New Delhi: Global sea levels are set to rise by at least 1m by 2100 if carbon emissions go unchecked, submerging hundreds of cities, including Mumbai and Kolkata, and in some cases entire countries, warned a UN report released on Wednesday.

Extreme events such as storm surges are likely to occur once a year rather than once a century because of accelerated global warming, said the report approved by the Intergovernmental Panel on Climate Change (IPCC).

The dire warnings are a call to action for governments and business leaders across the world to reduce carbon emissions that can jeopardize life on the only habitable planet known to humanity.

“The entire world needs to take responsibility and come together to fight climate change rather than debating it. India has at least taken a lead by accepting that these risks exist and has a climate change action plan in place. But it needs to be made a priority, which it is not at present," said Roxy Mathew Koll, climate scientist at the Indian Institute of Tropical Meteorology, Pune and one of the lead authors of the report.

The report was released two days after the end of the UN Climate Action Summit in New York. The summit, focused on convincing countries to scale up their climate action ambitions, failed to get higher pledges from most member nations.

The oceans have been warming unabated since 1970, but the rise in sea levels due to warming has been 2.5 times faster since 2005 than during the 20th century, according to the report.

“If carbon emissions continue unabated, the rate at which the waterline will rise will quadruple by 2100," it warned, adding that a mere 50cm rise in the sea level is enough to flood major port cities around the world and expose more than 150 million people to disasters.

By 2050, many coastal megacities and small island countries may face severe extreme weather events: tropical cyclones, heavy rain, floods, extreme waves and other coastal hazards. Four Indian cities—Mumbai, Kolkata, Chennai and Surat—are among places that could be the most severely impacted.

Mumbai, one of the world’s most populous cities and India’s financial capital, is already below the sea level at high tide and is flooded almost every monsoon season. It fails to drain regular floodwater during excess rainfall, a situation that could worsen due to the rising sea level.

The impact would also be felt on marine heatwaves and extreme El Niño and La Niña events, which are likely to become more frequent. These ocean phenomena are critically linked to the south-west monsoon that accounts for over 75% of the annual rainfall in India. El Niño’s impact on India became evident in 2015, when it faced a severe drought.

The oceans have absorbed 90% of the excess heat in the climate systems and make the planet livable. But this heat has accelerated the melt-off from glaciers, especially the ice sheets atop Greenland and Antarctica, to high levels driving the rise in sea levels.

Raising the red flag on loss of critical terrestrial and marine species to extinction, the report highlights that loss of oxygen has occurred from the surface of the sea to a depth of nearly 1,000m. Oceans have become more acidic. There has already been a shift in the distribution and abundance of fish from the equator to the poles, impacting income, livelihoods and food security of coastal cities. These changes would impact water resources and their uses, especially hydropower and irrigated agriculture in high mountain and downstream areas.

The release of the report was delayed as Saudi Arabia opposed a routine reference to the October 2018 IPCC report on the feasibility of capping global warming at 1.5 degrees Celsius. Under the 2015 Paris Agreement, countries including India committed to capping global warming to well below 2°C above pre-industrial levels, while making efforts to reduce the increase to 1.5°C.

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