Fascinating piece at www.sciencedaily.com/releases/2012/03/120316145802.htm … a paper in Geophysical Research Letters, 'Holocene Aridification of India' is based on a sediment core extracted from the Bay of Bengal where the Gondavari River drains the central Indian peninsular over which monsoon wind carry most of the precipitation. A 10,000 year reconstruction of climate in central India came about as a result of this core (close to a river mouth where it accumulates sediment very fast). Basically, in most of the Holocene era the climate of India was warm and tropical – and monsoon rains drenced the interior. As with Arabia and the Sahara changes in the track of Inter Tropical Convergence Zone (a band of heavy rain) and the monsoon wind system, began to falter in mid to late Holocene and by 2500-2000BC a distinct phase of aridity had set in. After 1700 years ago, AD300 roughly, the flora had switched entirely to arid adapted plants which implies that central India became even more drier. This is interesting as it coincides with increased aridity in North Africa in the Late Roman era. This situation has persisted, mostly so, down to present day. However, drought was a specific commonality in the 19th century – and less so in the 20th century (and the beginning of the 21st).
The authors therefore make the likely interpretation that it is warming in the 20th century that has brought this happy state about – particularly warm in the 1930s/early 40s and again in the 1980s/1990s (and still currently at a high level). This research more or less confirms a lot of earlier research on the subject, the Little Ice Age being a catalylyst for drought caused by a movement southwards of monsoon winds – and subsequent and preceding bouts of aridity that coincide with cold and wet weather in NW Europe. A similar situation prevailed across the Americas, cities being abandoned when the Inter Tropical Convergence Zone shifted (in the Maya lands for example). Even the Galapagos Islands have historically been affected by a similar shift in this tropical band of heavy rainfall. What intrigues me with this research is that a switch is catalogued in the 3rd century – and a few days ago, in a post on the dark ages in northern Europe we found that agriculture was forced to adapted at this time to a different cooler and wetter climate regime. This coincided with the 3rd century – not what is normally regarded as the dark ages, the 5th and 6th centuries. It happened long before the narrow growth tree ring event of Mike Baillie, at AD536-45. So how will India fare in the next couple of decades where global temperatures, it has been suggested, will fall – as a result of a less active Sun. Indeed, the authors say explicitly that the Sun has played a role by saying solar insolation affects the Inter Tropical Convergence Zone. Piers Corbyn has been saying for years that the Sun affects the Jet Streams, the bands of heavy rain that criss cross the earth nearer the Poles. Time will tell. Firstly, actual global temperatures have to fall a bit more dramatically than the recent plateau of the 2000s.