In a first direct observation of its kind, scientists have measured the increasing capacity of carbon dioxide (CO2) to absorb heat emitted from the Earth's surface across an 11-year period.
Atmospheric CO2 was responsible for an increasing amount of heat amounting to 0.2 Watts per sq mt per decade.
The study led by scientists from the US Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) attributed the upward trend to rising CO2 levels from fossil fuel emissions.
This is the first time measurements showing the difference between incoming solar energy and outgoing heat have been made outside the lab. They tally with the theoretical calculations, confirm the efficacy of current climate models.
While many studies have shown rising atmospheric CO2 concentrations, this study establishes the link between those concentrations and the addition of energy to the system.
The research was published online in Nature.
The scientists measured atmospheric carbon dioxide's contribution to radiative forcing at two sites, one in Oklahoma and one on the North Slope of Alaska, from 2000 to the end of 2010.
Positive radiative forcing occurs when the Earth absorbs more energy from solar radiation than it emits as thermal radiation back to space. It can be measured at the Earth's surface or high in the atmosphere.
In this research, the scientists focussed on the surface.
A significant rise in radiative forcing at both locations, about two-tenths of a Watt per sq mt per decade, was linked to the rise in carbon emissions by 22 parts-per-million between 2000 and 2010.
Much of this CO2 has been traced to the burning of fossil fuels based on data from the National Oceanic and Atmospheric Administration's Carbon Tracker system.
Increase in energy use by emerging economies sent carbon emissions to a record high in 2013 with fossil fuel use and cement production contributing 35.3 billion tonnes of carbon dioxide according to a recent report by PBL Netherlands Environmental Assessment Agency and the Joint Research Centre of the European Commission.
"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," says Daniel Feldman, a scientist in Berkeley Lab's Earth Sciences Division and lead author of the study.
The scientists used precise spectroscopic instruments operated by the Atmospheric Radiation Measurement (ARM) Climate Research Facility, a DOE Office of Science User Facility. The instruments measure thermal infrared energy that travels down through the atmosphere to the surface.
Using two other instruments, the heat emitted by clouds and water vapour was removed from the total observed to isolate the CO2 contribution to heat.
The time-series from two very different locations spans from 2000 to the end of 2010, and includes 3,300 measurements from Alaska and 8,300 measurements from Oklahoma obtained on a near-daily basis.
Both series showed the same upward trend.
The measurements showed that CO2 attributed radiative forcing dipped in the spring as flourishing photosynthetic activity pulled more of the greenhouse gas from the air.
The world has less than 1,000 billion tonnes of emissions that can be emitted before irreversible climate change sets in, scientists have suggested. This has prompted the IPCC to call for a total phase-out of fossil fuels before the end of the century.