Just a few minutes ago Chappellaz et al presented the deepest dregs of greenhouse gas concentration data from the EPICA ice core in Antarctica, extending the data back to 800,000 years ago. In Al Gore’s movie you saw what was at that time the longest record of atmospheric greenhouse gas concentrations, back to 650 kyr, and their astonishing correlation with Antarctic temperature. This iconic superstar record has probably consumed as many eyeball-hours as any in climate science, alongside other classics such as the Jones et al. global temperature trends, the Moana Loa recent CO2 record, and the hockey stick. The Antarctic CO2 record has spawned countless internet rants about the CO2 lag behind temperature, and the circle of cause and effect between CO2 and climate. And the new data say …
The first point to write home about is that the correlation between Antarctic temperatures and CO2 continues unabated. One could imagine a world in which CO2 had no impact on climate, although if you buy that Dick Lindzen has a bridge he wants to sell you. In such a world, it could be that the correlation between CO2 and temperature since 650 kyr was just a coincidence, Mother Nature playing a cruel joke, and maybe in that case a little more data would cause the spurious correlation to start to unravel. That didn’t happen. CO2 continues to be high in warm times and low in cold times. There were no gasps of astonishment from the audience at the continued striking correlation. Ho-hum, of course it still works.
That being said, there are some interesting subtleties in the latest data. The CO2 concentrations were generally lower than average between 800 and 650 kyr. The lowest CO2 value ever measured in an ice core is now 172 ppm, from 667 kyr ago. If you average over the glacials and interglacials, there appears to be a very long-term cycle in atmospheric CO2, low from 600-800, peaking around 400 kyr ago in stage 11, the 50 kyr-long “super interglacial” when the Earth’s orbit was nearly circular as it is now, and then subsiding a bit since then. Perhaps this variability is driven by variations in rock weathering, the longest-term geological carbon cycle. Interestingly, there is no corresponding million-year cycle in Antarctic temperatures, when they are averaged in a similar way. CO2 is a dominant controller of global climate, but it is not the only game in town.
There were also millennial timescale wiggles in atmospheric CO2 and methane concentration during the descent from the stage 19 interglacial to the stage 18 glacial time. There are strikingly similar to the Dansgaard-Oeschger wiggles from 30-70 kyr ago in isotope stage 3. The duration is similar, about a thousand years, and the trajectories of the events are the same, with sharp warming and associated rise in greenhouse gases, then slower recovery. As in stage 3, they occur through the slow cooling transition between extreme climates, not in the full-blown glacial or interglacial states. These abrupt climate changes appear to be business-as-usual for the global climate system.
What would be really cool is if there were enough ice to continue probing back into the deeper past. Around 800 kyr ago, the climate cycles on Earth switched from being dominated by 40 kyr cycles, to the stronger 100 kyr cycles of the more recent times. The time period from 800 – 1000 kyr ago is called the mid-Pleistocene transition, and since the rhythms of the Earth’s orbit didn’t change, it must have something to do with the way that climate on Earth operates, maybe something about the carbon cycle. What would be really cool is if we could get CO2 and methane records back to say 2 million years ago, through this crucial transition time. Alas, 800 kyr is as far back as the useful ice in this location goes. The bottom tens of meters of the ice, like the bottoms of all ice cores, are too scrambled to interpret.