Deep below the surface, boreholes offer clues to past warming in the Western Ghats

Mapping changes in temperature with depth in boreholes drilled in Maharashtra’s Koyna region has thrown light on past climate warming in the biodiversity-rich Western Ghats.

The results detailed in a study in the journal Theoretical and Applied Climatology show that the Western Ghats has experienced warming of about 0.8 degrees Celsius in the past 100 years due to climate change.

The authors underscore that despite limitations, digging into surface ground temperature in boreholes “provides a true estimate of climate change and can be used as a robust technique to reconstruct past climate warming”.

“Our study estimate is comparable with the warming estimates obtained from the meteorological data, which states that warming has been around 0.56 degrees Celsius in the past 100 years (1901 to 2001),” study author Srinidhi Jha of IIT-Indore told Mongabay-India.

The scientists used temperature-depth data recorded in 2014 from five deep boreholes (ranging in depth from 140 metres to 198 metres) in the Koyna region in the Western Ghats to quantify the climate warming in the chain of mountains running parallel to India’s western coast.

A borehole is a narrow vertical hole drilled in the ground and used for a variety of purposes such as extraction of groundwater, petroleum, mineral investigation and simple temperature-depth profiling in this case. In this process, the temperature records are collected at subsequent depths in a borehole with the help of a temperature probe attached to a string, said Jha.

These wells were drilled in the 1990s by National Geophysical Research Institute for examining the pore-pressure changes in the seismically active Koyna-Warna region. Jha said the study addresses the need for reliable climate warming estimates for the Western Ghats traversing the states of Kerala, Tamil Nadu, Karnataka, Goa.

“Ecologically rich Western Ghats play a crucial role in deciding the country’s climatological characteristics. Over the past few years, the estimation of climate warming in the world’s ecological hotspots has been gaining attention,” Jha added.

“In this study, we have considered a short period of 100 years for a particular place. It also depends on the rock type; how they absorb the heat. The area of the study site is basaltic area and it has a particular temperature gradient,” NGRI’s DV Reddy and study co-author told Mongabay-India.

Temperature-depth data from boreholes supplement the lack of long-term meteorological records and fulfil the need for accurate data in climate change studies. “Most of the meteorological records which are available in India date back to the 1900s. Further, longer historical data is not available for many places. The borehole profiling method allows us to reconstruct the proxy meteorological records of such regions,” said Jha.

This reconstruction is possible by unpacking how temperature changes down a borehole at present, which in turn offers clues to the changes in surface temperature in the past.

According to an explanation provided by the United States’ National Oceanic and Atmospheric Administration, deviations from the expected increase in temperature with depth can be interpreted in terms of changes in temperature at the surface in the past, which have slowly diffused downward, warming or cooling layers meters below the surface.

Jha stresses that one of the critical advantages of using this method is that it offers the ‘true’ estimate of past climate. “Since current modeling practices to build proxy meteorological data are based on many limitations and assumptions, these assumptions further produce inaccurate estimates of the historical data.”

“Also, the historical data of the past century may be imprecise due to computational and technical limitations. However, in this method, the temperature-depth profiles are present-day and real, produces a real assessment of past climate,” Jha said.

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The current study results are also in line with a 2018 research that indicated ground surface warming in peninsular India during the past three centuries.

The Western Ghats blocked the rain bearing clouds and caused them to rise and precipitate. Photo by S. Gopikrishna Warrier/Mongabay.

TV Ramachandra of the Indian Institute of Science, Bengaluru, who was not associated with the borehole temperature-depth profile study, said the observations of temperature increase align with their studies reporting annual temperature increase of 0.07 degree Celsius in the Western Ghats (from 2001 to 2016).

However, there are certain limitations to the borehole profiling method.

“Any error in the profiling may lead to inaccurate estimates because temperature-depth profiles provide the basis for most of the calculations. Most of the boreholes in India earlier were not drilled for climate change studies. Therefore, the availability of suitable data remains a big limitation,” said Jha.

Combining different kinds of data from different approaches offers a more robust understanding of the relations in variations in temperature and rainfall. These are the two most essential climate parameters, underscored earth scientist Atreyee Bhattacharya, research affiliate at CIRES at the University of Colorado, Boulder. Bhattacharya was not associated with the current study.

“Every approach has its limitations, challenges, and constraints. As we try to understand climate systems more and more, we realise that we can’t rely on the last 50 to 60 years of instrumental data that is available; we need to have comparable ​quality and quantity of data going back to at least a 100 to 150 years ​or longer because some of these climate cycles (temperature and rainfall cycles) of relevance to human dimensions operate at a timescale of 10, 40, 100 years. So, you need many of these cycles before you can think of the impacts,” Bhattacharya told Mongabay-India.

Different approaches allow one to go back further in time.

And paleoclimatic investigations ​usually apply a “multi-proxy” approach to understand a system; ​especially the processes that drive variations in ​climate variables (such as in temperature and rainfall) and “once you ​understand these processes, you can model climate and related societal outcomes more effectively.”

“We are currently living in the Anthropocene where we have impacted the Earth’s environment and the climate with our actions. We know from modeling and observational studies that the heat trapped, and then exchanged between different reservoirs of the Earth drives the response of the climate system and, ultimately, our ecosystem and environment. So, we would have to know the natural frequency of climate cycles and events, so that we understand how these cycles are changing, and assess the total amount of impacts, in terms of changes that we can expect in our human-environment systems,” said Bhattacharya.

Bhattarachya’s research focuses on applying geochemical and statistical techniques to characterising and quantifying responses of human-environmental systems to different scales of climate variability in semi-arid and coastal regions.

Intrigued by the human dimensions linked to Earth’s climate, she said paleoclimate provides the background for assessing and understanding technologies, vulnerabilities, and resilience.

Bhattacharya and co-authors have pointed out that there are major gaps in coverage in north-eastern, eastern, interiors of western regions, and central parts of India, in an initial review of the existing paleoclimate data. Except in the western, southern, and south-central parts of the country, where a combination of tree ring, stalactite and lake-based reconstructions provide critical information, paleo data on timescales of human interest- sub decadal, decadal, multi-decadal and centennial- are minimal, they said in the review.

She said efforts are on to create a paleoclimate database of publications and related datasets-specific to the Indian sub-continent so that investigators and planners interested in the issues of the region can access regional data efficiently.

Large parts of India, except for the Indo-Gangetic plains, have experienced significant warming in the last 60 years due to human-induced climate change, according to a 2020 study by IIT-Gandhinagar scientists. They noted a pronounced increase in the frequency of hot days in the last four decades. India’s first-ever climate change assessment report states that the country’s average temperature is expected to rise by 4.4 degree Celsius by the end of the year 2100.

The rise in temperature is also playing havoc with India’s rainfall, which is significant for India’s agriculture sector on which millions are dependent. The report highlighted that the summer monsoon precipitation (June to September) over India has declined by around six percent from 1951 to 2015, with notable decreases over the Indo-Gangetic Plains and the Western Ghats.


Tufted gray langur (Semnopithecus priam) crossing the road in Anaimalai hills, Southern Western Ghats, India. Photo credit: P Jeganathan/Wikimedia Commons [CC Attribution-SA 4.0 International Licence].

The forests of the Western Ghats include some of the best representatives of non-equatorial tropical evergreen forests anywhere and are home to at least 325 globally threatened flora, fauna, bird, amphibian, reptile and fish species. The Western Ghats, apart from being a storehouse of tropical biodiversity, is also the source of 38 east-flowing rivers and 27 rivers flowing into the Arabian Sea. The Godavari, Krishna, Mandovi, Kaveri and Zuari are some prominent rivers that originate in the Western Ghats.

Experts have consistently warned against rushed approvals to infrastructure projects in the Western Ghats that could spell doom for the ecosystem.

“Changes in climate (increase in temperature, or fluctuations in precipitation regime) would perturb the ecology and hence biodiversity and hydrologic regime of a region. Invasive species would take over, leading to the loss of native biodiversity. This would alter the hydrologic regime affecting the sustenance of water and threaten food security, which affects the livelihood of people,” said IISc’s TV Ramachandra.

“Loss of food and medicine (biodiversity) would affect us as it is now with COVID 19 pandemic. Degradation of landscape (deforestation) would lead to the escalation of carbon in the atmosphere and aggravate global warming. It would give way for zoonotic diseases (COVID, dengue, Kyasanur Forest Disease, etc.) – affecting human livelihood and the economy,” he elaborated.

“Our decision-makers are keen on lopsided development hoping for improvements in GDP, but end up paying a heavy price with the loss of ecosystems. Need to learn from past mistakes and respect mother nature,” he added.

This article first appeared on Mongabay.

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