How global warming fuelled extreme weather events

Extreme weather left its mark across the planet in 2016, the hottest year in recorded history. Record heat baked Asia and the Arctic. Droughts gripped Brazil and southern Africa. The Great Barrier Reef suffered its worst bleaching event in memory, killing large swaths of coral. Now climate scientists are starting to tease out which of last year's calamities can, and can't, be linked to global warming.

In a new collection of papers published in the Bulletin of the American Meteorological Society, researchers around the world analysed 27 extreme weather events from 2016 and found that human-caused climate change was a 'significant driver' for 21 of them. The effort is part of the growing field of climate change attribution, which explores connections between warming and weather events that have already 

happened.

To judge whether global warming made a particular extreme weather event more likely to occur, scientists typically compare data from the real world, where rising greenhouse gases have heated the planet over the past century, against a modelled counterfactual world without those rising emissions. This technique has gained broader acceptance among climate scientists in the last decade. Here are five extreme weather events from 2016 that scientists now think were made more likely by global warming:

Record temperatures around the world: Last year, earth reached its highest temperature on record, beating marks set in 2015 and 2014. While that partly reflected the influence of El Nino, a new study led by Thomas R Knutson of the National Oceanic and Atmospheric Administration (NOAA) concluded that the record warmth worldwide "was only possible due to substantial centennial-scale human-caused warming."

Two separate studies also found that unusually high temperatures across Asia and the Arctic in 2016 "would not have been possible without human-caused climate change." Such forceful assertions are rare: typically, scientists will only go so far as to say that global warming made an extreme weather event more likely to occur. In these cases, they went further, finding that such extreme warmth could not have happened in a world without 

rising emissions.

Coral bleaching in the Great Barrier Reef: Over the past two years, unusually warm waters in the Pacific have caused bleaching on the Great Barrier Reef, a phenomenon in which coral expel vital algae from their tissue and take on a ghastly white appearance. If the warm water temperatures persist, many corals can die off, with dire consequences for the marine ecosystems that depend on them.

Here, scientists were more measured in putting all the blame on global warming, in part because the impact of El Nino was tough to disentangle. A study led by Sophie C Lewis of Australian National University concluded that human greenhouse gas emissions "likely increased the risk of the extreme Great Barrier Reef event" by increasing thermal stress in the ocean.

Drought in Africa: In the first few months of 2016, severe droughts and heat waves spread across much of southern Africa, triggering local food and water shortages that affected millions. While such 'flash droughts' are often associated with El Nino, scientists now say climate change plays an important role, too.

A study led by Xing Yuan of the Chinese Academy of Sciences found that flash droughts had tripled in the region over the past 60 years, with global warming 'mainly responsible' for the trend. Climate change can't be blamed for all recent dry spells, however. In a separate study, researchers looked at a five-year drought in northeast Brazil, but "could not find sufficient evidence that human-caused climate change increased drought risk."

Wildfires in North America: In 2016, wildfires burnt around 8.9 million acres of western Canada and the United States. Here, climate change most likely played a supporting role. Researchers at the 

University of Edinburgh, UK found that global warming had made "extreme vapour pressure deficits" five times more likely across the region during the summer months - a measure of changes in atmospheric moisture that is associated with the drying of vegetation and wildfire risk.

The warm 'blob' in the Pacific Ocean: Over the past few years, a large patch of unusually warm water has appeared off the coast of Alaska, popularly known as 'the blob'. These warm waters have allowed toxic algae blooms to spread across the region, killing seabirds by the thousands and forcing local fisheries to close.

A new study, led by John E Walsh of the University of Alaska, USA, called the blob "unprecedented" and argued that it "cannot be explained without anthropogenic climate warming," although natural factors such as El Nino and atmospheric variability also played an important role. The study also concluded that more such blobs were likely to occur with further warming.

"A few events from this past year were judged to have been of such a magnitude that they would not have been possible in the climate of a few hundred years ago," said Martin P Hoerling, a meteorologist at NOAA who edited the collection. But, he added, "not everything is being made demonstrably more severe because of climate change."