But some events were so abnormal that they sent a wave of consternation through the scientific community. Antarctic sea ice is at a historically low level for this time of year, according to federal data. Sea surface temperatures across the North Atlantic have been “off the charts,” Europe’s Copernicus Climate Change Service reported, noting that the figures set records for this time of year “by a very large margin.” Water temperatures off the coast of South Florida rose to unfathomable levels in recent days, leading scientists to fear for the fate of the only living coral barrier reef in the continental United States.
“On the one hand, we knew these things were going to happen. These have been the predictions for a long time,” said Claudia Tebaldi, a scientist at the Pacific Northwest National Laboratory.
And yet, she said, “this year, in particular, has seemed so extreme. … The size of the anomalies is surprising.”
For years, climate scientists have detailed again and again the many impacts that are likely as the world grows steadily hotter, such as more intense storms, more torrential rainfall, fast-rising seas and melting ice caps.
But they also have been unequivocal that with more warming comes the possibility of unforeseen consequences — of rapid changes, irreversible collapses and other feedback loops.
More than a decade ago, a study from the National Academies of Sciences, Engineering and Medicine found that while many aspects of climate change and its effects “are expected to be approximately linear and gradual,” that won’t always be the case.
“It is clear that the risk of surprises can be expected to increase with the duration and magnitude of the warming,” the authors wrote.
That reality seems to be playing out.
“We have always said that the chances of the unexpected are growing with every increment of warming,” Tebaldi said. “The chance to trigger something [surprising] is in direct proportion to how much we warm the planet.”
That’s not to say that the alarming month that has just passed is solely a result of a hotter atmosphere.
Scientists say part of what could be driving some recent extremes is the weather pattern known as El Niño, marked by warmer-than-normal tropical Pacific waters, after three years of its counterpart, La Niña, which brings cooler-than-normal waters to the surface.
El Niño began developing this spring and probably won’t bring a crescendo of warmth to the Pacific until the end of the year. At the same time, there are other aspects of what scientists call natural variability, such as changes in wind patterns and ocean currents, that are also factors.
David Armstrong McKay, a research impact fellow at the University of Exeter, said El Niño and other natural variability probably play a role in this summer’s extreme events.
“But it’s all happening on this baseline of human-driven warming,” he said. “What used to be a rare event is becoming more common, and what used to be impossible in an unchanged climate is now becoming a real possibility.”
Gavin Schmidt, director of the NASA Goddard Institute for Space Studies, largely agrees. Conditions during what was Earth’s hottest month ever observed were “shocking, but not surprising,” he said.
But some data points — the massive surge in North Atlantic Ocean surface temperatures and minimal winter sea ice coverage around Antarctica, for instance — were unusual enough to surprise scientists.
From the British Isles to the Newfoundland coast, North Atlantic temperatures have surged nearly beyond scientists’ most extreme predictions, as much as 10 degrees Celsius (18 degrees Fahrenheit) warmer than normal last month.
Diminished cloud cover and an absence of Saharan dust plumes may be allowing more sunlight to reach the water’s surface, scientists say, though they don’t know for sure what has made temperatures surge so dramatically.
“That raises an eyebrow for me,” Schmidt said. “That seems to have happened very quickly.”
That was the most extreme example of a warming trend that extends across nearly half of the world’s ocean surface.
Global sea surface temperatures have risen about 0.15 degrees Celsius per decade, as the oceans have absorbed most of the warming that is a result of fossil fuel emissions and the greenhouse effect, said Gregory Johnson, an oceanographer at the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory.
In June and July, surface waters were closer to 0.25 degrees Celsius warmer than they were just last summer, he said.
“It’s about two decades’ worth of warming for the globe, year over year,” he said, a rapid surge that can’t fully be explained by El Niño.
“That,” he said, “makes this all the more alarming.”
Last September, McKay and other colleagues published a study in the journal Science, warning that allowing the world to warm more than 1.5 degrees Celsius (2.7 Fahrenheit) compared with preindustrial levels could trigger multiple “tipping points” around the globe.
Already, the planet has experienced more than 1.1 degrees Celsius (2 Fahrenheit) of warming, with few signs of slowing. If that trend continues, it could eventually lead to the vanishing of coral reefs, massive sea level rise due to collapsing ice sheets, widespread permafrost thaw or the demise of critical biomes such as the Amazon rainforest.
McKay said in an interview that the various anomalies on display this summer, while unsettling, do not mean major systems around the planet have crossed some unalterable threshold. Such critical shifts would probably become clear only over a long period of time.
“I’m not expecting these hot years to directly trigger climate tipping points,” he said.
At the same time, he said, that doesn’t mean certain regions or specific places aren’t already experiencing disastrous impacts that large-scale models have a hard time predicting.
“On a smaller scale, individual coral reefs or individual rainforests could tip a lot sooner,” he said. “I think it’s quite likely that these hot years are going to cause a bunch of damage and stress to a bunch of ecosystems.”
Tebaldi describes it this way: “Tipping points could happen for different people, for different communities, at different times.”
In its most recent assessment of the latest science, the Intergovernmental Panel on Climate Change detailed how different ecosystems, from boreal forests to permafrost, could change in “irreversible” ways at different levels of warming.
One system researchers are watching closely is Antarctic sea ice, which has been so slow to build up this year that it has led to questions about whether it is headed toward collapse.
Antarctic ice is always prone to fluctuation — it hit a record low in 2017 but then recovered close to its average extent. Over the past two years, however, it has hit repeated record lows during the Southern Hemisphere summer. Now, it is on pace toward a September maximum that will be by far its smallest ever — so small that scientists say it could be expected to occur just once in millions of years, if it were only a matter of natural variation.
“I can’t step out on a limb and say a tipping point has been passed,” said Marilyn Raphael, director of the UCLA Institute of the Environment and Sustainability. “What I can say is, everything is pointing in that direction.”
‘Will these impacts turn pressure up?’
As the sweltering summer of 2023 marches into August, with another round of triple-digit heat advisories, scientists and environmental advocates are hoping the recent extremes somehow spur the kind of global, collective action that has been largely absent.
“This could be an example of what the normal will be in a 1.5C world,” Tebaldi said. “Even if we stop [greenhouse gas] emissions tomorrow, we have to deal with this kind of climate. … This should be a stark reminder that we are living in a reality that has already changed, and if we want to be resilient, we need to invest on all sorts of fronts.”
McKay said the lack of sufficient climate action in recent years has not been for a lack of information about the problem, or examples of the damage wrought by a warming planet. Rather, it amounts to a paucity of political will.
“Will these impacts turn pressure up?” he said. “You’d hope some of these extremes would remind politicians and corporations that this is what we are starting to see at only about 1.2 Celsius of warming.”
António Guterres, the U.N. secretary general, is hoping for a similar realization among presidents, prime ministers and those in charge of the world’s largest industries.
“Leaders must lead. No more hesitancy. No more excuses. No more waiting for others to move first,” he said in a recent news conference on July’s extremes.
He cited positive signs, such as the ongoing growth of renewable energy and a recent international agreement to ensure that the shipping industry reaches net-zero emissions by mid-century.
But much more is needed, Guterres said, and if July is a harbinger of what lies ahead, there is not a moment to waste.
“The evidence is everywhere: Humanity has unleashed destruction. This must not inspire despair, but action,” he said. “We can still stop the worst. But to do so we must turn a year of burning heat into a year of burning ambition.”