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

Climate Change Driving Surge in ‘Day-Night Hot Extremes’ in Northern Hemisphere
Oil rigs seen at sunset off the southern California coast. Neil Nissing / The Image Bank / Getty Images

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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Warming temperatures on land and in the water are already forcing many species to seek out more hospitable environments. Atlantic mackerel are swimming farther north; mountain-dwelling pikas are moving upslope; some migratory birds are altering the timing of their flights.

Numerous studies have tracked these shifting ranges, looked at the importance of wildlife corridors to protect these migrations, and identified climate refugia where some species may find a safer climatic haven.

"There's a huge amount of scientific literature about where species will have to move as the climate warms," says U.C. Berkeley biogeographer Matthew Kling. "But there hasn't been much work in terms of actually thinking about how they're going to get there — at least not when it comes to wind-dispersed plants."

Kling and David Ackerly, professor and dean of the College of Natural Resources at U.C. Berkeley, have taken a stab at filling this knowledge gap. Their recent study, published in Nature Climate Change, looks at the vulnerability of wind-dispersed species to climate change.

It's an important field of research, because while a fish can more easily swim toward colder waters, a tree may find its wind-blown seeds landing in places and conditions where they're not adapted to grow.

Kling is careful to point out that the researchers weren't asking how climate change was going to change wind; other research suggests there likely won't be big shifts in global wind patterns.

Instead the study involved exploring those wind patterns — including direction, speed and variability — across the globe. The wind data was then integrated with data on climate variation to build models trying to predict vulnerability patterns showing where wind may either help or hinder biodiversity from responding to climate change.

One of the study's findings was that wind-dispersed or wind-pollinated trees in the tropics and on the windward sides of mountain ranges are more likely to be vulnerable, since the wind isn't likely to move those dispersers in the right direction for a climate-friendly environment.

The researchers also looked specifically at lodgepole pines, a species that's both wind-dispersed and wind-pollinated.

They found that populations of lodgepole pines that already grow along the warmer and drier edges of the species' current range could very well be under threat due to rising temperatures and related climate alterations.

"As temperature increases, we need to think about how the genes that are evolved to tolerate drought and heat are going to get to the portions of the species' range that are going to be getting drier and hotter," says Kling. "So that's what we were able to take a stab at predicting and estimating with these wind models — which populations are mostly likely to receive those beneficial genes in the future."

That's important, he says, because wind-dispersed species like pines, willows and poplars are often keystone species whole ecosystems depend upon — especially in temperate and boreal forests.

And there are even more plants that rely on pollen dispersal by wind.

"That's going to be important for moving genes from the warmer parts of a species' range to the cooler parts of the species' range," he says. "This is not just about species' ranges shifting, but also genetic changes within species."

Kling says this line of research is just beginning, and much more needs to be done to test these models in the field. But there could be important conservation-related benefits to that work.

"All these species and genes need to migrate long distances and we can be thinking more about habitat connectivity and the vulnerability of these systems," he says.

The more we learn, the more we may be able to do to help species adapt.

"The idea is that there will be some landscapes where the wind is likely to help these systems naturally adapt to climate change without much intervention, and other places where land managers might really need to intervene," he says. "That could involve using assisted migration or assisted gene flow to actually get in there, moving seeds or planting trees to help them keep up with rapid climate change."

Tara Lohan is deputy editor of The Revelator and has worked for more than a decade as a digital editor and environmental journalist focused on the intersections of energy, water and climate. Her work has been published by The Nation, American Prospect, High Country News, Grist, Pacific Standard and others. She is the editor of two books on the global water crisis. http://twitter.com/TaraLohan

Reposted with permission from The Revelator.

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The study, published in Current Biology, notes that the rhino population stayed fairly consistent for tens of thousands of years until 18,500 years ago. That means that people and rhinos lived together in Northern Siberia for roughly 13,000 years before rhinos went extinct, Science News reported.

The findings are an ominous harbinger for large species during the current climate crisis. As EcoWatch reported, nearly 1,000 species are expected to go extinct within the next 100 years due to their inability to adapt to a rapidly changing climate. Tigers, eagles and rhinos are especially vulnerable.

The difference between now and the phenomenon 14,000 years ago is that human activity is directly responsible for the current climate crisis.

To figure out the cause of the woolly rhinos' extinction, scientists examined DNA from different rhinos across Siberia. The tissue, bone and hair samples allowed them to deduce the population size and diversity for tens of thousands of years prior to extinction, CNN reported.

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