VELA LUKA, Croatia — The harvest in Mirjana Štimac's almond orchard usually starts in late July, but this year, the crop has failed, with only a few mature nuts per tree.

The trees are dying, and she blames a series of heat waves that have struck in recent years in an orchard that was already under stress from steadily rising global temperatures. Europe saw off-the-charts heat this summer: June was 2 degrees Celsius (3.6°F) above average and crushed the record for the month by a full 1°C (1.8°F), and a late July temperature spike set new record highs across the continent.

Both heat waves were made much more likely by human-caused global warming, scientists with the World Weather Attribution group announced in a study released Friday.

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These past two months have been extreme worldwide: It was Earth's hottest June on record, followed by a July that the World Meteorological Organization reported this week had at least tied for the hottest July since global record-keeping began and may have broken the record. [Update: NOAA later declared it the hottest July and hottest month on record.]

Štimac has been tending almonds, olives, grapes and figs on the north shore of Korčula Island for 25 years, and while her orchards didn't get the worst of the latest heat wave, they have been suffering. All of the plants are well-adapted to seasonal Mediterranean heat and dryness, yet the cumulative impacts of multiple extreme heat events are starting to add up, she says.

She snaps off a branch covered in dead leaves and shriveled nuts and points to something unexpected—a few bright green leaves at the tip.

"I've never seen that before in July," Štimac says. "I think climate change has shifted the seasons. It just keeps getting warmer. The last three years there was no spring, no fall, just long, hot summer. The trees aren't sure what to do. They need the seasons. They need a cool time to rest and rain in the spring."



Since the oldest trees in the orchard were planted about 100 years ago, the regional temperature has gone up by about 1.8°C (3.2°F), as has the sea surface temperature around the Dalmatian Islands off the coast of Croatia, and rainfall has steadily declined.

Against that backdrop, extreme temperature spikes have become more frequent, culminating in late July with the hottest temperatures ever recorded in several cities on the continent. The temperatures hit all-time highs in England, France, Belgium, the Netherlands and Germany, with several locations topping 40°C (104°F) for the first time on record.

'Extremely Unlikely Without Climate Change'

Altogether, 14 nations and territories have broken or tied all-time high temperature records this year.

The July heat wave was so extreme over continental Western Europe "that the observed magnitudes would have been extremely unlikely without climate change," the latest World Weather Attribution study concludes.

"It is noteworthy that every heat wave analysed so far in Europe in recent years (2003, 2010, 2015, 2017, 2018, June 2019, this study) was found to be made much more likely and more intense due to human-induced climate change," the scientists wrote.

In today's climate, that kind of heat wave in places like France and the Netherlands would be expected to happen only once every 50 to 150 years, and it would be nearly impossible there without human-induced global warming, the scientists wrote. In the United Kingdom and Germany, such heat waves can be expected every 10 to 30 years in the human-warmed climate, which has made them three to 10 times more likely, the study said.

Across the region, temperatures during the July heat wave were 1.5°C (2.7°F) to 3°C (5.4°F) warmer than they would have been in an unchanged climate, the attribution study found.

Mirjana Štimac's fig trees are also struggling. “Figs are well-adapted to the seasonal droughts and summer heat in the Mediterranean climate, so if they are being affected by climate change, we’re really in trouble,” she says. Credit: Bob Berwyn

In the Mediterranean region, the impacts of the long-term global warming trend are playing out as long-projected by climate scientists with great confidence. It's getting warmer and drier.

On Korčula, residents say the communal cisterns need more frequent refills with trucked-in water supplied from the mainland.

Mild late winters are increasingly morphing directly into hot summer-like temperatures, often without beneficial spring rains, and trees are having a hard time keeping up with the changes. Some are dying, reflecting the global warming damage seen in forests elsewhere.

Štimac stops beside a fig tree that's also struggling. She offers some of the succulent fruits, but points at other poorly developed clusters that formed at odd times and never fully matured.

"Figs are well-adapted to the seasonal droughts and summer heat in the Mediterranean climate, so if they are being affected by climate change, we're really in trouble," she says.

'All Living Things Are Vulnerable'

Global warming is fueling all climate extremes now, and withering summer heat events like we've witnessed now for the past several summers are particularly dangerous, said Pennsylvania State University climate scientist Michael Mann.

"All living things are vulnerable to heat extremes," he said. "The fields and crops, animals, plants and ecosystems, and our built environment, are vulnerable to ever-more extreme weather events."

Extreme heat waves in recent years have killed hundreds of people, with the most vulnerable, particularly the elderly, the most at risk. People need to be prepared for the risk that extreme conditions similar to this summer will happen about every two years from now on, said Omar Baddour, chief of the data management applications division with the World Meteorological Organization.

The July European heat wave was notable for its geographic scope, from the Mediterranean to the Arctic, with impacts that included accelerated sea ice, record wildfires and a record surface ice melt across Greenland.

Glaciers in the Swiss Alps, already melting rapidly, lost about 800 million tons of snow and ice during the June and July heat waves, Swiss glaciologist Matthias Huss wrote on Twitter. In England, the heat melted railway power lines. In Austria and Germany, some crops at sensitive stages of growth were killed or severely stunted by blistering temperatures.

Forest researchers rang alarm bells about large-scale tree die-offs, caused by bark beetles and repeated heat waves and droughts. "At least from some initial surveys, we know that mortality has increased last year, and we strongly expect it to further increase this year," said Cornelius Senf of the University of Natural Resources and Life Sciences, Vienna.

Hannah Cloke, a climate researcher at the University of Reading, said it's worrying for the future that temperatures during the recent heat waves in Europe soared beyond what climate models projected at this level of global warming, spiking to life-threatening levels across a section of Europe about as big as the eastern third of the United States, affecting more than 100 million people.

"There is a real sense of urgency now. We just have to make it clear that the heat will just keep getting worse and people are dying," Cloke said.

"We can see landscape and ecosystem changes, fishkills, songbird deaths, huge wildfires, melting ice sheets, on their own, as isolated incidents due to extreme weather, but when these same things happen year after year after year and there is no recovery then we are in a new place in our climate history and things are never going to be the same again," she said.

It's Getting Harder to Find Fish, Too

Heat waves also affect the ocean.

In the Vela Luka harbor on Korčula, boat captain Tonko Barčot steers his fishing boat alongside the pier and shrugs his shoulders as he tells waiting buyers that his holds are empty. A long night at sea in search of sardines brought up only empty nets, and the fish market vendors waiting at the pier grumble among themselves.

"It's usually in the winter that we have problems, but lately it's happening in the summer more often, too," says Krešimir Zuvela, a university student who works on fishing boats during school breaks.

Fishing boat captain Tonko Barčot has seen good years and bad in the Adriatic Sea, but says, “I think the big schools have moved farther out in the deeper channels where the currents are stronger and still colder, especially in summer." Credit: Bob Berwyn

Land heat waves can spill over to the ocean and affect marine ecosystems, said Dan Smale, an ecologist at Britain's Marine Biological Association in Plymouth. Under slow-moving summer weather patterns, the heat buildup over land areas can make the upper layers of the Mediterranean extremely hot, leading to mass die-off events that can ripple through the ecosystem, he said.

Its shape and geographic position make the Mediterranean especially sensitive to global warming, and cold water species are quickly being squeezed out of their habitat and replaced by warm-loving species, including invasive organisms from the tropics, said Joaquim Garrabou, a senior researcher with Spain's Institute of Marine Sciences.

Examples include the northward spread of bluefish, and the disappearance of cold-loving sprat from the northern Adriatic since the 1990s, concurrent with an increase in the rate of warming in the Mediterranean, he said.

If greenhouse gas emissions continue at their current pace, disruptive marine heat waves will happen every year, and they will be more intense and up to three months longer than under present conditions, he said.

Barčot says he's had some good years and bad fishing in the Adriatic Sea. At first, he blusters about global warming impacts: "It's the same as it was 50 years ago. The climate always changes. The ocean has cycles," he says.

But an hour later, as he helps unload sardines off his son's boat, he admits that it's been getting harder to find fish.

"I think the big schools have moved farther out in the deeper channels where the currents are stronger and still colder, especially in summer," he says. "I can find them with my big boat, but the smaller fishing boats that stay near the coast, many of them have given up. Some of my friends moved to the mainland."

And the bigger boats are leaving the harbors earlier because they have to go farther to find fish. That means higher fuel costs and more time away from family, Zuvela, the young crew member says. And sometimes, he says, there are no fish at all.