Sudden cooling in Greenland reduced summer monsoons in India 8,200 years ago: Study

Around 8,200 years ago, a drop in the temperature in Greenland, located at one end of the globe, triggered a decline in the intensity of the Indian Summer Monsoons (ISM), a study by Indian researchers has revealed.

The research shows that even in prehistoric times, India’s monsoon was sensitive to both high-latitude ocean changes and tropical Pacific variability, and highlight how major climate events in one part of the world can drastically affect weather systems thousands of kilometers away.

Between 8220 and 7600 calibrated years before present, tree savannah vegetation occupied the landscape under a cool-dry climate, indicating a reduced ISM rainfall. This phase of harsh climate broadly coincides, rather overlaps with the global ‘8.2 ka cooling event’ of the North Atlantic,” the researchers said.

The ‘8.2 ka cooling event’ is the largest climatic excursion of the Holocene from the perspective of Greenland temperature change. Temperature dropped by 3 degrees Celsius in Greenland, and methane declined by 80 parts per billion by volume, which suggests an important change in the hydrologic cycle, according to the study.

This was the result of a glacial outburst flood of freshwater from Lake Agassiz, a massive prehistoric pro-glacial lake that once existed in central North America, covering parts of modern-day Manitoba, Ontario and Saskatchewan in Canada, and Minnesota and North Dakota in the United States, thorough the Hudson Bay into the North Atlantic.

The Holocene is the current geological period that began about 11,700 years ago after the last major ice age, and is characterised by a stable, warm climate that resulted in the rise of human civilization, agriculture and significant environmental changes.

Scientists from the Birbal Sahni Institute of Palaeosciences and the Academy of Scientific and Innovative Research have found signature of this abrupt climate change (ACC) or rapid climate change (RCC) event of the North Atlantic in the core monsoon zone (CMZ) of India.

The team extracted a 1.2-metre long sediment profile from Tuman Lake in Korba District of Chhattisgarh, located within the CMZ and analysed the fossil pollen preserved in lake sediments, according to information shared by the Ministry of Science and Technology.

Each type of plant produces distinctive pollen grains. By identifying and counting 300 terrestrial pollen grains per sample, the researchers reconstructed past vegetation patterns, inferred past climate conditions and constructed a high-resolution climate archive written in microscopic grains of pollen.

The study, published recently in the Elsevier’s Quaternary International, an international peer-reviewed journal, interpreted that more tropical moist deciduous forest pollen indicated a stronger monsoon rainfall while drier deciduous or herbaceous pollen indicated weaker monsoon conditions, and identified weakened monsoon during the 8.2 ka interval. Using radiocarbon dating and statistical age-depth modelling, the researchers built a timeline stretching back over 8,200 years.

“The weakened monsoon during the 8.2 ka interval suggests a powerful tele-connection or an atmospheric and oceanic link between the North Atlantic and the ISM. It indicated that cooling in Greenland caused disruptions in Atlantic circulation that may have shifted global wind belts and weakened monsoons in the Northern Hemisphere, thereby reducing rainfall over India” the ministry said.