Tuesday, February 14, 2012

South Asian Paleoclimate Documented

How was the South Asian paleoclimate data gathered?

A new study provides about ten thousand years of continuous climate data for South Asia.

This research, lead by researchers from the Woods Hole Oceanographic Institute, analyzed evidence from a Bay of Bengal sediment core, which captures discharges from the large Godavari river system. The core data comes from carbon isotopes of leaf waxes, reflecting the amount of arid-adapted/savannah vegetation in the Godavari catchment, and oxygen isotopes from a marine microfossil that record salinity. This points to a general aridification trend over the course of the middle and late Holocene, supporting what we already would infer from pollen data in Rajasthan or monsoon proxies in the Arabian Sea, but this time providing more direct evidence from South India.

Given the strong role of seasonal monsoons in the climate of South India that is lacking in the Mediterranean basin and West Eurasia, it wouldn't have been surprising if South India's climate was quite distinct, even at the level of general trends, from the climate of places further to the West.

What does the study tell us about South Asian pre-history?

The two particularly big events in South Asian pre-history that one would like to establish the role of paleoclimate in motivating are the collapse of Harappan civilization in the Indus River Valley that preceded the rise of Indo-Aryan expansion from the same vicinity (ca. 1500 BCE, give or take), and the rise of agriculture in South India (ca. 2500 BCE, give or take), which is likely to be associated with the rise of the culture that led to the expansion of the Dravidian languages. Towards this end, the conclusion of the study states that:

The significant aridification recorded after ca. 4,000 years ago may have spurred the widespread adoption of sedentary agriculture in central and south India capable of providing surplus food in a less secure hydroclimate. Archaeological site numbers and the summed probability distributions of calibrated radiocarbon dates from archaeological sites, which serve as proxies of agricultural population, increase markedly after 4,000 BP in peninsular India. . .

In contrast, the same process of drying elicited the opposite response in the already arid northwestern region of the subcontinent along the Indus River. From 3,900 to 3,200 years BP, the urban Harappan civilization entered a phase of protracted collapse. Late Harrapan rural settlements became instead more numerous in the rainier regions at the foothills of the Himalaya and in the Ganges watershed.

There are some indications supported by Rig Vedic legend, archaeological ruin distribution, and satellite imagery that the event in the Indus River watershed may have concluded more dramatically, with one of the major tributaries to the Indus River shifting to a new course and leaving a huge expanse of the old Hakra river basin that was once at the heart of Harappan civilization suddenly dry in what is now the Cholistan Desert. For example, a recent search of sites in this desert found a Indus Valley Civilization seal from golden age of that culture.

The dates mentioned by the paper (ca. 2000 BCE for the South Indian Neolithic and ca. 1700-1200 BCE for the demise of Harappan civilization) are a bit later than the dates I have seen based on the oldest known remains in each region. I've seen estimates dating the oldest South Indian Neolithic sites to 2500 BCE, and there are traces of Indo-Aryan influence in the Cemetery H culture back to about 2000 BCE in the Northwest India/Northern Pakistan, and 1900 BCE is frequently used as an end point for Harappan civilization. But, the differences aren't huge and seem to be fairly consistent in magnitude and direction. They could simply reflect dates that are trying to capture the very earliest points at which a new civilization appears and the point in time at which it really starts to thrive.

Were there major droughts in South India, or was the trend a gradual one?

One curious point that isn't mentioned in the blog post on this study (but may be hidden in the data) is that in the Fertile Crescent region there were two very distinct severe droughts in the same general time period that punctuated the general trend towards aridity in the Holocene there.

One was happened at roughly 2000 BCE and is associated with the collapse of the Akkadian Empire in Mesopotamia, the First Intermediate Period in Egypt, and the beginning of the rise of Hittite power in Anatolia. The other was around roughly 1200 BCE is associated with the sweep of significant historical events across the ancient West Eurasian world known as Bronze Age Collapse. It remains unclear if these droughts extended as far east as South Asia, and this data set would be the obvious place to look for these punctuated period of drought as opposed to a more gradual trend towards aridity.

We also know from North American pre-historic paleoclimate correlations with archaeology that prolonged droughts are the sorts of things that can cause civilizations to fall and lead to dramatic upheavals in human affairs.

The answer to this question calls for a closer analysis of the data than I can do right now, and may require access to non-open access sources. Insights in the comments on this point would be greatly appreciated.

An Early Human Role In Climate Change?

The data from South India are also relevant to the extent to which Holocene climate change may have been driven by human activity. The new data tend to argue for human activity changes as an effect rather than a cause of early Neolithic era climate change.

The trend towards aridity in the Holocene corresponds with the rise of the Neolithic era when humans started farming and herding, activities that had significant ecological effects locally. But, the question of cause and effect arises.

Did farming and herding arise and start to become more important out of necessity because increased aridity made hunting and gathering lifestyles less viable, and perhaps favored agriculture in other ways, for example, by producing fewer deluge storms that could destroy crops)?

Or, did farming and herding disrupt ecosystems that were critical to maintaining regional homeostasis and stabilizing weather patterns, for example, by preventing soil from turning to dust and entering the atmosphere. This would be early human activity driven climate change, and in West Eurasia, where the prevailing winds can carry Fertile Crescent climate effects across the rest of the region, a causal connection isn't implausible.

But, South India developed agriculture about four or five thousand years after it appeared in the Indus River Valley and five or six thousand years after it arose in the Fertile Crescent, and aridity in South India is mostly driven by monsoon rains from the Southeast, so the human activity in the Fertile Crescent and Indus River Valley shouldn't have much impact on its aridity, and agricultural activity in China (both North and South) would have been sufficiently remote that it would be surprising to see early agriculture there influencing climate in South India whose whether patterns are more tied to conditions in Indonesia and Southeast Asia than to China.

If South India was seeing the same kinds of climate change trend towards aridity as West Eurasia at about the same times, the case that the rising trend towards aridity in West Eurasia was the product of human activity is weakened as a hypothesis.

Note: Minor updates to language and links added on February 20, 2012.

No comments: