A study published in Science found oceans to be warming at a rate 40% faster than that reported by a United Nations panel five years ago. The scientists also discovered that ocean temperatures have reached record highs for several years straight. The waters closest to the surface have heated up the most, and that warming has accelerated over the past two decades.

The heating of the oceans is closely linked to climate change. Excess greenhouse gases are added to the atmosphere primarily from burning fossil fuels, and 93% of this excess heat is stored in the oceans. In fact, ocean temperatures are often used to track the effects of greenhouse gas emissions. As the planet has warmed, the ocean has paid most of the price, and for this reason, some climate scientists feel that 93% of the climate change story is not being told. “If the ocean wasn’t absorbing as much heat, the surface of the land would heat up much faster than it is right now. In fact, the ocean is saving us from massive warming,” said Malin L. Pinsky from Rutgers University. Rising ocean temperatures are decimating marine ecosystems, however.

An increase in the baseline ocean temperature is one way in which oceans are affected. However, there has also been an increase in the duration and frequency of marine heat waves. The oceans are subject to heatwaves in much the same way as we are on land. A marine heatwave is said to have occurred when ocean temperatures are significantly warmer than usual for five consecutive days. One study found that marine heat waves became 34% more frequent and 17% longer between 1925 and 2016. Overall, there were 54% more days per year with marine heat waves globally. Such marine heatwaves threaten global biodiversity, according to a recent study published in Nature Climate Change, because organisms that form the bedrock of many ocean ecosystems are most affected.

As the water heats, it expands, taking up a larger volume. This is the primary reason responsible for rising sea levels, even more than melting ice caps

Foundational species like coral reefs, sea grasses, and kelp forests experience significant losses. In the last three years, 20% of coral reefs have died. Kelp provides habitat and food for ocean life, but tends to flourish in cooler waters. When the kelp goes, entire ecosystems can disappear with it. Aquatic life in parts of the Pacific, Atlantic, and Indian Oceans were found to be particularly vulnerable, as there are additional impacts like overfishing and pollution. Animals who depend on the sea for food, such as seabirds, are also a casualty.

The rising temperatures of the oceans is affecting humanity, too. People who depend on fishing for a living are likely to be adversely affected as heat waves increase. “Commercial fisheries, but also aquaculture facilities on the coast, will face challenges as ocean water changes. Heat waves are not great for aquaculture production,” said Gretchen Hofmann from the University of California. It is estimated that one billion people depend on coral reefs for food or income, and they are highly sensitive to temperature.

For populations that rely on fish for protein, warm oceans produce less food and that could lead to food insecurity. Marine life also moves as the oceans are disrupted and this had led to diplomatic disputes in some cases. “As the ocean heats up, it’s driving fish into new places, and we’re already seeing that that’s driving conflict between countries,” Pinsky said.

Sea temperatures have a large influence on climate and weather. For example, every three to seven years, a wide swath of the Pacific Ocean warms by 2°C to 3°C. This warming is a hallmark of the climate pattern El Niño, which changes rainfall patterns around the globe, causing heavy rainfall in the United States and severe drought in Australia, Indonesia, and southern Asia. There is also some indication that El Niños have been getting more extreme with climate change. While natural patterns like El Niño can affect the temperature on a yearly basis, it is worth noting a longer, decade-long warming trend is observed, which is attributed to humans.

As the water heats, it expands, taking up a larger volume. This is the primary reason responsible for rising sea levels, even more than melting ice caps. If global carbon emissions are not curbed, this thermal expansion alone would lead to a one foot rise in sea levels.

greenhouse gases are added to the atmosphere primarily from burning fossil fuels, and 93% of this excess heat is stored in the oceans

In order to accurately predict the future effects of climate change, the ocean temperatures need to be precisely known. However, tracking the ocean temperature over time is not straightforward. Before 2000, scientists relied on temperature sensors attached to ships. Since then, a network of drifting floats have been used. Satellites, such as the Moderate Resolution Imaging Spectroradiometer on NASA’s Aqua satellite, record the temperature of the top millimetre of the ocean surface.

There are many effects that result from warmer ocean waters. Marine species, such as coral, are at risk. Habitats are changing, which alter the behaviour of the marine life and, in turn, where people fish. Ice at the polar caps are melting and, along with thermal expansion, is leading to a rise in sea levels. While the rising baseline ocean temperatures represent gradual, albeit significant, changes the consequences of the marine heatwaves can have more dramatic short-term consequences. Robert Miller, University of California, said “In some cases, ecosystems might not be able to recover from these events. And so, the effects of climate change might happen a lot sooner than we expect.”