Pulse of the Planet: Can’t Turn Down the Heat

Imagine you’re in the shower and you turn up the hot water, but it gets too hot – scalding hot. What do you do? Turn the hot water down, of course. But what if you can’t? And what if you can’t even get out of the shower? You’d probably regret having turned up the hot water tap. A new paper shows that that kind of scenario is what we're facing in the case of global warming.

Since preindustrial times, carbon dioxide (CO₂) concentrations have increased from about 280 parts per million (ppm) to a little more than 385 ppm. Scientists have shown that much of this increase has resulted from the burning of fossil fuels. How have they reached this conclusion? In a variety of ways. Isotopes are one.

What Carbon-13 Tells Us About Atmospheric CO₂

Remember protons and neutrons? It’s the stuff the centers (or nuclei) of atoms are made of. The number of protons determines what kind of atom it is. For example, carbon has six protons. Any atom with six protons is carbon, regardless of the number of neutrons. Almost 99 percent of the carbon on Earth is carbon-12 – it has six protons and six neutrons. About one percent is carbon-13 with six protons and seven neutrons.

The carbon in fossil fuels is made up of less C-13 than the CO₂ in the atmosphere. As a result, one would expect that as fossil fuels are burned and the C-13-poor carbon in fossil fuels is converted into atmospheric CO₂, the C-13 abundance in atmospheric CO₂ would decline. And that is in fact exactly what is observed (see graphic). The conclusion: burning fossil fuels is increasing atmospheric CO₂ concentrations.

Changes in the relative abundance of C-13 in the atmosphere as reported by Oak Ridge National Lab’s Carbon Dioxide Information Analysis Center. Note: the more negative the numbers plotted on the left, the lower the relative abundance of C-13.

 It is now well-established that the increase in atmospheric CO₂ has already begun to disrupt the climate. In addition to rising average global temperatures, we are seeing melting glaciers, as well as melting sea ice, worrisome trends in rainfall with increasing variability favoring floods and droughts, rising sea levels, more wildfires, and dying forests.

At the same time this is occurring, global emissions of CO₂ have been increasing by about two percent a year. (That rate has undoubtedly slowed or maybe even reversed as a result of the global economic downturn, but will probably resume once the economy gets back in gear.) Increasing emissions lead to increasing CO₂ concentration and more climate disruption.

How to Fix the Problem?

What’s to be done? One approach is the one we often take in the shower. We turn up the hot water until it gets too hot and then turn it back down. In the case of climate change that would translate into continuing to emit CO₂ until the disruption gets so bad that we then just turn off the spigot – that is, we stop emitting CO₂ and let the climate cool. Right? Wrong. As shown by Susan Solomon of NOAA’s Earth Systems Research Laboratory and her colleagues in the Proceedings of the National Academy of Sciences, it will take more than 1,000 years for the climate to cool back to where we started in preindustrial times.

I have known Susan Solomon since the 1970s when she was a graduate student working with Paul Crutzen, winner of the 1995 Nobel Prize in Chemistry for his work on stratospheric ozone depletion.  In the 1980s Susan traveled to the Antarctic where she made the critical measurements showing that the Antarctic ozone hole was caused by reactions involving chlorine atoms from the breakdown of chorofluorocarbons. For that work, Susan was cited in the 1995 Nobel Prize presentation speech to Sherwood Rowland, Mario Molina, and Crutzen.

More recently Susan was the chairman of the Intergovernmental Panel on Climate Change’s Working Group 1 – the team charged with assessing the state of the science of the physical climate system. The group’s work was an arduous process: establishing a consensus among a broad spectrum of scientists and then, once the report was written to reflect that consensus, responding to literally hundreds of comments from peer reviewers.

Global Warming Is Forever

Now, Susan along with her colleagues has added to her impressive record with this new work showing in stark terms the consequences of waiting to act on climate change - for all intents and purposes, global warming is “forever.” There are two reasons and both arise from the slow mixing of the surface ocean with the deep ocean:

1. When CO₂ is emitted into the atmosphere, about half of it is almost immediately removed and added to the surface ocean and to forests. The remaining half, called the airborne fraction, stays in the atmosphere for many hundreds of years awaiting the very slow transfer to the surface ocean and then into the deep ocean. So even if we stopped emitting CO₂ today, it would take centuries on centuries (about 1,000 years), before CO₂ concentrations would approach that of preindustrial times.
2. One might expect that as CO₂ decreases (albeit slowly) from a cessation of CO₂ emissions, at least the climate would slowly cool, but it doesn’t. Some of the warming from the increased CO₂ has been stored in the ocean, and as CO₂ decreases this excess heat is slowly released from the ocean keeping temperatures elevated for a thousand years or so.

A thousand years is a long time to wait to cool down. Do we really want to keep turning up the hot water?