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31.07.2018

WMO: July sees extreme weather with high impacts

Extreme weather, including record temperatures and heatwaves, drought and disastrous precipitation, has marked the first half of summer in the northern hemisphere. This has had widespread impacts on human health, agriculture, ecosystems and infrastructure and led to devastating wildfires.

"2018 is shaping up to be one of the hottest years on record, with new temperature records in many countries. This is no surprise. The heatwaves and extreme heat we are experiencing are consistent with what we expect as a result of climate change caused by greenhouse gas emissions. This is not a future scenario. It is happening now,” said World Meteorological Organisation's (WMO) Deputy Secretary-General Elena Manaenkova.

A forest fire burning in Enskogen, Sweden, as seen by Copernicus Sentinel-2 on 16 July 
Quelle: Copernicus, Sentinel Hub A forest fire burning in Enskogen, Sweden, as seen by Copernicus Sentinel-2 on 16 July Quelle: Copernicus, Sentinel Hub

The persistency of high temperatures in some regions – including northern Europe - has been due to a stationary high pressure system. This is common in summer in both Northern and Southern Hemispheres. The jet stream, a core of strong winds around 10 kilometers above the Earth’s surface that blow from west to east and which steer weather around the globe – is generally slower in summer and occasionally become weaker than usual, then very settled weather occurs on the surface.

There is much scientific research into whether climate change and substantial changes to sea surface temperature, escalated by high multi-decadal natural variability, are contributing to more profound effect in altering the atmospheric circulation and so leading to more “blocking patterns.”

Drought and heat in northern Europe

WMO’s Regional Association for Europe’s Climate Centre on Climate Monitoring, operated by the German Weather Service, DWD, issued a second Climate Watch advisory with guidance on drought and above normal temperatures valid from 19 July until 6 August. The guidance product, used by National Meteorological Services to issue national warnings and forecasts, referred to "a continuation of the drought situation and above-normal temperatures for at least the next two weeks for northern Europe (from Ireland to the Baltic States and southern Scandinavia).

“The weekly temperature anomalies are forecasted with up to +3 to +6°C (in the first week in parts +6 to +10°C). The probability that precipitation will be below the lower tercile is more than 70% in the first week and more than 50% in the second week. This drought may be accompanied by water scarcity, local thunderstorms, risks of wildfires and harvest losses since soil moisture is reduced," it said.

Amid an extended heatwave in Scandinavia, temperatures topped 30°C in the Arctic Circle. Norway saw a record temperature of 33.5°C in Badufoss on 17 July, and it reached 33.4°C in Kevo, Finland. Influenced by a warm wind, the far north of Norway (Makkaur) saw a new record minimum overnight temperature of 25.2°C on 18 July.

The heatwave followed an exceptionally dry and warm May in northern Europe, leading to a a sustained elevated risk of forest fires throughout Scandinavia and the Baltic region. Sweden reported about 50 forest fires burning in mid-July.

Deadly fires fanned by extremely high winds killed dozens of people near Athens on the night of 24 July in one of Greece’s worst tragedies for years.

WMO reports many further extreme weather conditions (drought, extreme temperatures, heavy rain in many other countries, such as Ireland, Armenia, Japan, Korea, Algeria, Oman, Morocco, USA, Canada, Siberia (Russia), and more. Read the details in the report at WMO's website.

Relationship with Climate Change

Episodes of extreme heat and precipitation are increasing as a result of climate change. Although it is not possible to attribute the individual extreme events of June and July to climate change, they are compatible with the general long-term trend due to rising concentrations of greenhouse gases.

Many recent studies have found that the probability of the extreme event has been influenced by human activity, either directly or indirectly. Of a set of 131 studies published between 2011 and 2016 in the Bulletin of the American Meteorological Society, 65% found that the event’s probability was significantly affected by anthropogenic activities. In the case of some extreme high temperatures, the probability increased by a factor of ten or more.

It has been more difficult to identify anthropogenic influence in the attribution of precipitation extremes. Whilst some studies have found that the probability of some extreme precipitation events was increased, most often indirectly, by climate change, for many other studies the results have been inconclusive. This is because the underlying long-term climate signal in extreme precipitation is less clear than it is for temperature and, because extreme precipitation events typically occur on shorter spatial scales than extreme temperature events. At present, attribution studies are mostly carried out in research mode in peer-reviewed literature.

The IPCC 2012 Special Report on Extreme Events anticipates for example that “it is likely that the frequency of heavy precipitation or the proportion of total rainfall from heavy falls will increase in this century over many areas of the globe” and that “a 1-in-20 year annual maximum daily precipitation amount is likely to become a 1-in-5 to 1-in-15 year event by the end of the 21st century in many regions”.

More at the website of the World Meteorological Organisation WMO (more information, figures and links, …)

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