Tracking Temperature for a Better Look at Climate Change

By Alyson Kenward

All of the research presentations and news stories I’ve seen on climate change seem to be framed in terms of greenhouse gas (GHG) emissions and carbon dioxide (CO2) levels. Emissions levels have no doubt been the benchmarks of climate change: we’ve been marking increasing concentrations of atmospheric CO2 like we record a growing child’s height up the kitchen wall — albeit with less paternal pride.

But with the release last week of a new climate stabilization report from the prestigious National Research Council, there seems to be a new way to tell the climate change story: by defining it in terms of temperature change.

“The bottom line is that you could really understand climate impact more easily if you frame it in terms of number of degrees of global warming,” said Susan Solomon, who chaired the report titled Climate Stabilization Targets: Emissions, Concentrations and Impacts over Decades to Millennia, during a press conference on May 16, 2010.

It may seem obvious to think of global warming in terms of temperature change, but it hasn’t always been scientifically easy to do so. While measuring atmospheric CO2 levels is pretty easy, tracking global temperature change is trickier — particularly because there is a lag between the rise in GHGs and when the temperature begins to change noticeably. But drawing upon decades of research, including a lot of recent work, this new report concisely outlines the current thinking about the relationship between GHG emissions and global temperatures.

And with an idea of how temperature will vary if we can hold atmospheric CO2 concentrations at specific levels — for example, stabilizing emissions at 430 parts per million (ppm) will lead to a two degree Celsius rise in temperature, and at 540 ppm we can expect three degrees — it’s possible to estimate what might happen to the planet.

Because while it is the increasing concentration of greenhouse gases in the atmosphere (these days we’re up to about 390 ppm) that is causing the temperature to rise, it is really that higher temperature that is going have the most significant impact on the planet.

“The impacts don’t care how much CO2 is in the atmosphere or what time it is [in history],” said Solomon. “They only care about what the temperature is.” However, Solomon acknowledged that CO2 can affect changes in addition to temperatures, such as ocean acidification, but said that the majority of the risks are temperature-related.

The shift from using CO2 emissions to temperature increase as a barometer for climate change does more than just easily define future impacts. According to the report, describing global warming in terms of temperature change acknowledges the incredibly long-lasting impact that these increasing emissions will have.

Even if emissions are stabilized at, say, 540 ppm — and these days that seems to be a daunting “if” — the warming effects of that CO2 are going to persist for many centuries. Think of it as a time-delay for temperature. The thermometer rises somewhat in real time as emissions increase, which is the kind of warming we are currently observing. But if emissions level off, the temperature will keep escalating until the planet manages to equilibrate itself and learn to dispose of all that excess CO2.

Taking advantage of an apt but rarely applied analogy, the new report likens this phenomenon to a bathtub filling with water faster than it can empty (check out this nifty National Geographic rendition of this metaphor). In a similar way, humans are pumping CO2 into the atmosphere quicker than the overwhelmed planet can absorb it. In the event that emissions level off or, better yet, begin to decline, the planet will still be coping with the effects of the CO2 surplus as it slowly drains away over hundreds of years.

The crux of the report, 232-pages strong and chock-full of climate and risk analysis, is that significant and immediate emissions restrictions will need to be adopted in order to minimize the effects of global warming. Though the report plays through the consequences of settling on a number of different emissions targets, it is cautious about not saying exactly what level to aim for.

“How much risk is acceptable? Well that’s a value judgment we didn’t make,” said Solomon, explaining that the report adheres to identifying only what we are bound to expect depending on how we choose to proceed.

That being said, the report does draw a vivid picture that argues in favor of pursuing ambitious emissions cuts. The water level in the tub is rising. Quickly. Might it be time to turn off the faucet?