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.