The difference between climate change and global warming?
The terms ‘global warming’ and ‘climate change’ are often used interchangeably. But though they share many similarities, they are slightly different things. Understanding these subtle differences can help all of us plan the climate action we need to take in…
Comet C/2021 T4 Lemmon is sweeping through southern skies
Renowned British astronomer Guy Ottewell originally published this piece about Comet C/2021 T4 Lemmon on May 25, 2023. Reprinted with permission. Edits by EarthSky.
Comet C/2021 T4 Lemmon
Comet C/2021 T4 Lemmon was discovered on October 7, 2021, on images taken at the Mount Lemmon Observatory, northeast of Tucson in Arizona. T4 means the 4th discovery or recovery in the first half of October.
Mount Lemmon is the highest point of the Santa Catalina Mountains, one of four mountain ranges around Tucson. It’s not to be confused with Catilina, the conspirator who tried to seize power over the Roman republic in 63 BCE. I’m reminded of my speculation that the Navajos may have seen Canopus, the great star of the south, from one of the four sacred peaks surrounding their land. In fact, it’s shown as the cover picture for the Astronomical Calendar 2023.
When discovered, comet C/2021 T4, because of the geometry of its orbit, appeared quite northerly, at declination +12°.
Comet C/2021 T4 Lemmon is a long period comet
In fact, it’s a long-period comet; if it ever previously dropped from its remote home – at 44,000 AU out – to the inner solar system, it would have been millions of years ago. So during its present passage, it’ll feel gravitational perturbations from the planets that will shorten its period to merely thousands of years.
Its orbit is inclined about 20° to the ecliptic plane. However, it’s going in a retrograde direction, or opposite to the direction in which the planets revolve. The result is that it’ll make a very long rapid sweep across our southern sky.
At present the comet is 60° out in the morning sky, southerly (at declination -13°), 1.75 AU from the sun and 2 AU from Earth. However, it’s still at a dim magnitude of about 11. Then, on June 27, 2023, its distance from us will shrink to 1 AU.
On July 18, 2023, we will pass it at opposition. And around this time, it’ll be nearest to us, 0.54 AU, and brightest, perhaps about magnitude 8 or 7 but still below the unaided-eye limit. Its nearness will make it appear even farther south, at declination -56° on July 20.
Then in the following months it will climb north, becoming lower in the evening sky and more distant. At the same time it’ll be dimming by perhaps 2 or 3 magnitudes. It will reach perihelion, 1.48 AU from the sun, on July 31, 2023. Finally, it’ll ascend across the ecliptic on September 10, 2023, and be at conjunction behind and north of the sun on November 9, 2023.
Of course, we must remember that predictions of a comet’s brightness, and the size of their tails, can be unreliable. That’s because they depend on the melting of ice and release of dust in these lumpy spinning objects.
Comet-Hale Bopp still observable? Wow!
By the way, Alan Hale alerted us (Guy Ottewell) to this comet with a Facebook post on May 22. Alan was discoverer of the great comet Hale-Bopp (C/1995 O1). And, despite now being more than 47 AU away, it’s the first on the Minor Planet Center’s list of currently observable comets, not because of its present magnitude (about 20) but because it is the earliest-numbered non-periodic comet still considered observable at all.
Bottom line: Comet C/2021 T4 Lemmon was discovered from Mount Lemmon Observatory in 2021. It’s currently sweeping through the southern skies.
Biggest monster star! And the heaviest stars
The biggest monster star
There are very heavy stars. And there are gigantic stars. In terms of sheer size, the star UY Scuti is – as far as we know – the biggest star known. It’s only about 7 to 10 times the sun’s mass, but has a radius more than 1,700 greater than the sun.
While there is currently no competition for UY Scuti as the largest star, there is uncertainty about which star is the most massive star. All of the contenders are nearly twice as massive as what astronomers thought was possible.
Many sources continue to list R136a1 as the heaviest star known at 250 solar masses. However, a recent study in 2022 puts its mass between 170 and 230 times more massive than our sun. Thus, that enables two other stars to edge it out of the top spot on the massive star list. However, that list is dated 2016 and states that the masses listed on it are uncertain.
So, currently topping the massive star list at 250 solar masses is Westerhout 49-2. However, its mass may vary by as much as 120 solar masses – plus or minus – from that figure.
Another contender for the most massive star is BAT99-98. It’s estimated to be about 226 solar masses. And since its mass isn’t listed with a plus or minus range, it could easily be the most massive star.
Of course, regardless of which one tops the massive star list, all of them are very massive stars!
Read more about these monster stars below …
UY Scuti is just plain big
UY Scuti is located some 9,500 light-years away. And it’s the biggest star known, in terms of sheer physical size. The fact is that – for stars – mass and physical size don’t always go hand in hand. Consider that great mass means stronger gravity. And stronger gravity means a greater inward pull for a star. So being super massive might not correlate to being super big.
UY Scuti has a relatively modest mass. It’s only about 7 to 10 times more massive than our sun. But its radius is about 1,700 times greater than the radius of our sun. That would make this star nearly 8 astronomical units across. That’s eight times 93 million miles (150 million km), the distance between our Earth and sun. So, this single star is so large that its outer surface would extend far beyond the orbit of the planet Jupiter (which lies about five times farther from the sun than Earth).
Or look at it this way. More than a million Earths could fit inside the sun. But some 5 billion suns could fit inside a sphere the size of UY Scuti.
The other big stars
Who are the other candidates for the biggest star? They would include NML Cygni, whose estimated distance is about 5,300 light-years and whose radius is between 1,183 and 2,770 times greater than that of our sun. A recent study of this star suggested that it’s an unusual hypergiant star cocooned within a nebula and severely obscured by dust. So we don’t know its size exactly, and the upper part of the range would make it larger than UY Scuti.
Another hypergiant star is WOH G64, which is in the Large Magellanic Cloud and thus located at about 160,000 light-years from Earth. At an estimated 1,540 times the sun’s radius, this star is thought to be the largest star in the Large Magellanic Cloud in terms of sheer physical size. And, again, we’re talking size here, not mass. This star is only about 25 times the sun’s mass.
The heaviest stars
Currently topping the most massive star list at 250 solar masses is Westerhout 49-2. However, its mass may vary by as much as 120 solar masses – plus or minus – from that figure. It’s located 36,200 light-years away in the constellation of Aquila the Eagle. And it’s over 4 million times more luminous than our sun, with a surface temperature of 63,440 degrees F (35,226 C). However, it’s important to note, it could be a binary star system, so its estimated mass could be from a combination of two stars.
Second on the most massive star list is BAT99-98. It’s located 165,000 light-years distant in the Large Magellanic Cloud near the R136 star cluster. It’s estimated to be about 226 solar masses and is a Wolf-Rayet star. Also, it’s about 5 million times more luminous than our sun with a surface temperature of 80,540 degrees F (44,726 C).
Finally, the former champion, now third on the list, is R136a1. It’s located in the Large Magellanic Cloud at about 163,000 light-years away. R136a1 is what’s known as a Wolf–Rayet star. It has a mass between 170 and 230 times the mass of the sun. Its surface temperature is over 100,000 degrees F (55,538 degrees C). And it’s almost 5 million times more luminous than our sun
In addition to being on the massive star list, all three of these stars are among the most luminous stars.
How the most massive stars form
For decades, theories have suggested that no stars can be born by ordinary processes above 150 solar masses. So how did these stars grow so large? And why aren’t monster stars scattered throughout space?
One idea is that supermassive stars like R136a1 form through mergers of multiple stars. In 2012, astronomers at the University of Bonn suggested that the ultramassive stars in the Large Magellanic Cloud – such as R136a1 – were created when lighter stars in tight double-star systems merged.
Still, double-star systems are common. So why don’t we see more super-sized stars? The astronomers in Bonn say it’s because these stars formed under special conditions in a densely packed star cluster. And in a closely packed star cluster, double stars are more likely to encounter each other and merge.
But if these ultramassive stars form in this way, why don’t we see more of them? After all, multiple star systems are common throughout space, while monster stars are few and far between.
The answer may be that monster stars don’t live very long. They evolve very quickly in contrast to less massive stars like our sun. They end their lives in violent supernova explosions.
Imagine how bright they’d be nearby
As you can see, there are extremely heavy stars … and there are simply gigantic stars. What makes a star big might be its mass or its physical size. And either way, it’s fun to imagine what it would be like to have one of these stars relatively close to us in space … say, the distance to the nearest star system, Alpha Centauri, only four light-years away.
At that distance, any of these stars would blaze in our night sky!
Bottom line: Stars are considered big based on their sheer physical size or their mass. In terms of sheer size, UY Scuti is the biggest known star. As for the most massive, currently Westerhout 49-2 tops the list, but different sources vary on which star they list as most massive.
EarthSky | Will Betelgeuse explode in tens of years?
Betelgeuse due to explode soon?
Betelgeuse is the nearest red supergiant star to Earth. Distance estimates vary, but it’s probably within 1,000 light-years of Earth … a hop and a skip in galactic terms. Someday, Betelgeuse will explode as a supernova. When it does, it might become as bright as a full moon. It might even be visible in broad daylight! But when will Betelgeuse explode? A decade ago, this question was interesting but academic. The answer was: Maybe today. Maybe a thousand years from now. Few imagined it would be today. But now there’s been a noticeable uptick in the brightening and dimming of Betelgeuse. And a new paper – published this week (June 1, 2023) – suggests not thousands of years but “tens of years” as Betelgeuse’s explosion timescale.
The paper focuses on the concept of stellar nucleosynthesis, the process that enables stars to shine. Inside stars, simple atoms fuse to make more complex atoms, with energy as a by-product. It’s when a star’s nuclear fuel runs out, that a supernova occurs. Arxiv.org, an open-access repository, published the new study on June 1. It’s called The evolutionary stage of Betelgeuse inferred from its pulsation periods. The scientists said:
We conclude that Betelgeuse is … a good candidate for the next galactic supernova.
The first author is Hideyuki Saio from the Astronomical Institute, Graduate School of Science, at Tohoku University in Japan. The Monthly Notices of the Royal Astronomy Society has accepted the paper for publication.
It’s got people talking
Let’s be clear. The history of observations of supernovae within our own Milky Way is sketchy. But we’d surely be lucky to see any galactic supernova, much more one as nearby as Betelgeuse, in our lifetimes.
And Betelgeuse exploding in just tens of years? That’s an amazing thought, and has people talking!
But is it realistic?
Tens of years?!
What follows are a couple of tweets that have set off a new round of chatter on Twitter. The first – from Friday, June 2 – points to the “tens of years” scenario.
Yikes – serious scientific evidence that Betelgeuse might explode within “tens” of years.https://t.co/bgRqK9l97u
— Dr Jan Eldridge (@astro_jje) June 2, 2023
An almost 50% brightness increase
A second notable tweet – from @Betelbot on Twitter, which provides daily status reports on the star – is from May 18, 2023. It points to a recent almost 50% brightness increase for the star! Note that Betelgeuse is behind the sun in summer. So, until it emerges before dawn in late summer, we won’t know what it’s doing.
Now at 142% of my usual brightness! #Betelgeuse pic.twitter.com/S7TuFTcjdj
— Betelgeuse Status (@betelbot) May 18, 2023
The background buzz on Betelgeuse
Stars shine because they undergo thermonuclear fusion reactions in their interiors. Simply put, they fuse simple elements (like hydrogen) to create more complex elements (like helium), with energy as the by-product. As massive stars (eight or more solar masses) age, they run out of the simplest fuels, but progressively burn more complex fuels until ultimately their cores are made of iron … and then nuclear burning ceases. At that point, with no more fusion taking place, the high temperatures in a star’s interior drop. And that means the high pressures in the star’s interior drop too. The star begins to collapse on itself. It collapses … then rebounds in a terrific explosion, a supernova.
So, massive stars like Betelgeuse explode as Type II supernovae – collapsing rapidly and exploding violently – after they exhaust their fuel supply.
And so, when a star explodes depends on what’s going on inside the star, on how much fuel it has left, and on how close it is to collapse.
But what’s going on inside Betelgeuse?
The new online study said:
We conclude that Betelgeuse is in the late stage of core carbon burning …
And the carbon burning phase for a massive star like Betelgeuse lasts around 1,000 years. If we are “near the end” of that stage, then Betelgeuse has neared the end of its lifetime and may be about to explode, perhaps even in “tens of years.”
But are there other possibilities? Of course there are.
UniverseToday published a great story on Betelgeuse on Friday, June 2, 2023, that explains some of the science involved with drawing any conclusion about whether Betelgeuse will explode soon. The author pointed out that:
… What hasn’t attracted as much attention is the following part of the paper.
‘In fact, it is not possible to determine the exact evolutionary stage, because surface conditions hardly change in the late stage close to the carbon exhaustion and beyond,’ the researchers write. Astronomers can only see the surface, but it’s what’s happening deep inside the star that tells the tale.
The authors of the paper are really saying that according to observations, data, and modelling, Betelgeuse could explode sooner than thought. But – and this is critical – they don’t know what stage of core carbon-burning the star’s in. Carbon burning could go on for a long time, according to some of the models that fit the data.
So, basically, we’re back to square one. Betelgeuse might explode tomorrow. It might explode in “tens of years.” Or it might explode in a thousand years.
But why did Betelgeuse dim in 2019?
In late 2019, Betelgeuse sparked excitement around the world when it began dimming noticeably. Astronomers now refer to this event as the Great Dimming of Betelgeuse. As it was happening, many believed (and hoped!) the big event – the explosion of this relatively nearby star – was close at hand.
Of course – although Betelgeuse since regained brightness, then dimmed again, now brightened again, and so on – it has not exploded yet.
So why did it dim?
Analyzing data from NASA’s Hubble Space Telescope and several other observatories, astronomers concluded that the bright red supergiant star Betelgeuse quite literally blew its top in 2019. Betelgeuse lost a substantial part of its visible surface and produced a gigantic Surface Mass Ejection (SME). This is something never before seen in a normal star’s behavior.
Our sun routinely blows off parts of its tenuous outer atmosphere, the corona, in an event known as a coronal mass ejection (CME). But the Betelgeuse SME blasted off 400 billion times as much mass as a typical CME!
Read more: Betelgeuse is recovering from blowing its top
So, the Great Dimming of Betelgeuse in 2019 was apparently caused by a cloud of hot gas, expelled by the star, that temporarily blocked some of the star’s light.
Clearly, some game is afoot at Betelgeuse!
Will its supernova destroy Earth?
Whenever Betelgeuse does blow up, our planet Earth is too far away for this explosion to harm, much less destroy, life on Earth. Studies indicate we’d have to be within 160 light-years of a supernova for it to harm us. And Betelgeuse is perhaps four times this distance.
Instead, anyone alive on Earth when Betelgeuse does finally explode will see an amazingly beautiful sight in the night sky – a very, very, very bright star.
And professional astronomers will be happy to have an exploded Betelgeuse so close. They’ll be able to study the star post-supernova.
Meanwhile, amateur astronomers and casual stargazers will enjoy the explosion, too. But the many who enjoy seeing Betelgeuse as Orion’s bright red star will dearly miss it when it’s gone!
Betelgeuse in the night sky
At mid-northern latitudes, around the first of every year, Betelgeuse rises around sunset. The star is prominent on January and February evenings.
By the beginning of March, this star is due south in early evening. By mid-May, it is briefly visible in the west after sunset. Betelgeuse is traveling behind the sun in early summer, but it returns to the east before dawn by about mid- to late July. Certainly, by early August, you can see Betelgeuse in Orion in the east before sunrise, where the constellation is known as the ghost of the summer dawn.
The star Betelgeuse has a distinctive muted orange-red color. It’s ideal for convincing non-believers that stars do, in fact, come in colors.
Stars designated as Alpha are typically brightest in their constellations. But Betelgeuse is Alpha Orionis, despite the fact that it’s fainter than Orion’s other bright star, Rigel.
Betelgeuse is the 10th-brightest star in the sky overall, and it’s the 7th-brightest star visible from most of the U.S., Canada, Europe and the majority of the Northern Hemisphere.
Pop culture, history and mythology
Remember the movie Beetlejuice? This star’s name is similar to that.
The proper names of many bright stars are Arabic in origin. This fact reflects the dominance of Arabic astronomers and astrologers during Europe’s Dark Ages. The name Betelgeuse is derived from an Arabic phrase that is usually translated as The Armpit of the Giant. Of course, the Giant refers to Orion, but – rather than an armpit – some authors see Betelgeuse as representing a hand or sometimes a shoulder. While it is not entirely clear what the name means, Betelgeuse marks the right shoulder of Orion in many old star maps.
In the ancient myths, Orion is most often associated with a giant, a warrior, a hunter, a god or some other anthropomorphic or animal figure, so it is not surprising that most depictions of Betelgeuse have an anatomical connection. The Sanskrit name signified an arm, too, for example, although it likely was really the leg of a stag. In parts of Brazil, Betelgeuse was seen as the hind leg of a caiman (crocodilian) or the foreleg of a turtle. On the other hand, in ancient Japan, Betelgeuse was considered to be part of the rim of a ceremonial drum. In Peru, it was one of four vultures about to devour a criminal.
The position of Betelgeuse is RA 05h 55m 10.3053s, dec +07° 24′ 25.4″.
Bottom line: Betelgeuse is due to explode as a supernova someday, although maybe not soon on a human timescale. When it does explode, it’ll be bright enough from our earthly vantage point to shine during the day. But it’s far enough away that Earth won’t be in any danger.
Source of “tens of years” paper: The evolutionary stage of Betelgeuse inferred from its pulsation periods
Read more: Colors of Betelgeuse in a star collage
Full moon in June mimics path of December sun
When to watch in 2023: Overnight of June 3.
Where to look: lLook for the bright round moon in the east in the evening, highest in the sky around midnight, and in the west before sunrise.
Crest of the full moon falls at 3:42 UTC on June 4, 2023. That’s 10:42 p.m. CDT on June 3 in central North America. So if you live in either North or South America, your fullest moon hangs in the southeast after sunset on June 3.
In fact, all full moons rise along the eastern horizon near sunset, and set along the western horizon near sunrise. And they are visible all night as they trek across the sky. At full moon, the sun, Earth, and moon are aligned in space with Earth in the middle. That’s because the moon’s day side – its fully lighted hemisphere – directly faces us. That’s why the moon appears full. Also note that the moon will look full and round for a day or two around full moon.
It’s the Strawberry Moon
All full moons have names. In Europe, popular names for the June full moon include the Honey Moon and the Mead Moon. Some Native Americans used the name Strawberry Moon for the June full moon, and that name is still the most common in North America. The name Strawberry Moon highlights the time of year when many species of berries ripen, particularly sweet strawberries.
Read: Full moon names by month and by season
June full moon is in Scorpius in 2023
The June 2023 full moon can lie in front of one of three constellations of the zodiac. First, it can lie in the constellation Scorpius the Scorpion. The second is the lesser-known constellation Ophiuchus the Serpent Bearer. And the third is Sagittarius the Archer.
As seen from the Americas, the full moon on the night of June 3, 2023 will be located in the direction of Scorpius. You’ll notice a bright star nearby. It’s Antares, known as the Scorpion’s Heart.
When it rises on June 4, the moon will still appear full. On that night, from the Americas, it’ll be near the foot of Sagittarius the Archer and eastward of the previous night’s location. And it’ll be near the noticeable asterism called the Teapot.
June full moon mimics December sun
Because a full moon stays more or less opposite the sun, the full moon’s nighttime path mimics the sun’s daytime path from six months ago, or six months hence.
This full moon occurs close to the June solstice, so the moon follows nearly the same path across the sky as the December solstice sun. The December solstice is the Northern Hemisphere’s winter solstice. So the moon’s trek on the nights around the June full moon will resemble the low path of the winter solstice sun.
North of the Arctic Circle, the wintertime sun never climbs above the horizon. So neither will this June full moon.
Meanwhile – in the Southern Hemisphere – the June full moon’s flight across the sky will mirror that of the high summer solstice sun.
And, south of the Antarctic Circle, the moon will be out for 24 hours around the clock, simulating the midnight sun of summer.
Arc of the June full moon
The moon’s arc across our sky varies from month to month and season to season. Every full moon rises along the eastern horizon, opposite the sun as it sets in the west. And every full moon arcs across the sky throughout the night, and sets along the western horizon around dawn. So for viewers in the Northern Hemisphere, the arc of June’s full moon is lower than the paths of the full moons since December. This year, the July 3rd full moon arcs slightly lower because it occurs closer to the solstice on June 21 than the June 4th full moon.
For those in the Southern Hemisphere, the full moon’s arc across the sky is climbing higher with each successive month since December, reaching its highest at the full moon falling closest to the June solstice, which occurs sometime from mid June through early July. That’s because in 2023, even though the June 3rd full moon is close to the solstice, the July 3rd full moon falls closer.
Bottom line: The June full moon – the Strawberry Moon – occurs on the overnight of June 3, 2023.
Farmers’ Almanac Full Moon Calendar
Pandemic shutdowns lowered emissions but resulted in heightened climate warming: Study
The lockdown and related shutdowns in South Asia during the pandemic led to reduced emissions during a short period. Curiously, while this did lead to cleaner air, new research has discovered that it also led to increased climate warming in…
Venus after sunset: Greatest elongation on June 4, 2023
When to watch: Venus came into view after sunset in December 2022 and has been visible in the evening sky throughout the first half of 2023. Greatest elongation – when Venus will be farthest from the sunset – happens on June 4, 2023. Afterwards, Venus will quickly sink toward the sunset as it races toward its sweep between the Earth and sun around mid-August 2023.
Where to look: Look in the sunset direction while the sky is darkening. You can’t miss Venus as the dazzling evening “star.”
Greatest elongation is at 11 UTC on June 4. That’s a whole-Earth time … for all of us, Venus will still appear in our evening sky, in the west after sunset. At this elongation, the distance of Venus from the sun on the sky’s dome is 46 degrees.
Magnitude at greatest elongation: Venus shines at magnitude -4.4.
Through a telescope: Venus appears 49% illuminated, in a first quarter phase, 23.56 arcseconds across.
Note: As the sun’s 2nd planet, Venus is bound by an invisible tether to the sun in our sky. It’s always east before sunrise, or west after sunset (never overhead at midnight). Venus is the brightest planet visible from Earth and shines brilliantly throughout every morning or evening apparition. Greatest elongation happens when Venus is farthest from the sun on the sky’s dome.
For precise sun and Venus rising times at your location:
Old Farmer’s Almanac (U.S. and Canada)
Stellarium (free online planetarium program)
Venus after sunset in 2023 Northern Hemisphere
Venus after sunset in 2023 Southern Hemisphere
A comparison of elongations
Not all of Venus’s greatest elongations are created equal. That’s because the farthest from the sun that Venus can ever appear on the sky’s dome is about 47.3 degrees. On the other hand, the least distance is around 45.4 degrees.
Elongations are also higher or lower depending on the time of year they occur and your location on Earth.
Venus events from late 2022 into 2024
October 22, 2022: Superior conjunction (passes behind sun from Earth)
June 4, 2023: Greatest elongation (evening)
August 13, 2023: Inferior conjunction (races between Earth and sun)
October 23, 2023: Greatest elongation (morning)
June 4, 2024: Superior conjunction (passes behind sun from Earth)
Bottom line: At greatest eastern elongation on June 4, 2023, Venus is as far from the sunset as it will be for this evening apparition.
Why 40°C is bearable in a desert but lethal in the tropics
Heatwaves of similar magnitudes can have very different impacts depending on factors like humidity or how prepared an area is for extreme heat
This year, even before the northern hemisphere hot season began, temperature records were being shattered.
Spain for instance saw temperatures in April (38.8 degrees Celsius) that would be out of the ordinary even at the peak of summer. South and south-east Asia in particular were hammered by a very persistent heatwave and all-time record temperatures were experienced in countries such as Vietnam and Thailand (44°C and 45°C respectively).
In Singapore, the more modest record was also broken, as temperatures hit 37°C. And in China, Shanghai just recorded its highest May temperature for over a century at 36.7°C.
We know that climate change makes these temperatures more likely, but also that heatwaves of similar magnitudes can have very different impacts depending on factors like humidity or how prepared an area is for extreme heat.
So, how does a humid country like Vietnam cope with a 44°C heatwave and how does it compare with dry heat, or a less hot heatwave in even-more-humid Singapore?
Weather and physiology
The recent heatwave in south-east Asia may well be remembered for its level of heat-induced stress on the body. Heat stress is mostly caused by temperature, but other weather-related factors such as humidity, radiation and wind are also important.
Our bodies gain heat from the air around us, from the sun, or from our own internal processes such as digestion and exercise. In response to this, our bodies must lose some heat.
Some of this we lose directly to the air around us and some through breathing. But most heat is lost through sweating, as when the sweat on the surface of our skin evaporates it takes in energy from our skin and the air around us in the form of latent heat.
Meteorological factors affect all this. For example, being deprived of shade exposes the body to heat from direct sunlight, while higher humidity means that the rate of evaporation from our skin will decrease.
It’s this humidity that meant the recent heatwave in south-east Asia was so dangerous, as it’s already an extremely humid part of the world.
The limit of heat stress
Underlying health conditions and other personal circumstances can lead to some people being more vulnerable to heat stress. Yet heat stress can reach a limit above which all humans, even those who are not obviously vulnerable to heat risk — that is, people who are fit, healthy and well acclimatised — simply cannot survive even at a moderate level of exertion.
One way to assess heat stress is the so-called Wet Bulb Globe Temperature. In full sun conditions, that is approximately equivalent to 39°C in temperature combined with 50 per cent relative humidity. This limit will likely have been exceeded in some places in the recent heatwave across south-east Asia.
In less humid places far from the tropics, the humidity and thus the wet bulb temperature and danger will be much lower.
Spain’s heatwave in April with maximum temperatures of 38.8°C had WBGT values of “only” around 30°C, the 2022 heatwave in the UK, when temperatures exceeded 40°C, had a humidity of less than 20 per cent and WBGT values of around 32°C.
Two of us (Eunice and Dann) were part of a team who recently used climate data to map heat stress around the world.
The research highlighted regions most at risk of exceeding these thresholds, with literal hotspots including India and Pakistan, south-east Asia, the Arabian peninsula, equatorial Africa, equatorial South America and Australia. In these regions, heat stress thresholds are exceeded with increased frequency with greater global warming.
In reality, most people are already vulnerable well below the survivability thresholds, which is why we can see large death tolls in significantly cooler heat waves. Furthermore, these global analyses often do not capture some very localised extremes caused by microclimate processes.
For example a certain neighbourhood in a city might trap heat more efficiently than its surroundings, or might be ventilated by a cool sea breeze, or be in the “rain shadow” of a local hill, making it less humid.
Variability and acclimatisation
The tropics typically have less variable temperatures. For example, Singapore sits almost on the equator and its daily maximum is about 32°C year round, while a typical maximum in London in mid summer is just 24°C. Yet London has a higher record temperature (40°C vs 37°C in Singapore).
Given that regions such as south-east Asia consistently have high heat stress already, perhaps that suggests that people will be well acclimatised to deal with heat. Initial reporting suggests the intense heat stress of the recent heatwave lead to surprisingly few direct deaths — but accurate reporting of deaths from indirect causes is not yet available.
On the other hand, due to the relative stability in year-round warmth, perhaps there is less preparedness for the large swings in temperature associated with the recent heatwave.
Given that it is not unreasonable, even in the absence of climate change, that natural weather variability can produce significant heatwaves that break local records by several degrees Celsius, even nearing a physiological limit might be a very risky line to tread.
Don’t have time to read about climate change as much as you’d like?
Get a weekly roundup in your inbox instead. Every Wednesday, The Conversation’s environment editor writes Imagine, a short email that goes a little deeper into just one climate issue. Join the 10,000+ readers who’ve subscribed so far.
Alan Thomas Kennedy-Asser, Research Associate in Climate Science, University of Bristol; Dann Mitchell, Professor of Climate Science, University of Bristol and Eunice Lo, Research Fellow in Climate Change and Health, University of Bristol
This article is republished from The Conversation under a Creative Commons license. Read the original article.
We are a voice to you; you have been a support to us. Together we build journalism that is independent, credible and fearless. You can further help us by making a donation. This will mean a lot for our ability to bring you news, perspectives and analysis from the ground so that we can make change together.
SDG and climate change | The Express Tribune
Progress and modernity on the back of the industrial revolution came with a high price tag. The consumption of fuel and gas for transportation and to run factories contaminated the atmosphere to the extent that the naturally accumulated greenhouse gases…
Climate Change: What You Need to Know
Every year, the climate crisis intensifies — promising severe droughts, supercharged storms, blistering heatwaves, and other extreme weather events — threatening the lives and livelihoods of billions of people, and overwhelming the capacities of governments around the world. But there’s…