Extreme weather, migration and what else to expect due to ocean warming

In conversation with scientist Roxy Koll, whose study on rapid warming of the Indian Ocean holds alarming implications for the future.

India has witnessed intense heat waves this year with temperatures soaring above record levels, particularly in urban areas, sometimes even breaching the 50°C mark. The Indian Meteorological Department (IMD) has observed that the country is experiencing the effects of climate change as extreme weather events such as heavy rainfall, heat waves and intense tropical cyclones are occurring more frequently.

One contributing factor for these extreme weather patterns is the rapid warming of the Indian Ocean. A study Future projections for the tropical Indian Ocean, published by Elsevier and led by Roxy Mathew Koll, scientist with Climate Research Lab, Indian Institute of Tropical Meteorology (IITM), Pune, says that the heat content of the Indian Ocean, from surface to 2,000 meters deep, is currently increasing at the rate of 4.5 zetta-joules per decade. It is predicted to increase at a rate of 16–22 zetta-joules per decade in future.

To illustrate the magnitude of the issue, Roxy explains: “The future increase in heat content is comparable to adding the energy equivalent of one Hiroshima atomic bomb detonation every second, all day, every day, for a decade!”

In other reports: Climate migration hotspots and risks in South Asia

  • The southern Indian highlands, especially between Bengaluru and Chennai will be climate in-migration
    hotspots, as per the World Bank report, Groundswell: Preparing for Internal Climate Migration
  • According to report by Asian Development Bank (ADB), Climate Change and Migration in Asia and the Pacific, several mega cities of South Asia—such as Dhaka in Bangladesh, Karachi in Pakistan, and Kolkata, Mumbai, and Chennai in India—are at high risk of sea-level rise, prolonged cyclonic activity, and greater salt-water intrusion, which is likely to affect a large number of people due to high population density and poor urban planning

Impact of marine heat waves 

The warming in the Indian Ocean was strongest during the last seven decades, at a rate of 1.2°C per century. However, climate models predict warming at a rate of 1.7°C–3.8°C per century during 2020–2100, if emissions continue at the current rate. Marine heatwaves — periods of extremely high sea surface temperatures that occur sporadically — are expected to increase from 20 days per year to 220–250 days per year, pushing the tropical Indian Ocean into a near-permanent heatwave state.

“The Arabian Sea temperatures are generally between 26°C-29°C and the Bay of Bengal between 28°C and 30°C. During marine heatwaves, these can become 32-34°C. Even a 0.1°C increase in sea temperature can have significant ecological implications. Marine heatwaves can last from a few days to several months.” 

Marine heatwaves can impact monsoons and intensify cyclones. “Rapid intensification of cyclones demonstrates that consistent heat is necessary for their intensification.” Super cyclonic storm Amphan that hit West Bengal, Odisha and Bangladesh in 2020, was preceded by a marine heatwave, which caused considerable damage. Roxy, however, contends: “We need more on-site information to improve cyclone forecasting.” 

He adds that sometimes, marine heatwaves and terrestrial heatwaves (prolonged periods of excessively hot weather) can occur simultaneously, but again more research is needed regarding this.

Seasonal cycle and weather patterns have shifted

The study observes that seasonal cycle of surface temperatures is projected to shift, which may have implications of extreme weather events over the Indo-Pacific region. While the maximum basin-average temperatures in the Indian Ocean during 1980–2020 remained below 28°C (26°C–28°C) throughout the year, the minimum temperatures by the end of the 21st century will be above 28°C (28.5°C–30.7°C) year around, under high emission scenario.  Sea surface temperatures above 28°C are generally conducive for deep convection and cyclogenesis. Heavy rainfall events and extremely severe cyclones have already increased since the 1950s and are projected to increase further with increasing ocean temperatures.

Extreme Indian Ocean Dipole events 

The Indian Ocean Dipole (IOD) is getting extreme. The frequency of extreme dipole events are predicted to increase by 66% whereas the frequency of moderate events are to decrease by 52% by the end of the 21st century, the study notes.

Roxy says that ideally, extreme positive IOD events with warm anomalies in the west Indian Ocean / Arabian Sea should lead to extreme rains and cyclones impacting west coast cities. He adds, though, there are no studies exploring this in detail.”

What are IOD events?

Roxy explains that the IOD is the difference in sea surface temperatures between the western and eastern parts of the Indian Ocean. When the western Indian Ocean (northwest) has warmer temperatures and the eastern Indian Ocean (southeast) has cooler temperatures, this is known as a positive dipole. The converse is when the eastern Indian Ocean is warmer and the western Indian Ocean is cooler, it is called a negative dipole. The IOD phases occur aperiodically, which means it is not at regular intervals. Changes in the IOD can significantly influence the monsoons, with positive and negative dipole events affecting the intensity and distribution of monsoon rainfall.


Read more: Bengaluru climate action and resilience plan: Connecting the dots


Ocean acidification

The study observes that ocean acidification is predicted to intensify, with surface pH decreasing from a pH above 8.1 to below 7.7 by the end of the century. The projected changes in pH may be detrimental to the marine ecosystem since many marine organisms — particularly corals and organisms that depend on calcification to build and maintain their shells — are sensitive to the change in ocean acidity. “The change may be easier to fathom when we realise that a 0.1 fall in human blood pH can result in rather profound health consequences and multiple-organ failure,“ says Roxy.

Changes irreversible, but can be stopped from accelerating

We are already witnessing the impacts of the warming on fisheries along the Indian Coast. Effects are being observed on coral reefs and marine fisheries, including coral bleaching, mass fish mortality, and the collapse of marine aquaculture. But Roxy says that there is a lack of sufficient data on the effects of warming on fisheries along the Indian coast. “Targeted monitoring is needed but is currently inadequate. This is due to the different ministries not collaborating effectively.” 

Roxy emphasises that the impacts of these changes are not concerns of the future generations, but requires immediate attention as we are already seeing its effects, such as monsoon floods, droughts, cyclones, and heat waves over both land and ocean. And these changes are not reversible.

Roxy explains: “The ocean acts like a reservoir, with absorption being a slow process. Oceanic processes take hundreds to thousands of years, making changes largely irreversible.” However, we can stop these changes from accelerating, adds Roxy. “We can manage our lands, rivers, and mountains. Land-use changes, such as encroaching rivers and swamps, limit natural water absorption during floods. Water flushes into the sea, hindering groundwater recharge, resulting in floods and dry spells.”

Rising migrations to mega cities

With the increasing occurrence of cyclones, migrations from coastal areas to mega cities is rising. Roxy says it is the most vulnerable who are impacted the most.

“In India, there has been significant domestic migration from regions in Bengal and Uttar Pradesh to mega cities like Mumbai and Kolkata.,” says Roxy, “Migrants often face extreme weather events, such as heavy rains and agricultural changes, which drive their movement. In Mumbai, many migrants occupy low-lying areas and lack proper housing, making them the first to be affected by urban floods and more vulnerable to heatwaves. It is essential to consider urban migrant workers in our policy planning.” 

What needs to be done

Roxy says that a multifaceted approach is required to address the impending challenges in the Indian Ocean. The study states that by reducing global carbon emissions and investing in resilient infrastructure, the impacts of climate change on coastal communities can be mitigated. Also, conserving marine ecosystems through sustainable practices and enhancing forecasting capabilities can enhance the region’s resilience to extreme weather events.

Highlights from the study

  • The maximum warming is in the northwestern Indian Ocean, including the Arabian Sea.
  • Seasonal cycle and weather patterns have shifted: The projected shift may have implications of extreme weather events over the Indo-Pacific region. Heavy rainfall events and extremely severe cyclones are projected to increase further with increasing ocean temperatures.
  • Increase in the ocean heat content contributes to sea level rise.
  • The Indian Ocean is moving to a near-permanent marine heatwave state causing habitat destruction due to coral bleaching, seagrass destruction, and loss of kelp forests, affecting the fisheries sector adversely. And rapid intensification of cyclones.
  • Ocean acidification is predicted to intensify.
  • Marine productivity is going down.
  • Oxygen concentrations could also be declining.

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