Blue(ish) planet: How climate change will alter the colours of the Earth

A few weeks ago, Hollywood actor William Shatner became, at 90 years of age, the oldest man to go into space. Shortly after touching down in the Texan desert, with tears in his eyes, Shatner described the powerful emotions aroused by the adventure, reserving his greatest enthusiasm for the colours he had just witnessed:

“To see the blue colour go whip by, and now you’re staring into blackness – that’s the thing. The covering of blue – this sheet, this blanket, this comforter of blue that we have around, we think, ‘Oh it’s blue sky.’ And then suddenly you shoot through it all... and you’re looking into blackness. There is Mother Earth and comfort; and there is — is there death?”

Shatner wasn’t the first to feel this way. When astronauts and cosmonauts started going into space in the 1960s, they too were struck by the Earth’s unmistakable blue hue. Many more people became aware of the phenomenon in December 1968, when the crew of Apollo 8 took a photograph that became a global sensation. Earthrise, as it is now known, revealed the world as a glowing sapphire sphere suspended in the blackness of space. The image prompted the U.S. Poet Laureate James Dickey to coin the immortal term, ‘The Blue Planet’.

The Earth derives its blueness from a process called Rayleigh scattering. When sunlight travels through the planet’s atmosphere, its short blue wavelengths are disrupted by molecules in the air and scattered in all directions. This optical illusion makes the sky and sea look blue, as well as producing the cerulean nimbus that surrounds our globe like a glimmering gas flame.

But the blue planet isn’t exclusively blue. It also contains vast swathes of green, from the deep viridians of the taiga and the pea greens of the American Midwest to the deep olives that that run from one side of Central Africa to the other. All of these landscapes are packed with the most abundant pigment on Earth – chlorophyll – which lends trees, plants, grass and algae their characteristic green hues.

How will climate change alter these defining colours of the Earth? You’d be forgiven for thinking that rising temperatures will make our landscapes drier and browner, but the opposite, so far, is the case. Over the last 35 years, 18 million square kilometres of new vegetation has appeared on the planet – an area roughly twice the size of the United States – making the Earth greener today than it has been for millennia.

These new greens are, somewhat ironically, the result of destructive human activity. They have been caused, above all, by rising atmospheric carbon dioxide levels, which – despite posing a grave threat to us – are a significant catalyst of plant growth. In the short-term, our landscapes will likely become even greener, as nations embark on vast tree-planting programmes designed to soak up excess greenhouse gases.

What happens to these greens in the longer-term is much harder to predict – and will depend on how much, and in which ways, we alter our behaviour. If average global temperatures reach 3 or 4 degrees Celsius above their pre-industrial levels (which is not impossible by the end of the century if we don’t arrest the pace of change), those greens will indeed begin to fade, with dark green rainforests turning into sage-green savannahs and emerald grasslands becoming khaki deserts. Hotter summers and drier autumns, even in less affected parts of the world, could end the brilliant displays of autumn foliage to which we have grown accustomed, because leaves will die and fall before they have a chance to change colour.

Global heating is also likely to change the Earth’s signature blues. Scientists predict that by 2100 the oceans will have a very different complexion: subtropical seas will become bluer as warmer waters cause green-tinged phytoplankton to die out; polar seas, conversely, will get greener, as they become more hospitable to marine algae.

The brilliant blues of the sky might also be on borrowed time. If the initiatives announced at COP26 prove to be inadequate (as, alas, seems likely), geoengineers have proposed pumping vast quantities of sulphur dioxide into the upper atmosphere in order to reflect excess sunlight into space and thereby cool the planet. Their plan would almost certainly end up bleaching the blueness of the sky, making it a deathly shade of white.

When he had finished describing his gravity-defying exploit, William Shatner made a more fundamental observation about our planet:

“What I would love to do is to communicate, as much as possible, the jeopardy, the moment you see the vulnerability of everything. It’s so small. This air, which is keeping us alive, is thinner than your skin…”

Shatner had noticed something that many space travellers had grasped before him when they looked back at the Earth from a distance: He realised that our world, like the colours that ebb and flow across it, is not only beautiful but terrifyingly fragile.

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Image: Alexandra Francis for Penguin