Month after record-breaking month, 2023 is on track to be the hottest year measured in human history.

It has been a year of extraordinary drought, deadly rainfall, and searing heat waves. Extreme temperatures even reached underwater. Much of the southern hemisphere basked in summer-like weather through its winter, reaching all the way down to Antarctica.

Particularly notable is that 2023 may mark the first time global average temperatures have risen above a critical line, providing a glimpse into a world where humanity fails to get climate change under control. By the end of the year, some datasets may show the earth’s temperature on average was 1.5 degrees Celsius, 2.7 degrees Fahrenheit, warmer than temperatures before the Industrial Revolution.

That number — 1.5°C — stands out because it was a limit established as part of the 2015 Paris climate agreement. Almost every country in the world agreed that humanity needs to hold warming this century to “well below” 2°C, 3.6°F, above pre-industrial times, ideally limiting the rise to 1.5°C.

Global temperatures can rise and fall year to year due to natural variability, so the 1.5°C limit is calculated as an average over decades. “The broad takeaway is that virtually everyone agrees that a single year passing 1.5 degrees does not mean we’ve passed the Paris Agreement target,” said Zeke Hausfather, a research scientist at Berkeley Earth, an independent climate research group. He added that the accord doesn’t explicitly lay out how to measure these goalposts.

And in 2023, there were several natural forces converging on top of human-caused warming pushing up temperatures around the world. For example, in addition to heating caused by greenhouse gasses from burning fossil fuels, temperature cycles in the Atlantic Ocean and the El Niño pattern in the Pacific Ocean converged in their hot phases this year.

However, such a record-breaking year presents a vivid example of the conditions that may soon become typical in a warmer world, or even on the cooler end of possibilities. And for people concerned about the devastating effects of climate change, it’s ramping up the urgency to keep greenhouse gasses in check.

In 2018, the Intergovernmental Panel on Climate Change, the United Nations team of scientists, put out a special report examining the differences between staying below 2°C and staying below 1.5°C. They found that every fraction of a degree of warming had additional adverse consequences for the planet. Conversely, every bit of warming avoided yielded benefits to humanity. So 1.5°C is not intended as a threshold or a tipping point for the planet, but a practical target for countries.

Climate activists and many of the countries most vulnerable to rising seas and extreme weather seized on the findings. Their pressure has now turned 1.5°C into a rallying cry and a de facto limit for climate diplomacy. At the upcoming COP28 climate summit in the United Arab Emirates, where delegates will hammer out the details of meeting their climate promises, 1.5°C will take center stage.

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“Keeping 1.5 alive is a top priority and it will cut across everything I do,” Sultan al-Jaber, president of COP28 and the head of the UAE’s state-owned oil company, told Reuters in February. However, the UAE is facing criticism that its climate plan isn’t in line with this goal. And according to the UN, the rest of the world is also doing far too little, and time is running out.

To keep global average temperatures from rising beyond 1.5°C by 2100, greenhouse gas emissions need to fall 43 percent by 2030 relative to 2019 levels. Yet even taking into account commitments on paper, global emissions are poised to increase in the coming years. For climate negotiators, that raises an uncomfortable question: What’s the point of upholding this target if most indications show the world will miss it?

When asked this question, a US State Department official said they didn’t have a good answer, but noted that it would be very difficult to get countries to agree to a different goal.

With so much of the world reeling this year from disasters unlike any they’ve ever witnessed, the pressure is stronger than ever for countries to take bigger steps to stop climate change from getting worse. How much warmer it will get is now up for negotiation.

How 2023’s record temperatures fit into the big climate picture

To put this year into context, it’s worth unpacking how global temperatures are measured and calculated. The world has already warmed by about 1.1 degrees Celsius, 1.98 degrees Fahrenheit, compared to global average temperatures before the Industrial Revolution.

But that’s an average. Judging how much an individual year has warmed compared to the era before humans started burning coal, oil, and natural gas in gargantuan volumes is trickier. Many of the research groups that track global temperatures — the Met Office Hadley Centre, NOAA, the European Union’s Copernicus, Berkeley Earth — broadly agree on the temperature records over the past few decades.

Where they differ is in calculating the baseline in the 1800s, when there were far fewer thermometers and certainly no weather satellites. “The methodological choices the groups make of how to fill in those gaps affects temperatures a lot more back then than the last 50 years,” said Hausfather, who is also the climate research lead at Stripe. So, while Berkeley Earth’s data set is likely to show that 2023 was 1.5°C warmer than pre-industrial temperatures, other research groups may not come to the same conclusion. “In fact, we’re almost certainly going to have a split verdict,” Hausfather said.

Still, 2023 is on track to be the hottest year humans have ever experienced, and by a wide margin according to most data sets. September, for instance, was warmer by a larger amount than any month measured by NOAA in 174 years. “To put it another way, September 2023 was warmer than the average July from 2001-2010,” said NOAA chief scientist Sarah Kapnick in a press release.

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Several factors beyond human-produced carbon dioxide emissions are at work this year. The big one is that there’s a strong El Niño this year, which tends to drive up global temperatures. The Hunga Tonga-Hunga Ha‘apai volcano in Tonga that erupted last year may also be playing a role. Manoj Joshi, a professor of climate dynamics at the University of East Anglia in the United Kingdom, noted that, unlike most volcanic eruptions that spew aerosols that end up cooling the planet, the Tonga volcano sent an unprecedented amount of water vapor into the atmosphere. Water vapor can act as a heat-trapping gas.

There were also fewer cooling aerosols from other sources due to regulations to protect air quality, including rules to limit shipping pollution that went into effect in 2020. Weaker air currents over the Atlantic Ocean also led to less than typical amounts of dust from the Sahara desert getting whipped up and dimming the sun. That meant more of the sun’s rays hit the seas in some of the hottest times of year.

“When you add all these things up, you can start to explain what’s going on,” Joshi said.

However, these variables don’t say for certain whether 2023 is just a blip or the start of a trend, even though some months were warmer by huge, unprecedented amounts. “You need to look at ’24, ’25, ’26 and if it carries on like that, then you can say, right, it’s more likely to be an acceleration,” Joshi said.

Climate scientists prefer to track climate change using 20- to 30-year averages, which helps smooth over year-to-year variations. However, the window to act on climate change is closing and no one wants to wait a decade to find out whether the world already missed its target. So scientists use models and projections to estimate when that will happen. Right now, most models project that the world’s average temperature will rise 1.5°C above pre-industrial levels in the early 2030s, but some forecasts show that could occur as soon as 2027.

The strange, extreme weather of 2023 thus might best be interpreted as a window into the future. “In a warming world, these temperatures aren’t going to be exceptional in 10-20 years time,” Joshi said.

Does 1.5°C still matter?

Setting a climate change target has always been contentious. The countries losing land to rising oceans want a lower cap on warming than nations that depend on selling fossil fuels. There’s also a fundamental injustice behind climate change, that the countries that contributed least to the problem are usually among those that stand to suffer the most.

Since the 1970s, scientists and economists have tried to come up with an objective way to figure out how much warming humanity could tolerate. Looking at human history prior to the Industrial Revolution, researchers found that the planet’s temperature varied, but within a narrow band. It stood to reason that there was a point where the planet would get too hot for people to flourish or yield dangerous, unexpected effects.

“If there were global temperatures more than 2 or 3°C above the current average temperature, this would take the climate outside of the range of observations which have been made over the last several hundred thousand years,” wrote economist William Nordhaus in 1975. In his paper, he also noted that there are costs to keeping climate change in check, benefits from using fossil fuels, and damages incurred by warming.

Coming up with a target requires balancing these trade-offs, as well as anticipating other factors like advances in technology. For climate negotiators, it was still a fraught process to come up with a number that was defensible and palatable. After decades of analysis and wrangling, the 2009 Copenhagen agreement landed on 2°C, which was cemented in the 2015 Paris Agreement.

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“It’s both a political construct and a scientific construct,” Rachel Kyte, an adviser to the COP28 climate negotiations and dean emerita of the Fletcher School at Tufts. “It helps to know the point on the horizon to which everybody is pointed.”

But even at the time of the Paris Agreement, staying below 2°C was a long shot. Eight years later, with global emissions still rising, 1.5°C looks almost impossible. “1.5 is deader than a doornail and anybody who understands the physics knows that,” James Hansen, a former NASA climate scientist, told reporters earlier this month.

Every scenario chalked out by the IPCC in its most recent report shows that the world is likely to blow past the 1.5°C goal. The most optimistic pathway counts on the world falling below the line eventually as natural carbon sinks soak up emissions and technologies like capturing carbon dioxide directly from the air come online.

“We’re running out of budget really quickly. It’s looking more and more likely that we can’t get to 1.5 through the front door,” Kyte said. “We’re going to overshoot and have to come back down to it.”

But Kyte noted that for all the talk about specific temperatures, the cause of climate change isn’t lost if the planet does indeed warm up past 1.5°C. Every fraction of a degree of warming avoided is beneficial over the long term, while switching to cleaner energy and reducing pollution can have immediate benefits.

The basic formula for curbing climate change is the same as it was in Nordhaus’s 1975 paper: reduce greenhouse gas emissions, negate the effects of warming, and pull carbon dioxide out from the atmosphere. The good news is that there have never been more tools to limit emissions, they’ve never been cheaper, and they’re getting better all the time.

“We simply have to cut emissions faster than we are at the moment,” Kyte said.