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Postcards from Heatwave Europe

August 3, 2018

Low water levels on Lake Nisser in Norway. Image: Anne Lyche Solheim

Across the world, this summer’s weather has been characterised by extremes. The USA has experienced severe droughts and wildfires in the West, and flash flooding in the East, whilst an ‘unprecedented’ heatwave in Japan has been attributed as the cause of over 65 deaths.

Most of Europe has experienced an extended period of high temperatures and minimal rainfall, causing wildfires to spread in both the Arctic Circle and Greece. Meteorologists predict that the European temperature record of 48C (118.4F) set in Athens in July 1977 could be broken this weekend in Spain and Portugal.

In Europe, the heatwave has been caused by the stalling of the northern hemisphere jet stream, which generally brings cooler Atlantic weather over the continent. As a result, a huge ‘pocket’ of hot, dry air has been present over Europe (particularly northern regions) in recent months.

Analysis of the heatwave in northern Europe between May-July of this year by the World Weather Attribution group suggests that this extreme weather has been made at least twice as likely by ongoing climatic changes.  Similarly, speaking recently to The Guardian about this year’s global weather extremes, climate scientist Prof. Michael Mann, at Penn State University said, “This is the face of climate change. We literally would not have seen these extremes in the absence of climate change.”

The high air and water temperatures, low rainfall and flashy storms and flooding experienced across Europe are all key pressures on the health and status of freshwater ecosystems. To gain a picture of how this summer’s weather is affecting European waters, we put a call out to our network of aquatic scientists across the continent, asking them to send in brief ‘postcards’ of their observations.

The first responses are collected here, and more will be published in the coming weeks. If you would like to submit your own observational ‘postcard’, please email


Water levels in tributaries of Lake Nisser in Norway are extremely low, and in some cases completely dry. Image: Anne Lyche Solheim


I am currently on holidays close to Norway’s 10th largest lake (Nisser in Telemark) where the water level is extremely low, and most of the tributaries have dried out completely, or have become reduced to minor streams. Yesterday, a farmer in the local neighbourhood came with his tractor and a big water tank to get water from the lake for his cattle, who are suffering in a grazing area in the hills, where all the streams have tried out. This has never happened before.

The well we use for water supply to our summer house here have lost two-thirds of its water, so we don’t know how long we can stay here before the well runs dry. There is absolutely zero re-charge over the past weeks and months now. 12th of June was the last time we had any substantial rain in South-Eastern Norway.

The air temperature during daytime varies from 28 to 31C, and it’s been like that since mid-May. There are almost no insects, except a few bumble bees. The grass is mostly yellow and the grain crops are likely to be < 1/3 of normal. There is a crisis for cattle farmers, as they do not have enough fodder for them, so a lot has been slaughtered.

The surface water in the lake is now 25C, which is highly exceptional (although perfect for swimming). The littoral zone is completely dried out.

Dr. Anne Lyche Solheim, Niva


The weather has been completely weird this year! We had no winter: no snow, no rain, and very dry and warm weather. Between May and mid-June we had extreme rains with flash floods all around the country. Lakes and rivers across Turkey suffered from dissolved organic carbon problems, which are normally unheard of.

Now since mid-June, every day is record breakingly hot. In this heatwave, many small ponds are drying out, and lake levels are going down more than 1 cm/day. Evaporation is very high, which leads to an increase in salinity and conductivity in the water column. Of course, we’re seeing widespread algal and cyanobacteria blooms as water temperatures rise and nutrient concentrations increase.

Prof. Meryem Beklioğlu, Middle East Technical University


Huge blooms of blue-green algae in brackish coastal waters in the Gulf of Finland and Archipelago Sea. Image: ESA Copernicus Sentinel Data, 16 July 2018, processing by SYKE


With wildfires blazing in Greece and Sweden, the heatwave across Europe has not left Finland cold either. Temperatures were record-breaking both in May and July, with average temperatures 2–5 degrees above long-term mean values. This has not come without responses in Finnish freshwaters.

In Lake Enäjärvi in Southern Finland, mass deaths of lake mussels occurred, likely due to oxygen depletion in the exceptionally warm water. Also fish kills were observed in some lakes across Southern Finland, and extensive blooming of cyanobacteria occurred in many lakes. The hot weather also boosted a massive blue-green algae bloom in July in the Gulf of Finland and parts of the Archipelago Sea.

In inland fish farm ponds, salmonids have been suffering the prolonged heat. Across fishing sites in streams, there were pleas to temporarily cease salmonid fishing as catch-and-release might cause additional fatal stress to the fish already stressed by warm water.

Dr. Jukka Aroviita, Finnish Environment Institute (SYKE)


Flash flooding in Cardiff, Wales, after weeks of hot, dry weather. Image: Steve Ormerod


In Wales — as in much of the UK — we’ve been heading for what has looked like the driest summer since modern records began in 1961. River discharge levels have been very close to record lows throughout July, while air temperatures at or near 30C in mid and south Wales have meant the region’s rivers have been at 24C or higher. Parallels have drawn increasingly with the major drought of 1976, surpassing the extremes of more recent events in the early 1990s or 2003.

All of these past drought years brought significant ecological effects. In 1976, low water levels, high temperatures and attendant low oxygen concentrations in the River Wye (now a Natura 2000 river) were exacerbated by the dieback and decomposition of mats of Ranunculus fluitans causing extensive salmonid mortality. Here, at the south-west edge of the Atlantic Salmon’s European range, recruitment falls substantially following hot, dry summers like 2018, so we expect echoes of current conditions next year and beyond.

It’s very likely that the full effects of the 2018 drought will only be known retrospectively. As an example, droughts during several summers in the early 1990s were subsequently shown to have led to the widespread extinction across Wales of the cool-water flatworm, Crenobia alpina — and it hangs on now only where thermally damped subterranean discharge keeps water temperatures below 12-13C.

The current hot, dry conditions over Wales came to an abrupt end over the weekend of July 28th/29th, when 40-60 mm of rain fell over different parts of the nation. Although there was localised urban flooding, effects on river levels have been small or transient.

Hydrographs Wye tributaries

Hydrographs of tributaries of the River Wye, Wales, showing low water flows, even after heavy rain in late July 2018. Image:

These two 30 day hydrographs from the Ithon and Irfon – respectively left and right bank tributaries of the Wye – illustrate modest effects on discharge that also reflect local geological damping and slightly lower catchment altitude of the Ithon.

With a return to drier conditions forecast for much of July and August, the 2018 summer may yet break more records.

Prof. Steve Ormerod, Cardiff University

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