When it comes to understanding the Earth's past climates, we have to understand what the global temperatures were. Instrument readings only go back to the 1800s, so researchers have had to rely on proxies—things we can measure, like tree ring width or oxygen isotopes, that reflect the weather conditions at the time. This has been used to track as far back as the end of the last glacial period.

Beyond that, records are sparse and local. Ice cores, for example, go back over 800,000 years, but these only capture polar conditions. Now, Stanford's Carolyn Snyder has put together the longest global climate record we have for recent times, extending back two million years from the present. The record captures a key transition in the glacial cycles that dominate recent climates.

Snyder also used this record to calculate the sensitivity of the climate to carbon dioxide, coming up with an eye-popping number that bodes very poorly for our future. Several other climate experts, however, suggest that the number Snyder calculated isn't especially relevant.

A long record

To get a sense of the global temperature, you must have a set of proxy data taken from a geographically diverse set of locations. For her reconstruction, Snyder took data from 59 different ocean sediment cores, which were used to calculate 20,000 individual ocean temperature data points. While that's a tremendous amount of data, it only translates to one reading per century over two million years. As a result, the temporal resolution of the reconstruction is only about 1,000 years. That'll smooth over any sharp, sudden changes (like the one we're currently experiencing), but it's enough to easily pick out the glacial cycles.

For times where other reconstructions are available, the new one matches up well. For example, when compared with polar climates derived from ice cores, the correlation between the two records is 0.72 (where 1.0 would be a perfect match). While not an exact match, you wouldn't expect it to be, as polar temperatures respond more strongly to climate changes than the global average.

That's critical for a major scientific question. Somewhere around a million years ago, the climate underwent a transition. Earlier, it was going through glacial cycles every 40,000 years, but it shifted to taking 100,000 years to cycle (this shift is termed the mid-Pleistocene transition).

Snyder's new record shows that the planet was getting slowly but progressively colder for the first million years or so. But by 1.2 million years ago, the cooling trend began to slow down. After it flattened out, the overall global average temperature has remained stable through to the present, even as glacial cycles caused lots of fluctuations around that average.

The analysis can't separate cause and effect, so there are a number of possibilities here. One is simply that some external cause changed both the overall trend and the length of the glacial cycles. It's possible, however, that it simply became too cold for the 40,000 year cycle to register.

If the paper had stopped there, everyone seems to agree that it would have been a valuable contribution to the field. But Snyder went on to analyze how global temperatures compared to atmospheric levels of carbon dioxide over that period, using that to calculate what she termed the climate's sensitivity to greenhouse gasses. The IPCC's best estimate is that we'd see about a 3°C increase if carbon dioxide were doubled, although that number is based on time scales of a few centuries. Snyder's calculation focuses on the final state of the climate, and it comes up with a value that's much, much higher—at 9°C per doubling. That would mean that our current emissions have already committed us to radical changes:

This result suggests that stabilization at today’s greenhouse gas levels may already commit Earth to an eventual total warming of 5 degrees Celsius (range 3 to 7 degrees Celsius, 95 per cent credible interval) over the next few millennia as ice sheets, vegetation, and atmospheric dust continue to respond to global warming.

Probably not

The good news is that's probably not right.

We talked to Gavin Schmidt, head of NASA's Goddard Institute for Space Studies, and Penn State's Richard Alley. Both agreed that the reconstruction itself was significant. Schmidt said it probably deserved to be in a high-profile publication, and Alley said it "may prove to be of wide value." Still, both of them pointed out issues with the sensitivity analysis.

The problem is that glacial cycles are triggered by changes in sunlight caused by orbital forcings. This triggers changes in the amount of carbon dioxide in the atmosphere, but it also triggers changes in everything from ocean levels and atmospheric dust to the amount of sunlight that hits highly reflective ice when it reaches the Earth. All of this affects the global temperature significantly. But, for the purposes of this analysis, Snyder only compared the temperature and CO 2 , ensuring that all these other impacts were ascribed to that gas.

"This assumes that all of the temperature change over the ice-age cycles arose from the greenhouse-gas change," Alley told Ars. "But, we have high confidence that the ice ages were driven by features of Earth’s orbit and that the temperature would have changed (just not as much) if the greenhouse-gas forcing had not changed." Alley went on to say that "the 'sensitivity' calculated in the new paper is an upper limit, because we know that some of the temperature change was not caused by greenhouse gases."

Schmidt, for his part, focused on the consequences of what would happen if this sensitivity were right. "If I move the CO 2 levels today to glacial levels—180 [parts-per-million]—would I get an ice sheet the size of what we had at the height of the last glacial maximum?" he asked. "And the answer has to be no. The forcing is too small in the places where the ice would grow for that to happen."

So, most of the paper is solid, telling us new things about the recent history of our planet. But there appears to be some significant issues with one aspect of the analysis: the part that attempts to calculate how sensitive our climate is to rising greenhouse gasses. Unfortunately, that's the aspect that's most relevant to non-scientists, and it's the detail likely to attract the most attention.

Nature, 2016. DOI: 10.1038/nature19798 (About DOIs).