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expert reaction to the North American heatwave

Record-breaking high temperatures continue in western Canada and parts of the US Pacific northwest.


Dr Alexandre Koberle, Research Fellow at the Grantham Institute for Climate Change and the Environment at Imperial College London, said:

“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. Actual impacts will depend on local conditions of soil moisture and crop health before the heat wave, but many areas are reporting extremely low sub-soil moisture conditions. Irrigation helps reduce the impacts, but wind and heat reduce the effectiveness of irrigation. While all crops suffer in these conditions, fruits and vegetables that require good moisture may particularly at risk. 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. This heatwave is an example of the type of events agriculture will have to adapt to in the coming years, not decades. If North American farmers are struggling, imagine what happens in other regions where equipment, insurance and finance are lacking. Climate smart agriculture needs to be scaled up in the most vulnerable regions of the world.

“This heat wave is also affecting US prairie crops, much of which is exported. Extreme drought in Brazil, the other large agricultural exporter, is also putting harvests there at risk. While these events are not directly linked, higher probability of extreme events globally also increases the risk that they occur simultaneously in several regions. The global agricultural commodities market may face shortages putting food security at risk.”


Dr Emma Lawrance, Mental Health Innovations Fellow at the Institute of Global Health Innovation at Imperial College London and Climate Cares lead, said:

“Climate change is bringing higher temperatures and more frequent extreme heat in many places around the world. We are sadly seeing the impacts of extreme heat on physical health and deaths in Canada and the USA right now. Research tells us that it is not only physical health that suffers at such high temperatures, but also mental health. Instances of suicide and hospital visits for mental illness or extreme distress rise with increasing temperature. If climate change continues at its current rate, rising temperatures may lead to an estimated 22,000 extra suicides across the USA and Mexico alone by 2050. 

“While the elderly are a well-known at risk group in extreme heat, it is less well known that people with diagnosable mental disorders – particularly psychosis, dementia and substance abuse – are also more at risk, and by some estimates are two to three times more likely to die in a heatwave. Heatwaves can increase violence in the community, reduce economic outputs, and disrupt sleep, as well as induce physiological changes affecting mental function. The result is that population mental health suffers. For example, depressive language on Twitter rises at higher temperatures, and self-reported mental health and wellbeing declines.

“The effects of extreme heat on mental health is another example of how climate change exacerbates inequalities – poorer communities and individuals with experiences of mental illness are more vulnerable to the impacts of heat and may be less likely to have access to the cooling effects of green space, air conditioning and appropriate housing.  

“Health systems and public health must prepare now for the impacts of heatwaves to come. But 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 climate change. This is a win-win opportunity to turn a vicious cycle into a virtuous cycle and policy makers and communities must act now.”


Dr Amanda Maycock, Director of the Institute for Climate and Atmospheric Science and Associate Professor in Climate Dynamics, University of Leeds, said:

“These excursions are not that unusual – the 2010 heatwave in Russia was caused by a similar weather pattern — but we know that because of climate change the air trapped in this ‘block’ is hotter than it would otherwise be, and the associated heat stress is amplified leading to severe impacts on humans and animals. We are now at a point where we see record breaking heat extremes in many parts of the world year on year. This is a clear sign that climate change is already affecting our lives today.”


Dr Fredi Otto, Associate Director of the Environmental Change Institute and Associate Professor in the Global Climate Science Programme, University of Oxford:

“Every heatwave occurring today is made more likely and more intense by human-induced climate change. In some cases, local factors enhance or counteract this effect. In that part of the world there is nothing to suggest that the latter is happening, so climate change is definitely one of the significant drivers of the intensity. How big a driver it is, is a work in progress.”


Prof Andrew Charlton-Perez, Head of the Department of Meteorology at the University of Reading, said:

“Heatwaves have obvious impacts on human health. Even quite moderate temperatures, 20 degrees and above, are associated with increases in death rates and hospital admissions.

“Mostly these increases are driven by changes in respiratory and cardiovascular conditions. At the very high temperatures seen, for example, in the US and Canada recently, other more severe conditions where human thermoregulation starts to fail can occur.

“Heatwaves can also have far reaching effects on society in other ways, including on productivity and on infrastructure. It will be necessary for society to continue to adapt to more frequent and extreme temperatures as climate warms.

“As noted by the recent Committee on Climate Change report, progress on adapting buildings and cities more generally to heatwave conditions has been slow in the UK. Recent heatwaves around the world should remind us that this is challenge we need to act on quickly.”


Dr Paulo Ceppi, lecturer in climate science at Imperial College London, said:

“In terms of the causes of the North American heatwave, there are two key factors. First and foremost, the weather: the heatwave was associated with a very unusual weather pattern – a very strong anticyclone (area of high atmospheric pressure) causing the air to sink and warm up, described as a ‘heat dome’ in the media. 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.

“Then, there is the climate. Global warming is shifting the odds and making heatwaves increasingly likely, while making cold spells less likely. There’s no doubt that climate change has increased the severity of the current heatwave. If global warming continues amplifying, we can expect more and more of these unprecedented heatwaves to occur in different parts of the world.

“Humans have already caused substantial global warming (about 1.2°C since 1850). 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. If we are to comply with the Paris Agreement ambition to limit global warming to 1.5°C, we would need to take extremely rapid action. Scientists have calculated that we’d need to reduce emissions to net zero by 2050, so we only have 3 decades left. However, it’s also important to remember that it’s never too late to take action: every tenth of a degree makes a difference.”


Dr Karsten Haustein, Climate Service Center Germany, said:

“There are four aspects that contributed to this event: weather, timing, location and climate change.

“Every event is ultimately caused by random weather variability. But weather alone cannot explain the severity of this freak extreme event.

“Climate change has warmed land areas by almost 2°C, so almost every weather-induced temperature extreme will be warmer than it used to be (by approximately 2°C).  The warming associated with climate change could have contributed to altered circulation patterns as well, but currently we do not have enough understanding to infer causality.

“Weather and timing conspired in such a way that new temperature records were unavoidable. Those records have likely been further fuelled by favourable topographic features, and undoubtedly supercharged by climate change. Thus, new records were set even in terms of the difference between old and new records. 

“Whether the hypothesised jetstream shift due to global warming has added to the mix remains unclear for now. That is generally the hardest nut to crack in terms of attribution; yet a small dynamic change caused by human-induced warming could make a notable difference due to the ‘topographic amplification’ potential at the Pacific West coast. 

Additional heat dome information: 

“The heat dome analogy refers to a circulation anomaly at upper levels in the atmosphere, typically around 5-6 km altitude above sea level. It was caused by an anti-cyclonic wave-breaking event, which led to the rapid development of a large amplitude ridge. Its explosive development was fuelled by the topography, which led to rapid drying and warming of the air mass, which essentially intensified the ridge itself. Note that it wasn’t a ‘classic’ blocking event, as it was rather short-lived compared to ‘traditional’ heat waves that often last several weeks and are associated with true atmospheric blocking (often in the wake of cyclonic wave-breaking). Therefore the deliberate use of the term ‘ridge’.”


Prof Brian Hoskins, Chair of the Grantham Institute, Imperial College London, said:

“The temperature records in British Columbia are striking. The highest, 49.6C, breaks Canada’s record by 4.6C. It is very unusual for a record to be broken by this amount. The extreme heat is associated with a deep ridge of high pressure extending northwards from the very warm south-west of the US. One of the contributors to the extremely high temperatures is the very dry spring and early summer in Western N America. Most of the sun’s energy normally evaporates water from the ground. When it is so dry, this energy goes directly into heating the air.  

“The answer to whether  these record temperatures are caused by higher levels of greenhouse gases in the atmosphere is not straightforward. Certainly, global warming would lead to the same situation giving temperatures more than 1C higher. There is some indication that the drought in Western N America, and its contribution to the high temperatures, is more likely with climate change. It is currently uncertain whether the weather pattern itself is now more likely.”


Prof Hannah Cloke, a natural hazards researcher at the University of Reading, said:

“The deaths from extreme heat in Canada is both a tragedy and a warning to the world.

“The tragedy is that deaths from heatwaves are avoidable, but we will need better early warning systems and adaptations to our homes and lifestyles to avoid similar tragedies in the future.

“The warning is that these extreme weather events are just a precursor of what is still yet to come. Extreme heat is just one of the many impacts of a warming climate that will make the world more dangerous for people.

“We must recognise that heat, like disease, affects the most vulnerable people first. Those who cannot afford to adapt their homes or move to safety in a heatwave will be among the first to die.

“While temperatures of nearly 50 degrees in Canada are extraordinary, we will have to get used to the fact that an increasing number of regions across the world, particularly in Africa, will become almost uninhabitable due to heat in the future. This emphasises the need for action to curb climate change as a key part of global equity.”


Prof Richard Allan, Professor of Climate Science, University of Reading, said:

“Weather patterns that produce persistent high pressure systems, cloud-free conditions and dry continental winds can generate summer heatwaves. Climate change is intensifying these heatwaves as greenhouse gas increases raise temperatures and a warmer, more thirsty atmosphere rapidly dries out soils so that more of the sun’s energy heats the ground rather than evaporating water. Heatwaves have clear implications for human health as well as for plants and animals while conditions are often exacerbated by increased risk of fires, droughts and poor air quality as stagnant, sun-baked air traps and increases pollutants.”


Prof Julienne Stroeve, Professor of Polar Observation & Modelling, University College London, said:

“The unusual heat wave in the northwest of the US is caused by a an extremely high sea level pressure anomaly that some research has showed can be linked to the droughts in the West and Arctic sea ice loss. A study in Nature in 2017 showed that sea ice loss leads to persistent atmospheric ridging in the North Pacific, that in turn results in high sea level pressure over the North Pacific that dries out California.”


Prof Richard Betts, Head of Climate Impacts Research, Met Office Hadley Centre, said:

“Weather conditions leading to heatwaves are now happening on top of a background climate that has been warmed by humans adding more greenhouse gases to the atmosphere. So even though heatwaves happen naturally as part of the weather, they are now routinely hotter than they would have been without human-driven global warming. Extreme heat causes a risk to life, especially in places where people are not so used to hot weather so are not prepared to take action to keep cool, as seen in North America at the moment. The warming climate is also increasing the risk of wildfires in many regions around the world by drying out the landscape, including in the Pacific North-West.”


Prof Ilan Kelman, Professor of Disasters and Health, University College London, said:

“This heat wave is unusual, extremely dangerous, and unfortunately not surprising. It is exactly in line with what we expect due to climate change if we do not act, keeping in mind that we do not always have long datasets to be able to calculate how much climate change contributed to these specific temperatures in these specific places. Such terrible heat-related death tolls could be seen not only in western Canada and the USA, but also elsewhere around those countries and around the world from Melbourne to Manama–including London among many other European cities.

“The lethal danger particularly hits people who work outdoors or who do not have, or who cannot afford, indoor temperature control. Especially when it does not cool down sufficiently at night, not even fans help, because they end up just blowing hot air onto people. The deaths attributed to this heat wave is, sadly, just the beginning of what we will see if we do not act on averting climate change while helping people to deal with extreme heat and humidity.

“Heat and humidity combine to be one of the most immediate, dangerous health impacts from human-caused climate change, which we now witness in a horrible manner. We then have knock-on effects on food production, water availability, power outages, and fires, such as Lytton, British Columbia being alight.”



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