Emit some CO₂? Its warming influence should peak in about a decade

The time frames of climate change challenge us in a number of ways. For one thing, it’s difficult to personally experience changes in climate in an obvious and reliable way. (Our sense and memories are a little less precise than thermometers.) And it’s hard to feel a sense of urgency about something changing gradually, especially when the benefits of dealing with it also accrue gradually. Imagine convincing a four-year-old that cleaning his or her room would be a worthwhile investment of Saturday mornings if you had to add “but it’s going to take a couple years before you’ll notice a difference.” You might as well be pitching the joys of early bedtimes and adventurous diets.

If you believe you won’t live to see the benefits of any cuts made in greenhouse gas emissions today, a selfish case for action is harder to make. So will any of us see any difference as a result of cutbacks in emissions?

A popular 2010 post on the SkepticalScience.com website approached this question from the opposite direction, presenting an estimate for how long it takes to realize the warming from emitted CO₂. Largely because the ocean takes up heat slowly, the full force of that greenhouse warming isn’t felt immediately. That post threw out 40 years as an estimate for the lag between emissions and perceptible warming, based on a 1985 paper’s calculation for the time to reach 60 percent of the long-term warming. However, that number was based on a scenario in which CO₂ increases and then stays constant.

That number was in line with the impression of most climate scientists, though. A couple years ago, for example, one scientist said, “It takes several decades for the climate system to fully respond to reductions in emissions. If we expect to see substantial benefits in the second half of this century, we had better get started now.”

In a new paper published in Environmental Research Letters (open access), Carnegie Institution for Science climate scientists Katharine Ricke and Ken Caldeira beg to differ with that scientist—who happened to be Ken Caldeira himself.

The paper investigates a simple question: if you emit some amount of CO₂ and then stop emitting, how long does it take before temperature stops rising? The researchers used simplified, mathematical characterizations of climate sensitivity, the ocean’s “thermal inertia,” and carbon cycle activity from model comparison projects. Mixing and matching those three variables from a number of models, they generated 6,000 estimates of the temperature response to a pulse of 100 gigatons of CO₂ carbon, tracking it over the course of a century.

From the 2010 CO₂ background of 389 parts per million in the atmosphere, that’s a sudden increase of about 47 parts per million. (CO₂ is currently increasing by around 2 parts per million each year.) Between the ocean absorbing atmospheric CO₂ and terrestrial plants taking some up, that 100 gigaton addition of carbon drops roughly 40 percent within first two decades. From there, the rate slows considerably, with an extra 20 percent or so removed by the end of a century.

The oceans gradually take up the heat energy trapped by the addition of greenhouse gas. But as CO₂ drops (remember, the researchers are assuming zero human-caused emissions after the pulse), that heat trapping weakens, and there comes a point when the Earth’s surface temperature stops rising. For 90 percent of the simulations, that occurred about seven to 31 years after the pulse of CO₂, with a median of just over a decade.

While temperature peaks pretty quickly, the planet cools off very slowly. At the end of a century, the median temperature was still 82 percent of its post-pulse peak.