I have been continuing my attempts to understand why trends and predictions for global drying under global warming are in the opposite direction from the Pliocene climate. I have had a little email back and forth with Aiguo Dai (author of many of the PDSI papers I have been referring to in this series of blog posts) and he reminds me of the importance of the Hadley cell in driving the desert areas of the globe in this map from yesterday:
The Hadley cell is a name for the main climatic mechanism operating between the tropics and about 30o north and south of the equator:
figure from the Wiki:
Here the blue is ascending air velocity and the pink is descending.
Now, since the Pliocene climate apparently involved much less desert:
it seems that it must have had a weaker Hadley circulation. Why so? The best story I've been able to find is in Brierley et al Greatly Expanded Tropical Warm Pool and Weakened Hadley Circulation in the Early Pliocene. This 2009 Science paper argues based on evidence from fossil fauna that the SST distribution in the Pliocene involved warm surface temperatures 15o further north and south than at present:
The main point here is the difference between the soft grey line (postulated Pliocene) and modern (thin solid black line). Note the huge shift - we are talking about 10o-15o of latitude so in the ballpark of a thousand miles in both directions. Obviously this chart relies on a rather small number of data points and the need to translate from the Atlantic speaks volumes - so it's probably indicative rather than dispositive.
Anyway - when they put this much larger tropical warm pool into an atmosphere-only climate simulation the Hadley circulation decreases substantially:
The Northern Hemisphere branch of the Hadley circulation weakens on average by roughly 30%. During boreal winter, the reduction reaches nearly 40%. The center of the Hadley cell moves northward by ~7°, whereas the cell’s latitudinal extent (26) increases by 3° to 4°. These changes of the Northern Hemisphere circulation are a robust response to changes in meridional SST gradient as evinced by additional sensitivity calculations (table S2). The volume transport of the circulation’s southern branch nearly halves, making the southern Hadley cell even weaker than the northern cell. Whether this is a genuine feature of the early Pliocene climate remains to be seen, because there are only a few SST data points currently available in the Southern HemisphereThis makes sense as much warmer water at the bottom (descending) leg of the Hadley cell is going to reduce the temperature difference between northern and southern ends of the cell that drives the circulation. And in their simulations, this indeed shows up as fewer subtropical dry zones:
So again this points me in the direction of thinking about the difference in response times here - the ocean takes around a thousand years to completely turn over so presumably it's going to take around that amount of time before it has fully responded to the changes humans are making in the atmosphere - the present deep ocean water knows nothing about what we've done and it will presumably continue to well up into (and thus cool) the surface ocean for hundreds of years to come. Look at this last map which comes from NASA GISS and shows the estimated temperature change between 1880-1900 and 2000-2010:
Clearly the oceans have warmed much less than the land - and a lot of the eastern Pacific has barely warmed at all (presumably because this is the site of a lot of upwelling of deep old ocean water).
Thus even if we've created a Pliocene concentration of carbon dioxide in the atmosphere, it will be a long time before we have a Pliocene ocean and thus a Pliocene weaker Hadley cell and gentler distribution of wet and dry (assuming this provisional picture of the Pliocene climate turns out to be right).