Last weeks Reads

[SteamyTea]

Been a while since I have done some of these, but then it has been August.

 

The colour of your car has a big impact on urban heat

Dark-coloured cars can make a measurable difference on nearby air temperature, and in cities of millions the effect can add up and noticeably increase how hot it feels

By Jeremy Hsu

21 August 2025

Lighter-coloured cars could mean cooler streets

Olena Polkovnykova/Alamy

 

The colour of a car can make a discernible difference in the surrounding air temperature, as dark cars absorb and emit more heat than lighter vehicles when sitting on the street or in a parking lot. The collective impact from hundreds of thousands or millions of cars in cities could significantly influence urban heat island effects, and even intensify heat stress for pedestrians during sunny days.

 

“You know when you walk past a parked car on a hot day and feel the heat radiating off it?” says Márcia Matias at the University of Lisbon in Portugal. “That’s real! It’s not your imagination.”

Matias and her colleagues measured the air temperature around two cars – one black and one white – parked outside for more than 5 hours during daylight. Their measurements showed the black car raised the local air temperature by as much as 3.8°C compared to the adjacent asphalt under a clear and sunny summer sky with temperatures of 36˚C. Meanwhile, the white car had much smaller impacts on the surrounding air temperature.

The reason for such temperature differences is white vehicle paint reflects between 75 and 85 per cent of incoming sunlight, whereas black paint absorbs most incoming sunlight, reflecting just 5 to 10 per cent. A car’s thin steel or aluminium skin can also heat up quickly under strong sunlight, unlike even very dark asphalt that is thicker and warms more slowly. “Now picture thousands of cars parked across a city, each one acting like a little heat source or a heat shield,” says Matias. “Their colour can actually shift how hot the streets feel.”

The researchers calculated repainting parked cars from dark to lighter colours could create cooler surfaces and lower near-surface air temperatures on sunny, low wind days. Using the city of Lisbon as an example, the change could effectively raise street-level reflectance of incoming sunlight from just 20 per cent to nearly 40 per cent in areas where parked cars cover more than 10 per cent of the road.

 

Harnessing light-coloured vehicles as a “mitigation strategy for urban heat is particularly novel”, says Sarah Berk at the University of North Carolina. Previous studies have focused on modifying roofs and pavement to reflect more sunlight.

Fleets of municipal vehicles, taxis and delivery trucks or vans are obvious candidates for getting light-coloured makeovers, says Matias.

 

Journal reference

City and Environment Interactions DOI: 10.1016/j.cacint.2025.100232

 

 

Super-cool cement could stop buildings trapping heat inside

A new formulation of cement reflects and emits heat more effectively than normal Portland cement, so it stays much cooler on a hot day

By Alex Wilkins

20 August 2025

Concrete buildings absorb heat in hot weather

Panther Media Global/Alamy

 

Cement that can cool itself by reflecting light on the outside and releasing heat from its surface could help buildings stay comfortable without needing air conditioning.

Normal cement tends to absorb infrared radiation from the sun and store it as heat, which can increase the temperature inside cement buildings as well as that of the surrounding air.

 

So Fengyin Du, then at Southeast University in Nanjing, China, and her colleagues decided to address this by creating a cement in which tiny reflective crystals of a mineral called ettringite collect on the surface.

The team’s cement emits infrared light from its surface, rather than storing it, and so loses heat quickly. “It works like a mirror and a radiator, so it can reflect sunlight away and send heat out into the sky, so a building can stay cooler without any air conditioning or electricity,” says Du.

To make it, the researchers first produce tiny pellets from common minerals like limestone and gypsum. These are ground to dust and mixed with water before being poured into a silicone mould covered in small holes. Air bubbles passing through the holes create slight depressions in the cement’s surface, where the reflective ettringite crystals can then grow, while an aluminium-rich gel in the set cement lets infrared light pass through the material.

 

This process is easily scalable, says Du, and the cement is $5 per tonne cheaper than regular Portland cement because it can be produced at lower temperatures.

Du and her team tested how their cement kept cool on a hot roof at at Purdue University in Indiana, which jointly hosted Du’s PhD project, finding that the surface temperature was 5.4°C (9.7°F) lower than the air and 26°C (47°F) lower than the Portland cement.

 

Dimples in the surface of the cement, viewed with an electron microscope

Guo Lu/Southeast University

 

“It’s a useful material,” says Oscar Brousse at University College London. “You increase the reflective capacity as well as increasing the emissivity, so any energy that is captured or conducted to the material is emitted efficiently back.”

However, measuring only the surface temperature of the material doesn’t tell us how it will perform in the real world, says Brousse, so it should undergo further testing. “It doesn’t mean that because the surface is 5°C lower, that the air temperature will be 5°C lower around it. The effect locally may be greatly limited.”

 

Journal reference:

Science Advances DOI: 10.1126/sciadv.adv2820

 

Rare 'triple-dip' La Niña may explain why 2023 was so hot

The record-breaking global temperatures seen in late 2023 may have emerged partly because of unusual conditions in the Pacific Ocean in the preceding years

By Madeleine Cuff

18 August 2025

The Pacific Ocean released heat into the atmosphere in 2023

blickwinkel/Alamy

 

An unusual “triple dip” La Niña that suppressed ocean temperatures in the Pacific Ocean for three years running may have primed the planet for the dramatic surge in global heat experienced in 2023.

While global temperatures had been expected to increase around this time, due in part to greenhouse gas emissions and warm surface waters in the Pacific, they weren’t anticipated to peak until early 2024. As it was, record-breaking heat emerged from September 2023, months ahead of schedule.

 

Julius Mex at the University of Leipzig in Germany and his colleagues set out to explore what exactly happened in late 2023 to trigger the onset of this extraordinary heat. “What we’re trying to explain is why the change in temperature in boreal fall was so extreme,” he says.

The team used datasets that combine historical weather observations with climate models to investigate circulation, temperature, cloud cover, radiation and precipitation in the Pacific during 2022 and 2023.

They conclude that the background state of the Pacific, which unusually had been stuck in cooler La Niña conditions since 2020, was a key factor. That suppressed ocean heat and encouraged the development of low-lying clouds, helping to reflect more of the sun’s radiation back into space.

 

When the El Niño weather pattern finally emerged in 2023, the swing from La Niña to El Niño was so dramatic that it produced unusual effects on air circulation and rainfall over the western Pacific Ocean, allowing the ocean to release even more heat than anticipated into the atmosphere.

In parallel, the shift to El Niño also triggered a sudden and dramatic fall in cloud coverage in the eastern Pacific, allowing Earth to absorb much more radiative heat. “This is something that can drive the annual temperature change,” says Mex.

Karsten Haustein, also at the University of Leipzig, wasn’t involved in the work but says he broadly agrees with the analysis. “If you have a triple dip La Niña, then you are not allowing the ocean to release heat,” he says. “So you build up heat deeper in the ocean basin, and eventually it has to come out.”

 

Mex says the findings are in line with research published in recent months suggesting the disappearance of ocean cloud cover was a key driver of the rapid jump in temperatures beginning in 2023. “I think it’s a perfect fit,” he says.

Richard Allan at the University of Reading in the UK says the work improves understanding of how cloud cover changed in the Pacific during 2022 and 2023. But he stresses that human-caused climate change, alongside cuts to planet-cooling aerosol pollution, were also major factors in reducing ocean cloud cover and driving warming.

“The size of the global temperature rise in 2023 was only possible due to the rising overall heating of the planet caused by rising greenhouse gases, but also reducing and dimming clouds related to the warming and also declining aerosol particle pollution,” says Allan.

 

Reference:

Research Square DOI: 10.21203/rs.3.rs-6958463/v1

 

 

Earth's carbon sinks are being eroded by climate change feedback loops

Carbon dioxide levels in the atmosphere have risen an extra 15 parts per million since 1960 due to the declining ability of the land and sea to soak up excess CO2

By Michael Le Page

19 August 2025

Wildfires, like this one in Greece, are blunting Earth’s natural carbon sink

Thanassis Stavrakis/AP Photo/Alamy

 

Climate change is increasingly affecting the ability of Earth’s natural carbon sinks to soak up excess carbon dioxide, and this means more of this greenhouse gas emitted by human activity is staying in the atmosphere, leading to further warming.

These feedback effects are responsible for about 15 per cent of the increase in CO2 levels since 1960, according to Pierre Friedlingstein at the University of Exeter at the UK.

 

The land and oceans have been acting as carbon sinks, soaking up nearly half of all the excess CO2 humans have been pumping out. For instance, higher CO2 can boost plant growth, meaning more CO2 is taken up by vegetation. But as the world warms, extreme heat, droughts and wildfires can increasingly counteract this CO2 fertilisation effect.

Friedlingstein is involved in the Global Carbon Project, which is trying to work out exactly how much CO2 is being emitted, how much is absorbed by various sources and how this is changing over time. He and his team had previously used climate models to estimate that the land sink would be taking up 27 per cent more CO2 were it not for feedbacks such as droughts.

Their latest estimate now puts it at 30 per cent, Friedlingstein told the Exeter Climate Conference last month. Meanwhile, the ocean sink would be taking up 6 per cent more CO2 were it not for feedbacks, he said.

 

Put together, the land and oceans would be taking up 15 per cent more CO2. Since the level of CO2 in the atmosphere has risen by around 100 parts per million (ppm) since 1960, 15 ppm is due to feedback effects hitting the sinks. “The sink is not collapsing, but it is slowly [coming] down,” Friedlingstein said.

The sinks have still grown in absolute terms, just not as much as they would’ve done otherwise, says David Armstrong McKay at the University of Sussex in the UK.

“It’s broadly in line with expectations, but still not great news that it’s a bit more than we thought,” says McKay. “The more warming there is, the harder it gets for the land sink to keep pace with rising CO2, as the CO2 fertilisation effect on vegetation growth is increasingly limited by extreme events like the recent El Niño-enhanced droughts.”

 

The big question is what happens next. There has been much concern about studies suggesting the land sink has hardly taken up any net CO2 in the past two years because of warming-fuelled droughts and fires.

That has led to suggestions that there could be a sudden, massive decline in the relative capacity of the land sink, rather than the slow decline most climate scientists expect.

But Friedlingstein described these short-term events as “blips” that aren’t necessarily the best guide to the future, because the land sink can vary greatly from year to year. “The long term is what we should be looking at,” he said.