We began this study because 2023 and 2024 were exceptionally warm years, both globally and specifically in the oceans. While to some degree that was expected due to the El Niño phenomenon being in a positive phase, which tends to make the world and the oceans warmer, the warming exceeded what we would typically expect from an El Niño of that strength. So we decided to have a closer look at what might be the explanation for that extra warmth, because it’s been observed for a number of years now. We found that the Earth’s energy imbalance, which is the rate at which energy accumulates in the climate system, has become increasingly positive. This imbalance is measured from space, much like the ocean observations we use. Since energy accumulates as heat within the climate system, this higher imbalance has been driving an accelerated warming of the oceans. Our study showed that over the past 40 years, the long-term trend of ocean warming closely tracks this energy imbalance. Underneath the year-to-year fluctuations, the overall warming trend has been accelerating. Particularly in the last ten years, the Earth’s energy imbalance has increased significantly, which has led to a more rapid rise in ocean temperatures. You can’t really explain 2023 and 2024 without taking into account that acceleration as well as the El Niño. We also explored what these trends might look like if projected into the future. Our findings suggest that the current rate of ocean temperature rise is at the very high end of what climate models have predicted.

The main datasets we used are sea surface temperature records, which my expertise is in. I’ve run projects for measuring the global sea surface temperature using satellite data, which has allowed us to track changes over the past 45 years. These satellite sensors provide detailed, high-resolution observations, but we put a lot of effort into ensuring they accurately reflect long-term trends. My collaborator, Richard Allan, professor in climate science at the University of Reading Department of Meteorology, focuses on the Earth’s energy imbalance. This is also partially observed from space, and it’s more challenging to measure, although capabilities have improved over the decades. You need to combine multiple satellite datasets and reference them against the total heat content of the ocean. Since most of the heat accumulating in the climate system ends up in the oceans and goes quite deep, we calibrate these measurements against deep ocean heat content data. While we care about surface temperatures because they interact with the atmosphere and weather, those are only a portion of the total heat content of the ocean. These datasets require significant analysis to get what we need from the initial observations.

Ocean warming often follows a ratcheting pattern. It goes along fairly steadily, then an El Niño event resets the system to a higher level, and it goes along at that level until the next El Niño. One of the ecosystems most affected by this is coral reefs, particularly tropical reefs. Corals are long-lived animals that don’t move, so they’re very adapted to the climate in their location, making them highly sensitive to temperature changes. The more the ocean warms, the more reefs approach or exceed their temperature limits, leading to bleaching and ultimately killing them. At the current warming rate, the future of most tropical coral reefs is not looking very positive. Some regions, like much of the Caribbean, are basically devastated. Similar coastline ecosystems, like seagrasses and mangroves, are equally susceptible to warming because they’re tied to one place, as opposed to fish, which can migrate. But you do also see fish and sea birds dying in marine heatwaves.

There are also large-scale ocean circulation changes to consider. One major uncertainty is the dynamics in the North Atlantic. As it warms, it could disrupt the formation of deep cold water that drives the Atlantic Meridional Overturning Circulation (AMOC). AMOC is already weakening, and if it were to collapse it could cause regional cooling in Europe even as the rest of the planet continues warming. That’s a big uncertainty, and teams like those led by Professor Christine Gommenginger at the National Oceanography Centre are looking at it in depth.

One thing that hasn’t happened to the degree that we expected is that the oceans haven’t cooled down as much after the recent El Niño phase. Typically, we would expect La Niña to bring a noticeable drop in temperatures, but that hasn’t happened to the usual extent. The Earth’s energy imbalance appeared to dip slightly at the end of last year, but since it’s a highly variable quantity, you can’t really judge it over a few months. We’ll be looking at how sea surface temperatures and the energy imbalance unfold over the next year. We’re also trying to work out the role of sea surface temperature patterns in influencing the energy imbalance. Over the past 10-15 years, the Earth’s energy imbalance has been more positive than expected, probably due to two main factors. One is that various forms of pollution have been cleaned up. Industrial pollution, particularly in China, and the removal of sulphur from marine shipping fuel have reduced aerosols in the atmosphere. These aerosols previously brightened clouds, making the Earth more reflective. With less aerosols, the planet absorbs more sunlight, mostly warming the oceans. The other thing that can happen is that clouds respond to the circulation of the atmosphere, which responds to the warmth of the ocean. In certain parts of the North Pacific, we’ve seen decreases in cloud reflectivity in response to ocean surface temperature patterns. If a warmer world means less reflective clouds, this would amplify global warming. These two sources of decreased albedo , resulting in more absorption of heat from the sun, are big uncertainties, and we’ll be looking at the feedback effect over the longer term.

These interventions won’t fully compensate for the natural diversity that’s already been lost. Coral ecosystems are highly complex and have evolved over long periods, and artificial interventions can’t fully replace that biodiversity. Corals are very much adapted to the environment of their past. For example, corals in the Red Sea, which experience some of the highest ocean temperatures around the world, may have the potential to survive in other parts of the world that are warming up. But introducing species to new environments is risky, and we know from history that it has to be done cautiously to avoid unintended ecological consequences.

There are also groups trying to work with corals by identifying and supporting naturally resilient coral populations in their native regions and selectively cultivating corals that can better withstand higher temperatures. That’s a good thing, but it can’t completely prevent biodiversity loss. Ultimately, no outcome will be as good as limiting the damage of global warming in the first place.