For almost a year now, the planet has been in uncharted territory. Since June 2023, every consecutive month has been the hottest on record. Antarctic sea ice has been tracking at near-record lows for months. Sea surface temperatures have . And in the midst of this, the world has been gripped by floods, heatwaves, droughts and storms.
With each consecutive month of record-breaking temperatures, concerns are growing that the world has tipped into a new phase of warming. But is climate change really accelerating? Like everything to do with climate change, it is complicated. What is clear is that the extraordinary heat of 2023 has climate scientists rattled 鈥� and even, in a rare example of disagreement across the field, butting heads.
鈥�2023 was unusually warm, and we still don鈥檛 have a good explanation for why,鈥� says at US non鈥憄rofit Berkeley Earth. 鈥淭hat is worrying in its own right.鈥�
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鈥淚鈥檝e been using the word 鈥榙isquieted鈥�,鈥� says at NASA. 鈥淲e like to have answers, we like to be able to explain things, especially important things like the climate. And up until last year, we were pretty good at explaining things.鈥�
The basic facts aren鈥檛 in contention: Earth鈥檚 climate is warming, mostly due to the greenhouse gases we have emitted, and it will continue to do so until we reach net zero. But because the climate operates on the scale of decades, not years, interpreting 2023 has caused a schism in climate science circles.
鈥淭he sharp uptick in global temperatures last year cannot yet be used as proof of an even steeper acceleration in the rate of climate change,鈥� says at the Met Office, the UK鈥檚 national weather service. 鈥淥ne year does not make a climate.鈥�
So what is going on? that the world will indeed warm faster in current and future decades compared with the rate of warming experienced since the 1970s. That is because while planet-warming emissions of carbon dioxide are still rising, emissions of planet-cooling aerosol gases have been falling dramatically in recent years, as part of a global crackdown on air pollution. Since 2006, China鈥檚 emissions of sulphur dioxide have fallen by more than 75 per cent, for example.
To be clear, this means that climate scientists are in agreement that we should expect an acceleration. The latest assessment from the Intergovernmental Panel on Climate Change suggests the rate of warming up to 2050 will be about 26 per faster than that from 1970 to the present day.
Where the disagreement arises is on whether we can already detect an increase in the rate of warming from real-world observations. 鈥淚t鈥檚 not necessarily that warming is happening faster than climate scientists expected,鈥� says Hausfather. 鈥淏ut it may well be that warming is happening faster than we have seen historically.鈥�
He says other climate data beyond just average temperatures, such as ocean heat figures, indicate an acceleration is already under way. 鈥淭here鈥檚 a consilience of evidence across multiple lines.鈥�
at the University of Reading, UK, says from the sun penetrating Earth鈥檚 atmosphere since 2001. 鈥淚f we are increasing the amount of shortwave radiation that鈥檚 coming into the climate system, we would expect that to cause a warming.鈥�
But there is a catch. Enter El Ni帽o, the weather pattern that develops when waters in the Pacific Ocean warm, which tends to push up global temperatures. 鈥淭he 2023 warming spike is due to El Ni帽o,鈥� says at the University of Pennsylvania.
Jones suggests the record heat in 2023 could be a result of the El Ni帽o pattern increasingly acting as a release valve for excess heat stored in the ocean.
鈥淭he ocean has always been accumulating heat, but it鈥檚 been accumulating more and more,鈥� he says. As the ocean becomes more stratified due to colder waters sinking, more of that heat is held in surface waters rather than penetrating into the deep ocean. 鈥淭hat might mean the heat that is going into the ocean on a slow timescale tends to be residing incrementally nearer the surface. Therefore, when you have an event like an El Ni帽o, which allows heat back to the surface, there鈥檚 more of it to more readily come back,鈥� says Jones.
As a result, these temperature 鈥渂umps鈥� from El Ni帽o might become larger in future years. 鈥淲e鈥檙e going into difficult territory, there鈥檚 no question about that,鈥� he says. 鈥淎nd it鈥檚 the heat in the ocean that I think is the primary driver.鈥�
But others are sceptical that El Ni帽o explains all of 2023鈥檚 record warmth. For a start, the current El Ni帽o doesn鈥檛 fit the pattern of previous events, says Schmidt, with temperature records broken even before it had fully developed. 鈥淲e saw record warm temperatures in the North Atlantic starting in March, record warm temperatures globally starting in June, record low sea ice starting in July, in Antarctica,鈥� he says. El Ni帽o wasn鈥檛 formally declared until June 2023.
If El Ni帽o can鈥檛 explain the extraordinary warmth of 2023, something else may be going on. Research is under way to assess the impact of additional cooling from volcanic eruptions, for example, and the effects of tougher aerosol pollution rules for shipping. As well as uncertainty about the volume of aerosols in the atmosphere, there could be errors in current assumptions about their cooling impact on Earth, which could account for some of 2023鈥檚 unexplained heat.
In a press briefing last month, at the Copernicus Climate Change Service said that even accounting for impacts from shipping, volcanoes and other factors, researchers can only partially explain the year鈥檚 record heat.
鈥淲hen we look at these different contributions, when we look at greenhouse gas concentrations, we can say that has contributed about half of the warming that we saw in 2023, from the previous warmest year,鈥� she said. 鈥淲hen we look at other contributing factors, including natural variability, including aerosols, including the solar cycle, that also contributes about a third of the remainder. So we are missing somewhere between 0.05 to 0.1 of a degree of the additional heat that we saw in 2023, compared with the expectations from where we were in 2022.鈥�
The puzzle may not be resolved until later this year, when El Ni帽o is expected to fade. If temperatures also dip, that will be a strong hint that El Ni帽o was ultimately to blame 鈥� but if not, there is the possibility that the world has entered a new phase of climate change with impacts beyond what scientists have predicted.
鈥淚f the anomaly does not stabilize by August 鈥� a reasonable expectation based on previous El Ni帽o events 鈥� then the world will be in uncharted territory,鈥� Schmidt this year. 鈥淚t could imply that a warming planet is already fundamentally altering how the climate system operates, much sooner than scientists had anticipated.鈥�
To date, climate models have been remarkably accurate at forecasting how much global warming a given level of greenhouse gases will produce. However, there is still a lot of uncertainty about climate feedbacks, which could result in more greenhouse gases in the atmosphere than currently expected 鈥� for instance, if forests die off at lower levels of heat and drought than predicted.
However, scientists are nervous about sounding the alarm too early. They stress that even though 2023 was an unusual year, it still lies within the range predicted by long-term models. And it will take years before any long-term trend becomes clear.
鈥淭here is a risk of jumping to conclusions here,鈥� says Hausfather. 鈥淲e don鈥檛 have much data yet, and we don鈥檛 have great answers. We can鈥檛 rule out the possibility that 2023 was just a short-lived internal variability that is not going to persist.鈥�
Mann goes further, drawing parallels between the 2023 puzzle and the now-debunked scientific speculation in the early 2000s that climate change had 鈥減aused鈥�. 鈥淭his is simply the flip-side of that, an overreaction in the other direction that I fear will, too, call into question the credibility of climate science and scientists,鈥� he says.
