Data collected by Argo floats, such as this one, helped Hansen’s team improve the calculation of Earth’s energy imbalance. Credit: Argo Project Office |
A
new NASA study underscores the fact that greenhouse gases generated by
human activity—not changes in solar activity—are the primary force
driving global warming.
The
study offers an updated calculation of the Earth’s energy imbalance,
the difference between the amount of solar energy absorbed by Earth’s
surface and the amount returned to space as heat. The researchers’
calculations show that, despite unusually low solar activity between
2005 and 2010, the planet continued to absorb more energy than it
returned to space.
James Hansen, director of NASA’s Goddard Institute for Space Studies (GISS) in New York City, led the research. Atmospheric Chemistry and Physics published the study last December.
Total
solar irradiance, the amount of energy produced by the sun that reaches
the top of each square meter of the Earth’s atmosphere, typically
declines by about a tenth of a percent during cyclical lulls in solar
activity caused by shifts in the sun’s magnetic field. Usually solar
minimums occur about every eleven years and last a year or so, but the
most recent minimum persisted more than two years longer than normal,
making it the longest minimum recorded during the satellite era.
Pinpointing
the magnitude of Earth’s energy imbalance is fundamental to climate
science because it offers a direct measure of the state of the climate.
Energy imbalance calculations also serve as the foundation for
projections of future climate change. If the imbalance is positive and
more energy enters the system than exits, Earth grows warmer. If the
imbalance is negative, the planet grows cooler.
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Hansen’s
team concluded that Earth has absorbed more than half a watt more solar
energy per square meter than it let off throughout the six year study
period. The calculated value of the imbalance (0.58 W of excess energy
per square meter) is more than twice as much as the reduction in the
amount of solar energy supplied to the planet between maximum and
minimum solar activity (0.25 W per square meter).
“The
fact that we still see a positive imbalance despite the prolonged solar
minimum isn’t a surprise given what we’ve learned about the climate
system, but it’s worth noting because this provides unequivocal evidence
that the sun is not the dominant driver of global warming,” Hansen
said.
According
to calculations conducted by Hansen and his colleagues, the 0.58 W per
square meter imbalance implies that carbon dioxide levels need to be
reduced to about 350 parts per million to restore the energy budget to
equilibrium. The most recent measurements show that carbon dioxide
levels are currently 392 parts per million and scientists expect that
concentration to continue to rise in the future.
Climate
scientists have been refining calculations of the Earth’s energy
imbalance for many years, but this newest estimate is an improvement
over previous attempts because the scientists had access to better
measurements of ocean temperature than researchers have had in the past.
The
improved measurements came from free-floating instruments that directly
monitor the temperature, pressure and salinity of the upper ocean to a
depth of 2,000 m (6,560 feet). The network of instruments, known
collectively as Argo, has grown dramatically in recent years since
researchers first began deploying the floats a decade ago. Today, more
than 3,400 Argo floats actively take measurements and provide data to
the public, mostly within 24 hours.
Hansen’s
analysis of the information collected by Argo, along with other
ground-based and satellite data, show the upper ocean has absorbed 71%
of the excess energy and the Southern Ocean, where there are few Argo
floats, has absorbed 12%. The abyssal zone of the ocean, between about
3,000 and 6,000 m (9,800 and 20,000 feet) below the surface, absorbed
five percent, while ice absorbed eight percent and land four percent.
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The
updated energy imbalance calculation has important implications for
climate modeling. Its value, which is slightly lower than previous
estimates, suggests that most climate models overestimate how readily
heat mixes deeply into the ocean and significantly underestimates the
cooling effect of small airborne particles called aerosols, which along
with greenhouse gases and solar irradiance are critical factors in
energy imbalance calculations.
“Climate
models simulate observed changes in global temperatures quite
accurately, so if the models mix heat into the deep ocean too
aggressively, it follows that they underestimate the magnitude of the
aerosol cooling effect,” Hansen said.
Aerosols,
which can either warm or cool the atmosphere depending on their
composition and how they interact with clouds, are thought to have a net
cooling effect. But estimates of their overall impact on climate are
quite uncertain given how difficult it is to measure the distribution of
the particles on a broad scale. The new study suggests that the overall
cooling effect from aerosols could be about twice as strong as current
climate models suggest, largely because few models account for how the
particles affect clouds.
“Unfortunately,
aerosols remain poorly measured from space,” said Michael Mishchenko, a
scientist also based at GISS and the project scientist for Glory, a
satellite mission designed to measure aerosols in unprecedented detail
that was lost after a launch failure in early 2011. “We must have a much
better understanding of the global distribution of detailed aerosol
properties in order to perfect calculations of Earth’s energy
imbalance,” said Mishchenko.
