Climate Change Driving Surge in ‘Day-Night Hot Extremes’ in Northern Hemisphere

By Daisy Dunne
Deadly "day-night hot extremes" are increasing across the northern hemisphere due to climate change, a new study finds.
And the number of people exposed to such events, also known as "compound hot extremes," is likely to increase "several-fold" as temperatures continue to climb in the coming decades, the study authors tell Carbon Brief.
If global temperatures reach 2 C — the upper limit set by countries in the Paris agreement — the frequency of compound hot extremes could more than double across the northern hemisphere, when compared to 2012, the research finds.
However, if greenhouse gas emissions are not curbed, compound hot extremes could become eight times more frequent by the end of the century.
The study sets out "clear evidence" that human-caused climate change is leaving its mark on extreme heat events, another scientist tells Carbon Brief.
Day and Night
The new study, published in Nature Communications, looks specifically at "compound hot extremes" — a 24-hour period in summer where hot daytime temperatures are followed by similar nightime temperatures. (Temperatures are considered "hot" if they are in the top 10% of temperatures experienced by a region from 1960-2012.)
These kinds of events pose a particularly high danger to human health, explain study authors Dr Yang Chen, a climate extremes scientist from the Chinese Academy of Meteorological Sciences, and Dr Jun Wang, a climate and meteorological scientist from the Institute of Atmospheric Physics in China. In a joint interview, they tell Carbon Brief:
Simply put, compound hot extremes deprive humans of the valuable chance of relief, which could have been provided by the 'cooling-off' effects of a nighttime low.
Such conditions occurred during the 2003 summer heatwave in Europe, which saw 70,000 deaths across 16 countries, the authors say. Another example is the 1995 Chicago heatwave, which led to more than 700 heat-related deaths in just five days.
The study is the first to present "a complete storyline on compound hot extremes" — investigating how they have changed, the role of climate change in this and how they might increase in the future, the authors say.
The results show that compound hot extremes "are significantly increasing and will continue to increase in frequency and intensity" across the northern hemisphere, say Chen and Wang:
These increases in heat hazards will translate into several-fold increases in population exposure to them. The rise of anthropogenic emission of greenhouse gas emissions is to blame for these increases.
Burning Up
For the first part of their study, the authors analysed the "fingerprint" of human-caused climate change on compound hot extremes to date. To do this, they conducted an "attribution" analysis.
This involves using climate models to produce two sets of simulations: one including all the factors that affect the climate, including human-caused greenhouse gas emissions, volcanic eruptions and solar variability, and one including all of these factors except for greenhouse gas emissions.
The researchers then compared the frequency and intensity of compound hot extremes in both of these scenarios.
They found that only the scenario including human-caused greenhouse gas emissions could closely reproduce the pattern of compound hot extremes observed from 1960 to 2012. In their research paper, the authors write:
We find that the summer-mean warming over 1960-2012 largely dictates the past increases in frequency and intensity of compound hot extremes during that period in both observations and simulations.
The maps below show observed changes in summertime compound hot extreme frequency (left) and intensity (right) across the northern hemisphere from 1960-2012.
The left-hand map shows changes in the number of compound hot extreme days per decade (yellow to red for increases; light to dark blue for decreases), while the right-hand map shows changes in the average temperature of compound hot extremes per decade (same color scale).
Contributions from changing temperature mean and variability. Wang et al. (2020)
The map shows that increases in the frequency and intensity of compound hot extremes are widespread across the northern hemisphere, with parts of continental Europe and China particularly affected.
(Gaps in the data prevented the researchers from analysing changes in the most southern parts of the northern hemisphere, the authors say in their research paper.)
While the global pattern of increases is best explained by human-caused global warming, it is possible that some regional differences may be explained by other factors, the authors say.
For example, the drying of soils could help to explain local variation of heat extremes, the authors say in their research paper.
This is because dry soils accumulate heat during the day and release it at night, Wang and Chen say, making night hot extremes and, therefore, compound hot extremes, more likely.
Furnace Forecast
The authors also used climate models to project possible future changes to compound hot extremes until 2100. They investigated two scenarios: one "intermediate mitigation" pathway with moderately high greenhouse gas emissions ("RCP4.5") and one with very high greenhouse gas emissions ("RCP8.5").
Within each emissions scenario, they also looked at the changes to compound hot extremes expected if the world reaches 1.5 C and 2 C of global warming, which are the temperature limits set by the Paris agreement.
The charts below show the average expected change in the number of summertime compound hot extreme days (purple line), as well as independent hot days (blue line) and independent hot nights (turquoise line) across the northern hemisphere under RCP4.5 (top) and RCP8.5 (bottom) until 2100. (Compound extremes are where a hot day is followed by a hot night, whereas an "independent hot day" is when a hot day is not followed by a hot night.)
On the charts, red circles point out when the temperature limits of 1.5 C and 2 C will be breached in each scenario. The bottom chart also highlights when 4C could be breached. The various data points represent results from different climate models.
(It is worth noting that events are considered to be compound or independent. So, a 24-hour period where a hot day is followed by a hot night would be considered a compound extreme, but not an independent hot day or hot night.)
Constrained projections of summertime hot extremes. Wang et al. (2020)
The results show that the average number of compound hot extreme days across the northern hemisphere in summer would more than double if temperatures reach 2 C, when compared to 2012.
Keeping temperatures at 1.5 C could see five fewer compound hot extreme days across the northern hemisphere, on average, when compared to 2 C, the research adds.
If greenhouse gas emissions are extremely high (RCP8.5), the number of summertime compound hot extremes could increase eight-fold by 2100, when compared to 2012, the results show.
The charts also show that compound hot events are expected to increase at a much more rapid rate than independent hot day or hot night events.
This is chiefly because climate change is known to have a larger effect on nightime temperatures than daytime temperatures, the authors say.
Therefore, as the chances of hot nights become higher, the chances of compound hot events also increase — and, so, the chances of a hot day or night occurring independently decreases, explain Chen and Wang.
‘Clear Evidence’
The findings reinforce "the urgency in reducing emission of greenhouse gases" for policymakers, say Chen and Wang:
We should keep the point in mind that as the globe warms, future summers are increasingly dominated by compound hot extremes and become more uncomfortable. Namely, a hot day accompanied by a hot night without a relief window for humans might become a 'new norm'. As a result, vigilance against excess heat should be kept through day and night.
The study is "impressively comprehensive," says Dr Eunice Lo, a research associate in climate extremes from the University of Bristol, who was not involved in the research. She tells Carbon Brief:
I think the main take home message from this study is that we should use consecutive day-night hot extremes as a major heat-health indicator for policymaking, as compound hot extremes are projected to have larger future increases in frequency and intensity then hot days or nights.
The findings produce "clear evidence" that human-caused climate change is leaving its mark on extreme heat events, says Prof Peter Stott, who leads on climate monitoring and attribution at the Met Office Hadley Centre. Stott, who was also not involved in the research, tells Carbon Brief:
I don't find the conclusions of the study very surprising, but I do like the way the authors have comprehensively set out the implications – the clear evidence that the changes to date are driven by human emissions and the clear evidence that future changes will result in significant increases in the frequency and intensity of these compound extremes worldwide.
Reposted with permission from Carbon Brief.
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By Michael Svoboda, Ph.D.
Despite a journey to this moment even more treacherous than expected, Americans now have a fresh opportunity to act, decisively, on climate change.
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EcoWatch Daily Newsletter
By Katy Neusteter
The Biden-Harris transition team identified COVID-19, economic recovery, racial equity and climate change as its top priorities. Rivers are the through-line linking all of them. The fact is, healthy rivers can no longer be separated into the "nice-to-have" column of environmental progress. Rivers and streams provide more than 60 percent of our drinking water — and a clear path toward public health, a strong economy, a more just society and greater resilience to the impacts of the climate crisis.
Public Health
<img lazy-loadable="true" data-runner-src="https://assets.rebelmouse.io/eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpbWFnZSI6Imh0dHBzOi8vYXNzZXRzLnJibC5tcy8yNTUyNDY3MC9vcmlnaW4uanBnIiwiZXhwaXJlc19hdCI6MTY2MDkxMTkwNn0.pyP14Bg1WvcUvF_xUGgYVu8PS7Lu49Huzc3PXGvATi4/img.jpg?width=980" id="8e577" class="rm-shortcode" data-rm-shortcode-id="1efb3445f5c445e47d5937a72343c012" data-rm-shortcode-name="rebelmouse-image" data-width="3000" data-height="2302" />Wild and Scenic Merced River, California. Bob Wick / BLM
<p>Let's begin with COVID-19. More than <a href="https://www.nytimes.com/interactive/2020/us/coronavirus-us-cases.html?name=styln-coronavirus&region=TOP_BANNER&block=storyline_menu_recirc&action=click&pgtype=LegacyCollection&impression_id=2f508610-2a87-11eb-8622-4f6c038cbd1d&variant=1_Show" target="_blank">16 million Americans</a> have contracted the coronavirus and, tragically,<a href="https://www.nytimes.com/interactive/2020/us/coronavirus-us-cases.html?name=styln-coronavirus&region=TOP_BANNER&block=storyline_menu_recirc&action=click&pgtype=LegacyCollection&impression_id=2f508610-2a87-11eb-8622-4f6c038cbd1d&variant=1_Show" target="_blank"> more than</a> <a href="https://www.nytimes.com/interactive/2020/us/coronavirus-us-cases.html?name=styln-coronavirus&region=TOP_BANNER&block=storyline_menu_recirc&action=click&pgtype=LegacyCollection&impression_id=2f508610-2a87-11eb-8622-4f6c038cbd1d&variant=1_Show" target="_blank">300,000 have died</a> due to the pandemic. While health officials encourage hand-washing to contain the pandemic, at least <a href="https://closethewatergap.org/" target="_blank">2 million Americans</a> are currently living without running water, indoor plumbing or wastewater treatment. Meanwhile, <a href="https://www.theguardian.com/us-news/2020/jun/23/millions-of-americans-cant-afford-water-bills-rise" target="_blank">aging water infrastructure is growing increasingly costly for utilities to maintain</a>. That cost is passed along to consumers. The upshot? <a href="https://research.msu.edu/affordable-water-in-us-reaching-a-crisis/" target="_blank">More than 13 million</a> U.S. households regularly face unaffordable water bills — and, thus, the threat of water shutoffs. Without basic access to clean water, families and entire communities are at a higher risk of <a href="https://www.americanprogress.org/issues/green/news/2020/08/05/488705/bridging-water-access-gap-covid-19-relief/" target="_blank">contracting</a> and spreading COVID-19.</p><p>We have a moral duty to ensure that everyone has access to clean water to help prevent the spread of the coronavirus. Last spring, <a href="https://nymag.com/intelligencer/2020/03/coronavirus-stimulus-bill-explained-bailouts-unemployment-benefits.html" target="_blank">Congress appropriated more than $4 trillion</a> to jumpstart the economy and bring millions of unemployed Americans back to work. Additional federal assistance — desperately needed — will present a historic opportunity to improve our crumbling infrastructure, which has been <a href="https://www.theguardian.com/us-news/2020/jun/23/millions-of-americans-cant-afford-water-bills-rise" target="_blank" rel="noopener noreferrer">grossly underfunded for decades</a>.</p><p>A report by my organization, American Rivers, suggests that <a href="https://s3.amazonaws.com/american-rivers-website/wp-content/uploads/2020/07/09223525/ECONOMIC-ENGINES-Report-2020.pdf" target="_blank" rel="noopener noreferrer">Congress must invest at least $50 billion</a> "to address the urgent water infrastructure needs associated with COVID-19," including the rising cost of water. This initial boost would allow for the replacement and maintenance of sewers, stormwater infrastructure and water supply facilities.</p>Economic Recovery
<p>Investing in water infrastructure and healthy rivers also creates jobs. Consider, for example, that <a href="https://tinyurl.com/y9p6sgnk" target="_blank">every $1 million spent on water infrastructure in the United States generates more than 15 jobs</a> throughout the economy, according to a report by the Value of Water Campaign. Similarly, <a href="https://tinyurl.com/yyvd2ksp" target="_blank">every "$1 million invested in forest and watershed restoration contracting will generate between 15.7 and 23.8 jobs,</a> depending on the work type," states a working paper released by the Ecosystem Workforce Program, University of Oregon. Healthy rivers also spur tourism and recreation, which many communities rely on for their livelihoods. According to the findings by the Outdoor Industry Association, which have been shared in our report, "Americans participating in watersports and fishing spend over <a href="https://s3.amazonaws.com/american-rivers-website/wp-content/uploads/2020/06/30222425/Exec-summary-ECONOMIC-ENGINES-Report-June-30-2020.pdf" target="_blank">$174 billion</a> on gear and trip related expenses. And, the outdoor watersports and fishing economy supports over <a href="https://s3.amazonaws.com/american-rivers-website/wp-content/uploads/2020/06/30222425/Exec-summary-ECONOMIC-ENGINES-Report-June-30-2020.pdf" target="_blank">1.5 million jobs nationwide</a>."</p><p>After the 2008 financial crisis, Congress invested in infrastructure to put Americans back to work. The American Recovery and Reinvestment Act <a href="https://thehill.com/blogs/congress-blog/economy-a-budget/25941-clean-water-green-infrastructure-get-major-boost" target="_blank">of 2009 (ARRA) allocated $6 billion</a> for clean water and drinking water infrastructure to decrease unemployment and boost the economy. More specifically, <a href="https://www.conservationnw.org/news-updates/us-reps-push-for-millions-of-restoration-and-resilience-jobs/" target="_blank" rel="noopener noreferrer">an analysis of ARRA</a> "showed conservation investments generated 15 to 33 jobs per million dollars," and more than doubled the rate of return, according to a letter written in May 2020 by 79 members of Congress, seeking greater funding for restoration and resilience jobs.</p><p>Today, when considering how to create work for the <a href="https://www.bls.gov/news.release/pdf/empsit.pdf" target="_blank" rel="noopener noreferrer">10.7 million</a> people who are currently unemployed, Congress should review previous stimulus investments and build on their successes by embracing major investments in water infrastructure and watershed restoration.</p>Racial Justice
<p>American Rivers also recommends that Congress dedicate <a href="https://s3.amazonaws.com/american-rivers-website/wp-content/uploads/2020/07/09223525/ECONOMIC-ENGINES-Report-2020.pdf" target="_blank" rel="noopener noreferrer">$500 billion for rivers and clean water over the next 10 years</a> — not just for the benefit of our environment and economy, but also to begin to address the United States' history of deeply entrenched racial injustice.</p><p>The <a href="https://www.epa.gov/npdes/sanitary-sewer-overflows-ssos" target="_blank" rel="noopener noreferrer">23,000-75,000 sewer overflows</a> that occur each year release up to <a href="https://www.americanrivers.org/2020/05/fighting-for-rivers-means-fighting-for-justice/#:~:text=There%20are%20also%2023%2C000%20to%2075%2C000%20sanitary%20sewer,to%20do%20with%20the%20mission%20of%20American%20Rivers." target="_blank" rel="noopener noreferrer">10 billion gallons of toxic sewage</a> <em>every day</em> into rivers and streams. This disproportionately impacts communities of color, because, for generations, Black, Indigenous, Latinx and other people of color have been <a href="https://www.scientificamerican.com/article/flooding-disproportionately-harms-black-neighborhoods/" target="_blank" rel="noopener noreferrer">relegated</a> to live in flood-prone areas and in neighborhoods that have been intentionally burdened with a lack of development that degrades people's health and quality of life. In some communities of color, incessant flooding due to stormwater surges or <a href="https://www.ajc.com/opinion/opinion-partnering-to-better-manage-our-water/7WQ6SEAQP5E4LGQCEYY5DO334Y/" target="_blank" rel="noopener noreferrer">combined sewer overflows</a> has gone unmitigated for decades.</p><p>We have historically treated people as separate from rivers and water. We can't do that anymore. Every voice — particularly those of people most directly impacted — must have a loudspeaker and be included in decision-making at the highest levels.</p><p>Accordingly, the new administration must diligently invest in projects at the community level that will improve lives in our country's most marginalized communities. We also must go further to ensure that local leaders have a seat at the decision-making table. To this end, the Biden-Harris administration should restore <a href="https://www.epa.gov/cwa-401#:~:text=Section%20401%20Certification%20The%20Clean%20Water%20Act%20%28CWA%29,the%20United%20States.%20Learn%20more%20about%20401%20certification." target="_blank">Section 401 of the Clean Water Act</a>, which was undermined by the <a href="https://earthjustice.org/news/press/2020/tribes-and-environmental-groups-sue-trump-administration-to-preserve-clean-water-protections#:~:text=Under%20Section%20401%20of%20the%20Clean%20Water%20Act%2C,seeks%20to%20undermine%20that%20authority%20in%20several%20ways%3A" target="_blank">Trump administration's 2020 regulatory changes</a>. This provision gives states and tribes the authority to decide whether major development projects, such as hydropower and oil and gas projects, move forward.</p>Climate Resilience
<p>Of course, the menacing shadow looming over it all? Climate change. <a href="https://media.ifrc.org/ifrc/wp-content/uploads/2020/11/IFRC_wdr2020/IFRC_WDR_ExecutiveSummary_EN_Web.pdf" target="_blank">More than 100 climate-related catastrophes</a> have pummeled the Earth since the pandemic was declared last spring, including the blitzkrieg of megafires, superstorms and heat waves witnessed during the summer of 2020, directly impacting the lives of more than <a href="https://media.ifrc.org/ifrc/wp-content/uploads/2020/11/IFRC_wdr2020/IFRC_WDR_ExecutiveSummary_EN_Web.pdf" target="_blank">50 million people globally</a>.</p><p>Water and climate scientist Brad Udall often says, "<a href="https://www.youtube.com/watch?v=xQhpj5G0dME" target="_blank">Climate change is water change</a>." In other words, the most obvious and dire impacts of climate change are evidenced in profound changes to our rivers and water resources. You've likely seen it where you live: Floods are more damaging and frequent. Droughts are deeper and longer. Uncertainty is destabilizing industry and lives.</p><p>By galvanizing action for healthy rivers and managing our water resources more effectively, we can insure future generations against the consequences of climate change. First, we must safeguard rivers that are still healthy and free-flowing. Second, we must protect land and property against the ravages of flooding. And finally, we must promote policies and practical solutions that take the science of climate disruption into account when planning for increased flooding, water shortage and habitat disruption.</p><p>Imagine all that rivers do for us. Most of our towns and cities have a river running through them or flowing nearby. Rivers provide clean drinking water, irrigate crops that provide our food, power our homes and businesses, provide wildlife habitat, and are the lifeblood of the places where we enjoy and explore nature, and where we play and nourish our spirits. Healthy watersheds help <a href="https://news.un.org/en/story/2020/03/1059952" target="_blank" rel="noopener noreferrer">mitigate</a> climate change, absorbing and reducing the amount of carbon in the atmosphere. Healthy rivers and floodplains help communities adapt and build resilience in the face of climate change by improving flood protection and providing water supply and quality benefits. Rivers are the cornerstones of healthy, strong communities.</p><p>The more than <a href="https://archive.epa.gov/water/archive/web/html/index-17.html" target="_blank" rel="noopener noreferrer">3 million miles</a> of rivers and streams running across our country are a source of great strength and opportunity. When we invest in healthy rivers and clean water, we can improve our lives. When we invest in rivers, we create jobs and strengthen our economy. When we invest in rivers, we invest in our shared future.</p>Trending
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