The fingerprint of human-caused climate change has been found on heatwaves, droughts and floods across the world, according to scientists.
The discovery indicates that the impacts of global warming are already being felt by society and adds further urgency to the need to cut carbon emissions. A key factor is the fast-melting Arctic, which is now strongly linked to extreme weather across Europe, Asia and north America.
Rising greenhouse gases in the atmosphere have long been expected to lead to increasing extreme weather events, as they trap extra energy in the atmosphere. But linking global warming to particular events is difficult because the climate is naturally variable.
The new work analysed a type of extreme weather event known to be caused by changes in “planetary waves” – such as California’s ongoing record drought, and recent heatwaves in the US and Russia, as well as severe floods in Pakistan in 2010.
Planetary waves are a pattern of winds, of which the jet stream is a part, that encircle the northern hemisphere in lines that undulate from the tropics to the poles. Normally, the whole wave moves eastwards but, under certain temperature conditions, the wave can halt its movement. This leaves whole regions under the same weather for extended periods, which can turn hot spells into heatwaves and wet weather into floods.
This type of extreme weather event is known to have increased in recent decades. But the new research used observations and climate models to show that the chances of the conditions needed to halt the planetary waves occurring are significantly more likely as a result of global warming.
“Human activity has been suspected of contributing to this pattern before, but now we uncover a clear fingerprint of human activity,” said Prof Michael Mann, at Pennsylvania State University in the US and who led the study published in the journal Scientific Reports.
Kai Kornhuber, at the Potsdam Institute for Climate Impact Research (PIK) in Germany and another member of the research team, said: “We looked into dozens of different climate models, as well as into observational data, and it turns out that the temperature distribution favouring planetary wave stalling increased in almost 70% of the simulations.”
Large scale wind patterns are largely driven by the temperature difference between the poles and the tropics. But global warming is altering this difference because the Arctic is heating up faster than lower latitudes and because land areas are heating up faster than the oceans.
Recent changes in the Arctic are particularly striking, with record low levels of ice cover and extremely unusual high temperatures. “Things in the Arctic are happening much faster than we expected,” said Prof Stefan Rahmstorf, also at PIK.
“It is not just a problem of nature conservation or polar bears, it is about a threat to human society that comes from these rapid changes,” he said. “This is because it hits us with increasing extreme events in the highly populated centres in the mid-latitudes. It also affects us through sea level rise, which is hitting shores globally. So these changes that are going on in the Arctic should concern everyone.”
Other climate research, called attribution, is increasingly able to calculate how much more likely specific extreme weather events have been made by global warming. For example, the heatwave in south-eastern Australia in February was made twice as likely by climate change, while Storm Desmond, which caused heavy flooding in the UK in 2015, was made 40% more likely.
The record-breaking heat that made 2016 the hottest year ever recorded has continued into 2017, pushing the world into “truly uncharted territory”, according to the World Meteorological Organisation.
The WMO’s assessment of the climate in 2016 reports unprecedented heat across the globe, exceptionally low ice at both poles and surging sea-level rise.
Global warming is largely being driven by emissions from human activities, but a strong El Niño – a natural climate cycle – added to the heat in 2016. The El Niño is now waning, but the extremes continue to be seen, with temperature records tumbling in the US in February and polar heatwaves pushing ice cover to new lows.
“Even without a strong El Niño in 2017, we are seeing other remarkable changes across the planet that are challenging the limits of our understanding of the climate system. We are now in truly uncharted territory,” said David Carlson, director of the WMO’s world climate research programme.
“Earth is a planet in upheaval due to human-caused changes in the atmosphere,” said Jeffrey Kargel, a glaciologist at the University of Arizona in the US. “In general, drastically changing conditions do not help civilisation, which thrives on stability.”
The WMO report was “startling”, said Prof David Reay, an emissions expert at the University of Edinburgh: “The need for concerted action on climate change has never been so stark nor the stakes so high.”
The new WMO assessment also prompted some scientists to criticise Donald Trump. “While the data show an ever increasing impact of human activities on the climate system, the Trump administration and senior Republicans in Congress continue to bury their heads in the sand,” said Prof Sir Robert Watson, a distinguished climate scientist at the UK’s University of East Anglia and a former head of the UN’s climate science panel.
“Our children and grandchildren will look back on the climate deniers and ask how they could have sacrificed the planet for the sake of cheap fossil fuel energy, when the cost of inaction exceeds the cost of a transition to a low-carbon economy,” Watson said.
Trump is aiming to cut climate change research. But the WMO’s secretary-general Petteri Taalas said: “Continued investment in climate research and observations is vital if our scientific knowledge is to keep pace with the rapid rate of climate change.”
2016 saw the hottest global average among thermometer measurements stretching back to 1880. But scientific research indicates the world was last this warm about 115,000 years ago and that the planet has not experienced such high levels of carbon dioxide in the atmosphere for 4m years.
2017 has seen temperature records continue to tumble, in the US where February was exceptionally warm, and in Australia, where prolonged and extreme heat struck many states. The consequences have been particularly stark at the poles.
“Arctic ice conditions have been tracking at record low conditions since October, persisting for six consecutive months, something not seen before in the [four-decade] satellite data record,” said Prof Julienne Stroeve, at University College London in the UK. “Over in the southern hemisphere, the sea ice also broke new record lows in the seasonal maximum and minimum extents, leading to the least amount of global sea ice ever recorded.”
Emily Shuckburgh, at the British Antarctic Survey, said: “The Arctic may be remote, but changes that occur there directly affect us. The melting of the Greenland ice sheet is already contributing significantly to sea level rise, and new research is highlighting that the melting of Arctic sea ice can alter weather conditions across Europe, Asia and North America.”
Global sea level rise surged between November 2014 and February 2016, with the El Niño event helping the oceans rise by 15mm. That jump would have take five years under the steady rise seen in recent decades, as ice caps melt and oceans get warmer and expand in volume. Final data for 2016 sea level rise have yet to be published.
Climate change harms people most directly by increasing the risk of extreme weather events and the WMO report states that these raised risks can increasingly be calculated. For example, the Arctic heatwaves are made tens of times more likely and the soaring temperatures seen in Australia in February were made twice as likely.
“With levels of carbon dioxide in the atmosphere consistently breaking new records, the influence of human activities on the climate system has become more and more evident,” said Taalas.
Record-breaking climate change pushes world into ‘uncharted territory’ Damian Carrington 21 March 2017