
to happen in those that do not.
enough, the greenhouse-gas filled computer simulations looked much
more like the satellite observations than did simulations that only
included natural variability. The study therefore concludes that
“the trend of strengthened northern vegetation greening … can be
rigorously attributed, with high statistical confidence, to
anthropogenic forcings, particularly to rising atmospheric
concentrations of greenhouse gases.”
this formal detection and attribution appears to be new, the report
is at least the thirdstudy in
the past several months alone to highlight northern hemisphere
greening and to reinforce this basic conclusion. It’s one that has
often been celebrated by climate change skeptics and contrarians who
have long contended that global warming won’t be all bad, and that
plants might help offset any global warming trend.
first find this kind of human fingerprint … particularly the
greenhouse gas impact, on this kind of enhanced vegetation growth,”
says Mao, the study’s lead author.
researchers also dove in more closely to try to determine precisely
why so much greening is happening in the Northern Hemisphere’s
colder latitudes. Sure enough, the overall trend was tied to warmer
temperatures, although in areas where greening was missing, declining
precipitation trends seemed to partly explain the result as well.
(The study did not discuss whether growing wildfires in northern
forests may also be countering greening in some areas.)
his accompanying essay evaluating the research, Aarhus University’s
Buitenwerf praises the study but notes that in some ways, the
current satellite-based approach represents a blunt instrument. An
examination of total leaf area, he notes, doesn’t record
whether a particular area has changed its type of vegetation, say
tundra shrubs to small trees.
also notes that the science is less clear about what is happening in
the global tropics and in the Southern Hemisphere, where greening
trends are either less apparent, or more difficult to explain.
the subtext of all of this research is clear — a key fraction of
all the greenhouse gases that humans pour into the atmosphere each
year is pulled back into plants through the process of
photosynthesis. This is happening even as the overall warming of the
planet may, by lengthening growing seasons and moistening the
atmosphere, further stoke plant growth. They don’t call it the
“greenhouse effect” for nothing.
key question then becomes how much this process can offset overall
global warming over time. And that’s quite unclear.
a lot, after all, that we don’t understand. For instance, a recent
National Oceanic and Atmospheric Administration report found
that while Arctic tundras have indeed been greening over the past 30
years, in the past two to four years, that trend has reversed itself.
we already identify this kind of human impact on this historical
vegetation growth, for the future, it’s hard to predict,” says
Mao. He said he is not sure to what extent the greening trend will
continue, as “disturbances” like wildfires might counteract it,
or plants may become “acclimated to this kind of high temperature.”
the trend is already prompting more optimistic assessments of our
climate future in some quarters. Arctic greening was recently
in a major report by the U.S. Geological Survey, as the central
reason that the state of Alaska, despite worsening wildfires and more
thaw of permafrost, might still be able to stow away more carbon than
it loses over the course of the 21st century.
is clear, then, that greening is emerging as a factor with the
potential to blunt some of the worst impacts of human greenhouse gas
emissions. But thus far, researchers do not seem to be arguing that
it’s enough to counterbalance the entire human-induced warming
we are perhaps lucky that CO2 has this effect on plant physiology, in
addition to being a greenhouse gas, it is not our ‘get out of jail
free’ card when it comes to our ongoing emissions of CO2,”
climate scientist Richard Betts of the U.K.’s Hadley
global greening is perhaps better taken as yet another indicator,
visible from space, of how much we are causing the Earth to change.
the strong evidence provided here of historical human induced
greening in the northern extratropics, society should consider both
intended and unintended consequences of its interactions with
terrestrial ecosystems and the climate system,” the new study