Scientists have directly confirmed what they have long assumed to be true: Increasing amounts of greenhouse gases in the atmosphere, such as carbon dioxide, are trapping heat from escaping back into space and are thereby causing global warming.

The observations of what is known as radiative forcing were made over the course of 11 years between 2000 and 2010 from two locations in North America, in Oklahoma and the North Slope of Alaska. Highly specialized instruments in both locations were used to measure thermal infrared energy fluctuations and analyze the source of such changes.

The study, published Wednesday in the advance online edition of the journal Nature, explores the Earth's energy account balance. It found that over time, the planet is running a surplus of energy at the surface, causing global air and ocean temperatures to increase with a wide variety of mostly negative impacts.

Before this study, scientists already knew that the energy balance was tilted in the direction of a growing surplus, but they lacked precise measurements at the surface. The researchers were also able to trace this energy surplus mainly to manmade emissions of carbon dioxide and other greenhouse gases through the burning of fossil fuels such as coal and oil, as well as forest fires.

The research provides observational evidence that the increased heating of the atmosphere during the period was due in large part to the increase in carbon dioxide concentrations at the time. The study found that the 22 parts per million increase in carbon dioxide during this period caused the amount of energy absorbed at the Earth's surface to increase by about two-tenths of a Watt per square meter per decade.

"We see, for the first time in the field, the amplification of the greenhouse effect because there's more carbon dioxide in the atmosphere to absorb what the Earth emits in response to incoming solar radiation," Daniel Feldman, a scientist in Berkeley Lab's Earth Sciences Division and lead author of the study, said in a press release.

"Numerous studies show rising atmospheric carbon dioxide concentrations, but our study provides the critical link between those concentrations and the addition of energy to the system, or the greenhouse effect," Feldman added.

Earth's energy surplus is growing

The study's findings confirm longtime predictions as well as observations of a manmade enhancement of the greenhouse effect, and also help to reinforce the results of many climate models that are predicated in part on accurately simulating the effects of carbon dioxide in the atmosphere.

Carbon dioxide concentrations from historical data and modern measurements. Image: Scripps Institute of Oceanography

Scientists have been monitoring Earth's changing heat balance at the top of the atmosphere via satellite; they are also measuring changes in ocean heat content, which can also indicate a growth or decline in heat energy absorption.

Kevin Trenberth, a climate researcher at the National Center for Atmospheric Research in Boulder, Colorado who was not involved in the new research, said the study is valuable because it "validates what we had assumed."

"It is good to have validation," he said in an email to Mashable. Trenberth says he was "impressed" by the study, particularly since the authors were up front about the difficulties in accurately measuring small changes in radiative heating. "I like the fact that they talk about some of the difficulties, such as contamination of the instrument readings from heat of the instrument itself," he said.

The global average carbon dioxide concentration is close to 400 parts per million, up from 280 parts per million at the start of the industrial revolution. Carbon dioxide concentrations are now higher than they have been throughout all of human history, and they are increasing at a rate of about 2.1 parts per million per year, according to the study.

The observations from the two monitoring sites showed an increase in the heat-trapping ability of the Earth's atmosphere that moved roughly in tandem with the rise in carbon dioxide levels during the period, with seasonal variations. Each spring, plants pulled more carbon dioxide from the atmosphere, temporarily lowering amounts detected at each station, only to release more of the gas back into the air in the fall.

Trenberth faulted the researchers for not separating out the seasonal variations in carbon dioxide concentrations from the longer-term trend, but said this likely did not significantly affect the results.

The study found that the "climate perturbation" from the increase in heat-trapping gases during the observation period would likely be larger than the observed effects, since feedbacks in the climate system tend to accelerate the warming kicked off by the uptick in greenhouse gases.