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.
Makarieva A.M., Gorshkov V.G., Li B.-L. Revisiting forest impact on atmospheric water vapor transport and precipitation. Theoretical and Applied Climatology, doi: 10.1007/s00704-012-0643-9.
Water cycle on land owes itself to the atmospheric moisture transport from the ocean. Properties of the aerial rivers that ensure the “run-in” of water vapor inland to compensate for the gravitational “run-off” of liquid water from land to the ocean are of direct relevance for the regional water availability. The biotic pump concept clarifies why the moist aerial rivers flow readily from ocean to land when the latter gives home to a large forest — and why they are reluctant to do so when the forest is absent.
Compared to our previous studies, in the new paper we used a global (rather than land only) precipitation database that allows one to compare precipitation patterns on land to those over the adjacent ocean. We extended our previous approach to analyze seasonal (rather than annual only) changes in the spatial precipitation patterns in world’s major forest regions. Apart from the tropical rainforests, we analyzed precipitation distribution across world’s longest (>7,000 km) forest belt, the Eurasian boreal forest. The data describe how the active summer forest wins the water “tug-of-war” with the Atlantic Ocean. Indeed, in summer the forest steals most moisture inland and depletes the oceanic precipitation. The dormant winter forest loses this war to the ocean, such that precipitation over the Atlantic Ocean in winter, despite the oceanic evaporation is minimal, rises threefold compared to summer months. Analyzed for comparison, the unforested Australia is unable to draw moisture far inland in either wet or dry season, i.e., irrespective of moisture availability over the neighboring ocean.
While it is increasingly common to blame global change for any regional water cycle disruption, the biotic pump evidence suggests that the burden of responsibility rather rests with the regional land use practices. On large areas on both sides of the Atlantic Ocean, temperate and boreal forests are intensely harvested for timber and biofuel. These forests are artificially maintained in the early successional stages and are never allowed to recover to the natural climax state. The water regulation potential of such forests is low, while their susceptibility to fires and pests is high. The exploited forests are degrading; the relatively undisturbed old-growth forests are shrinking in size. A conflict (rarely appreciated or discussed) exists between the modern commercial value of a forest and the forest’s ability to regulate the regional water cycle and to be self-sustainable: these parameters cannot be maximized simultaneously. Therefore, the regional water safety is not about keeping the live forest biomass stationary. It is about keeping it stationary in an environmentally competent condition.
You are welcome to visit http://www.bioticregulation.ru/ab.php?id=taac to download the paper, the Electronic Appendix, as well as to have a look at some animated graphs.
Full text with editorial summary:
Jeremy Hance mongabay.com (February 01, 2012).
New meteorological theory argues that the world’s forests are rainmakers.
- 1.>> Will you tell us how the biotic pump works?
- 2.>> Why do you associate the biotic pump with natural forests rather than with individual tree species? Cannot a tree plantation act as biotic pump?
- 3.>> Have there been any significant changes to your biotic pump theory over the last couple of years?
- 4.>> Have you seen wider acceptance in the scientific community for your theory?
- 5.>> Can you give an example of why the current understanding of condensation and precipitation is wrong?
- 6.>> Recent evidence has linked the decline and fall of the Maya civilization to deforestation leading to less precipitation. How could the biotic pump theory connect to this?
- 7.>> How do you see deforestation in the Amazon as impacting regional precipitation?
- 8.>> How do you think widespread deforestation will effect the hydrological cycle of places like the Indonesian islands? Given their smaller size, do they need the biotic pump?
- 9.>> Does the biotic pump theory apply to boreal forests, such as those in Russia, as well?
- 10.>> Does biotic pump theory modify our current understanding of global climate change?
- 11.>> What policy changes does the biotic theory suggest for governments worldwide?
The condensation-induced atmospheric dynamics posits condensation rather than a temperature gradient to be the driver of winds (see here for a more detailed discussion). The conventional explanation holds it rains during the wet season because land is warmer than the ocean and the moist air comes from the ocean to ascend over land (because the warm air rises). When moist air rises, it rains. It does not rain during the dry season because the land is colder and the cold air does not ascend.
Let us now look at this excerpt discussing the causes of floods in Thailand:
Other than looking at precipitation, one way to determine the difference between the wet and dry season is to look at air and water temperatures relative to each other. The dry season is characterized by cooler temperatures over the land, and warmer temperatures over the ocean. GLOBE schools in this region could verify this by examining both the Max/Min/Current Air Temperature protocol as well as finding a data source that provides sea surface temperatures from ships or buoys. Because of these temperature differences, a gradient forms, and the winds will blow from the Northeast, bringing cooler air in from China.
In other words, one thinks it is dry because winds do not come from the warm ocean to the cold land. In the meantime,
In early 2011, an unusual cold wave hit the region, and temperatures were below average for a few days. This caused the normal temperature gradient seen in the dry season to be even more pronounced. While this doesn’t sound like something that would cause major flooding, more convection formed due to the different amounts of heating and caused this devastating flooding.
In other words, it became even colder over land, but now it suddenly rained heavily.
Food for thought.
See also “Why the heat?” for an opposite case, when the land became abnormally hot, but not a single raindrop fell down (the 2010 heat wave in Russia).