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[Nagraj Adve]

Enough excess energy - over half a watt per metre squared - to light up a
40 watt bulb in a middle class home.
Naga


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*National Aeronautics and Space Administration <http://www.nasa.gov/>*

*Goddard Institute for Space Studies <http://www.giss.nasa.gov/>*

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*Goddard Space Flight Center <http://www.nasa.gov/centers/goddard/>
Sciences and Exploration Directorate <http://sciences.gsfc.nasa.gov/sed/>
Earth Sciences Division <http://sciences.gsfc.nasa.gov/610/>*

*Research News*

*Earth's Energy Budget Remained Out of Balance Despite Unusually Low Solar
Activity*

Jan. 30, 2012

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.

[image: conceptual image of the
sun]<http://www.giss.nasa.gov/research/news/20120130b/619626main_solarcycle0281.jpg>

A prolonged solar minimum left the sun's surface nearly free of sunspots
and accompanying bright areas called faculae between 2005 and 2010. Total
solar irradiance declined slightly as a result, but the Earth continued to
absorb more energy than it emit throughout the minimum. An animation of a
full solar cycle is available
here<http://svs.gsfc.nasa.gov/vis/a020000/a020100/a020133/>.
(Credit: NASA Goddard's Scientific Visualization Studio)
+ View larger image<http://www.giss.nasa.gov/research/news/20120130b/619626main_solarcycle0281.jpg>|
TIFF
format<http://www.giss.nasa.gov/research/news/20120130b/619627main_solarcycle0281.tif>

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.

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 Watts 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 Watts 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
Watts 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.

[image: graph of the sun's total solar
irradiance]<http://www.giss.nasa.gov/research/news/20120130b/619624main_solar_irradiance_graph.jpg>

A graph of the sun's total solar irradiance shows that in recent years
irradiance dipped to the lowest levels recorded during the satellite era.
The resulting reduction in the amount of solar energy available to affect
Earth's climate was about .25 Watts per square meter, less than half of
Earth's total energy imbalance. (Credit: NASA/James Hansen)
+ View larger image<http://www.giss.nasa.gov/research/news/20120130b/619624main_solar_irradiance_graph.jpg>

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 meters (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.

[image: Argo float and
ship]<http://www.giss.nasa.gov/research/news/20120130b/619622main_Arvor_pourquoi_pas05b.jpg>

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 <http://www.argo.ucsd.edu/pictures.html>)
+ View larger image<http://www.giss.nasa.gov/research/news/20120130b/619622main_Arvor_pourquoi_pas05b.jpg>

Hansen's analysis of the information collected by Argo, along with other
ground-based and satellite data, show the upper ocean has absorbed 71
percent of the excess energy and the Southern Ocean, where there are few
Argo floats, has absorbed 12 percent. The abyssal zone of the ocean,
between about 3,000 and 6,000 meters (9,800 and 20,000 feet) below the
surface, absorbed five percent, while ice absorbed eight percent and land
four percent.

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.

[image: map showing global reach of Argo
floats]<http://www.giss.nasa.gov/research/news/20120130b/619618main_Argo_chart.jpg>

A chart shows the global reach of the network of Argo floats. (Credit: Argo
Project Office <http://www.argo.ucsd.edu/index.html>)
+ View larger image<http://www.giss.nasa.gov/research/news/20120130b/619618main_Argo_chart.jpg>

"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.

*Reference*

Hansen, J., Mki. Sato, P. Kharecha, and K. von Schuckmann, 2011: Earth's
energy imbalance and implications<http://pubs.giss.nasa.gov/abs/ha06510a.html>.
*Atmos. Chem. Phys.*, *11*, 13421-13449, doi:10.5194/acp-11-13421-2011.