mumbai

Updated: Nov 19, 2017 17:50 IST

Temperatures in Greenland, a frigid island located to the northwest of Europe, may have an effect on India’s southwest monsoon and likely nullify the impact of El Nino, a climate cycle in the Pacific which affects rain in India. This has been revealed in a new study, done by a group of institutes led by the Wadia Institute of Himalayan Geology (WIHG), Dehradun, based on 8000-year-old peat deposits in the Himalayas.

The study analysed a 5.2-metre-long thick peat sequence – partially decayed vegetation – in Kedarnath. Comparing the peat with similarly old ice cores from Greenland, the 13-member team found that temperature in the world’s largest island – which is mostly covered in ice – influences India’s summer monsoon. Wetter rainfall events coincide with a warm Greenland, and a cooler Greenland resulted in less rainfall over the country, the study said.

Scientists not involved in the study said linking the monsoon to temperatures in Greenland supports the activities taken up Goa-based National Centre for Antarctic and Ocean Research to study the effect of melting Arctic on the Indian monsoon.

“The findings brings out an important fact that all the changes in Indian monsoon are not only due to increase in anthropogenic greenhouse gases, but also have major contribution from the naturally occurring changes on the earth,” JV Ratnam, senior scientist, Japan Agency for Marine-Earth Science and Technology, who was not involved in the study, told HT. “The study will be important for improving modelling studies of paleo-climate.”

The peat was a chance discovery after a team of geologists found it exposed between two moraines – deposits of past glacial advances – after the 2013 deluge in Uttarakhand. Professor YP Sundriyal, HNB Garhwal University, Srinagar, who was member of the team got the peat dated using 14C-AMS technique.

Researchers said meteorologists in India predict monsoon based on sea surface temperature, land temperature, land vegetation and snow cover over Tibet and also temperature in north Atlantic region.

“We have tried to establish that this forcing factor for the Indian summer monsoon is valid for at least the past 8,000 years. What’s new about this finding is that Indian summer monsoon gets strengthened when temperature in North Atlantic is warmer,” said Pradeep Srivastava, lead author, WIHG.

The team also found a correlation between temperatures in Greenland influencing the Indian monsoon during an El Nino year. El Nino is a weather phenomenon caused when warm water from the western Pacific Ocean flows east. If the Pacific warms up, the precipitation shifts in that direction, weakening monsoon currents and causing below-normal rainfall in other parts of the world, including India.

“The [drought-inducing] impact of El Nino nullifies over India if Greenland is warm. But a cold Greenland during an El Nino year will lead to severe drought,” said Srivastava.

The team said this is the first radiocarbon dated continuous Holocene climate record from Indian higher Himalaya that reveals the basic ramp of natural variability of India monsoon during the last 8,000 years. Holocene is the current geological epoch that started when glaciers began to retreat from lower altitudes around 11,500 years ago.

“Our findings are important for climate models that are being developed to predict monsoons especially in the era of climate change caused by anthropogenic [manmade] warming. It will also help in contextualising Indian history in the realm of this newly available Indian monsoonal record,” said Dr Rajesh Agnihotri, co-author, Birbal Sahni Institute of Palaeosciences, Lucknow.

Peat records from Kedarnath are important because the temple town, which lies in the northern extremity of the Indian summer monsoon, logs small fluctuations in the monsoon pattern.

“If there is slight increase in monsoon, Kedarnath becomes wet, and the town becomes dry with a decrease in rainfall activity. This is in contrast to the Ganga plains where small variations in monsoon are not recorded because the plain is already humid,” said Srivastava.

Researchers said the study finding is therefore significant because Kedarnath with its 8,000-year-old history of climate has helped in understanding past variability, past vegetation cover, glacial period, and interglacial Holocene period which co-relate with human evolution in India.

The US-based National Centres for Environmental Information describes glacial period as a time with large ice sheets – also called the ice age – while an interglacial timescale is one without large ice sheets. The earth is currently in the interglacial period called the Holocene.

Till date, most Indian summer monsoon records spanning the Holocene have used sediments from the Arabian Sea, cave deposits from peninsular India, lake sediments and peat sequences from central and north-western India. The Kedarnath peat, comprising more than 50% organic matter, contained pollen grains belonging to broad leaf trees and conifers.

The team found wet and dry climate records preserved in the peat – so 5,000 years to 6,000 years ago was the warmest. At 5,400 to 3,800 years ago, climate varied between extreme dry and extreme wet. “This was the time when the Harappan (Indus) civilisation was facing adversaries of climate that may have led to significant change in style of subsistence,” said Agnihotri.

These climate variations in India gave way to a higher monsoon phase, which researchers describe as the Vedic Age (2,900 to 3,700 years ago), followed by a dry phase, then another wet phase during the Gupta empire regarded as the golden age of Indian history (900 to 1400 AD). The Kedarnath peat also recorded a distinct colder phase that is globally called as Little Ice age (14th century to mid-19th century).

“The ratio of these two plants was suggestive of varying climate. We inferred the climate was warmer if the proportion of broad leaf trees in the peat increased significantly. A rise in the conifers was representative of cold climate,” said Srivastava. “Broad leaf trees are not found in Kedarnath today, and hence the town was far warmer in the past.” The studies on pollens were conducted by Dr Anju Saxena, BSIP.