Smoke from wildfires in Red Square last week (Image: Natalia Kolesnikova/AFP/Getty)

Russia has sweltered under an intense heatwave since mid-July, recording its highest ever temperatures. The heat has caused widespread drought, ruined crops and encouraged wildfires that have blanketed Moscow in smog and now threaten key nuclear sites. According to the head of Moscow’s health department, the city’s daily death rate has doubled – up to 700 from the usual average of 360 to 380.

What caused the heatwave?

The primary cause was a “blocking event” – a static atmospheric pattern that has trapped a high-pressure bubble over western Russia since mid-July, pulling in hot air from Africa.


Blocking events naturally occur from time to time. There is evidence that low solar activity increases their numbers , and the sun is currently in a period of minimum activity.

Jeff Knight of the UK Met Office says that the climatic pattern known as El Niño was also a factor. Around the new year, the eastern tropical Pacific heated up, sending a slow-moving wave of heat around the globe – conditions that are characteristic of El Niño. “It warms the global mean temperature with a delay of about six months,” says Knight. This extra packet of heat will have increased the likelihood of heatwaves around the world.

Is climate change to blame?

Computer models of climate are not detailed enough at present to reproduce blocking events, making it impossible to say whether rising greenhouse gas concentrations makes them more likely to happen.

However, whatever the mechanism, there is a large body of evidence to suggest that climate change increases the number of heatwaves and make them longer. Since 1880 the frequency of extremely hot days has nearly tripled and the length of heatwaves across Europe has doubled. Models also predict that climate change will push up peak temperatures faster than average temperatures.

This is an example of climate change’s tendency to increase the likelihood of extreme weather events. The number of very hot days is forecast to increase fivefold by 2100. One model study has suggested that Paris, France, will see the frequency of heatwaves grow by 31 per cent over the century, and that by 2100 they will last twice as long.

The consequences will be widespread. Agricultural yields are likely to drop, and summer death rates will rise worldwide. True, winter death rates will drop during milder winters, but this will not offset the extra summer deaths.

However, it is important to bear in mind that no single weather event can be reliably linked to climate change. “It’s a statistical tendency, a push in one direction,” says Joanna Haigh of Imperial College London. The Russian heatwave might have occurred anyway, without help from greenhouse gases. All we can say for sure is that such events are more likely in a warmer world.

What are the health consequences?

The Russian heatwave is blamed for nearly 800 wildfires, more than 500 of which are still burning. Moscow’s health department chief said earlier this week that the heat and smoke from the fires has doubled the city’s daily death rate.

Burning peat and vegetation produces smoke rich in tiny airborne particles. However, Frank Kelly, director of environmental research at King’s College London, says the smoke isn’t as dangerous as the industrial smog seen in most cities. Traffic pollution tends to contain more metals, which react to produce damaging free radicals in the lungs.

Although some of the pollutants, such as benzene, cause cancer, they are in tiny amounts in this type of smoke, says Kelly. What’s more, cancer becomes a concern only after months or even years of exposure. Still, the sheer volume of pollutants, including benzene, means they pose a real risk – of worse symptoms and even death – to asthmatics and those with chronic obstructive pulmonary disease.

Kelly thinks the scorching temperatures are behind around three-quarters of the reported deaths. With temperatures hitting 40 °C, the elderly and very young, who find it harder to control body temperature, are particularly at risk: when blood vessels expand to lose heat, blood pressure drops, decreasing blood supply to the brain. In extreme heat, vulnerable people are at risk of fainting or even slipping into a coma.

How much longer will it last?

We just don’t know. “These events can last for many days, or even many weeks in exceptional cases,” says Paul Williams of the University of Reading, UK. “For example, the 2003 European heatwave lasted from June until August.”

The end could be sudden. “If the weather changes its mind, the pattern can break just like that,” says Knight.

Even if the current blocking event breaks sooner rather than later, there is always the possibility that another will come along. They are more common in summer than in winter, and difficult to predict. “They are like buses,” says Williams. “Sometimes a long time will go by without one, and then two will come along in quick succession.”

Will it happen again?

The same exact circumstances may not recur any time soon, but more heatwaves are inevitable, even without climate change. “Variations in atmospheric circulation happen spontaneously,” says Knight. “It will happen again.”

Blocking events are not restricted to Russia. This one also helped trigger the floods in Pakistan and may even be behind the torrential rain in China that led to a deadly landslide this week. Another blocking event triggered the unusually hot temperatures in the eastern US in early July.

Climate change will continue to push temperatures upwards and make heatwaves both more frequent and more severe. Knight notes that, while western Russia has roasted in record-breaking temperatures, more easterly regions like Siberia have been subjected to continual flows of cold air and have actually been several degrees below the average temperature for the time of year (as shown in the temperature data in this graphic).