The oceans are heating up as the planet warms.

This past year, 2024, was the warmest ever measured for the global ocean, following a record-breaking 2023. In fact, every decade since 1984, when satellite recordkeeping of ocean temperatures started, has been warmer than the previous one.

A warmer ocean means increased evaporation, which in turn results in heavier rains in some areas and droughts in others. It can power hurricanes and downpours. It can also harm the health of coastal marine areas and sea life – coral reefs suffered their most extensive bleaching event on record in 2024, with damage in many parts of the world.

Warming ocean water also affects temperatures on land by changing weather patterns. The EU’s Copernicus Climate Change Service announced on Jan. 10 that data showed 2024 had also broken the record for the warmest year globally, with global temperatures about 2.9 degrees Fahrenheit (1.6 Celsius) above pre-industrial times. That would mark the first full calendar year with average warming above 1.5 C, a level countries had agreed to try to avoid passing long-term.

Climate change, by and large, takes the blame. Greenhouse gases released into the atmosphere trap heat, and about 90% of the excess heat caused by emissions from burning fossil fuels and other human activities is absorbed by the ocean.

But while it’s clear that the ocean has been warming for quite some time, its temperatures over the past two years have been far above the previous decades. That leaves two mysteries for scientists.

It’s not just El Niño

The cyclic climate pattern of the El Niño Southern Oscillation can explain part of the warmth over the past two years.

During El Niño periods, warm waters that usually accumulate in the western equatorial Pacific Ocean move eastward toward the coastlines of Peru and Chile, leaving the Earth slightly warmer overall. The latest El Niño began in 2023 and caused global average temperatures to rise well into early 2024.

But the oceans have been even warmer than scientists expected. For example, global temperatures in 2023-2024 followed a similar growth and decline pattern across the seasons as the previous El Niño event, in 2015-2016, but they were about 0.36 degrees Fahrenheit (0.2 Celsius) higher at all times in 2023-2024.

Scientists are puzzled and left with two problems to solve. They must figure out whether something else contributed to the unexpected warming and whether the past two years have been a sign of a sudden acceleration in global warming.

The role of aerosols

An intriguing idea, tested using climate models, is that a swift reduction in aerosols over the past decade may be one of the culprits.

Aerosols are solid and liquid particles emitted by human and natural sources into the atmosphere. Some of them have been shown to partially counteract the impact of greenhouse gases by reflecting solar radiation back into space. However, they also are responsible for poor air quality and air pollution.

Many of these particles with cooling properties are generated in the process of burning fossil fuels. For example, sulfur aerosols are emitted by ship engines and power plants. In 2020, the shipping industry implemented a nearly 80% cut in sulfur emissions, and many companies shifted to low-sulfur fuels. But the larger impact has come from power plants reducing their emissions, including a big shift in this direction in China. So, while technologies have cut these harmful emissions, that means a brake slowing the pace of warming is weakened.

Is this a warming surge?

The second puzzle is whether the planet is seeing a warming surge or not.

Temperatures are clearly rising, but the past two years have not been warm enough to support the notion that we may be seeing an acceleration in the rate of global warming.

Analysis of four temperature datasets covering the 1850-2023 period has shown that the rate of warming has not shown a significant change since around the 1970s. The same authors, however, noted that only a rate increase of at least 55% – about half a degree Celsius and nearly a full degree Fahrenheit over one year – would make the warming acceleration detectable in a statistical sense.

From a statistical standpoint, then, scientists cannot exclude the possibility that the 2023-2024 record ocean warming resulted simply from the “usual” warming trend that humans have set the planet on for the past 50 years. A very strong El Niño contributed some natural variability.

From a practical standpoint, however, the extraordinary impacts the planet has witnessed – including extreme weather, heat waves, wildfires, coral bleaching and ecosystem destruction – point to a need to swiftly reduce carbon dioxide emissions to limit ocean warming, regardless of whether this is a continuation of an ongoing trend or an acceleration.

This article has been updated with Copernicus Climate Change Service’s global 2024 temperature data.

Annalisa Bracco, Professor of Ocean and Climate Dynamics, Georgia Institute of Technology

This article is republished from The Conversation under a Creative Commons license. Read the original article.