Europe is facing another spell of dangerous heat, with temperatures crossing 40 degrees Celsius in parts of France and Spain, heat alerts issued across large parts of the continent and authorities warning of serious risks to life and infrastructure.
France, currently at the centre of the heatwave, recorded its hottest nationally averaged day on Tuesday, while temperatures in one town crossed 44 degrees Celsius. In southern Spain, temperatures exceeded 45 degrees Celsius in Andújar. Heat alerts were in place across 23 European countries, with Germany, France, Spain, Switzerland and Luxembourg placed under the most severe red-level warnings.
The heat has also proved deadly. French Prime Minister Sébastien Lecornu said 40 people had drowned since
June 18 and linked the deaths to the high temperatures, calling them a “grim scourge”. Three elderly people died near Bordeaux, while two children, aged two and four, were found dead in a hot car in southern France.
The immediate weather pattern behind this prolonged spell of heat is known as an Omega Block. But what does that mean, and why is it keeping temperatures so high?
What Is An Omega Block?
An Omega Block is a weather pattern in which a large area of high pressure becomes trapped between two areas of low pressure.
It gets its name from the Greek letter Omega, written as Ω. On a weather map, the arrangement of the pressure systems resembles the shape of the letter: a bulge of high pressure in the middle, with low-pressure systems on either side.
The word “block” is important. Normally, weather systems move from west to east across Europe, carried along by a band of strong winds high in the atmosphere known as the jet stream.
During an Omega Block, that normal movement is disrupted. The jet stream bends sharply northwards and southwards instead of flowing in a relatively steady direction. This leaves the pressure systems cut off and slows their movement.
As a result, the high-pressure area can remain stuck over the same region for several days. Omega Blocks generally last between three and 10 days, but in some cases, they can remain in place for weeks.
How Is It Trapping Heat Over Europe?
The high-pressure area at the centre of the Omega Block is creating hot, dry and settled conditions over parts of Western Europe.
High pressure limits the formation of clouds. With fewer clouds to block the Sun, the ground receives intense sunshine for long periods during the day. Winds also remain weak, which means the hot air is not quickly replaced by cooler air.
The heat therefore remains over the same area and continues to build.
This central high-pressure zone is also described as a heat dome because it behaves like a lid over a pot, holding hot air in place. The heat dome is one part of the wider Omega Block, which also includes the two low-pressure systems on either side.
Richard Allan, a professor of climate science at the University of Reading, told CNN that a heat dome over Europe in summer was not unusual, “but the temperatures are”.
The effects of the block are different across the continent. Countries beneath the central high-pressure system face clear skies, dry conditions and extreme heat. Areas under the low-pressure systems on either side are more likely to experience cooler and wetter weather.
Britain lies near the boundary between the intense heat and cooler air to the northwest. According to the UK Met Office, this is producing very hot conditions in southern and eastern Britain, while northern and western areas remain relatively cooler and wetter.
Why Is The Current Heat So Extreme?
The Omega Block explains why the weather has become stuck, but climate change explains why the heat trapped beneath it is now reaching much higher levels.
The burning of coal, oil and gas has increased greenhouse gases in the atmosphere and warmed the planet by about 1.3 degrees Celsius compared with pre-industrial times.
This means every heatwave now begins from a warmer starting point. When a weather pattern such as an Omega Block holds hot air over one region, temperatures can climb far beyond those seen during similar events in the past.
Clair Barnes, a research associate in extreme weather and climate at Imperial College London, told Reuters that heatwaves in Europe are now 2 to 4 degrees Celsius hotter than they would have been without human-caused warming.
Scientists have not reached a firm conclusion on whether climate change is making Omega Blocks more frequent. However, there is clear scientific agreement that it is making heatwaves more frequent and more intense.
Peter Thorne, director of the ICARUS Climate Research Centre at Maynooth University in Ireland, told CNN that changes in weather circulation “appear to be making blocking events with heat domes more frequent and long-lived”.
Is El Niño Behind The Heatwave?
A strengthening El Niño is beginning to develop in the tropical Pacific Ocean. El Niño can raise global temperatures and increase the likelihood of extreme heat in different regions.
However, scientists say it is not responsible for the current European heatwave because it has only recently begun.
Liz Bentley, chief executive of the Royal Meteorological Society, told CNN it was having “little or no impact” on the current heatwave, although it could contribute to stronger heat next summer.
Climate change remains the main factor behind the severity of the present temperatures.
“Yes it’s climate change, yes it’s us, no it’s not El Niño,” Friederike Otto, a professor of climate science at Imperial College London, told CNN.
Europe Is Now Warming Up To 3x Faster Than Global Average
Europe is warming around two to three times faster than the global average, making it the fastest-warming continent.
One reason is its proximity to the Arctic, which is heating faster than any other part of the world. As rising temperatures melt snow and ice, darker surfaces beneath them are exposed. These darker surfaces absorb more heat from the Sun, which causes further warming.
Cleaner air has also had an unintended effect. European anti-pollution laws have reduced harmful particles in the atmosphere. These particles were dangerous to health, but they also reflected some sunlight away from the Earth. With fewer such particles in the air, slightly more solar energy can reach the surface.
Changes in atmospheric circulation are also influencing how heat remains over Europe.
Scientists say the broad rise in the severity and spread of European heatwaves is in line with long-standing climate predictions. What remains striking is how early such heat is now arriving.
Europe is experiencing its second record-breaking heatwave in two consecutive months, even before July, which is usually the continent’s hottest month.
“This isn’t the new norm at all; this is the foothills of absolute catastrophe,” Hugh Montgomery, professor of intensive care medicine at University College London, told CNN.
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