17 October, 2019
17 October, 2019
Clouds
Empty skies ahead
There seems to be a moment in everyone's childhood when they are watching the clouds go past, imagining something familiar in the cloud's shape.
What's harder to imagine, and much less enjoyable, is that climate change could deny that experience from future generations.
Unfortunately, that is exactly what Caltech scientists, Tapio Schneider and colleagues, are predicting.
The scientists are saying that one of the most common kind of clouds — stratocumulus clouds — are likely to disappear in a warmer climate.
It's a change which may have dire consequences for the planet.
Clouds are already thinning
For several years now, scientists have known that clouds have been thinning. Records of satellite images show clouds have been moving south or north away from the tropics, leaving the hottest parts of the planet exposed. Clouds have also been moving higher into the sky, away from the Earth's surface.
To understand what was happening in more detail, scientists looked closely at one special type of cloud — stratocumulus cloud.
Computer simulation of stratocumulus clouds. Tapio Schneider
Stratocumulus clouds are the most common clouds on Earth. They are the clouds that you see on satellite photos stretching thousands of kilometres across the ocean.
The clouds are also important for keeping the Earth cool. They form a protective layer, shading the Earth from the direct sun.
The scientists created a high resolution simulation of stratocumulus. The model included over 10 million variables, representing the cloud over a 5 square kilometre patch of ocean. They hoped it would resemble California or Peru in the summertime.
Dr Schneider proudly describes it as "comparable with that of the most advanced of today's atmospheric general circulation models."
8 °C hotter
With a realistic simulation, the researchers could then see what happens when more CO2 is added into the atmosphere.
The reseachers found that, when CO2 levels reach 1200 parts per million (ppm), the stratocumulus clouds thin and break up. The clouds disappear. And the clouds don't come back until CO2 levels fall far below the CO2 levels we have currently.
What's more, without the protection of the clouds, the Earth then gets about 8 °C (14.4 °F) hotter.
Right now the Earth is 1 °C (1.8 °F) hotter than normal and already experiencing melting icecaps, drought and heatwaves.
An 8 °C increase would be impossible for most life to survive.
Hot, dry days without clouds, Al Mireles, Pixabay
Some scientists disagree
Some scientists disagree with the results, arguing that the assumptions are flawed.
Dr Roy Spencer says the model is inaccurate. He argues that moisture in the clouds will not be much warmer as a result of global warming. The ex-NASA scientist maintains "these clouds exist where cold water is upwelling from the deep ocean".
Another issue with Dr Schneider's research is that it is based on a five square kilometre patch of the ocean. It is a big assumption to say the results are then correct for the whole of the globe, says Bjorn Stevens, a climate scientist at Max Planck Institute for Meterology.
Joel Norris, a cloud scientist at Scripps Institution of Oceanography, says it's unlikely that all stratocumulous clouds will disappear simultaneously as if turned off by a switch. He says Dr Schneider's model "essentially has two settings", but "it is very likely that the Earth has more knobs than that".
How likely is this scenario?
Regarding his own predictions, Dr Schneider says that "it is unlikely that we will reach such high CO2 levels". This could be because we do the sensible thing and act now to reduce CO2 emissions, or it could be because the decision is made for us, as "climate change would have severe and dangerous global impacts well before the loss of stratocumulus decks."
An IPCC (Intergovernmental Panel on Climate Change) report shows possible CO2 levels depending on climate scenarios.
The top curve is the most negative scenario — called RCP8.5. By 2100, this curve shows CO2 at around 1000 ppm CO2 and continuing well past it in the following decades. On the RCP8.5 scenario, if we miss 1200 ppm CO2 in 2100, we will get there not long after that.
CO2 forecast for 2100. IPCC long-term climate change: Projections, commitments and irreversibility
The likelhood that we can avoid the hot, cloudless future forecast by Dr Schneider's team depends on the decisions we make. The year 2100 is not unreachable for today's children. If we continue to ignore CO2 levels, then they will experience CO2 levels that will dissipate clouds and bring about a dramatic 8 °C (14.4 °F) increase in global average temperatures.
Has it happened before?
It's difficult to find out anything about the clouds that existed millions of years ago. But Dr Schneider believes there is a clue in something called the climate sensitivity.
Climate scientists define climate sensitivity as how much the temperature increases each time CO2 levels double. So if raising CO2 levels from 400 ppm to 1600 ppm means a temperature increase of 10 °C (9 °F), then that suggests a climate sensitivity of 5 °C (18 °F).
This is close to what happened around 55 million years ago. At that time, known as the Paleocene-Eocene Thermal Maximum (PETM), temperatures were much higher than today, about 10 °C (18 °F) higher, and CO2 levels reached about 1600 ppm, 4 times higher than now.
A climate sensitivity of 5 °C (18 °F) is way above previous estimates, with most coming in at 2 or 3 °C. Only recently have estimates come closer to a climate sensitivity of 5 °C or more.
The difference from earlier estimates? This time the modelling of clouds was more accurate.
Dr Schneider's predicts an 8 °C jump in global temperatures when stratocumulus clouds disappear. This could be a reason why climate sensitivity has been so high over the last 55 million years. If his forecast is correct, the bad news is that global temperatures are going to go much higher, much faster than expected.
Watch: Clouds and the climate tipping point | T. Schneider
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