Cold signatures in wood

BETWEEN 1400 AD and 1725 AD, Europe experienced what has come to be known as the Little Ice Age: exceptionally cold winters, with temperatures plunging as low as 5 degree C. below normal. But whether the Little Ice Age was purely a European phenomenon has for long been warmly -- if not hotly -- debated. Now, there is evidence to show that the cold wave had indeed travelled as far away as India.

The witness to this unusual phenomenon is a 300-year-old fir tree (Abies pindrow) in Jammu and Kashmir, which has preserved the signatures of temperature changes in the region in its annual rings. Scientists at the Physical Research Laboratory in Ahmedabad analysed the fir tree's rings' ratio of hydrogen (H) to deuterium (D), which is essentially a heavier form of hydrogen. The D/H value is sensitive to temperature changes -- the greater the drop in temperature in a year, the more the deuterium to be found in that year's tree ring. By looking at the profile of the tree rings, the climate of a bygone era can be deciphered, says R Ramesh, a palaeoclimatologist at PRL.

Explains Ramesh, "The concentration of hydrogen in a tree is directly related to the D/H ratio of the water it has used to grow -- groundwater, rain or snow. The ratio is more or less constant for groundwater, hence trees that take up only groundwater, for example teak, do not show the same deuterium level response to temperature changes. Deuterium in rainwater and snow on the other hand, is sensitive to temperature changes, and this shows up in the cellulose extracted from trees like fir."

The PRL team found that the deuterium concentration in the fir had increased between 1600 and 1700. This, says Ramesh, amounted to a drop of about 4-5 degree C., suggesting Europe's Little Ice Age was not localised, but spread as far as Kashmir.

Science & Technology