What happens when once reliable indicators of the world’s climate future suddenly seem not so dependable?
That’s one startling puzzle from 2023, when scientists’ predictions based on past observations and trends failed to warn of a huge spike in land and ocean surface temperatures. So far, in 2024, predictions for global temperatures are aligning with real world observations, suggesting last year may have been an anomaly.
Going into 2023, prominent climatologists gave relatively low odds that the year would be the hottest on record, which it was. The world was coming out of three years of La Niña, a climate pattern marked by cooler than usual temperatures in the eastern Pacific Ocean around the equator, which affects weather both in nearby South America and far away.
“Drawing on precedents when similar conditions prevailed at the beginning of a year, several climate scientists, including me, put the odds of 2023 turning out to be a record warm year at just one in five,” writes Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies, in a March 2024 essay in Nature.
The margin of warming also surprised climatologists — Earth in 2023 was about 0.2 degrees Celsius warmer than predicted, Schmidt writes in Nature.
The 10 warmest years on record have all happened from 2014 to 2023, according to the National Oceanic and Atmospheric Administration.
“Most records are set on the order of a few hundredths of a degree, so this is a really big jump,” said Russ Vose, chief of monitoring and assessment at the National Centers for Environmental Information, part of NOAA, during a media briefing earlier this year.
And 2023 wasn’t just the hottest year on record.
The summer of 2023 may have been the hottest summer in the past 2,000 years in the Northern Hemisphere, according to estimates in a May 2024 paper in Nature.
The best temperature measurements available today began in the U.S. and the U.K. in 1880. Climatologists use this data to extrapolate global temperatures. For earlier estimates, scientists rely on other measurements, such as tree rings, which is what the authors of the Nature paper used.
Certain trees grow slowly during colder years and faster during warmer years, meaning the size of their rings is determined by the climate at the time.
Roughly 90% of people live in the Northern Hemisphere.
“The year 2023 may well come to be seen as a watershed year for the climate,” writes Joy Merwin Monteiro of the Indian Institute of Science Education and Research in a May 2024 commentary in Communications Earth & Environment.
How yearly temperature records get broken
There are two things audiences should know about yearly warming records. The first is that humans are warming the planet, in large part by burning fossil fuels. The ongoing warming of the planet since the industrial revolution is a long-term, scientifically observed trend, and its rapid pace is caused by humans. Humans are responsible for a roughly 1 degree Celsius increase in global temperatures compared with pre-industrial levels, according to the UN’s Intergovernmental Panel on Climate Change.
“There is unequivocal evidence that Earth is warming at an unprecedented rate,” according to NASA. “Human activity is the principal cause.”
The second is natural yearly climate variability — such as whether an El Niño or La Niña climate pattern is present. When the overall, long-term warming trend caused by humans coincides with natural climate patterns that tend to warm the planet, such as El Niño, that is usually when climatologists predict warmer-than-usual years.
The heat has persisted in 2024, with June 2024 marking the 13th month in a row that broke global temperature records, according to Berkeley Earth, a nonprofit climate data organization founded by Richard Muller, a professor emeritus of physics at the University of California, Berkeley.
July 22 was also the hottest day on record, according to the NASA Goddard Institute. The previous single-day record was set in July 2023.
None of that is necessarily surprising, as the tail end of El Nino patterns tend to warm the planet going into a new year.
“Things do seem to be acting pretty much according to expectations, and the expectation is that 2024 will be a record-breaking year,” Schmidt says.
Put another way, the warmth in 2024 is something climatologists saw coming, as El Nino is a reliable driver of global temperature increases. One paper, published in February 2024 in Nature’s Scientific Reports, put the odds at 90% that global surface air temperatures would beat historical records over the year spanning July 2023 to June 2024. Climate models are again meeting expectations.
“If things are now back to — not normal, there’s no normal anymore, everything is shifting — but things are back to within what we are predicting or what we expect, then it makes it look like 2023 is more blip-like,” Schmidt says. “Which means that it isn’t going to have a sustained systemic shift going forward, which would be good news. But the whole situation is extremely unsatisfactory and I want us to have a better understanding of what’s actually going on.”
Understanding what happened in 2023 is important because climate change is affecting every municipality, state and country — and nearly every news beat. Here are some examples:
Agriculture: Farmers worldwide use seasonal climate predictions to decide what to grow and when. More broadly, global warming could drastically reduce yields across the world for staple crops such as corn and wheat as soon as 2030, according to research published in Nature Food.
Emergency response: Federal and local officials are incorporating climate change into their planning, including for ongoing sea-level rise due to melting glaciers and preparedness for more frequent and intense coastal storms.
Aviation: Commercial airliners use weather predictions to manage flights, and most flight delays are weather-related. Weather conditions can be affected by climate patterns, such as El Niño and La Niña. Burning jet fuel drives climate change, research has shown. Research also suggests warmer temperatures and other consequences of climate change have made turbulence worse in recent decades.
Migration: Environmental disasters and long-term warming and drought may displace millions of people globally, though researchers caution that current predictions of climate-related migration are unreliable. That’s partly because people move for a variety of reasons, sometimes but not always because of climate factors.
But the beginning of 2023 saw the winding down of three straight La Niña years, which led climatologists to predict a not-quite-so-hot 2023. Climatologists have suggested several factors that could explain the annual record set in 2023. The most researched among these are:
- The 2022 eruption of the Hunga Tonga-Hunga Ha’apai volcano in the Southern Pacific Ocean.
- An international regulatory change on shipping emissions.
- The El Niño climate pattern.
Overall, research indicates it’s still an open question as to why 2023 was so unexpectedly warm. Still, Schmidt says, it’s important that climatologists figure out why 2023 was so hot to rule out some unknown systemic change in the climate that is accelerating warming — or, more troubling, to pinpoint such a change.
That’s why there will be a session dedicated to in-progress research on the unexpected warmth in 2023 at the annual convening of the American Geophysical Union in mid-December in Washington, D.C.
One area Schmidt singles out for further research: how social and economic shifts related to the COVID-19 pandemic, such as the rise of work-from-anywhere and reduced commuting to city centers, could be affecting the climate.
Below we cover what the research says (so far) about why 2023 was so hot — plus, 3 tips for journalists covering climate change.
The Hunga Tonga-Hunga Ha’apai volcanic eruption
The Hunga Tonga-Hunga Ha’apai volcano erupted in January 2022, spewing water vapor and sulfur dioxide into the stratosphere. The stratosphere is the second-lowest layer of Earth’s atmosphere, located about 7.5 to 31 miles above its surface.
The plume reached nearly 36 miles up and into the next atmospheric layer, the mesosphere, according to a November 2022 paper in Science. It’s likely the highest volcanic plume on record, according to NASA.
When sulfur dioxide interacts with other atmospheric chemicals, sulfate aerosols form that scatter light and prevent the sun’s energy from reaching Earth. This is why volcanic eruptions usually cool the planet, despite expelling super-hot gases.
But Hunga Tonga-Hunga Ha’apai was also the biggest underwater eruption on record, according to NOAA, sending out 150 million tons of water vapor, the most prevalent greenhouse gas. Globally, the eruption led to up to a 5% increase in water vapor in the stratosphere, according to a September 2022 paper in Science.
The question for climatologists: Which effect would win out? The cooling aerosols or the warming water vapor? Initially, scientists seriously considered the possibility that the eruption could warm the planet, according to a paper published in January 2023 in Nature Climate Change.
The key was where the volcano spewed the water vapor — and the answer is the mid-stratosphere, says Andrew Dessler, a professor of atmospheric sciences at Texas A&M University. The amount of energy greenhouse gases trap depends on the difference between the temperature of the gas and the temperature of the atmospheric layer it’s in.
The mid-stratosphere is fairly warm, Dessler says. This is where the ozone layer forms, and ozone traps ultraviolet light from the sun and radiates heat.
By contrast, the upper part of the troposphere is quite cold. Climbers atop Mount Everest reach the upper troposphere. The lower troposphere is where earthbound creatures live and where weather occurs.
If water vapor had been mostly sent into the upper troposphere, the warming effect would have been greater, Dessler says, because of the greater difference between the temperature of the vapor and the upper troposphere. Because the vapor spread mostly throughout the mid-stratosphere, the temperature difference was not as wide.
While the volcanic plumes persisted into 2023, by the end of the year any climactic effects from the eruption were gone, according to a July 2024 paper published in the Journal of Geophysical Research: Atmospheres by Dessler and several coauthors. In fact, there may have been an overall slight cooling effect in the Southern Hemisphere due to the eruption, the paper finds.
“We can confidently rule it out as a large contributor,” Dessler says of the Hunga Tonga-Hunga Ha’apai eruption’s effect on global warmth in 2023.
Shipping emissions
Somewhat ironically, an international shipping rule aimed at improving air quality by reducing sulfur emissions is one hypothesis climatologists have considered as a possible driver of the 2023 warming. In January 2020, the International Maritime Organization reduced the allowable sulfur content in fuel oil for ships from 3.5% to 0.5%. The IMO is the United Nations agency that regulates international shipping.
The sulfur content of fuels used in shipping vessels was previously higher than for fuel used on land, according to a 2023 report from the U.K.-based climate news and data analysis site Carbon Brief. Maritime sulfur emissions dropped sharply once the IMO rule took effect.
“Sulfur burnt in ships produces sulfur dioxide,” Schmidt says. “So that’s the same mechanism as the volcano.”
In other words, the aerosols resulting from burning relatively sulfur-rich maritime fossil fuels reflected light from the sun and held down the global temperature. Low-lying sulfur aerosols also increased the reflectivity of clouds, which prevented the sun’s energy from reaching Earth, according to an October 2022 paper in the Proceedings of the National Academies of Sciences.
By 2021, the rule had led to a 70% drop in sulfur oxide emissions from shipping, according to the IMO. Most of that decrease would have occurred in the Northern hemisphere where ships travel, Schmidt explains — across the Northern Atlantic Ocean, for example, through the Panama Canal, around Africa and through the Indian Ocean.
Current research suggests the shipping emissions regulation is unlikely to have had enough of an effect to explain the 2023 warming. The Carbon Brief analysis, for example, estimates a global temperature increase of only 0.05 degrees Celsius by 2050 related to the rule change.
Meanwhile, a May 2024 paper in Communications Earth & Environment suggests the rate of global warming will increase rapidly throughout the 2020s because of the emissions rule change. But a preprint paper by several British climatologists suggests warming in line with the Carbon Brief estimate.
Climatologists are still researching the effects of the IMO’s emissions rule change on the 2023 warming. While the emissions regulation theory “seems to be a little bit more well supported” than the volcano theory, it “doesn’t quite match up with what’s going on,” Schmidt says. “And, of course, if the volcano was actually having a cooling effect, then we need more warming effects to kind of cancel out the cooling effect of the volcano.”
Environmental events can indeed cancel each other out. For example, the El Niño pattern that persisted into 1992 should have led to higher global temperatures. But it was preceded by the 1991 eruption of Mount Pinatubo in the Philippines, which sent out sulfate aerosols that cooled the earth. In effect, the events neutralized one another and 1992 was not a record warm year, according to the May 2024 paper in Nature.
El Niño
El Niño climate patterns are typically a reliable indicator of global warmth, but “El Niño usually exerts a bigger impact in its decaying year,” according to an April 2024 paper in Communications Earth & Environment.
With the most recent El Niño pattern arising in mid-2023 and dissipating in 2024, 2024 is the year where El Niño should be affecting global temperatures. And 2024 does indeed have the potential to surpass 2023 as the hottest year on record, according to climatologists. But that doesn’t explain what happened in 2023.
La Niña patterns dominated the previous three years which “may have hindered increases of global temperatures” and “probably contributed to the relatively abrupt warming in 2023,” according to the April 2024 paper.
The author, Seung-Ki Min of the Pohang University of Science and Technology in South Korea, uses computer simulations to compare the percentage of Earth affected by hotspots and the strength of El Niño patterns. Min estimates that the strong 2023 El Niño would have affected a “very small” fraction of the planet — about 3.4% — though he writes that figure represents “a tentative first estimate.”
Meanwhile, a July 2024 preprint paper by researchers at the University of Miami and Princeton University notes that global temperature spikes in 1957 and 1977 also featured a prolonged La Niña followed by an El Niño, suggesting that it is not only the presence of an El Niño that matters, but what came before it.
Still, this remains another area where early research suggests no obvious, major driver of the unexpected warmth in 2023, though the body of analysis on El Niño is small in this instance.
3 tips for covering and localizing global warming
1. Remind audiences of the big picture: Intergenerational global warming is happening, along with lobbying from the fossil fuel industry.
What happened in 2023 remains a riddle that has piqued intense curiosity from climatologists, but Dessler says finding an answer won’t change the fact that global warming, caused by human activity, is still happening with no end in sight.
“We now have the technology to stop doing it, to stop warming the climate mostly,” Dessler says. “The other thing that matters is the reason we’re not doing it is because of the politics, and because of how immensely powerful fossil fuels are as a special interest.”
Fossil fuel companies are more likely to spend money lobbying lawmakers in Congress and government agencies when major news organizations, such as The Wall Street Journal, The New York Times, the BBC and CNN, report on the negative effects of climate change, finds a March 2023 paper published in Global Finance Journal.
The top five biggest spenders and the most active corporate lobbyers are oil companies: Exxon Mobil, Chevron, Royal Dutch Shell, ConocoPhillips and BP, according to an analysis of 476 global and regional energy firms from 2009 to 2018.
Generally, firms in the dataset that have shown little progress toward decarbonization “are spending most heavily on lobbying and preventing the enactment of urgently needed climate policies,” the authors write.
2. Seek sources that can offer a view of the past.
Local weather won’t noticeably change because a particular year was the hottest on record by a tenth or hundredth of a degree.
“That’s not really relevant for your local story,” Schmidt says. “But is your weather different now than it was 50 years ago? The answer is absolutely yes.”
Reporting on local effects of climate change should involve talking to people who remember what regional weather was like decades ago and can describe specific ways it has changed.
Is there a lake where people ice fished last century, but where now that’s not possible? Do people remember being fine during the summer using window fans or letting the natural breeze cool them, instead of air conditioners? Are there mountains that used to be reliably snowcapped in the winter or spring, where that’s no longer the case? Do streets now flood every few months instead of once or twice every few years?
Those are just a few questions journalists can pursue to develop a compelling story or series based on community members’ remembrance of climate past.
3. Convey that exact global temperature measurements are not possible.
Like all science, climatology primarily operates within ranges of probability rather than precision or absolute proof. While many governmental and non-governmental organizations release seemingly exact measurements of the global average temperature, or precisely how much the planet has warmed since the industrial revolution, journalists should treat those figures with some caution, Schmidt says.
It’s not that they’re wrong, but rather they are estimates, interpolated from incomplete data. They’re also not the most important story to report, Schmidt says.
“Who lives in the global mean temperature? Nobody,” he says. “It’s a story because records are being broken. It’s not a story because it’s exactly 17.09 degrees Celsius.”
Surface temperature measurement has markedly improved over the past century. There are tens of thousands of temperature stations globally. But from 1850 to 1900, there were only 58 and most of those were in Europe, according to the May 2024 paper in Nature.
Still, even with exponentially more data, climatologists don’t have readings for every point on the planet. They use statistical modeling to fill in the gaps.
It’s similar to the difference between a poll and a census. Pollsters take responses on questions from a sample of a population, then interpolate and extrapolate to say something about the sentiment of the population. A census, on the other hand, is a form of data collection where every person in a population is asked the same questions.
Climate data and temperature measurements are like global polls, rather than a census.
“One has to respect what we know, but not go beyond what we know,” Schmidt says.
Research and analysis to inform your coverage
Evolution of the Climate Forcing During the Two Years After the Hunga Tonga-Hunga Ha’apai Eruption
M. R. Schoeberl, Y. Wang, G. Taha, D. J. Zawada, R. Ueyama and A. Dessler. JGR Atmospheres, July 2024.
The authors write: “Enhanced tropospheric warming due to the added stratospheric water vapor is offset by the larger stratospheric aerosol attenuation of solar radiation.”
The 2023 Global Warming Spike was Driven by El Niño/Southern Oscillation
Shiv Priyam Raghuraman, et. al. Preprint, July 2024.
The authors write: “Here we show that climate models that are subject only to internal variability can generate such spikes, but they are an uncommon occurrence. However, when a prolonged La Niña immediately precedes an El Niño in the simulations, as occurred in nature in 1956-57, 1976-77, 2022-23, such spikes become much more common.”
2023 Summer Warmth Unparalleled Over the Past 2,000 years
Jan Esper, Max Torbenson and Ulf Büntgen. Nature, May 2024.
The authors write: “Observational data from around the world show that the 2023 summer temperatures were extremely warm across the Northern Hemisphere landmasses and that these conditions continued globally until the end of the year.”
Abrupt Reduction in Shipping Emission as an Inadvertent Geoengineering Termination Shock Produces Substantial Radiative Warming
Tianle Yuan, et. al. Communications Earth & Environment, May 2024.
The authors write: “The magnitude of the aerosol cooling effect has significant implications for estimating how sensitive our climate is to greenhouse gas forcing and the amount of expected future warming for a given increase of greenhouse gas concentrations.”
The Year 2023 May Afford Us a Peek at a Warmer World
Joy Merwin Monteiro. Communications Earth & Environment, May 2024.
The author writes: “While a single year is by no means representative of changes in extreme events, 2023 underscores the importance of understanding how changes in the mean translate to changes in extremes for both global and regional climates.”
Human Influence Can Explain the Widespread Exceptional Warmth in 2023
Seung-Ki Min. Communcations Earth & Environment, April 2024.
The author writes: “This quick assessment using an area-based metric suggests that anthropogenic warming had [a] dominant role in the exceptional widespread heat recorded in 2023. This metric captures the observed abrupt expansion of unusual heat into both land and ocean in line with global warming. Yet, the mechanisms responsible for regional and seasonal warming patterns remain to be determined.”
Climate Models Can’t Explain 2023’s Huge Heat Anomaly — We Could Be In Uncharted Territory
Gavin Schmidt. Nature, March 2024.
The author writes: “Much of the world’s climate is driven by intricate, long-distance links — known as teleconnections — fueled by sea and atmospheric currents. If their behavior is in flux or markedly diverging from previous observations, we need to know about such changes in real time.”
Enhanced risk of record-breaking regional temperatures during the 2023-24 El Niño
Ning Jiang, et. al. Scientific Reports, February 2024.
The authors write: “[I]mpending warmth heightens the risk of year-round marine heatwaves and escalates the threat of wildfires and other negative consequences in Alaska and the Amazon basin, necessitating strategic mitigation measures to minimize potential worst-case impacts.”
How Well Can We Predict Climate Migration? A Review of Forecasting Models
Kerilyn Schewel, Sarah Dickerson, B. Madson and Gabriela Nagle Alverio. Frontiers in Climate, January 2024.
The authors write: “We show why, at this stage of development, forecasting models are not yet able to provide reliable numerical estimates of future climate-related migration. Rather, models are best used as tools to consider a range of possible futures, to explore systems dynamics, to test theories or potential policy effects.”
The Rising Risks of Fossil Fuel Lobbying
Viktoriya Lantushenko and Carolin Schellhorn. Global Finance Journal, May 2023.
The authors write: “Our analysis focuses on corporate lobbying, which the extant literature has recognized as being more prominent than other forms of political spending, such as campaign contributions; thus, corporate lobbying is more impactful in shaping public policy.”
Analysis: How Low-Sulfur Shipping Rules are Affecting Global Warming
Zeke Hausfather and Piers Forster. Carbon Brief, July 2023.
The authors write: “Carbon Brief’s analysis suggests that the additional warming due to the IMO regulations on marine fuel is approximately equivalent to two additional years of global greenhouse gas emissions from human activity at their current rate.”
Global Response of Upper-Level Aviation Turbulence from Various Sources to Climate Change
Soo-Hyun Kim, Jung-Hoon Kim, Hye-Yeong Chun and Robert Sharman. NPJ Climate and Atmospheric Sciences, July 2023.
The authors write: “[T]hese results imply that we can expect that climate change will cause bumpier flights with respect to all possible types of upper-level turbulence. In this regard, flight routing may become more complicated to avoid and detour around turbulent air, and this may cause extended flight times and increased fuel consumption and emissions from commercial aircraft. This in turn, may provide positive feedback, whereby these additional emissions could actually accelerate global warming.”
Tonga Eruption Increases Chance of Temporary Surface Temperature Anomaly Above 1.5 °C
Stuart Jenkins, Chris Smith, Myles Allen and Roy Grainger. Nature Climate Change, January 2023.
The authors write: “Most large volcanic eruptions are notable for their negative perturbation on global surface temperatures, since they emit large quantities of [sulfur dioxide], an aerosol particulate which scatters incoming solar radiation. However, it is possible that over a multiyear period [this eruption] will cause a temporary increase in global surface temperatures due to this large water vapor increase and lack of a large counterbalancing sulfate aerosol perturbation.”
The January 2022 Eruption of Hunga Tonga-Hunga Ha’apai Volcano Reached the Mesosphere
Simon Proud, Andrew Prata and Simeon Schmauss. Science, November 2022.
The authors write: “[W]e find that the volcanic plume produced by this volcano reached an altitude of 57 kilometers at its highest extent. This places the plume in the lower mesosphere and provides observational evidence of a volcanic eruption injecting material through the stratosphere and directly into the mesosphere.”
Water Vapor Injection into the Stratosphere by Hunga Tonga-Hunga Ha’apai
Holger Vömel, Stephanie Evan and Matt Tully. Science, September 2022.
The authors write: “This event raised the amount of water vapor in the developing stratospheric plume by several orders of magnitude and possibly increased the amount of global stratospheric water vapor by more than 5%. This extraordinary eruption may have initiated an atmospheric response different from that of previous well-studied large volcanic eruptions.”
Shipping Regulations Lead to Large Reduction in Cloud Perturbations
Duncan Watson-Parris, et. al. Proceedings of the National Academies of Sciences, October 2022.
The authors write: “Global shipping accounts for 13% of global emissions of [sulfur dioxide], which, once oxidized to sulfate aerosol, acts to cool the planet both directly by scattering sunlight and indirectly by increasing the albedo of clouds. This cooling due to sulfate aerosol offsets some of the warming effect of greenhouse gasses and is the largest uncertainty in determining the change in the Earth’s radiative balance by human activity.”
Climate Impacts on Global Agriculture Emerge Earlier in New Generation of Climate and Crop Models
Jonas Jägermeyr, et. al. Nature Food, November 2021.
The authors write: “While future yield estimates remain uncertain, these results suggest that major breadbasket regions will face distinct anthropogenic climatic risks sooner than previously anticipated.”
The Contribution of Global Aviation to Anthropogenic Climate Forcing for 2000 to 2018
D.S. Lee, et. al. Atmospheric Environment, January 2021.
The authors write: “The largest contribution to anthropogenic climate change across all economic sectors comes from the increase in [carbon dioxide] concentration, which is the primary cause of observed global warming in recent decades.”
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