Also posted on Climate411.
There are lots of reasons for passing good climate legislation as soon as possible. One reason that adds urgency but gets little attention is this: even if we could stabilize greenhouse gas concentrations at today’s levels, some global temperature increase is already locked into the system. This is sometimes called the climate system's "warming commitment" or the "warming in the pipeline".
What’s behind this phenomenon? The short answer is: water, water everywhere.
Earth’s vast oceans buffer the atmosphere from large temperature changes because it takes a lot of energy to change water’s temperature. (In scientific parlance, water has high heat capacity.) You know this from everyday life: if you put a pot of cold water on a stove and turn on the burner, you can hold your finger in the water for quite a while before the water starts to warm up.
The same principle applies at the global scale. Oceans cover 71 percent of Earth’s surface. It takes an enormous amount of energy to heat that much water, especially since ocean circulation causes warm surface water to sink and cold deep water to return to the surface. As a result, there is a lag time between when greenhouse gases are emitted to the atmosphere and when temperatures start to rise.
How much warming is in the pipeline? It depends on the level at which greenhouse gases are stabilized. For example, the latest IPCC report calculates that if we held greenhouse gas concentrations steady at 2000 levels, average global temperature would go up another degree Fahrenheit. Concentrations have, to put it mildly, gone up since then, so the warming commitment for today’s greenhouse gas level is slightly higher than one degree.
The implication of this ocean-induced lag time is that we have less time to act than it first appears. The tipping point for losing the Greenland ice sheet may be just 2.3 degrees Fahrenheit above today. If you factor in the warming in the pipeline, about half of that amount is already gone.
In addition to the oceans' role, there’s another factor that makes it so important to act today: the long lifetimes of greenhouse gases. How much of an effect does this have? Well, if we cut global emissions deeply enough, the amount of greenhouse gases in the atmosphere would begin to decline. (A couple decades later, temperatures would decline, too.) Unfortunately, it could take centuries for greenhouse gas concentrations to fall all the way back to today’s levels.
The bottom line is that, thanks to the ocean-induced time lag, for the next few decades we’re committed to additional warming. In addition, because greenhouse gases stay in the atmosphere for so long, every ton of global warming pollution we emit today will affect the climate for decades to come, even if we don’t see the effects immediately.
All the more reason to get started now!
Further Reading
* The mother of all climate science, the IPCC Synthesis Report.
* A much more technical explanation of climate time scales from RealClimate.