Humans held responsible for the meanders of climate change since 1900 | Science

By Paul VoosenCan. 23, 2019, 12:15

While industry and agriculture were reducing greenhouse gases at an increasing rate during the twentieth century, global temperature has been on an erratic trajectory, increasing for three decades starting in 1915 and then stabilizing in the 1950s at the end of the 1970s and then resume its ascent. For decades, scientists have linked these beginnings to the internal variability of the planet, in particular a climatic stimulator called the Multidimensional Atlantic Oscillation (AMO), characterized by long-term variations in ocean temperature. But researchers are increasingly wondering if the AMO played the dominant role that was thought. The oceanic pacemaker seems to be pulsating.

It is now possible to explain the twists almost without the AMO, says Karsten Haustein, a climatologist at the University of Oxford in the United Kingdom and lead author of a new study published this month in the Climate Journal. After correcting for the distinct effects of pollution on land and oceans, as well as the flaws in temperature readings, Haustein and his colleagues calculated that greenhouse gas and air pollution play alone on the climate of the twentieth century. "It is very unlikely that this oceanic leprechaun produces an unknown cyclicity," says Haustein. It also means that it is also unlikely that a fresh movement in OMA will mitigate man-made warming.

Others are not convinced that the "leprechaun" is completely defeated. "They're probably right in this [the AMO] is not as important globally as we sometimes thought, "says Kevin Trenberth, a climatologist at the National Center for Atmospheric Research in Boulder, Colorado. "But I guess they're underestimating her role a bit."

The OMA was created following observations that the sea surface temperature in the North Atlantic seemed to change from an unusually warm temperature to a cold one and to return between 20 and 60 years ago; the old climate seems to have had similar oscillations. The researchers hypothesized that periodic changes in the conveyor belt of the currents of the Atlantic Ocean are at the origin of this variability. But the reason why the conveyor accelerated and slowed down on a regular basis was a mystery, and the evidence of regular large oscillations has slowly eroded, says Gabriele Hegerl, a climatologist specializing in statistics at the University of Edinburgh. "Those are more difficult to defend."

The new skepticism began with work led by Ben Booth, a climatologist at the Met Office Hadley Center in Exeter, UK. In 2012, he reported in Nature this pollution, or aerosols, began to thicken the clouds over the Atlantic in the 1950s, which could have cooled the ocean without reduced help due to an oscillation internal. Last year, several independent models gave similar results. Meanwhile, most global climate models have been unable to replicate AMO-type oscillations unless researchers include the influence of pollutants, such as soot and sulphates produced by the combustion of fossil fuels, says Amy Clement, a climatologist at the University of Miami Florida.

Now, it seems plausible that such human influences, with the help of aerosols spewed by volcanic eruptions, have resulted in virtually every twentieth century climate change. Haustein and his co-authors have tweaked a relatively simple climate model to account for the fact that most of the pollution comes from the earth, which heats up and cools faster than the ocean – and that's it. there is much more land in the northern hemisphere. And they recalled the cooling effect of volcanic eruptions – a reasonable move, says Booth, who is not affiliated with the study. "We know that models respond too strongly to volcanoes."

They also adjusted the global temperature record to account for a change in the way ocean temperatures are measured; During the Second World War, the British practice of measuring water samples in buckets gave way to ever warmer US readings of water passing through the intake valves of ships. Haustein and his team discovered that earlier efforts to compensate for this change had been in vain. They used data from weather stations on the coast and on the islands to correct the facts.

The team used greenhouse gas and aerosol data developed for the next US climate report, as well as data on historical volcanic eruptions, solar cycles, and El Niño warming of the Pacific. . Comparing the simulated climate with the temperature-adjusted record, they found that multi-dimensional variability could account for only 7% of the recording. The soot from the industry caused the warming of the early 20th century as it was heading to the Arctic, darkening the snow and absorbing sunlight. After World War II, sulphate mist reflecting light from power plants increased, preventing the potential warming of increased greenhouse gases. Then, pollution control happened in the 1970s, allowing warming to accelerate.

This is a compelling picture, but it could have been very different if the team had used other equally valid assumptions about the impact of aerosols on the climate, Booth explains. Trenberth believes that the adjustments made by the team had the effect of adding the model to an uncertain record. "There is considerable room for maneuver in what the disc really is," he says.

Haustein disputes that the team adapted the model to explain the warming of the 20th century. "We only used the available data in the most physical way possible," he says. The researchers used the model from 1500 to 2015, and he says it fits well with paleoclimate records, including the Little Ice Age in Europe.

If a large oceanic oscillation does not change the climate, a future cooling of the oceans will probably not give society time to cope with global warming. But the disappearance of the OMA could also facilitate the prediction of what is in store. "All we are going to have in the future," says Haustein, "is what we do."