Record highs reached for ocean temperatures

According to three vital global metrics for ocean temperatures, 2024 was the warmest year on record. The coincidence of all three global metrics being highest on record is unusual. The last time this happened was 2016.

The three metrics are the global mean surface temperature (GMST), the global sea surface temperatures (SST), and the global ocean heat content (OHC) from the surface to 2000 metres depth.

The most reliable indicator of the warming planet is the OHC, which acts as an integrator of all heating. It continues to break records year after year. From 2023 to 2024, the global ocean heat content showed an increase of 15 zettajoules (1021 joules), energy equivalent to 140 times the world’s total electricity generation in 2023.

The SST varies quite a lot from year to year, especially with El Niño events. The GMST also varies with El Niño and is affected by weather variability as well. The latest El Niño began in April 2023 and continued for a year.

From April 2023 until June 2024, the global SSTs were the highest on record, but as the El Niño has waned, global SSTs have declined. As is typical with El Niño events, the biggest anomalous SST value in 2024 was in February, which exceeded 19.1 degrees Celsius, although the highest actual GMST values occurred in July 2024.

The differences relate to the dominance of the southern hemisphere ocean areas for SST versus the land areas in the northern hemisphere, which heat up more, for GMST.

At the same time, the concentrations of carbon dioxide in the atmosphere have been at record high values. Allowing for the seasonal cycle, the values are now about 425 parts per million (ppm) by volume, more than 50 percent above the preindustrial values of 280ppm.

Indeed, the values are running over 3ppm above last year, indicating that global heating from increased greenhouse gases continues unabated. These increases are mostly from human activities burning fossil fuels, although wildfires have also contributed. As Earth-warming emissions keep rising it shows that the efforts to transition away from fossil fuels faltered in 2024.

Higher atmospheric temperatures mean that the atmosphere sucks more moisture out of the surface of the land, leading to drying, where it is not raining. Everything dries out quicker, leading to wilting plants, heatwaves, and increased risk of wildfire.

The consequences of increased ocean temperatures are much more water vapour in the atmosphere, at about 7 percent per degree Celsius warming. Water vapour is also a powerful greenhouse gas and adds to heating.

The increased water vapour, as atmospheric moisture, gets caught up in all storms, fuelling their developments and leading directly to heavier rains (and snows). This includes hurricanes, typhoons, cyclones and atmospheric rivers. The result is increased risk of severe weather, heavy rains, and flooding extremes.

Many in the public and even many scientists have been puzzled by the record high global temperatures experienced over the past two years. But this is not a puzzle to those of us who follow the changes in ocean heat content that have been at record highs year after year.

Nevertheless, the effects of the heating climate and higher ocean temperatures are pronounced regionally and quite uneven. This is because of patterns of weather related to El Niño and marine heat waves. The latter also profoundly affects marine life, including fish, mammals, sea birds, and plants such as kelp forests.

Drought, crop failure, heat waves and wildfires became common in many areas of the world in 2024, from southern Africa in February, southern Asia and the Philippines by April, the Pantanal in Brazil in June, widespread heatwaves and wildfires in Europe in August, and in the northeast United States in November.

Mass bleaching occurred in the Great Barrier Reef in March, and multiple hurricanes in the Caribbean in July and southeast United States (especially Helene) caused havoc in September. Yagi was a deadly and destructive super typhoon in early September in Southeast Asia and South China and then flooded Myanmar, Laos and Thailand.Major floods in Chad and Nigeria also occurred in September, and a monstruous deluge in Valencia, Spain, in October led to major devastation.

As well as loss of life and livelihood, the costs and disruption locally were shattering.No doubt there are many down-stream effects in all the above events. These include erosion, especially in coastal regions where storm surges occur. Downed trees and power poles cause electricity outages and trigger fires.

Extreme shifts in weather patterns, from very dry sunny conditions to very wet conditions and back again increase vulnerability, because soils shrink in dry conditions, opening cracks, and then rain waters flow into the cracks and enlarge them. The result is that severe weather and record heat powered by climate change unleashed havoc around the globe. Widespread adverse health effects occur in many animals and humans, especially from smoke.

Prospects then are for more warming although, given the record increases in the past two years, 2025 is apt be a tad cooler in GMST (global temperatures) even as ocean heat continues to increase. It does not mean that global warming is not happening. It does mean that there is a lot of variety in how it is manifested. Increased awareness of the risks, and how to build resilience to weather extremes is always a good investment.