This story has been updated.
Sometimes, on environmental issues, we make things too complicated. Global warming is an example. We go on and on about emissions targets and thresholds and offsets — when really, it’s all about something much simpler: heat.
But burning fossil fuels also gives off carbon dioxide, which makes its way into the atmosphere and gives us heat in a different way — through the greenhouse effect, in which this gas traps infrared heat radiation that would otherwise escape the planet to space. Instead, that heat sticks around and makes Earth a little bit hotter. Moreover, carbon dioxide stays in the atmosphere and keeps on having this effect for a very long time — for a given atmospheric pulse of carbon dioxide, some portion of it will still be there in a thousand years.
Now, in a recent study in Geophysical Research Letters, Xiaochun Zhang and Ken Caldeira of the Carnegie Institution for Science in Stanford, Calif., compare these two kinds of heating that result from the burning of fossil fuels. And they show that it only takes a relatively brief period of time for the long-term heating caused by the atmospheric carbon dioxide to exceed the short-term heating that we get from burning the fossil fuel in the first place.
“The total amount of heat released upon burning a lump of coal is dwarfed by the total amount of energy trapped in the atmosphere by the CO2 that burning releases,” Caldeira said by e-mail. (Here’s a video that he made to explain the result further.) Moreover, the difference is not small. As Caldeira put it in the video: “Over its lifetime in the atmosphere, if you burn a lump of coal, the greenhouse warming exceeds the direct thermal warming by a factor of 100,000 or more.”
A key reason the differential is so large has to do with the notoriously long atmospheric lifetime of carbon dioxide. “The fossil fuel is burned in an instant, but some of the CO2 remains in the atmosphere for many thousands of years,” note Zhang and Caldeira in their paper.
A recent paper unpacking this in more detail, by University of Chicago geoscientist Raymond Pierrehumbert, reaches a staggering conclusion. It considers a gigantic hypothetical pulse of carbon dioxide large enough to increase atmospheric concentrations to 1250 parts per million over pre-industrial levels (which were 280 parts per million). In 1,000 years, Pierrehumbert finds, concentrations will still be 675 parts per million. The stuff stays up there for many, many human lifetimes. This is why some have suggested that substantial part of global warming is “irreversible.”
Granted, coal, oil and natural gas are all different. The new study by Zhang and Caldeira found that in about 34 days, the warming caused by the greenhouse effect of burning a given quantity of coal has exceeded the heat given off in that burning. For oil, that number was 45 days, and for natural gas, 59 days.
“This won’t come as a surprise to climate scientists, but there’s just never been a peer-reviewed citation for these kinds of numbers, so it’s just good to get it into the literature,” Caldeira said.**
I asked a few scientists to comment on the new paper by Zhang and Caldeira to make sure I was properly understanding its implications. Atmospheric scientist Kerry Emanuel of MIT noted that the paper is “just showing that the heat energy trapped by greenhouse gases exceeds that directly produced by fossil fuel combustion after only a very short time.” However, he added, “if greenhouse gas warming were less by a factor of, say, 10, then the heat trapped would still vastly exceed the heat of combustion after a short time, but we would be less worried about the consequences.”
Michael Mann, a climate researcher at Penn State, made a similar point: “In one case we’re talking about net energy released to do work/generate power/heat, etc. In the other case we’re talking about a redistribution of heat within the atmosphere that isn’t simply related to the former quantity — it is a subtle feature of the particular pollutant being released.” For this reason, Mann wondered about “what that comparison tells us physically.”
Maybe nothing super profound from a physics standpoint. Nonetheless, the comparison between how much energy we get from fossil fuels, and how much heating they leave behind once they’re burned, is hard to forget.
Granted, we shouldn’t neglect what we owe to fossil fuels — a vast amount. And those industrious humans who, in the 1800s, first started the mass burning of coal, driving up atmospheric carbon-dioxide concentrations, could hardly be expected to have known about numbers like these.
We’re still wrapping our minds around them ourselves, and it’s 2015.
** After publication of this article, we became aware that a prior paper, published in 2014 in the journal Fuel by Roger Sathre, actually did perform a similar analysis of the direct heat produced by burning fossil fuels, compared with the greenhouse warming consequences of burning them. Ken Caldeira acknowledged that his literature search missed this prior paper.