Using spectroscopic instruments operated by the U.S. DoE’s Atmospheric Radiation Measurement Climate Research Facility, scientists have directly observed an increase in carbon dioxide’s (CO2) greenhouse effect at the planet’s surface.
These graphs show carbon dioxide’s increasing greenhouse effect at two locations on the Earth’s surface, in Oklahoma and on the North Slope of Alaska; as the atmospheric concentration of CO2 (blue) increased from 2000 to the end of 2010, so did surface radiative forcing due to CO2 (orange), and both quantities have upward trends; this means the Earth absorbed more energy from solar radiation than it emitted as heat back to space. Image credit: Lawrence Berkeley National Laboratory.
The influence of atmospheric CO2 on the balance between incoming energy from the Sun and outgoing heat from the Earth – also called the planet’s energy balance – is well established.
But this effect has not been experimentally confirmed outside the laboratory until now.
A team of scientists led by Dr Daniel Feldman of the Lawrence Berkeley National Laboratory measured atmospheric CO2’s increasing capacity to absorb thermal radiation emitted from the Earth’s surface over an eleven-year period (2000 – 2010) at two locations in North America – one in Oklahoma and one on the North Slope of Alaska near the town of Barrow.
“We measured radiation in the form of infrared energy. Then we controlled for other factors that would impact our measurements, such as a weather system moving through the area,” said Dr Feldman, who is the first author of the paper published in the journal Nature.
The result is two time-series from two very different locations. Each series includes 3,300 measurements from Alaska and 8,300 measurements from Oklahoma obtained on a near-daily basis.
The scientists found that CO2 was responsible for a significant uptick in radiative forcing – a measure of how much the planet’s energy balance is perturbed by atmospheric changes – at both locations, about 2/10 of a Watt per square meter per decade.
They linked this trend to the 22 parts-per-million increase in atmospheric CO2 between 2000 and 2010.
Much of this CO2 is from the burning of fossil fuels, according to a modeling system that tracks CO2 sources around the world.
“We see, for the first time in the field, the amplification of the greenhouse effect because there’s more CO2 in the atmosphere to absorb what the Earth emits in response to incoming solar radiation,” Dr Feldman said.
“Numerous studies show rising atmospheric CO2 concentrations, but our study provides the critical link between those concentrations and the addition of energy to the system, or the greenhouse effect.”
The measurements also enabled the team to detect, for the first time, the influence of photosynthesis on the balance of energy at the surface.
The scientists found that CO2-attributed radiative forcing dipped in the spring as flourishing photosynthetic activity pulled more of the greenhouse gas from the air.
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D. R. Feldman et al. Observational determination of surface radiative forcing by CO2 from 2000 to 2010. Nature, published online February 25, 2015; doi: 10.1038/nature14240
