London School of Hygiene & Tropical Medicine

What is going on with the weather in the Northern Hemisphere at the moment?

The main reason is a wind pattern in the upper atmosphere – known as the jet stream - which is blocked or rather stationary. This is causing very hot, dry conditions over southern Europe, as well as southern US, north Africa and parts of Asia. In the UK it’s bringing cooler, wetter weather.

At the same time, El Nino – a weather phenomenon which brings warmer sea surface temperatures in the equatorial Pacific roughly every 3-7 years – arrived in 2023.

What is the jet stream and what happens if it is blocked?

The jet stream is a fast-moving wind in the upper atmosphere that flows from west to east at mid-latitudes in the northern hemisphere.

Currently, the jet stream from North America to Europe is stuck in a position south of the English Channel. To put differently, we have a “traffic jam” in the upper atmosphere over these regions causing a stalled, rather warm, weather pattern.

To the north, we have cooler Arctic air and areas of low pressure, which is bringing rain spells to the UK. To the south of the jet stream, we have an opposite weather pattern where it’s stalled in a high-pressure system and drawing hot air from north Africa, maintaining the high temperatures – and associated wildfires - in Greece and Italy.

A similar pattern currently exists over the US with the jet stream stalled in a high-pressure weather pattern over large parts of California, Texas and Florida, creating abnormally high temperatures.

Parts of Asia are also experiencing blocking patterns with hot air being blown to specific mainland regions such as China, and increasing the temperatures to higher than expected for this time of year.

Do we know why the jet stream is stuck in this position? Has it happened before?

Our weather system is complex and ever-changing, driven largely by the difference in near-surface temperature between the equator and higher latitudes, as well as between land and ocean.

Small changes in temperature, whether due to natural climate variability or human-induced climate change, can have larger effects in different parts of the world.

The current blocking pattern is not uncommon in historical climate records for the regions, but its strength and how long it lasts could indicate changes over time. As scientists from the World Weather Attribution note: “Without human induced climate change these heat events would however have been extremely rare.”

Are ‘blocking events’ becoming more frequent or lasting longer in more parts of the world, and is climate change playing any role in that? 

Evidence from recent years including the current heatwave suggests that blocking events are becoming more persistent in nature, but more studies are needed to confirm this hypothesis.

Researchers are using climate model simulations to explore whether climate change can make these blocking patterns more frequent, erratic, and last longer, but we need more research in this area.

What is El Nino and is it also involved in the heatwaves?

The El Nino Southern Oscillation (ENSO) is a natural variability in our climate system in the tropical Pacific which occurs roughly every 3-7 years. It has two phases – the cooler La Nina and warm El Nino. There is no scientific consensus on what causes it or why it becomes stronger in some years.

We are seeing an El Nino gathering momentum since this May, and its effects have already kicked in with significant anomalies in upper-surface sea temperatures in the Pacific. This in turn is known to disturb wider weather patterns not only in the Pacific, but also globally such as over central America and parts of Europe, Africa and Asia, something that climate scientists refer to as “teleconnection”.

According to the current seasonal forecasts, the El Nino is expected to strengthen and affect the global weather pattern for the rest of the year.

Is there a link between climate change and the fact we have heatwaves affecting Europe, the US and Asia at the same time?

While we cannot attribute a single specific extreme weather event to climate change, a warming climate may increase the likelihood of such an event occurring more often. Scientists from the World Weather Attribution used published methodologies to examine how likely such extreme weather events would be in a natural climate system (ie in the absence of human-induced climate change). Their preliminary analysis suggests that the present extreme heatwaves are unlikely to occur in the absence of anthropogenic global warming.

Why are these heatwaves so unusual and alarming?

The present heatwaves are alarming in terms of its intensity and duration. The temperatures in some locations have either touched their historical high records or are forecasted to be surpassed. While there have been heatwaves in the past, they have occurred for other reasons and have been largely localised (eg the 1930s dust bowl in the US). It is the concurrent occurrences of the current heatwave in four different continents in the Northern Hemisphere – North America, (southern) Europe, (north) Africa, and (east) Asia – which is quite alarming.

While there is no reason to say that such heatwaves are unusual in a natural climate system such as in a strong El Nino year, it is reasonable to expect that the additional heat from our warming climate can add extra thermal energy in our weather system. And the recent preliminary findings of the scientists from the World Weather Attribution suggests the same.

What are the main effects of these heatwaves on the environment and health?

Agriculture: Heat stress has a detrimental effect on crop productivity. Most cereal crops require an optimum range of temperatures between 10-30 deg C (along with moisture or water either as natural rainfall or irrigation) to reach nutritious levels.

Heatwaves can have a damaging effect on the crops irrespective of the timing of the crop growing season. Similarly, it can also exacerbate soil moisture and droughts, thus affecting the wider flora and fauna.

Energy: Increased use of air conditioning gives rise to a sudden spike in energy demand, which in turn can result in potential blackouts if the energy infrastructure is incapable of coping. Moreover, recent land surface temperatures reached such high values (eg in Sicily, Italy) that the power transmission cables/lines were directly affected.

Health: Heatwaves create added pressure on ambulance callouts, hospitalisations and public health infrastructure. Excess deaths from heatwaves have been well documented in recent years.

What health conditions are you most worried about during heatwaves, and who is most at risk?

Heat strokes, cardiovascular disease, and renal disorders are common causes of deaths during heatwaves.

In addition, in regions with increased pollution levels, heatwaves can compound the effects of air pollution on health and put people at added risk.

People with heart conditions and other co-morbidities, pregnant women, and elderly people (generally aged over 65) should take extra precautions to minimise the risks of heatwaves to their health.

What measures can help prepare for and mitigate the effects of these heatwaves?

Many countries in southern Europe are adapting to extreme heat through more air conditioning in residential spaces. Another shift has been to build or transform cities with additional green spaces. The heat effect can be more severe in urban areas due to the urban heat island effect so more green spaces can help offset the heat to some extent by providing a natural shield from the direct sunlight.

Architects, town planners, engineers and climate scientists need to work together to address the challenges in infrastructure. Cities will need to be designed to cope with extreme weather events (be it heatwaves or flooding), with modern designs and materials to withstand these forces of nature.  

Countries that are dependent on tourism and at higher risk from heatwaves – such as Greece, Spain and Italy - will need to find the right balance to maintain public health facilities and resources for their local population, while at the same time not having a negative impact on their tourism industry.

One way could be to provide incentives to tourists to avoid visiting tourist attractions during peak hours of the day (or even outside summer seasons) when the temperatures are likely to be at high levels.

People generally need to be better prepared for the prospect of more extreme weather events such as heatwaves occurring more often and lasting longer.