Oceans have heatwaves too

Come summer, heat waves resulting from a changing climate hit the headlines across the world. While their ramifications on land are visible, we often don’t hear much about heatwaves in the oceans.

Marine heatwaves occur when sea surface temperatures are hotter than a threshold for more than five consecutive days. They are recorded in all of the world’s oceans, including the Southern Ocean near Antarctica.

Many factors contribute to their occurrence, frequency and intensity—ocean currents, winds, and climate patterns like El Niño. But the biggest contributor to marine heatwaves in the recent past is human-caused climate change.

Oceans across the globe are warming. Since 1880, sea surface temperatures have increased at a rate of 0.6°C per century, and it is projected to touch 2°C very soon.

Warm oceans increase the likelihood, duration and intensity of marine heatwaves. Globally, marine heatwaves have doubled in frequency between 1982-2016, and eight of the 20 most extreme heatwaves in the oceans have occurred after 2010.

Scientists estimate that almost all (84-90%) marine heatwaves that occurred between 1986-2005 are due to anthropogenic climate change.

Oceans at risk

Akin to how disastrous they are on land, heat waves are catastrophic for life in the oceans too. They bleach corals, kill kelp and seagrass, disrupt and damage the range of fish in the oceans and cause widespread mortalities of all marine life.

For instance, a huge marine heatwave called ‘The Blob’, which occurred in the Pacific Ocean off the coast of North America during 2014-2016, killed almost a million seabirds, thousands of California sea lions, seals and other marine invertebrates.

It also shifted kelp forests, bleached corals, caused harmful algal blooms and increased the abundance of warm water species like tuna, whales and orcas.

Marine heatwaves also cause an enormous decline in fisheries revenue and loss of livelihoods. Recent studies suggest marine heatwaves can lead to tropical storms, hurricanes, disrupt monsoon winds, and cause floods, droughts and wildfires on land. Cutting down on greenhouse gas emissions to tackle climate change is our only way out.

Spoorthy Raman