Science

European summer droughts are more severe than in the past 2,100 years

Droughts in Europe during the summer are more severe now than at any time in the past two millennia, a new study reveals.

Researchers studied ‘chemical fingerprints’ – carbon and oxygen isotopes – in European oak trees to reconstruct summer climate over the last 2,110 years.  

They found that drought conditions suddenly intensified in 2015 beyond anything in the past two thousand years, likely due to climate change.

Europe experienced severe summer heat waves and drought spells in 2003, 2015 and 2018, which affected agricultural sectors and the wine and forestry industries. 

Europe’s recent summer droughts have had ‘devastating ecological and economic consequences’ – and are set to worsen as the global climate continues to warm. 

Pictured, landscape of dry earth ground and viaduct, extreme drought in Entrepenas reservoir, in Guadalajara, Castilla, Spain. Recent summer droughts in Europe are far more severe than anything in the past 2,100 years, according to the new study

ISOTOPES IN TREE RINGS 

Tree rings hold great potential for reconstructing past climates.

Stable isotopes of carbon, hydrogen and oxygen within the wood of trees record changes in the isotope composition of the source materials used by the tree during wood formation. 

Suitable trees are widespread, and by piecing together the records from living, dead and subfossil wood it is possible to produce ‘long and continuous chronologies’.

Scientists avoid cutting trees down to access tree rings by collecting a sample with an instrument called an increment borer. 

The borer extracts a thin strip of wood that goes all the way to the centre of the tree. 

Source: Isotopes in Palaeoenvironmental Research/NASA 

The research has been conducted by an international team, led by experts from the University of Cambridge, who think studying trees can provide more accurate results than ‘limited’ computer modelling simulations. 

‘We’re all aware of the cluster of exceptionally hot and dry summers we’ve had over the past few years,’ said study author Professor Ulf Büntgen from the University of Cambridge’s Department of Geography.

‘But we needed precise reconstructions of historical conditions to see how these recent extremes compare to previous years. 

‘Our results show that what we have experienced over the past five summers is extraordinary for central Europe, in terms of how dry it has been consecutively.’

Most studies attempting to reconstruct past climates are restricted to temperature – but this team analysed stable isotopes in tree rings. 

Chemical characteristics of the rings inside a tree can reveal what the weather conditions were like during each year of that tree’s life.  

For the study, Büntgen and his colleagues from the Czech Republic, Germany and Switzerland studied more than 27,000 measurements of carbon and oxygen isotopic ratios from 147 living and dead European oak trees.

The trees, from the genus Quercus, covered a period of 2,110 years, ranging from 75 BC to 2018.

The samples came from archaeological remains, subfossil materials, historical constructions and living trees from what is now the Czech Republic and parts of south-eastern Bavaria.

Example of a polished cross-section of an oak from the Czech Republic. Researchers studied 'chemical fingerprints' – carbon and oxygen isotopes – in 147 living and dead European oak trees to reconstruct summer climate over the last 2,110 years

Example of a polished cross-section of an oak from the Czech Republic. Researchers studied ‘chemical fingerprints’ – carbon and oxygen isotopes – in 147 living and dead European oak trees to reconstruct summer climate over the last 2,110 years 

‘Generally, our understanding is worse the further back we go back in time, as datasets looking at past drought conditions are rare,’ said Büntgen. 

‘However, insights before medieval times are particularly vital, because they enable us to get a more complete picture of past drought variations, which were essential for the functioning and productivity of ecosystems and societies.’ 

For each ring in each tree, researchers extracted and analysed carbon and oxygen isotopes independently.

Carbon values depend on the photosynthetic activity, which is influenced by the amount of sunlight present, while oxygen values are affected by the source water.

Fossil fuel emissions since the industrial era have also affected the isotopic composition of carbon dioxide (CO2). 

Together, carbon and oxygen isotopic values closely correlate with the conditions of the growing season, the researchers say. 

‘These tree-ring stable isotopes give us a far more accurate archive to reconstruct hydroclimate conditions in temperate areas, where conventional tree-ring studies often fail,’ said co-author Professor Jan Esper from the University of Mainz, Germany.

Typically, trees are aged by measuring their girth – specifically the rings that develop over time that increase that girth.  

Graph shows when 147 living, historical, archaeological and subfossil oaks (green bars). The photographs at the bottom show examples of archaeological remains, subfossil materials, historical constructions and living oaks, and the grey shading on the right refers to the industrial period during which human-made fossil fuel emissions affect the isotopic composition of carbon dioxide (CO2)

Graph shows when 147 living, historical, archaeological and subfossil oaks (green bars). The photographs at the bottom show examples of archaeological remains, subfossil materials, historical constructions and living oaks, and the grey shading on the right refers to the industrial period during which human-made fossil fuel emissions affect the isotopic composition of carbon dioxide (CO2)

But trees involved in this study had already been aged. Looking at the isotopes within them revealed more about the climate at the time they were alive.    

Over the 2,110-year period, the tree-ring isotope data showed there were very wet summers, such as AD 200, 720 and 1100, and very dry summers, such as AD 40, 590, 950 and 1510. 

Despite these ‘out of the ordinary years’, the results show that for the past two millennia, Europe has been slowly getting drier.

The samples from between 2015 and 2018, however, revealed that drought conditions in recent summers have far exceeded anything in the 2,110 years.

‘We’ve seen a sharp drop following centuries of a slow, significant decline, which is particularly alarming for agriculture and forestry,’ said co-author Professor Mirek Trnka from the CzechGlobe Research Centre in Brno, Czech Republic. 

The magnified inset shows the anatomical structure of large earlywood vessels and homogeneous latewood fibres

The magnified inset shows the anatomical structure of large earlywood vessels and homogeneous latewood fibres

‘Unprecedented forest dieback across much of central Europe corroborates our results.’

The researchers say that the recent cluster of abnormally dry summers is most likely the result of human-caused climate warming, and the associated changes in the position of the jet stream.  

The jet stream is a band of strong winds between five and seven miles above the Earth blowing from the west to the east. 

Atmospheric circulation over Europe and the position of the jet stream represent the dominant drivers of historical drought occurrence in the region, the team say. 

A further increase in the frequency and severity of heat waves under projected global warming will likely result in a multitude of harmful direct and indirect impacts on human health’, they warn. 

Professor Büntgen also pointed out that climate change does not necessarily mean it will get drier everywhere.

‘Some places may get wetter or colder, but extreme conditions will become more frequent, which could be devastating for agriculture, ecosystems and societies as a whole,’ he said. 

The results have been published in the journal Nature Geoscience

Summer 2019 was hottest on record for Northern Hemisphere 

Summer 2019 was hottest on record for Northern Hemisphere, according to the US’s National Oceanic and Atmospheric Administration.

The heat impacted Arctic sea ice coverage, shrinking it to the second smallest for the month on record.

The Northern Hemisphere summer (June through August) record high temperature also ties with the same period in 2016. 

The following are some of the specific causes. 

WHAT CAUSED THE HEATWAVE?

The heatwave was triggered by the build-up of high pressures over Europe over, leading to the northward movement of warm air from Europe over the UK.

‘At this time of year southerly winds will always lead to above average temperatures,’ said University of Reading meteorologist Peter Inness.

‘Air from continental Europe, the Mediterranean and even North Africa is brought over the UK.’

‘The eastward passage of weather fronts and low pressures from the North Atlantic are currently being blocked by the high pressure over Europe,’ added University of Reading climate scientist Len Shaffrey. 

WAS IT RELATED TO THE US HEATWAVE?

The US’s warm weather in 2019 was caused by a high-pressure dome building up over much of the country, trapping the summer heat.

This had wider-reaching effects.

‘Heatwave conditions in the U.S Midwest and the East coast have strengthened the jet stream,’ said environmental scientist Kate Sambrook of the University of Leeds.

‘The resulting thunderstorms occurring on the continent have helped the jet stream to meander and move to the north of the UK.’

‘As a result of this shift, hot air has been drawn up from Europe causing the high temperatures we are experiencing this week.’

The US's warm weather last summer was caused by a high-pressure dome building up over much of the country, trapping the summer heat

The US’s warm weather last summer was caused by a high-pressure dome building up over much of the country, trapping the summer heat

 

IS CLIMATE CHANGE CAUSING HEATWAVES?

‘The fact that so many recent years have had very high summer temperatures both globally and across Europe is very much in line with what we expect from man-made global warming,’ said Dr Inness.

‘Changes in the intensity and likelihood of extreme weather is how climate change manifests,’ said environmental scientist Friederike Otto of the University of Oxford.

‘That doesn’t mean every extreme event is more intense because of it, but a lot are. For example, every heatwave occurring in Europe today is made more likely and more intense by human-induced climate change.’

However, local factors also play a role, with each extreme weather event being influenced by the location, season, intensity and duration.

The present heatwave is not the only notable indicator of climate change, experts note, with ongoing droughts — such as those being experienced in many parts of Germany — also being in line with scientific predictions.

Research into the 2003 European heatwave suggested at the time that human activity had more than doubled the risk of such warm summers — and that annual heatwaves like we are experiencing now could become commonplace by around the middle of the century.

‘It has been estimated that about 35,000 people died as a result of the European heatwave in 2003, so this is not a trivial issue,’ said Dr Inness.

‘With further climate change there could be a 50% chance of having hot summers in the future,’ agreed Dr Finney.

‘That’s similar to saying that a normal summer in future will be as hot as our hottest summers to date,’ he added. 


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