By Tim Radford
Global heating may be to blame for the fact that Europe has grown drier over the last 2,000 years to a new high in 2015.
LONDON, 17 March, 2021 − Europe has grown drier, an outcome shown by the continent’s last five summers, which have been marked by drought that has no parallel in the last two millennia.
Researchers studied two kinds of evidence delivered by 27,000 measurements taken from 21 living oak trees and 126 samples from ancient beams and rafters, to piece together a precise picture of the climate of Germany, Switzerland and the Czech Republic over the last 2,110 years.
They report, after 2015, that drought conditions intensified suddenly, in ways that were beyond anything over that entire 2000-year tract of time. And, they add, “this hydroclimatic anomaly is probably caused by anthropogenic warming.”
Europe is also getting hotter. In 2003, 2015 and 2018 it was hit by severe summer heat waves and spells of drought that damaged plantations, crops and vines; the damage from drought was intensified by more virulent attacks from pathogens, insect outbreaks and tree death.
“Extreme conditions will become more frequent, which could be devastating for agriculture, ecosystems and societies as a whole”
In the baking summer of 2003, an estimated 70,000 people died because of extremes of heat. And, the researchers say, “a further increase in the frequency and severity of heat waves under projected global warming implies a multitude of harmful direct and indirect impacts on human health.”
In other words, things are bad now and are likely to get worse, according to a report by 17 British, European and Canadian researchers in the journal Nature Geoscience.
Dendrochronologists can and do routinely build up a picture of bygone temperatures by measuring the growth rings in trees: enough old living trees, and reliable knowledge about the felling of oaks for chateaux, cathedrals, sailing ships, fortresses and stockades can help pinpoint seasonal change on an annual basis.
But trees are also living chronicles of changes in carbon and oxygen isotope ratios − tiny atomic variations in the plant’s biochemistry − which provide evidence of rainfall and therefore a more precise picture of any growing season.
Wandering jet stream
The trees delivered mute evidence of very wet summers in 200, 720 and 1100 AD, and very dry summers in the years 40, 590, 950 and 1510 of the Common Era. But overall the big picture emerged: for the years 75 BC to 2018, Europe has slowly been getting drier.
Even so, the evidence from 2015 to 2018 shows that drought conditions in the area from which the trees were taken far exceeds anything in the previous centuries. The mostly likely explanation is the impact of ever-rising temperatures, driven by ever-higher greenhouse gas emissions from the ever-more profligate combustion of fossil fuels.
These temperatures are now considered high enough to affect the course of the stratospheric jet stream in ways that alter the long-term pattern of temperature and rainfall that defines a region’s climate.
“Climate change does not mean it will get drier everywhere,” said Ulf Büntgen, who holds research posts in the University of Cambridge, UK and the Czech Republic and Switzerland. “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.” − Climate News Network