Cause and Effect | 2024 may well break the 1.5°C-barrier for the full year
2023 broke all weather records: The ocean heat content is at an all time high. Expect global temperatures overall to continue to spike in 2024.
The phrase “troubled waters” is taking on a new, literal meaning for the planet’s oceans: sea surface temperatures (SST) this month reached a high of 20.9°C on December 17 and stayed there till December 24.
Seen in isolation, this number would not mean much. But when compared with the 1982-2011 mean temperature — the standard used by the University of Maine’s Climate Reanalyser — it means that the world’s oceans were 0.8°C warmer for this time of the year. This was a 5.3 sigma event, which roughly translates into a once-in-200-year occurrence.
“The most recent forecasts and expert assessment suggest a high likelihood of continued warming in the central-eastern equatorial Pacific for at least the next four overlapping three-month seasons: November-January, December-February, January-March, and February-April 2024,” the World Meteorological Organisation said in its El Nino-La Nina update in November, which predicted the ongoing El Nino to continue at least till April 2024.
The record for December isn’t the warmest the oceans have been in 2023. For five days in April, the SST touched single-day highs of 21.1°C which surpassed the previous record of 21°C set in March 2016, the last time the world was in the grip of an El Nino event.
Only set to get warmer
The oceans are a thermostat for the planet. They soak up excess heat and contribute to sending it back to the atmosphere. In doing so, they drive weather patterns, act as a carbon sink and — thanks to the plankton that float on its surface — produce half of the Earth’s oxygen. Put simply, the cool air blowing off the sea helps make land temperatures more bearable.
But the warmer the water gets, the lower its ability to absorb heat as well as the planet-warming carbon dioxide, which leaves more greenhouse gases in the atmosphere. This, in turn, accelerates the melting of glaciers that flow into the ocean, leading to more sea-level rise.
This year, for instance, temperatures in the Mediterranean Sea, the Atlantic Ocean off the coast of Florida, and the North Atlantic Ocean region were higher than normal throughout the year. These were then linked to several extreme weather events, particularly extreme rains and intense tropical cyclones.
Reading the data
The global sea surface temperature data over the year has clearly reflected an alarming departure from what was observed by the mean over the 1982-2011 period. That 2023 would appear to be among the biggest deviations was always a given, considering each month broke a weather record. It is the magnitude and consistency of the rise that underscores the intensifying impact of climate change on our oceans.
The minimum temperature in the long period average (the 1981-2011 baseline) was 19.99°C in mid-November, while the maximum was 20.41°C around mid-March observed by the NOAA Optimum Interpolation SST V2, a dataset providing daily mean sea surface temperature and maintained by University of Maine's Climate Reanalyzer.
The reason for the timing is simple enough: March is the time of the year when oceans in the southern hemisphere get the warmest, and because the southern hemisphere has more oceans, it tends to be the highest peak of the year.
In 2023 though, the coldest period turned out not only warmer but also arrived at a different time — temperatures hit 20.54°C at the very start of the year, in the first week of January. The year’s hottest period saw a similar deviation on both fronts, moving to August end from the typical March, where it hit 21.08°C.
Was El Niño to blame?
Looking at the numbers, temperatures were elevated in the second half of the year after the World Meteorological Organization, a United Nations agency, announced the onset of El Niño in July.
The first day of January 2023 itself saw a spike at 20.55°C, setting the tone for a persistent warming trend — and one that nearly consistently worsened as the days progressed.
“Sea surface temperatures in certain regions were already well above normal before El Nino, most notably in the North Atlantic in June and July. At the time there was no good explanation – some were blaming it on the lack of Saharan dust, the change in shipping fuel regulations, and Hunga-Tonga [volcanic eruption]. However, El Nino was not mentioned at the time. Of course, now the temperatures have spiked in the El Nino regions, but sea surface temperatures nearly everywhere remain at record levels,” Eliot Jacobson, retired professor of mathematics and computer science, said in an email interview.
Notably, this upward trajectory shows no signs of relenting.
One of the most alarming signs of the nature of the temperature increase in the oceans is how the 2023 figure has proceeded to run away from the 1982-2011 period. The gap between the two, which was around 0.44 degrees at the start of the year, stood at around 0.79 degrees at the end of 2023 (the latest data which was available).
At certain points in the year, particularly in August and September, it touched 0.85 degrees. The monthly average of this deviation presents this quite clearly. While the average deviation through January and February were 0.44 degrees and 0.45 degrees, this gap widened to 0.54 degrees in March, 0.65 in April and 0.64 in May, data shows. By August, it hit a peak of 0.82 degrees, then 0.81 degrees in September, finally dropping slightly to 0.80 degrees in October and 0.79 degrees in November.
Based on data till December 23, this figure stood at 0.78 degrees for the month of December — which presents an altogether new problem for 2024. The new year will start off around 0.79 degrees or so warmer (a period which had the least deviation in 2023).
The threat this poses
The effects on marine life are likely to be more visible, with the high temperature bringing about the extinction of several species, including the corals. Higher temperatures can lead to stress, reproductive failure and increased mortality in marine species, leading to the collapse of entire ecosystems.
“Marine species have evolved over millions of years to occupy specific thermal windows. When temperature thresholds are exceeded, this can lead to high stress, reproductive failure and widespread mortality, with implications for food webs and the entire ecosystem,” Dr Dan Smale, senior research fellow at the Marine Biological Association, UK, told the Science Media Center in June.
Jacobson said the rising temperatures could trigger multiple tipping points — critical thresholds in the climate system that when breached could lead to large, cascading and irreversible changes.
“Coral bleaching is a big issue, and it’s going to possibly lead to massive die-offs of coral in 2024 as we hit new highs. Also, deoxygenation and acidification of the world’s oceans are ongoing and getting worse, leading to the regional die-off of many fish species and even the possibility of extinction for some species. Warmer oceans mean more methane released as frozen methane hydrates melt. Finally, the AMOC will continue to degrade, posing an existential risk to European countries,” he said.
Future tense
The year is virtually certain to end 1.5°C higher than the pre-industrial average, making it, by far, the warmest year since measurements began nearly 8 decades ago.
The past six months in a row – June, July, August, September, October, and November – have all set new records for monthly average temperature in 2023, and mostly by large margins.
“Unless sharp reductions in man-made greenhouse gas emissions occur soon, the long-term average is likely to pass 1.5°C during the 2030s,” California-based Berkeley Earth said in its November 2023 temperature update.
All forecasts for 2024 predict a much bleaker picture. “The ocean heat content is at an all-time high. As that heat is released into the atmosphere, expect global temperatures overall to continue to spike in 2024. It is almost certain that 2024 will break the 1.5C barrier for the full year, and may even breach 1.6C. Temperatures at this level can lead to a cascade of tipping points, most notably, the melting of Greenland and Antarctic ice sheets,” Jacobsen said.
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