How clouds complicate global warming
After decades of uncertainty, scientists are getting better at predicting whether clouds will help to cool or cook the planet. Michael Lucy explains.
Clouds are wispy evanescent things, beloved of poets and daydreamers. They may also determine whether civilisation as we know it survives the 21st century.
Depending on how clouds react to global warming, they could cool or cook the planet. But, so far, we haven’t been able to predict which way they will swing. That’s a big problem for our ability to prepare for what’s coming.
“When we look into the future, the most uncertain part of the climate models is what will happen to the clouds,” says Christian Jakob, an atmospheric scientist at Monash University. “It’s a devilishly intricate problem.”
Understanding clouds, circulation and climate sensitivity is one of several grand challenges that the Swiss-based World Climate Research Programme (WCRP) is focusing on. The programme brings together scientists from all over the world to tackle big questions in climate science, and it considers cloud feedback “the intellectual and experimental challenge of our lifetime”.
Clouds depend on a huge number of processes that act on scales from the sub-millimetre-sized world of water droplets and ice crystals to the kilometres-wide spread of a thunderstorm and beyond. Limits on computing power, however, mean that climate models are often ‘low res’. Their blocky simulations chop the planet up into 100 km square pixels.
“So what we try to do is build mathematical models of how clouds work in one of those 100 by 100 kilometre areas,” says Jakob. These small-scale results are then averaged out and plugged into the larger-scale models.
When it comes to warming, clouds acts in three ways. They act like silvery shields reflecting away incoming sunlight; they act like insulators trapping heat on the planet (recall how much cooler it gets on a cloudless night); and they act like radiators sending heat out into space. Whether a cloud acts more like a shield, a blanket or a radiator depends on where it lies and what it’s made of.
And here things get diabolically complex. Though they are a small piece of the overall climate puzzle, solving the behaviour of clouds is a problem with many, many moving parts.
“The most uncertain part of the climate models is what will happen to the clouds.”
It’s understood that low clouds tend to radiate more heat back to space, because they are warmer. High clouds are cooler, and radiate less heat. Bottom line: low clouds will help cool the planet; high clouds will push temperatures further up.
Worryingly, there are signs that high clouds are getting higher. But altitude is just the first entry on this list of moving parts.
If a warming climate makes clouds more watery and less icy, they will become more shiny and help cool things down.
At the same time, changing weather patterns may move clouds around the globe. For instance, banks of low, reflective cloud are already shifting away from middle latitudes towards the poles, which leaves less shielding against the brightest equatorial light. The net effect is to speed up warming.
Even the shape of the water droplets or ice crystals in the cloud influence their climate impact. “It can be hexagons, it can be needles, it can be all sorts of things. It matters,” explains Jakob.
Beyond water molecules, the aerosol particles that trigger the condensation of droplets are another key variable. “That’s where chemistry comes in,” says Robyn Schofield, an atmospheric chemist at the University of Melbourne. “A sulfur particle or sea salt is pretty good at taking water up. Black carbon is less good.”
Aerosols can affect both the amount of cloud and the size of the droplets inside it. Smaller droplets reflect more light, making for better shields.
To gather data on cloud chemistry, Schofield’s team has built a mobile lab in a shipping container that goes by the acronym AIRBOX (Atmospheric Integrated Research facility for Boundaries and OXidative experiments).
Last summer, AIRBOX instruments were aboard the CSIRO ship RV Investigator and the Australian Antarctic Division’s Aurora Australis as they headed south to a data desert: the Southern Ocean. “That’s the area where there is the most uncertainty about clouds and aerosols,” says Schofield. The study was part of a major US and Australian collaboration.
“The US brought in their Gulfstream V research plane to sample clouds, the RV Investigator was out there with cloud radar and lidar. There was some equipment on Macquarie Island and on the icebreaker Aurora Australis,” says Steve Siems of Monash University, who participated in the effort from Hobart. “We’re overwhelmed. We’ll be looking at these data for the next five years.”
The new data will help constrain the moving parts in our models of cloud formation and, in due course, narrow down the size of the likely global temperature increase.
At issue is climate sensitivity, which measures how much the temperature changes if the amount of carbon dioxide (CO2) in the atmosphere doubles. In 2014 the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) estimated this number somewhere between 1.5 and 4.5°C. This huge range comes down almost entirely to the effect of clouds.
“We haven’t really uncovered any other possible major uncertainty,” says Steven Sherwood, a climate scientist at the University of New South Wales.
By amplifying warming or reining it in, clouds could make the difference between a 2°C increase – where sea levels rise a metre or more and deadly heatwaves are predicted – and a truly catastrophic 4.5°C – which, according to the IPCC, means widespread extinctions, global food supplies at risk and many parts of the planet too hot to live in.
The latest data suggest clouds will amplify warming, though not as severely as some models had implied.
That means more than 3°C of warming by 2100 if the world’s countries make the emissions cuts promised in the Paris Agreement, which is well beyond the agreement’s target of less than 2°C.
But however much CO2 we put into the atmosphere, the precise effect will depend on clouds. To get a better sense of the details, we still have quite a wait: the next IPCC assessment is due out in 2021.