New record extends global temperatures back two million years
<![CDATA[By John Timmer
Sees major transition at 1.2 million years, questionably high climate sensitivity.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 recordTo 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.