Europe’s freak weather, explained

Europe’s freak weather, explained

17 August 2018

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EUROPE – BERLIN — We’ve all become increasingly used to reports of extreme weather over the past few years. But this summer’s raft of dramatic weather events is significant: Not only does it show what warming can do, it points to the potential large-scale trouble that lurks in the disruption of the planet’s winds and ocean currents.

In the past few months alone, we’ve seen extreme heat in Western Europe, Canada, Alaska, the western United States, Texas, Japan and Algeria, which set a new temperature record for Africa. Greece, Scandinavia, California and Siberia all suffered through drought and wildfires, while Japan, the U.S., Europe and India were hit with devastating floods. The human toll and harvest losses are still being tallied.

That global warming leads to more heat extremes is not rocket science and has been confirmed by global data analysis. We’re seeing five times more monthly heat records — such as “hottest July on record in California” — now than we would in a stable climate.

As part of this pattern, we can expect more heat drying out soils and causing more drought and wildfires. We also expect to see more extreme rain, given that a warmer atmosphere can take up and then release more moisture. A global increase in rainfall records has also been documented in weather station data.

But there is something more interesting going on here too.

2018 was a whopping 4.3 degrees above the average value of the first 30 years in which data was measured.

It’s not just that the weather is doing what it always does, except at a higher temperature level. Rather, there is growing evidence that the dynamics of weather itself are changing.

Let’s take a look at a concrete example. In my home town Potsdam, near Berlin — which boasts a high-quality weather station with uninterrupted homogeneous data since 1893 — April was the warmest April since measurements began, and May was the warmest May. Although June and July did not set any new records — those were recorded in 2003 and 2006 — they were also among the warmest. Just how extraordinary the current hot weather anomaly really is can best be seen when looking at the period between April and July.

We see a steady climate warming of around 2 degrees Celsius in the smooth climate curve since 1980, in parallel to global warming but twice as fast. This is typical of continental areas; ocean areas warm less due to heat storage and evaporation. We also see that 2018 was a whopping 4.3 degrees above the average value of the first 30 years in which data was measured, and nearly 2 degrees above the smoothed climate curve. This is by far the largest outlier relative to the climate curve. What’s going on?

A naive way to estimate the contribution of climate change to the high temperatures goes something like this: The smoothed curve shows the effect of global warming, and the scattering of the grey bars around this curve is the random variations of the weather. Accordingly, slightly more than half of the 4.3 degrees would be due to global warming, the rest to weather.

That’s not a bad first estimate, but it likely underestimates the contribution of climate change.

Not only is the current outlier by far the biggest, there is growing evidence that the “rest of the weather” is not just random but has already been altered by climate change too.

This is currently one of the hottest topics in climate research. The basic idea is that the jet stream — a band of high winds around the Northern Hemisphere that significantly influences our weather in the mid-latitudes — is changing.

This phenomenon has been confirmed by data: Researchers showed in 2015 that the jet stream has actually slowed down significantly in recent decades and undulates more. The cause is probably the strong warming of the Arctic, as the jet stream is driven by the temperature contrast between the tropics and the Arctic. Because this temperature difference is getting smaller and smaller, the jet stream is weakening and becoming less stable.

The weaker summer circulation means fewer weather changes, so the weather is becoming more persistent.

A certain wave pattern in the jet stream, meandering from north to south, settles for a long time and brings heat and drought or continuous rain, depending on where you are in this pattern. Such a persistent jet stream pattern has played an important role in the weather extremes of recent weeks, connecting the extremes around the Northern Hemisphere.

But the atmosphere is not the only player that can change its flow patterns. The ocean circulation may also have played a role, in particular the Gulf Stream System.

Researchers have shown that particularly cold surface water in the subpolar North Atlantic favors summer heat in Europe, again by changing the pattern of highs and lows in the atmosphere and thus the undulations of the jet stream. This happened in the “summer of the century” in 2003 and the heat wave of 2015.

The reality of global warming is catching up with us fast, and no longer an issue for future generations.

That year even saw the coldest temperatures on record in the subpolar Atlantic — the only region on Earth that has defied global warming and cooled instead. Such cold in the North Atlantic is occurring more and more frequently because the Gulf Stream System is weakening, as has been predicted by climate models in response to global warming.

Climate change does not just mean that everything is gradually getting warmer: It is also changing the major circulations of our atmosphere and ocean. This is making the weather increasingly weird and unpredictable.

The reality of global warming is catching up with us fast, and no longer an issue for future generations. We will need to prepare for more unpleasant surprises in the coming years, and we need to urgently cut down emissions to prevent further destabilizing our climate system.

Stefan Rahmstorf is professor of physics of the oceans and head of Earth System Analysis at the Potsdam Institute for Climate Impact Research. He is the recipient of the 2017 Climate Communication Prize of the American Geophysical Union.

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