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Thursday, August 2, 2018
A scientist's final paper looks toward Earth's future climate
From space, satellites can see Earth breathe. This visualization
shows 20 years of continuous observations of plant life on land and at
the ocean’s surface, from September 1997 to September. 2017. On land,
vegetation appears on a scale from brown (low vegetation) to dark green
(lots of vegetation); at the ocean surface, phytoplankton are indicated
on a scale from purple (low) to yellow (high). Credit: NASA
By Patrick Lynch,
NASA's Goddard Space Flight Center
A NASA scientist's final scientific paper, published posthumously
this month, reveals new insights into one of the most complex challenges
of Earth's climate: understanding and predicting future atmospheric
levels of greenhouse gases and the role of the ocean and land in
determining those levels.
A paper published in the Proceedings of the National Academy of
Sciences was led by Piers J. Sellers, former director of the Earth
Sciences Division at NASA's Goddard Space Flight Center, who died in
December 2016. Sellers was an Earth scientist at NASA Goddard and later
an astronaut who flew on three space shuttle missions.
The paper includes a significant overarching message: The current
international fleet of satellites is making real improvements in
accurately measuring greenhouse gases from space, but in the future a
more sophisticated system of observations will be necessary to
understand and predict Earth's changing climate at the level of accuracy
needed by society.
In a 2016 interview, Piers Sellers talked about his enthusiasm and
appreciation for working at NASA’s Goddard Space Flight Center. Credit:
NASA’s Goddard Space Flight Center
Sellers wrote the paper along with colleagues at NASA's Jet
Propulsion Laboratory and the University of Oklahoma. Work on the paper
began in 2015, and Sellers continued working with his collaborators up
until about six weeks before he died. They carried on the research and
writing of the paper until its publication this week.
The paper focuses on the topic that was at the center of Sellers'
research career: Earth's biosphere and its interactions with the
planet's climate. In the 1980s he helped pioneer computer modeling of
Earth's vegetation. In the new paper, Sellers and co-authors
investigated "carbon cycle–climate feedbacks" – the potential response
of natural systems to climate change caused by human emissions – and
laid out a vision for how to best measure this response on a global
scale from space.
The exchange of carbon between the land, ocean and air plays a huge
role in determining the amount of greenhouse gases in the atmosphere,
which will largely determine Earth's future climate. But, there are
complex interactions at play. While human-caused emissions of
greenhouses gases are building up in the atmosphere, land ecosystems and
the ocean still offset about 50 percent of all those emissions. As the
climate warms scientists are unsure whether forests and the ocean will
continue to absorb roughly half of the emissions – acting as a carbon
sink – or if this offset becomes lower, or if the sinks become carbon
sources.
Paper co-author David Schimel, a scientist at JPL and a longtime
scientific collaborator of Sellers', said the paper captured how he,
Sellers and the other co-authors saw this scientific problem as one of
the critical research targets for NASA Earth science.
"We all saw understanding the future of carbon cycle feedbacks as one
of the grand challenges of climate change science," Schimel said.
Scientists' understanding of how Earth's living systems interact with
rising atmospheric levels of greenhouse gases has changed tremendously
in recent decades, said co-author Berrien Moore III, of the University
of Oklahoma. Moore has been a scientific collaborator with Sellers and
Schimel since the 1980s. He said that back then, scientists thought the
ocean absorbed about half of annual carbon emissions, while plants on
land played a minimal role. Scientists now understand the ocean and land
together absorb about half of all emissions, with the terrestrial
system’s role being affected greatly by large-scale weather patterns
such as El Niño and La Niña. Moore is also the principal investigator of
a NASA mission called GeoCarb, scheduled to launch in 2022, that will
monitor greenhouse gases over much of the Western Hemisphere from a
geostationary orbit.
NASA launched the Orbiting Carbon Observatory-2 (OCO-2) in 2014, and
with the advancement of measurement and computer modeling techniques,
scientists are gaining a better understanding of how carbon moves
through the land, ocean and atmosphere. This new paper builds on
previous research and focuses on a curious chain of events in 2015.
While human emissions of carbon dioxide leveled off for the first time
in decades during that year, the growth rate in atmospheric
concentrations of carbon dioxide actually spiked at the same time.
This was further evidence of what scientists had been piecing
together for years – that a complex combination of factors, including
weather, drought, fires and more, contributes to greenhouse gas levels
in the atmosphere.
However, with the new combination of OCO-2 observations and
space-based measurements of plant fluorescence (essentially a measure of
photosynthesis), researchers have begun producing more accurate
estimates of where carbon was absorbed and released around the planet
during 2015, when an intense El Niño was in effect, compared to other
years.
The paper follows a report from a 2015 workshop on the carbon cycle
led by Sellers, Schimel, and Moore. Schimel and Moore both pointed out
that every one of the more than 40 participants in the workshop
contributed to a final scientific report from the meeting – a rare
occurrence. They attributed this, in part, to the inspirational role
Sellers played in spurring thought and action.
"When you have someone like Piers in the room, there's a magnetic
effect," Moore said. "Piers had his shoulder to the wheel, so everyone
had to have their shoulders to the wheel."
Schimel and Moore said the workshop paper lays out a vision for
what's needed in a future space-based observing system to measure,
understand, and predict carbon cycle feedbacks: active and passive
instruments, and satellites both in low-Earth and geostationary orbits
around the world. In the coming years, NASA and space agencies in
Europe, Japan, and China, will all launch new greenhouse-gas monitoring
missions.
"Piers thought it's absolutely essential to get it right," said
Schimel, "and essential to more or less get it right the first time."
The authors dedicated the paper's publication to Sellers, and in
their dedication referenced a Winston Churchill quote often cited by the
British-born scientist. They wrote: "P.J.S. approached the challenge of
carbon science in the spirt of a favorite Churchill quote,
'Difficulties mastered are opportunities won,' and he aimed to resolve
the carbon–climate problem by rising to the difficulties and seizing the
opportunities."
