Continuing the holiday theme of drought/climate explorations, I found some more vegetation maps for ancient climate states, which I'd like to document here for future reference. First let me borrow a few temperature history graphs from Wikipedia to orient readers who may not be familiar with major features of the global temperature history. The first one shows the last 65 million years of estimated temperature:
here and it's highly recommended to read the article at the link for a flavor of the Herculean scientific labors that are involved in constructing a map like this.
The top is modern, and the bottom is Miocene. The striking thing about the Miocene is that there is basically no desert anywhere - in contrast to the Pliocene (discussed yesterday and Friday) where there are small deserts (by modern standards):
So the Miocene seems to have been both warmer and wetter than the Pliocene.
This next temperature graph shows the last 5.5 million years of estimated temperature history based on sediment cores.
You can see the peaks of interglacials and the troughs of the ice ages. I have marked the LGM=Last Glacial Maximum - the height of the most recent ice age about 18,000 years ago. So that was a climate much colder than today in which Canada, parts of the Northern US, Britain, and Scandinavia were all covered in permanent ice sheets. And the vegetation map looked as follows:
In these simplified maps the white is the grassland/scrubland/savannah that represents regions too dry to support forest but not so dry as to be deserts. Clearly the LGM climate was very dry compared to modern climates with much shrunken forests and massive deserts. If we focus in on the last 12000 years (what is known as the Holocene - the current interglacial during which agriculture and civilization have arisen) the temperature history looks as follows:
Note the temperature was a bit warmer in the early Holocene - about 8000 years ago was the "Holocene Climate Optimum" when the vegetation map was as follows:
Note the absence of a Sahara desert - the Holocene optimum was a wetter world than the modern one:
It would appear that it didn't take all that large a temperature increase to cause a weaker Hadley cell and result in far less desert at mid-latitudes.
Again it seems to me that it's a critical point to look at the temperature graph:
and note that the temperature is only changing by about 0.1oC - 0.2oC in a millenium. This is in contrast to the modern rate of about 1.6oC/century. We are moving the climate about two orders of magnitude faster than pre-industrial holocene change. In particular, with a turnover time of around a thousand years, the ocean in the mid-Holocene optimum was presumably far closer to equilibrium with the rest of the climate system than it is in the twenty-first century.