Heat Domes in Canada, Flooding in Germany, Droughts in Africa and Central Asia:  Have We Finally Broken the Climate? By Francesco Collini, Johann Grolle, and Thomas Milz Der Spiegel, August 18, 2020

Forest fires everywhere	Melting glaciers

 

Summer of Anxiety: Have We Finally Broken the Climate?

Heat domes in Canada, flooding in Germany, droughts in Africa and Central Asia: This summer has seen some worrying weather. How closely is it tied to climate change? Scientists around the world are trying to find out.

The television interview given by Jan Polderman on the Monday before the fire laid bare just how suddenly things changed. A smile on his face, the mayor of Lytton told a reporter that his counterpart from the neighboring town of Lillooet owed him a beer. The two had made a bet over which of the two towns in the Canadian province of British Columbia would experience the hotter temperatures, and Polderman was proud to have won. He had no way of knowing that just 48 hours later, his community would no longer exist.

Lytton is known for high temperatures, with a sign on the Trans-Canada Highway welcoming visitors to "Canada’s hotspot.” The town is located on the floor of Fraser Canyon, where heat tends to collect in the summer months. Still, nobody there was prepared for what happened at the end of June: On Sunday, June 27, the local weather station registered a value that was two degrees warmer than the 80-year-old record, topping out at 46.6 degrees Celsius (115.9 degrees Fahrenheit).

It was even hotter the next day, reaching 47.6 degrees Celsius, and the temperature still wasn’t done climbing. That Tuesday, it was 49.6 degrees Celsius (121.3 degrees Fahrenheit). A canyon in Canada was suddenly experiencing desert-like temperatures and weather experts around the world took notice. "What is going on?" wondered climate researcher Jochem Marotzke of the Max Planck Institute for Meteorology in Hamburg. 

Then came the fire, nourished by extremely dry conditions. Later, some conjectured that a spark from a passing train set the bushes along the tracks on fire. At 6 p.m., the mayor signed the evacuation order.

The flames sped through the dried-out town. "It took, like, a whole 15 minutes from the first sign of smoke to, all of a sudden, there being fire everywhere," Polderman told Canadian broadcaster CBC. Those fleeing the flames filmed the inferno through their car windows, with the world sharing their misfortune via YouTube. The clips show smoke, ash and embers, with the curtain of soot occasionally revealing a house, a tree or a car. Just a few hours later, nothing was left of most of the buildings.

The extremity of the heat and drought on the West Coast of North America made a mockery of the weather forecasts. Dozens of new heat records were reported in British Columbia and in the U.S. states of Washington and Oregon, with some of the old records being smashed by up to five degrees – a temperature shock not accounted for in any of the prediction models developed by climate researchers. If desert-like temperatures can suddenly appear in the mountains of Canada, what might we be facing next?

Overnight, the town of Lytton, with its 250 residents, became a symbol for climate change. Many people are aware that the Pacific island nation of Kiribati is sinking into the ocean and that Siberian homes are growing unstable because of thawing permafrost. But a mountain town in Canada? If Lytton can be destroyed, then we are all at risk.

Just two weeks after Lytton went up in flames, the water started rising in Hagen and Wuppertal. Heavy rains – up to 150 liters per 24 hours in some places – poured down on western Germany. Climate change had arrived in the heart of Europe. But even as the bulldozers were clearing the region’s streets of household belongings and mud, Europe’s next weather debacle got underway. In the Mediterranean, temperatures began climbing early this month in Greece and Turkey's Antalya region to well over 40 degrees Celsius (104 degrees Fahrenheit). In some places, it stayed over 30 degrees through the night. Firefighters are having a tough time battling the flames in many areas, and meteorologists are unanimous in calling it an historic heatwave.

Our planet’s weather is changing, something that is becoming clearer with each passing year. Extreme weather phenomena have been reported far beyond western Canada and Germany.

The situation, to be sure, has since calmed down in British Columbia, but further south – in California, Arizona, Nevada and Utah – the heat and aridity have settled in for the long haul. The American southwest has been suffering a "megadrought" for the last two decades. In California, the weather is even driving rattlesnakes into populated areas, and Oregon is experiencing a grasshopper plague.

"Climate change causes heatwaves. That is as well proven as the fact that smoking causes cancer." Friederike Otto, climate researcher at the University of Oxford

The peak of the fire season hasn’t even arrived yet, but already, the amount of CO2 that has been released by fires in the American West is approaching the total of emissions saved in California during the coronavirus pandemic. The fires, which are growing worse with every passing year of drought, are more than surpassing efforts to reduce greenhouse gas emissions.

Places like Helsinki and Moscow have also recorded new highs, seeing unprecedented temperatures for the month of June. In Siberia, thousands of firefighters are trying to control wildfires. More than a million hectares of forest are in flames, and in some areas, the peat soil now smolders all year long.

A "heat dome" in the Middle East drove temperatures to over 50 degrees Celsius (122 degrees Fahrenheit) in June. Afghanistan is facing food shortages for millions of people due to a drought. The World Food Program has warned of an unprecedented famine on Madagascar, with people in some places forced to rely on locusts for food. And Angolans are also facing a drought – the worst in four decades. A large part of the country’s harvest has already been spoiled and cattle are dying of thirst.

Brazil, meanwhile, is facing possible power shortages, with the Iguaçu River pushing only a fifth of its normal flow over its large waterfalls and the Paraná River, which drives one of the largest hydroelectric power stations in the world, experiencing historic lows.

In Taiwan, normally home to plenty of precipitation, typhoons have been absent this year, leading to a dry spell not seen since 1964. Indeed, computer producers could run into problems if Taiwan’s reservoirs dry up, since they supply chip manufacturers with the water they need.

Four-thousand kilometers further to the west, in China’s largest desert, more precipitation has fallen to earth than a normal two-year total. In the city of Zhengzhou, on the Yellow River, subway tunnels filled with water, trapping hundreds inside the trains. At least 108 people died in the city.

Weather, to be sure, tends to be moody, and the Earth has always been visited by natural catastrophes. This year, though, many regions of the world are facing such phenomena at the same time. Furthermore, the number and the severity of the weather events have left the impression of a planet out of balance. The Earth’s climate seems to be shifting.

Heat waves

June produced extreme heatwaves on all continents around the world. This map shows deviations from average temperatures (1981 to 2010) in June 2021.

Canada

British Columbia was hit by a once-in-a-millennium heatwave. The town of Lytton was lost to fire.

Northwest Africa

Temperatures in the region have never been as high as they were in June.

Europe

The continent experienced its second-warmest June on record. Records were broken in both Helsinki and Moscow.

Afghanistan

Heat and drought have made life difficult. One-third of the populace is facing food shortages.

Siberia

A million hectares of forestland are on fire. In many places, temperatures have climbed above the 30-degree Celsius threshold.

Sources: KNMI, WMO

As such, representatives of the Intergovernmental Panel on Climate Change are appearing before the public at a critical moment. On Monday, they presented the first part of their new report, the sixth. The reports in the series are a compendium of the most current research into climate change, and they form the foundation of the climate strategies pursued by countries around the world. In many countries, this sixth report will be received by an audience that is far more sensitive to global warming than the previous report, which was released in 2013.

The period since then saw the Paris climate summit, with heads of government in more than 190 countries committing to keeping the global average temperature rise to well below 2 degrees Celsius, and under 1.5 degrees Celsius if possible. Early signs have pointed to those commitments being followed up by action. The share of renewable energy is finally rising noticeably, and more and more electric cars are plying the world’s roadways. Moreover, after four years of preposterous climate policy under President Donald Trump, the U.S. has also returned to the Paris Agreement.

The U.S., Europe and China have committed to strict emissions reduction goals. The EU has set a goal of reducing greenhouse gas emissions by 55 percent relative to 1990 by 2030. U.S. President Joe Biden has set his country on a course of carbon neutrality by 2050 and even China wants to become carbon neutral by 2060, despite its booming industrial sector. South Korea, Japan, Canada and Britain have likewise all made substantial commitments. But will all that be enough to avoid the worst?

The IPCC report isn’t particularly optimistic. Climate change has sped up, and the opportunity to meet the goal set by the Paris Agreement is slipping. Furthermore, because the Earth’s climate only shifts slowly, it will take some time before even a radical reduction in greenhouse gases translates into lower temperatures. The situation, in other words, won’t improve for quite some time, and is likely to grow worse.

The new IPCC report describes in minute detail what the world is facing according to the climate models. The dominating trend is that of rising temperatures. Heatwaves are likely to increase in frequency far more than all other extreme weather events. Summers that seem extreme today could become normal within just 10 years.

But it’s not just the temperatures that are climbing. Because a warmer world means that more water evaporates and condenses in clouds, there will be more precipitation, and that precipitation will be distributed differently. Torrential downpours that can lead to flooding are on the rise in many countries. At the same time, dry regions might see no precipitation at all. The basic rule that now applies to the planet is the following: Extreme events will become more frequent.

Flooding in Germany and on the Yellow River, droughts in Afghanistan and Madagascar, fires in California, Greece and Siberia: The weather seen worldwide in the last several weeks seems to have matched up well with the forecasts. Yet it is questionable whether all of it can really be explained by the models commonly used to evaluate climate change. Some scientists are concerned that the human influence on global climate developments has triggered new processes that are insufficiently understood.

The extreme heat that afflicted the Pacific Coast of North America in late June has proven particularly puzzling to researchers. How could such a thing be possible? Could it be that the atmosphere there has begun changing in ways that the computer models have missed?

To approach an answer to such questions, a new branch of research has been developed: Called attribution studies, it seeks to determine whether individual weather events are within the margin of error of existing prognoses and how likely it is that they are the consequence of anthropogenic climate change.

When the temperatures in British Columbia reached record highs in June, members of World Weather Attribution (WWA), an association of scientists from around the world, flipped on their high-powered computers. One week later, the results of their rapid analysis were ready.

The report determined that according to existing models, such heat events should be seen in British Columbia at most once a millennium – and even that is only true of today’s conditions, with an atmosphere that has already been heated by human greenhouse gas emissions. Without anthropogenic climate change, such an event would be "virtually impossible."

The tone being used by researchers to discuss climate change has shifted. For a long time, each extreme weather event – be it a flood, a storm or a heatwave – led to questions as to whether it was just the product of the world’s capricious weather or if it was because of climate change. And their answer was consistently the same: Certainly, the frequency of such events is on the rise, but causal links cannot be established for individual weather phenomena. Nobody could exclude the possibility, they would argue, that such an event might be just a matter of chance, even if such an explanation wasn’t likely.

Given the heatwave in British Columbia and the Northwest of the U.S., the WWA team has abandoned such precaution. They exclude the possibility of a random event. In other words: Without human-caused climate change, the town of Lytton would still exist.

The question of causality, says Friederike Otto, a researcher from the University of Oxford and one of the authors of the WWA study, has been answered. "Climate change causes heatwaves. That is as well proven as the fact that smoking causes cancer."

Less clear is whether the record temperatures seen on the Pacific Coast are still consistent with the climate models being used. It is possible that it could simply be an extremely unusual weather phenomenon, say the WWA researchers. But it is also possible, they write, that "non-lineal interactions” are in play. In other words, it is possible that anthropogenic climate change is triggering novel and unexpected behaviors. The Canadian heatwave raises questions "as to whether we have really understood how climate change makes heatwaves hotter and deadlier," says Otto’s WWA colleague Geert Jan van Oldenborgh.

The IPCC report also addresses the question as to whether instabilities in the climate system could emerge. Scientists around the world have set off on a search for places where global warming has triggered accelerated processes, such that they magnify global warming itself. The term for such phenomena is "tipping points," meaning a shift that could provoke sustained and potentially irreversible changes.

The fear is that such feedback loops could throw important ecosystems permanently out of balance.

Permafrost in the Arctic is thawing, for example, releasing the methane sequestered within. This greenhouse gas leads to a further heating of the atmosphere and rising temperatures. The thawing of the permafrost accelerates.

The Arctic sea ice, meanwhile, is melting, which transforms reflective ice into heat-absorbing water. Because less sunlight is being reflected, the temperature of the water increases. More sea ice melts as a result.

The Greenland ice sheet is also melting, which means the thickness of the ice is continually falling. The altitude of the ice surface, in other words, is dropping to where temperatures are warmer. The ice then melts even faster.

The West Antarctic ice sheet is flowing into the ocean, which leads to water finding its way under the ice. This makes it easier for the ice to glide, which allows even more water to seep under the ice. The glacier begins to move faster.

In the Amazon rainforest, heat and aridity is damaging the trees, which means they release less water into the atmosphere. That leads to fewer clouds and to a stagnation of the "flying rivers.” Damage to the trees accelerates.

If, for example, thawing permafrost releases the potent greenhouse gas methane, it would further warm the atmosphere, thus accelerating the thawing of the permafrost. Or if the Arctic Sea ice melts, it would transform a vast, white surface into a darker surface of water, which absorbs more of the sun’s heat. The water would become warmer, which would accelerate the loss of ice.

Some researchers believe that such feedback loops can be found everywhere in the Earth’s climate system. "Climate tipping points – too risky to bet against," was the headline on an article in the journal Nature that attracted significant attention. The authors, which included experts from the Potsdam Institute for Climate Impact Research (PIK), wrote that "the stability and resilience of our planet is in peril."

Tipping points, according to the analysis, can be found in the North Atlantic and in the Amazon rainforest, in the Siberian taiga and in coral reefs. And once one tipping point is reached, it could trigger a series of others, the researchers argue, since they are all linked. Researchers speak of the possibility of a "cascade of tipping points" and of "toppling dominoes."

"We are still living in a Garden of Eden," says Johan Rockström, co-director of the PIK, referring to the stable climatic conditions that have dominated since the last ice age. "In the last 10,000 years, global temperatures have only varied by plus/minus one degree from a mean of 14 degrees Celsius," Rockström says. This extreme stability has made it possible for humanity to develop agriculture and advanced cultures and societies.

"It could be that our models are more stable than reality." Jochem Marotzke, climate researcher at the Max Planck Institute for Meteorology in Hamburg

Such climate consistency, though, is in no way a given. On the contrary, in the past, the climate was often marked by drastic swings. Ice cores from Greenland provide a kind of climate record of the past couple hundred thousand years, and they show that at times, temperatures have made swings of 5 degrees up or down within just a few decades. Sediment cores from the oceans suggest that such abrupt changes have not just been limited to the Arctic. The Earth, it would seem, used to be something of a climate madhouse. No wonder Homo sapiens didn’t even try agriculture for a long time: The weather would simply have destroyed the harvest.

Rockström warns that climate chaos of the kind seen in the last ice age could return in the future. By continuing to pump greenhouse gases into the atmosphere, he says, we are putting the current stability of our climate at risk. Rockström is concerned that the global balance is beginning to wobble.

That, though, is a concern that isn’t shared by all of his colleagues. "There has been too much talk of tipping points," says Otto, the researcher from the University of Oxford. Marotzke, the Max Planck expert from Hamburg, is also skeptical. He says that the self-reinforcing effects that make tipping points so dangerous sound plausible, but they are usually based on simplified assumptions. "We don’t see most of the tipping points in our climate models," he says. Marotzke doesn’t have much time for apocalyptic scenarios: "Climate change is scary enough without tipping points," he says.

But even if he believes that the alarmism from his colleagues in Potsdam is overwrought, Marotzke does admit that there are potentially vital processes that have not thus far been correctly depicted by the supercomputers. "It could be that our models are more stable than reality," he says. The peculiar temperature jumps that took place during the ice age provide particular cause for wariness. Thus far, no climate model has been able to accurately reflect those jumps. And if the models are unable to account for the past, how reliable are they when it comes to predicting the future?

There is much indicating that the biggest surprises for the Earth’s climate system are hiding in the polar regions. The ice that dominates the landscape there has precisely the characteristics needed to produce deep changes to the climate: It melts slowly, yet by disappearing, it triggers permanent changes to the entire system.

The Greenland ice sheet is the perfect example of a tipping point. It is five times the size of Germany and up to 3.5 kilometers thick, and it lies just off Europe. If we aren’t successful in eliminating greenhouse gas emissions, it is destined for permanent collapse.

Even those scientists who tend to be skeptical of tipping points on the whole believe that a tipping process is underway in Greenland. At some point, the melting of the ice sheet on Greenland will develop a momentum that can no longer be stopped until the vast majority of the ice has disappeared.

The reason is the so-called melt-elevation feedback. It is the consequence of a temperature effect that every hiker is familiar with: The higher you climb, the colder it gets. The same effect plays a role in Greenland: Every meter of ice that melts means a meter of altitude lost. And every bit of altitude lost means warmer temperatures and more rapidly melting ice.

The only controversial element of the debate is when global warming will have reached the critical threshold at which this phenomenon takes hold. "Climate models indicate a range from 0.8 to 3.2 degrees," says Ricarda Winkelmann, a researcher at the Potsdam Institute for Climate Impact Research who is trying to figure out how long we have before the tipping point is reached. Already, the Earth is an average of 1.2 degrees warmer than it was in the pre-industrial era. That means that the critical threshold may already have been reached and the fate of the Greenland ice sheet is sealed.

If all of the ice in Greenland melts, it would translate to a seven-meter rise in sea levels. "That is a reason for climate protection measures, but not for panic," says Anders Levermann, who works together with Winkelmann at PIK. After all, nobody is expecting the ice to plunge into the ocean overnight. The process, rather, will take decades, centuries, or even millennia – with more ice melting in the summer than is produced in the winter. An inexorable, slow-motion catastrophe.

No other place lends itself to observing this process better than the mouth of the Ilulissat Icefjord. A steady stream of huge icebergs, many of them hundreds of meters tall, pass by on the way from the Sermeq-Kujalleq Glacier to the ocean. Around 10 percent of all Greenland ice that reaches the ocean comes from here, likely including the legendary iceberg that sank the Titanic.

The Sermeq Kujalleq is the fastest glacier in the world, flowing toward the sea at speeds of up to 46 meters per day. The Ilulissat Icefjord’s uniqueness, combined with its importance to climate research, led UNESCO to grant it World Heritage status in 2004.

Danish researchers recently reported that the ice sheet is again experiencing "massive" melting. Temperatures in northern Greenland have climbed to over 20 degrees Celsius (68 degrees Fahrenheit), a record that has resulted in the disappearance of eight cubic kilometers of ice each day. In total, the Greenland ice sheet is currently losing an average volume of 268 cubic kilometers of ice per year. That is the equivalent of roughly five-times the volume of Lake Constance. Or, to put it differently, that amount of water would be sufficient to cover all of Germany with water, 75 centimeters deep.

But as huge as such a volume of water might seem, it isn’t even enough to raise global sea levels by a single millimeter. The decisive question is: How quickly will the thawing process accelerate?

Droughts and Floods

The trend can be seen in many areas around the world: Dry areas are becoming more arid while places with lots of precipitation are getting even more rain. This map shows deviations from average precipitation levels (1986 to now) during the period from July 2020 to June 2021.

U.S.

The Southwest is suffering from an extended drought. California experienced the worst fires in the state’s history in August and September 2020.

Angola

There was hardly any precipitation in the country during the rainy season from November to April. Now, the country is facing a famine.

Sudan

Strong rainfall led to floods in August 2020. The Blue Nile crested at 17.5 meters (57 feet), the highest level ever measured.

Taiwan

A lack of typhoons has meant insufficient rainfall, leading to the worst drought since 1964.

China

The entire country has experienced an unusual amount of rain. The torrential rains experienced in July 2020 were particularly bad, forcing 2.8 million people to flee rising floodwaters.

Sources: KNMI, WMO

Thus far, there is reason for cautious optimism. Simulations indicate that the melting of Greenland's ice sheet will produce no more than an 18-centimeter increase in sea levels.

"We won’t have to rapidly evacuate any coastal cities as a result, but in the long term, we might have to give up a couple of them," says Levermann. He is, however, concerned about the variables that could be hiding in the ice. The findings of Jörg Schäfer, from Columbia University in New York, are particularly disturbing. Schäfer drilled through Greenland’s ice sheet to take samples of the rock below. By analyzing the rock’s isotopic composition, he was able to determine that it was exposed to sunlight for long periods within the last few million years.

"The ice sheet must have almost completely melted on several occasions during warmer periods,” Levermann concludes. "And that means: Perhaps it is far less stable than it seems."

Another key element in the global climate system lies some 7,500 kilometers to the south – and there, a potential tipping point is approaching much more rapidly. "As rapidly as trees can die," says Levermann laconically.

The future of the Brazilian rainforest is at stake. If the worst predictions come true, then a large part of the Amazon will turn into a savannah landscape, resulting in the extinction of thousands of animal and plant species and the release of vast amounts of carbon dioxide into the atmosphere.

"But the situation is much more complex than it is in Greenland. Here, it’s not just about physics, but also about ecology,” says Kirsten Thonicke, a scientist at the Potsdam Institute for Climate Impact Research who is examining the interrelation between fires, root depth, stress, microclimate, species diversity and the macro weather situation in the rainforest.

"The next 10 years will show whether we will be successful in changing course." Johan Rockström, co-director of the Potsdam Institute for Climate Impact Research

The more closely scientists look, the clearer it becomes that the rainforest is changing, and that the effects of that change can be felt deep into the agricultural regions of South America. "This year was completely atypical," says Gilberto Rodrigues, who runs an agricultural cooperative in Munhoz de Melo, a municipality in the southeastern Brazilian state of Paraná. January saw an unusual amount of rain, but the dry period started already in April. There has been no water since, and then came a bout of frost just over a month ago. "We’ve never seen such a thing before," Rodrigues says, adding that most of the corn harvest has been destroyed.

The village elders in particular can remember what the weather used to be like. "When we came here in 1951, it would often rain for weeks at a time," says Antonio Moncalvo, 82. "Now, there might be an afternoon shower before it stops again."

Tasso Azevedo, the coordinator of the initiative MapBiomas, which maps the deforestation of the rainforest, confirms the trend. "For the areas south of the Amazon region, the dry period expands by seven days every decade,” he says. If things continue like that for another 10 years, he adds, the "safrinha,” the second corn harvest, will no longer be possible.

Azevedo believes there is a direct connection to the destruction of the rainforest through slash-and-burn practices, settlement expansion and mining. The majority of the rain that falls in Brazil’s agricultural regions, after all, comes from the rainforest. Water vapor continually rises out of the jungle and collects in clouds, which are then driven south by the wind: "Flying rivers" which, taken together, carry about the same amount of water as the Amazon River itself.

But these flying rivers are threatening to dry up. And if that happens, the farmers in Brazil’s south won’t just face a lack of water, but the forest will as well. At some point, researchers fear, the process will become self-reinforcing: Once the trees begin to wither, the flying rivers could stop altogether, which would further promote desertification. That would mark the completion of one of the dreaded feedback loops, and the rainforest would essentially destroy itself.

Antonio Donato Nobre, a climate researcher at Brazil’s National Institute for Space Research (INPE), is the one who promulgated the theory of flying rivers. "If the clearcutting isn’t stopped soon, the climatic changes will lead to enormous losses in agricultural production not only in Brazil, but also in neighboring countries like Bolivia, Paraguay and Argentina," he says.

His brother and colleague, Carlos Nobre, is responsible for the numbers that are today seen as the benchmark for the rainforest’s tipping point: Once 40 percent of the original forested area in the Amazon is destroyed, he calculated, then the further desertification will be unstoppable. A 4-degree rise in temperatures would have the same effect, Nobre believes. But because clearcutting and warming are hitting the forest at the same time, combined with the additional stress of road construction, selective logging and mining, Carlos Nobre believes the critical benchmark is closer to 25 percent. The forest cannot be allowed to shrink more than that, otherwise its very existence is threatened. Already, though, close to 20 percent of the rainforest has been destroyed, meaning that the critical benchmark isn’t far away.

A recently published analysis of satellite images tells an even more ominous story. For the first time, the images don’t just show areas that have been deforested, but also regions where the forest has been damaged. And the findings are concerning: "The swaths of deforestation create a herringbone pattern, through which the adjoining forested areas are heavily damaged," says PIK researcher Thonicke. That results in a loss of both species diversity and biomass, reducing the resiliency of the ecosystem.

As a consequence of this development, the tropical forest is undergoing a dramatic role reversal in the global carbon dioxide cycle. Particularly in years of El Niño, which are especially stressful for the Amazon rainforest, the jungle is releasing far more carbon dioxide than it absorbs. What was once one of the world’s largest carbon reservoirs is becoming a source of greenhouse gas emissions.

What does all this mean for Europe? What will happen if the ice in Greenland and the forest in Brazil disappear? Are we perhaps heading for the kind of weather chaos last seen in the last ice age?

The answers to these questions depend on who you ask. Climate scientists are essentially divided into two camps.

The majority is adamant about leaning exclusively on established facts. And they say that the best tool available for predicting future climate developments – despite their deficiencies – are the computer models. These models show no indication that the climate will begin changing uncontrollably should temperatures continue to rise. "Experience has taught me that nonlinear developments grow less visible the more complex our models become,” says Marotzke, the climate researcher in Hamburg. That means: The more complex the system being simulated on the computer, the more stable it seems to be.

The other faction warns that the risk of an out-of-control climate is too great to ignore. Systemic feedback loops, they say, are real and they have the potential to throw everything out of balance. They also point out that climate history clearly demonstrates that the current stability can in no way be taken for granted. Furthermore, they argue, once the cascade of tipping points begins, it will be too late to do anything about it. "The next 10 years will show whether we will be successful in changing course," says PIK co-director Rockström.

The competing views have been on full display with the recent extreme weather events – the Canadian heatwave and the flooding in Germany. The more concerned group believes that both catastrophes can be traced back to the same root cause that they blame for almost all extreme weather phenomena that occur in the temperate latitudes of America, Europe and Asia: the jet stream.

It is indisputable that this air current, which circles the earth at an average altitude of 10 kilometers, plays a huge role in the weather of Central Europe. Driven by temperature differences between the Arctic and subtropical regions, the jet stream consistently blows from the west to the east, pulling the high- and low-pressure regions that move toward Europe from the North Atlantic and the Azores. The jet stream also holds the polar vortex at bay, the cold air that rotates above the Arctic Sea, thus sealing it off from the more moderate latitudes further south.

Just how strongly the jet stream is affected by climate change is a matter of some debate among scientists. Stefan Rahmstorf from the PIK in Potsdam points out that the temperature difference between the polar region and the subtropics is falling as a result of global warming because the Arctic is heating up much more rapidly than the desert belt that stretches across much of the Northern Hemisphere. The result is a weakening jet stream, with the effects of the low- and high-pressure areas occasionally expanding far to the north and south. More than anything, though, they move more slowly in summer. In the worst case, their eastward movement comes to a stop and weather systems stagnate – resulting in the kind of heat dome seen in British Columbia and the extended rainfall experienced by Germany and neighboring countries this summer.

None of that has been proven, notes Marotzke. When he ups the amount of greenhouse gases in the atmosphere in his models, they don’t show any uniform change to the jet stream, which he finds unsurprising. It is true, he allows, that ground temperatures in the Arctic are rising faster than in the subtropics, but at higher altitudes, in the troposphere, the situation is precisely the opposite, and the jet stream is affected by temperature differences at all altitudes. It isn’t yet clear, Marotzke says, which effect will ultimately prove dominant.

For that reason, Marotzke leans toward viewing the extreme weather events that have occurred in recent weeks as statistical anomalies rather than as signs of a new, insufficiently understood weather pattern. "Before we begin preparing for future, nonlinear climate developments, we should be sure that they actually exist,” he says.

But when it comes to the necessary political response to climate change, the two camps are in agreement: Global carbon dioxide emissions must be reduced – as quickly as possible. The changing weather will prove increasingly challenging to people in the future, even if no domino effect appears in the immediate future.

Analyses performed by the attribution studies experts reveal just how dramatic the temperatures will increase. They didn’t just calculate the likelihood of a super-heatwave like the one seen in Canada given the current climate conditions. They also examined how rapidly that likelihood will climb in the future.

Their findings are troubling. In their simulations, the researchers assumed an additional rise to average global temperatures of 0.8 degrees Celsius, such that the benchmark of a 2.0-degree Celsius increase was reached. Under that scenario, the kind of once-in-a-millennia heatwave seen in Lytton would hardly be remarkable. According to the calculations, desert-like climate events like those seen this summer would occur every five to 10 years in southwestern Canada.

In that future, which may not be all that far away, one doesn’t even want to imagine what a once-in-a-millennia summer might look like.

Summer of Anxiety: Have We Finally Broken the Climate? - DER SPIEGEL

***

Share the link of this article with your facebook friends