When there's more energy radiating down on the planet than there is radiating back out to space, something’s going to have to heat up. (This is the first of a two-part "Explained" on the scientific concepts underlying the concept of the greenhouse effect and global climate change. Part two deals with climate sensitivity.)

When people talk about global warming or the greenhouse effect, the main underlying scientific concept that describes the process is radiative forcing. And despite all the recent controversy over leaked emails and charges of poorly sourced references in the last Intergovernmental Panel on Climate Change report, the basic concept of radiative forcing is one on which scientists — whatever their views on global warming or the IPCC — all seem to agree. Disagreements come into play in determining the actual value of that number.

The concept of radiative forcing is fairly straightforward. Energy is constantly flowing into the atmosphere in the form of sunlight that always shines on half of the Earth's surface. Some of this sunlight (about 30 percent) is reflected back to space and the rest is absorbed by the planet. And like any warm object sitting in cold surroundings — and space is a very cold place — some energy is always radiating back out into space as invisible infrared light. Subtract the energy flowing out from the energy flowing in, and if the number is anything other than zero, there has to be some warming (or cooling, if the number is negative) going on.

It's as if you have a kettle full of water, which is at room temperature. That means everything is at equilibrium, and nothing will change except as small random variations. But light a fire under that kettle, and suddenly there will be more energy flowing into that water than radiating out, and the water is going to start getting hotter.

In short, radiative forcing is a direct measure of the amount that the Earth’s energy budget is out of balance.

While the concept is simple, the analysis required to figure out the actual value of this number for the Earth right now is much more complicated and difficult. Many different factors have an effect on this balancing act, and each has its own level of uncertainty and its own difficulties in being precisely measured. And the individual contributions to radiative forcing cannot simply be added together to get the total, because some of the factors overlap — for example, some different greenhouse gases absorb and emit at the same infrared wavelengths of radiation, so their combined warming effect is less than the sum of their individual effects.

In its most recent report in 2007, the IPCC produced the most comprehensive estimate to date of the overall radiative forcing affecting the Earth today. Ronald Prinn, the TEPCO Professor of Atmospheric Science and director of MIT's Center for Global Change Science, was one of the lead authors of that chapter of the IPCC's Fourth Assessment Report. Radiative forcing "was very small in the past, when global average temperatures were not rising or falling substantially," he explains.

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