Marco Magrini considers the climate consequences of a dimming planet
The Earth is slowly losing its shine. Its reflectivity – or albedo – is declining, mostly due to vanishing ice cover and fewer clouds. So few, in fact, that 2023 stood out as the year with the lowest planetary albedo on record – a change now believed to have contributed to the sudden surge in global temperature.
Albedo measures the fraction of light reflected by a surface, on a scale from 0 (a perfect absorber) to 1 (a perfect reflector). To give a few examples: fresh snow has an albedo of 0.8; desert sand, around 0.4 depending on its colour; bare soil, 0.17; and the open ocean, a mere 0.06, as water strongly absorbs solar radiation. Crucially, high clouds reflect between 30 and 50 per cent of sunlight, but lower clouds are far more reflective – bouncing back 60–80 per cent.
That makes a significant difference. Low clouds – such as stratocumulus and stratus – are generally thicker and more efficient at reflecting incoming shortwave solar radiation back into space. At the same time, they allow most outgoing longwave infrared radiation to escape – the key factor in the greenhouse effect. High clouds, such as cirrus, are thinner and composed of ice crystals. They reflect less solar radiation and trap more outgoing heat, thus exerting a greater warming effect.
Clouds act like floating mirrors, bouncing sunlight into space. However, recent observations suggest that their total albedo is weakening. ‘The decline is apparently caused largely by a reduced low-cloud cover in the northern mid-latitudes and tropics, in continuation of a multi-annual trend,’ reads a recent paper compiled by the Alfred Wegener Institute and published in Science.
As the atmosphere heats up, warmer air inhibits the formation of extensive, stratified lower cloud layers. At the same time, regulations have reduced the number of sulfate aerosols – highly reflective particles once emitted from ship exhausts – making skies more transparent. These twin effects may help explain the 0.17°C temperature rise that shocked climatologists (and, notably, only them) two years ago.
A diminishing albedo is another signal of the planet’s deepening climate afflictions. Changes in reflectivity don’t just accompany warming – they amplify it. When bright surfaces darken or disappear, more sunlight is absorbed, driving further warming in a self-reinforcing cycle: a classic positive feedback loop. Retreating glaciers and disappearing summer Arctic sea ice – replacing white with dark rock or ocean – are among the most potent albedo spoilers. Our expanding urbanisation plays a part, too. But as the case of vanishing low cloud cover shows, the web of climatic causes and effects is complex.
Take soot. Also known as black carbon, it’s derived from diesel engines, wildfires and industrial processes. It can travel through the atmosphere to remote regions and settle on snow and ice, darkening their surfaces. In parts of the Arctic, this has reduced albedo by up to 15 per cent. These darker patches absorb more heat, accelerating melt. Worse still, airborne black carbon itself absorbs sunlight and heats the atmosphere. According to the IPCC, it’s the second-largest human-made contributor to climate change – another unwanted gift from fossil fuels.
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Earth’s reflectivity has been declining since the 1970s, with an accelerating drop in the past decade. ‘However, our analysis of the datasets shows that the decline in surface albedo in the polar regions only accounts for roughly 15 per cent of the most recent decline in planetary albedo,’ says Helge Goessling, lead author of the study published in Science.
To calculate the potential impacts of reduced global albedo, researchers applied an energy budget model designed to simulate climate fluctuations. They determined that without the decrease in planetary reflection, the average temperature in 2023 would have been 0.23°C cooler.
This may help laypeople like me understand why climate scientists were right to be alarmed by the 2023 temperature spike: it’s yet another sign that positive feedback loops are kicking in.
A dimmer Earth is a warmer Earth.
