Pederson et al. 2012 investigated the recurrence of droughts in some parts of the U.S. starting from the end of the 17th century. The study was motivated by the recent increase in drought frequency — the researchers attempted to look at it from a historical perspective. It was concluded that the period from approx. 1700 to 1820 saw particularly many drought events, while the 20th century as a whole was more comfortable.
It is instructive to compare this result with the dynamics of forest cover change in some states:
Fig. 3 of Fitzjarrald et al. 2001 Journal of Climate 14: 598. Dynamics of forest cover in some states.
One can see that starting from 1700 and until approximately the middle of the 19th century forest cover in the U.S. was being eradicated most rapidly. Then forests started to regrow and had been increasing in size for more than a century. Data for the last twenty years are missing, but one can notice a declining trend in forest cover to be there again starting from the 1990s. And during the last twenty years the droughts came back.
This is consistent with the biotic pump concept. Degradation of forest cover in Eurasia and North America make the two continents drier. As the remaining forests compete for moisture of the Atlantic, the growing drought zone migrates from one continent to another. For example, when in 2010 there was a disastrous drought in Russia, in the U.S., on the contrary, the percentage of land experiencing drought was exceptionally low.
It would be interestig to analyze similar data for Russian droughts and Russian forests.
Stormy Science 