Imperial College London

"Raspberries baking on the vine" - Heatwave spells disaster in North America

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Fully dressed people walking under water mist spray system to cool off in a sunny street during a heatwave

People cool off in water mist during heatwave in Montreal, Canada.

Imperial experts talk long-term drought, climate change and dire consequences of the 'heat dome' for people and planet

"The temperature records in British Columbia are striking. The highest, 49.6°C, breaks Canada’s record by 4.6°C. It is very unusual for a record to be broken by this amount," says Professor Sir Brian Hoskins, Chair of the Grantham Institute - Climate Change and the Environment at Imperial College London.

Temperatures in the North-Western United States and Western Canada have been unusually high this year in June and early July, leading to media reports of a vast heatwave referred to as a 'heat dome'.

Many locations reached around 45°C or higher, making them as hot as some of the hottest places on Earth, although only for a few days. The peak temperature of 49.6°C was recorded in Lytton, British Columbia, which is at the same latitude as Lizard Peninsula in Cornwall and Prague in the Czech Republic. For comparison, the highest temperature ever recorded in Australia is 50.7°C.

Satellite image map shows US-Canada border including Vancouver and Calgary, Seattle and Portland, and heat map shows land surface temperatures up to 65 degrees
Satellite map contains modified Copernicus Sentinel data (2021), processed by ESA, CC BY-SA 3.0 IGO

People at these northern latitudes are not widely prepared for such temperatures. Officials reported 116 deaths in Oregon and 78 in Washington according to The Guardian. Vancouver police reported 130 sudden deaths in five days, with elderly people or those with underlying health conditions accounting for the majority of lives lost, according to The Independent.

The village of Lytton, which recorded the peak temperature, was later destroyed by a wildfire.

Beyond the present threat to human health and scorched landscapes, experts at Imperial College London are concerned about the impact of climate change on food production, wildlife and peoples' basic needs.

Dr Alexandre Koberle, Research Fellow at the Grantham Institute, says:

"The extreme heat wave roasting Western North America is linked to a long-term drought affecting the region. Hot droughts such as this compound the impact on agricultural productivity and have been shown to lead to deeper and wider yield losses.

"Raspberries are 'baking on the vine' and strawberries are ripening too quickly before they reach size. Forage crops are also impacted, putting livestock at risk."

Dr Bonnie Waring, Senior Lecturer at the Grantham Institute and Department of Life Sciences, says:

"Research across the world has consistently shown that trees also experience elevated mortality due to heat stress and the drought that often accompanies it. Not only does this imperil plant biodiversity, it also weakens forest carbon uptake and further accelerates rising atmospheric carbon dioxide and climate change."

Firefighter in protective clothing treads through a burnt forest
Firefighter picks through scorched forest remains Credit: CALFIRE_Official / Flickr (CC BY-NC 2.0)

The so-called 'heat dome' was caused by a combination of several factors and, while experts cannot pin the blame for an individual heatwave event on climate change, it is agreed that the chances of such an event occurring have hugely increased - by 150 times according to World Weather Attribution scientists, as reported by BBC News.

Scientists are also united in calling for steep and equitably managed cuts to greenhouse gas emissions across society to prevent the further global heating.

Dr Paulo Ceppi, Lecturer in Climate Science at the Grantham Institute at Imperial College London, says:

"If global warming continues amplifying, we can expect more and more of these unprecedented heatwaves to occur in different parts of the world.

"With current carbon emissions we expect this warming to proceed at a fast pace. The only way to stop this trend is by drastically reducing our emissions of carbon dioxide, ideally to net zero. It’s also important to remember that it’s never too late to take action: every tenth of a degree makes a difference."

What conditions led to the 'heat dome'?

Weather patterns have trapped hot air over huge swathes of the continent, affecting northern California, Idaho, Western Nevada, Oregon, and Washington State in the United States, as well as British Columbia, and, in its later phase, Alberta, Manitoba, the Northwest Territories, Saskatchewan, and Yukon, all in Canada.

If North American farmers are struggling with the heat, imagine what happens in other regions where equipment, insurance and finance are lacking. Extreme drought in Brazil, the other large global agricultural exporter, is also putting harvests there at risk. Dr Alex Koberle Research Fellow

There are two key factors in how the 'heat dome' and its effects have occurred: First and foremost, weather patterns.

Dr Ceppi explains: "The heatwave was associated with an unusually large and persistent weather pattern – a very strong anticyclone (area of high atmospheric pressure). In such a high pressure, the air sinks and warms up, leading to the description as a 'heat dome' in the media. The sinking also leads to strong sunshine, making the near surface air hotter and hotter. Also, the winds associated with this anticyclone pushed hot air from inland areas toward the coast, displacing the cooler marine air that usually keeps this coastal region temperate."

Professor Hoskins adds: "One of the contributors to the extremely high temperatures is the very dry spring and early summer in Western North America. Most of the sun’s energy normally evaporates water from the ground. When conditions are so dry, this energy goes directly into heating the air."

Did climate change cause this heatwave?

The current North American heatwaves are not isolated incidents. With recent years seeing record temperatures broken in the United Kingdom, Europe, the Arctic circle, and across the world, data is building up a picture that points to the disruptive effects of a warmer world.

Researchers agree that climate change is a second key factor in causing the extreme conditions of the 'heat dome', drought and wildfires seen this summer in North America, as well as the incidence of more deadly tropical storms.

Photo: At night, a wildfire rages in the distance, with a lake in the foreground
Wildfires rage across Northwest America. Credit: CALFIRE_Official / Flickr (CC BY-NC 2.0)

The planet is warming due to greenhouse gases created by human activities that burn fossil fuels, such as petrol and diesel-powered transport, coal, oil and gas-fired electricity and heating, agriculture and manufacturing, as well as deforestation and land use change.

Humans have already caused substantial global warming (about 1.2°C since 1850), and this is shifting the odds to make heatwaves increasingly likely, while making cold spells less likely.

Professor Sir Brian Hoskins says: "To what extent a particular record temperature is caused by higher levels of greenhouse gases in the atmosphere is not straightforward; Nevertheless, it is certain that global warming would lead to the same 'heat dome' weather pattern giving temperatures at least 1.2°C higher than they would otherwise be."

Disastrous for food supplies and peoples' basic needs

As a result of climate change, this heatwave is an example of the type of event that people will have to adapt to in the coming years, not decades. The heatwave is also affecting US prairie crops, many of which are exported to feed people and livestock abroad.

Dr Koberle says: "If North American farmers are struggling with the heat, imagine what happens in other regions where equipment, insurance and finance are lacking.

Photo of dry flat farmland, with sparse bushes growing, with blue sky, electricity pylons in distance
Dry Farm Land, San Joachin County, California. Credit: Eric Sonstroem / Flickr (CC BY 2.0)

"Extreme drought in Brazil, the other large global agricultural exporter, is also putting harvests there at risk. While these two are not directly linked, a globally higher probability of extreme weather events increases the risk that they occur simultaneously in several regions."

This situation means the global agricultural commodities market may face shortages, putting people at risk of food shortages or rising prices; a threat not all people can afford.

Dr Koberle and colleagues at Imperial College Business School's Centre for Climate Finance and Investment published Climate Change and the Future of Food, a report that looks deeper into the physical risks that climate change presents to agriculture, exploring how even small changes to the climate in the near-term may spell disaster due to radical changes in weather patterns and severe implications for financial risk assessment.

The good news is that actions to reduce the impacts of heat in our cities, such as increased tree cover, more equitable green space access and improved housing, can also help reduce further impacts from climate change. Food production can benefit from climate-smart agriculture solutions that build resilience, including better soil management, increasing pasture productivity, and drought-resistant crops, according to the report.

"Climate smart agriculture needs to be scaled up in the most vulnerable regions of the world. These are win-win opportunities to turn a vicious cycle into a virtuous cycle and policy makers and communities must act now," Dr Koberle says.


Simon Levey

Simon Levey
The Grantham Institute for Climate Change

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Tel: +44 (0)20 7594 5650

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Hana Amer

Hana Amer
Business School

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Climate-change, Environment, Strategy-decision-makers, Food-security, Comms-strategy-Real-world-benefits
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