The internet is abuzz with a new climate study that seemed to deliver a bombshell conclusion: The amount of carbon humans have put into the atmosphere may have already committed the planet three to seven degrees Celsius of global warming. Image: Micolo J/Flickr
Three to seven degrees Celsius is a big amount of warming — an apocalyptically big amount. The Paris climate accord, adopted less than one year ago by nearly 200 nations, resolved to slash global carbon emissions so that we don't exceed two degrees Celsius of warming overall. If we've already blown that target, not only is humanity's landmark climate agreement depressingly obsolete, coral reefs, low-lying island nations and many of our planet's coastal cities are doomed.
Here's the good news: Prominent experts are calling this a load of malarkey. "This is simply wrong," Gavin Schmidt, director of NASA's Goddard Institute for Space Studies, told Gizmodo. "The actual committed warming is only 0.5 to perhaps 1 [degree Celsius] — and nothing in the study changes that."
The new study, which appears today in Nature, is a reconstruction of Earth's average surface temperature over the past two million years. To produce it, Stanford's Carolyn Snyder assembled dozens of sea surface temperature records that researchers around the world developed from ocean sediment cores. Doing so was no easy feat: "The communities that produce these data are notoriously cagey about releasing it," Schmidt said, adding that Snyder "did a big amount of work putting together disparate data" in what will no doubt prove a valuable resource for the climate science community.
And yet, "Scientist Compiles Dozens of Datasets to Reconstruct Two Million Years of Global Average Surface Temperatures" is hardly a sexy headline. The study might have flown under the public radar, except for a section toward the end, where Snyder used her new dataset to estimate the sensitivity of Earth's surface temperature to levels of carbon dioxide in the atmosphere.
When Synder compared her temperature record with a record of atmospheric CO2 over the last two million years, she found a strongly positive, linear relationship between the two. From the slope of that relationship, she determined that a doubling of atmospheric CO2 levels translates to a nine degree Celsius change in global average surface temperatures, give or take four degrees. Snyder dubbed this amount the "Earth system sensitivity" (ESS).
"This research follows previous work in the literature that defined the correlation relationship between global temperature and greenhouse gas radiative forcing changes as ESS or S[ghg] as a way to summarise patterns in the Earth's past climate," Snyder told Gizmodo.
Here's where things get hairy. Since humans have increased global CO2 levels roughly 40 per cent since the start of the industrial revolution, Snyder decided to use her correlation from the past to project forward in time: "This [Earth system sensitivity] result suggests that stabilisation at today's greenhouse gas levels may already commit Earth to an eventual total warming of 5 degrees Celsius," she writes in the study's abstract.
Using a past relationship to say something about the future is not illogical. The problem lies in the details of the complex relationship between climate and CO2.
We know that as atmospheric CO2 levels have risen and fallen throughout Earth's history, the planet's temperature has changed. We know that when temperatures change, ice sheets to retreat and advance. Animals and plants flourish and die off; the amount of dust in the atmosphere rises and falls. All of these changes, which climate scientists call "Earth-system feedbacks" trigger more climate change, further altering the level of CO2 in the atmosphere.
"You have this chicken and egg situation, where ice changes, which causes CO2 to change, which causes ice to change, and so on and so forth," Schmidt said. When you do a correlation between a temperature and CO2 over a long enough timescale, all of these feedbacks get smeared together. As Schmidt put it, "you're mixing the impact of CO2 on climate, and climate on CO2."
In other words, the "Earth system sensitivity" cannot be interpreted as the direct impact of atmospheric CO2 on global temperatures, as some media outlets are now reporting. In principle, CO2 doesn't have to impact climate at all in order to see a positive correlation.
"Simply correlating the temperature with the CO2 level would tell you climate sensitivity only if you knew that CO2 changes explained the vast majority of the temperature changes," Ken Caldeira, a climatologist at the Carnegie Institution for Science told Gizmodo. "However, we don't know that. So, the climate sensitivity numbers reported in this paper should be regarded as an upper bound on possible 'Earth System' climate sensitivity. Real 'Earth System' climate sensitivity is likely substantially lower."
In all fairness, Snyder said that it was not her intention to make a prediction about how human carbon emissions will impact future climate. "This research cannot and does not provide a forecast or prediction for future climate change," she said. "All we can say is, if we take the past relationship [between temperature and CO2] and translate it forward, this is what we get."
Unfortunately, the nuances and criticisms I've outlined above weren't made clear enough in the study's abstract, nor did they make it into a Nature press release issued under embargo last week, which repackaged what was a minor point of the new research into a dramatic punchline.
The planet is definitely warming, and human carbon emissions are definitely to blame. But seven degrees or more of guaranteed future heat? We still have time to prevent that.