The record temperatures recorded never cease to make an impression. The last nine years have been, one after another, the hottest ever recorded. Despite this, even in such a context, 2023 has been an extraordinary year. Climatologists call last year's events "strange" and say they will spend the next few months or perhaps years unraveling what exactly drove such extreme values.
Data published this week by American and European public organizations and international research groups have confirmed that 2023 has been by far the hottest year ever recorded. According to the World Meteorological Organization (WMO), it was 1.45°C warmer, plus or minus 0.12°C, than pre-industrial temperatures. That is touching the symbolic threshold of 1.5°C warming. The year brought a series of regional records and extreme events, such as floods, forest fires, droughts and heat waves. Below, eight graphs help understand the situation.
In the northern spring, researchers observed the first signs of the arrival of El Niño, a natural climate cycle capable of temporarily raising temperatures between 0.1°C and 0.2°C. El Niño events are gradual processes that spread heat and energy from the tropical Pacific to other parts of the planet. They usually cause a rise in air temperatures the year after their appearance. However, last year's surprising rise caught climatologists off guard. Each of the last seven months of 2023 has been considerably warmer than the corresponding months of any other year.
The second graph shows that almost half of the days of the year were at least 1.5°C warmer than pre-industrial averages. Two days in November were more than 2°C warmer. Days above 1.5°C were recorded for the first time in 2015, but have never been so numerous before; and days above 2°C had never been recorded.
Each year, climate modelers are able to project how warm temperatures will be over the next 12 months (based on the cyclical nature of El Niño and the long-term warming trend of greenhouse gas emissions). Taking into account the margins of uncertainty, the projections are usually quite good. "In 2023, the model is shattered," says Zeke Hausfather, a modeler at Berkeley Earth. The only other year in which actual temperatures were outside the error range of the projections was 1992, when the eruption of the Pinatubo volcano in the Philippines injected particles of soot and pulverized rock into the stratosphere that acted as a natural parasol and caused a brief cooling of the planet.
Next, let's consider which regions caused an increase in average annual temperatures. El Niño is characterized by warmer than usual surface waters in the tropical Pacific (see gray area). It is observed that its pattern increases from the second quarter of the year. However, El Niño cannot fully explain the warm rarity of 2023. Anomalous and extreme temperatures in the oceans and on land were reached before the WMO formally declared the onset of El Niño in July and before its effects had been felt. spread from the Pacific to other parts of the world.
The warming of the northern subtropical oceans was especially unusual and notable. So were the warm temperatures in the subtropical continental masses, which caused extreme regional values in populated areas (pink band).
A closer look at the subtropical oceans shows that the North Atlantic had an especially strange year. Surface temperatures soared. There is no doubt that this increase was an important factor in raising the world average for the year. However, its cause is still debated. One possibility is that it was linked in some way to the heat that has been accumulating for decades in deeper layers of the ocean (up to 2,000 meters). However, researchers are still not sure about this.
It was also a very unusual year on the other side of the planet. The ring of sea ice surrounding Antarctica shrank to record lows in its winter, when sea ice should be at its greatest extent. "What is happening in Antarctica causes fear," says Francesca Guglielmo, a climatologist at the Copernicus Climate Change Service, an agency of the European Union. Some scientists worried that Antarctica had entered a new phase in 2016, when its sea ice began to show signs of decline. That is also the subject of debate. The situation seemed to recover in December; Researchers are watching how things will evolve in 2024.
The specific geophysical processes that created all that weirdness will keep climatologists busy for months to come. However, the long-term warming trend is clearly driven by industrial greenhouse gas emissions. In 2023, as at regular intervals in the past, this warming was amplified by a relatively strong El Niño, especially after three consecutive years of La Niña, its cooling counterpart.
Three other factors may have contributed marginally to the onset of warmer temperatures. The first is the 11-year cycle in the sun's activity and radiation. The solar cycle influences the Earth's climate (although only to a very small degree compared to greenhouse gases). Currently, the cycle is in a growing phase and raises temperatures a little. It will continue to do so until it peaks and enters the waning phase sometime around 2025.
Secondly, on January 15, 2022, an eruption of the Hunga Tonga-Hunga Ha'apai volcano, in the southern Pacific Ocean, threw gases, soot and water vapor into the atmosphere. Large volcanic eruptions of this type usually cool the climate for a few months or years. (Soot and pulverized rock particles produce an umbrella effect). Now, since Hunga Tonga is a seamount, it also generated an enormous amount of water vapor, a greenhouse gas. According to one study, around 146 million tons of water vapor ended up in the stratosphere, which saw its water content increase by approximately 10%. The degree of warming caused by this phenomenon in 2023 is still not known exactly. The good news is that the steam will end up falling back to Earth and its warming effects will disappear.
A third factor will have longer-term consequences. Regulations that came into force in 2020 have begun to clean the fuel used by ships of sulfur. Historically, suspended particles in the exhaust gases of diesel engines have had a sunshade, and therefore cooling, effect on the Earth. So cleaning up emissions from shipping contributes, perversely, to warming the planet. That effect will persist. Some believe it has contributed to the year 2023 being especially warm. Others point out that emissions from shipping constitute a small fraction of the total aerosols generated by other industrial facilities, such as factories and power plants. The oceans would also have been expected to warm sooner.
What does all this mean for 2024? Water vapor from Hunga Tonga will continue to warm the climate for some time. Likewise, the soot of shipping will remain absent. Greenhouse gas emissions will continue to accumulate in the atmosphere. El Niño is expected to peak in the first half of the year, but it is too early to know whether the cycle will head toward a neutral phase or a cooling La Niña phase.
However, annual "one year ahead" projections agree that 2024 is likely to be slightly warmer than 2023 (see the eighth and final graph). Asked how last year's turn of events affects his confidence in the projections, Hausfather smiles. "I definitely think it makes me trust a little less," he says. "Maybe what happened in 2023 is a strange combination of factors that will not continue in 2024. Or maybe it will. Time will tell."
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Translation: Juan Gabriel López Guix
