Although some people continue to question whether Earth's temperatures have actually warmed significantly over the last century, the focus of many has shifted to cause: while the world has clearly warmed, a lot of people have suggested that the cause of that warming is "up for debate." Many studies have looked at that question and determined that human activity is the primary driver, as the 2007 IPCC report concluded. A paper published this week in Nature Geoscience makes a novel contribution to this question, even as it comes up with the same answer.

Previous work has primarily used observations of temperature trends (from the deep ocean or the lower atmosphere) combined with computer modeling to assess the cause of warming. For example, the latest IPCC report showed climate models run with and without the effect of greenhouse gases to illustrate that temperatures would have been stable without anthropogenic warming.

The new study, published by researchers from the Institute for Atmospheric and Climate Science at ETH in Switzerland, directly examined Earth’s energy balance instead.

The global energy balance has two primary terms—incoming, shortwave radiation from the Sun, and outgoing, longwave radiation from the Earth. Any imbalance between those terms will cause the planet to warm or cool by driving the total heat content of the atmosphere and oceans to increase or decrease. There are several "control knobs" that can alter that balance: the amount of incoming radiation that is reflected back into space, the amount of outgoing radiation that is absorbed by greenhouse gases, and changes in the output of the Sun itself.

Turning one of those knobs acts as a "climate forcing"—a push that moves the energy balance in one direction or the other. Forcings are expressed in terms of energy per unit area of the Earth’s surface, or watts/m2.

For this new work, researchers used observations (and historical reconstructions) of various climate forcings from 1780 to present. The list of forcings considered is pretty extensive: carbon dioxide, methane, nitrous oxide, CFCs, ozone, aerosols (particles <1 micron in diameter), black carbon, stratospheric water vapor, volcanic gases, and solar activity. They took these forcings and fed them into a climate model to simulate the Earth’s energy balance through time.

Their results showed that, between 1950 and 2004, the increasing concentration of carbon dioxide in the atmosphere has added a total of about 9 x 1023 joules of energy to the global energy balance. Other greenhouse gases have added another 7 x 1023 joules. In contrast, changes in solar activity added about 2 x 1023 (and the authors note that their calculated solar contribution is likely to be overestimated). On the flip side, approximately two-thirds of that added energy was offset by the cooling effects of aerosol emissions.

That means that most of the effect of anthropogenic greenhouse gas emissions has been masked by aerosols, without which we would have seen even greater warming. But those are just the current dynamics; while the residence time of aerosols in the atmosphere is fairly short, CO 2 hangs around for a very long time. As the authors write, "The warming induced by CO 2 will also persist for at least a thousand years as a result of the slow ocean carbon uptake, far longer than the warming from most other forcing agents."

This interplay leads to a sort of trap—aerosols will continue to mask some portion of warming until fossil fuel use seriously declines, at which point the full CO 2 forcing will start to be felt.

By looking at the temperature trend associated with this time period, the authors conclude, "Our results show that it is extremely likely [>95 percent confidence] that at least 74 percent of the observed warming since 1950 was caused by anthropogenic radiative forcings, and less than 26 percent by unforced internal variability” such as oceanic oscillations. If you’re wondering where that leaves natural forcings, they calculate that solar activity is only responsible for about 0.07°C of the warming since 1950.

Their analysis also allows them to estimate of the climate sensitivity—the magnitude of warming a given increase in CO 2 will lead to. The 2007 IPCC report put the likely range for warming caused by a doubling of the (current) CO 2 concentration at 2.0 to 4.5°C, with a most likely value of 3.0°C. An interesting study we covered last week estimated a somewhat lower sensitivity of 2.3°C. This new study adds another number to the stack: a mean sensitivity of 3.6°C (with 90 percent of the probability distribution falling between 1.7 and 6.5°C).

Nature Geoscience, 2011. DOI: 10.1038/NGEO1327 (About DOIs).