Global Warming Preceded by Increasing Carbon Dioxide Concentrations during the Last Deglaciation

So says a paper published last week in Nature by Jeremy Shakun et al. The authors construct new temperature and CO2 series for the last deglaciation period between 22 and 6000 years ago. This is the key figure from the paper:



Clearly the rise in CO2 preceded the main rise in temperature at the end of the ice age. CO2 in the atmosphere also stabilizes before temperature does and most convincingly CO2 falls before the Younger Dryas cold period. I'm actually surprised by the length of the lag.

But the picture is confused by the different behavior of the northern and southern hemispheres. Temperature actually increased in the southern hemisphere before CO2 began to rise:



So based on this data alone perhaps both CO2 and northern hemisphere temperature were driven at different speeds by southern hemisphere temperature.* The authors address this issue with a simulation exercise using an NCAR model. They find they can reproduce the paleotemperature data quite well using greenhouse gases, orbital effects on insolation (amount of sunlight) in the simulation but not so well without the greenhouse gases. This suggests that greenhouse gases do amplify the warming effect due to increased insolation but that they don't initiate deglaciation. This model based evidence is usually considered stronger evidence than empirical time series evidence by climate scientists. But it is the kind of evidence that non-scientists are especially skeptical of.

* Our own paper in Nature in 1997 showed that southern hemisphere temperature Granger caused northern hemisphere temperature in the observational record.