Why clouds are the greatest source of uncertainty in how the planet will warm.
What is a cloud? At the smallest scale, it’s simple: just moisture condensed onto a tiny particle — a speck of dust, a grain of pollen, salt spray from the ocean, or a mote of soot.
But as soon as more than one of these cloud droplets get together, things get chaotic, quickly. Scientists describe clouds as an emergent phenomenon, where smaller constituent parts give rise to sophisticated, self-organized patterns, like a school of fish swimming together or a murmuration of starlings.
This chaos is why clouds are so difficult to predict. But the consequences of this inability to see through clouds go beyond sunshine and shade; it’s also obscuring our understanding of climate change.
“How clouds change determines how warm it gets in response to a certain amount of greenhouse gas forcing,” said Angeline Pendergrass, an assistant professor of atmospheric science at Cornell University. And the stakes of how this relationship plays out are high.
Whether a given area sees more rainfall, drought, heating, or cooling in the coming years hinges on what kinds of clouds are present. And right now, scientists are still struggling to understand how this will unfold. Part of this is due to a lack of data about the myriad cloud varieties that are out there, part is due to a lack of computing power, and part is due to a spotty historical record.
In this episode of the Unexplainable podcast, we talk to researchers about why it’s so hard to understand these ubiquitous accoutrements of weather, why it’s so easy to underestimate their power, and why it’s worth taking a moment to appreciate their expressions in the sky.
As the planet reaches record-high carbon dioxide concentrations in the atmosphere, bringing higher temperatures and more dangerous weather extremes, scientists are using every tool in their box — weather balloons, satellites, simulations, aircraft, and even sailor’s logs — to try to see through clouds and into the future of the world as we know it.
Why clouds are clouding our picture of climate change
The largest source of uncertainty in our understanding of the future under climate change is what humans will do.
After that, it’s clouds.
The basic mechanism of climate change is pretty simple: Heat-trapping gases such as carbon dioxide are emitted into the atmosphere as humans burn fossil fuels and damage natural stores of carbon. The more greenhouse gases accumulate in the atmosphere, the more the planet heats up.
So, how much people actually work to curb fossil fuels and reduce greenhouse gas emissions will radically shape how much Earth warms up in the coming century.
Of course, there’s more to climate change than the planet heating up by a couple degrees. Not every part of the world is warming at the same rate, and a shift in the average temperature has important knock-on effects, like melting ice, rising sea levels, and weather events pushed to greater extremes.
These effects are what end up being the most consequential aspects of climate change for humans, altering where we can live, how much food we can grow, and whether we can continue to afford our lifestyles.
Clouds are critical to all of these impacts, but how they factor in can be complicated and confusing.
They behave as distinct units with unique properties, spreading out into thin layers or piling up into heaps, rising or falling in the sky. And when it comes to the climate, one of the most significant attributes of clouds is they can either cool an area or trap heat.
“The way in which they behave depends on where they sit in the atmosphere,” said Scott Collis, an atmospheric scientist at Argonne National Laboratory. The puffy cumulus clouds at low altitudes, for instance, tend to bounce sunlight back into space, increasing the albedo, or reflectiveness, of the Earth. That has a cooling effect. Wispy, high-altitude cirrus clouds, on the other hand, bounce back infrared radiation coming up from the ground, warming the surface. And many clouds can do both, to varying degrees.
Now the entire planet is warming up, and for every degree Celsius the air warms, it can absorb about 7 percent more water. More water in the air could lead to more clouds, but which ones? Another effect to consider are feedbacks. The heat-trapping clouds could amplify the warming caused by greenhouse gases, leading to more water evaporation and creating even more of these clouds.
And the effects aren’t uniform across the world; some places may see far more reflective clouds while others may experience more warming clouds, and others still may see more, or less, of both. How these effects align will change how the planet warms in the coming decades and the practical consequences thereof.
“If we overestimate the degree to which clouds cool the planet in response to greenhouse gas forcing, then we’ll underestimate how warm it gets in response to certain amounts of greenhouse gases,” Pendergrass said.
Figuring this out is difficult because scientists have only recently been able to sharpen their picture of clouds. Ground-based radar and satellite images have helped researchers gain insight into the broad patterns of clouds across the planet, while weather balloons and aircraft have yielded narrow but detailed pictures of their inner workings.
But many of these techniques have only been deployed in the past half-century. Prior to that, observations of clouds were far more coarse. And unlike historical changes in temperature and rainfall, which can leave behind clues in sediment, ice cores, tree rings, and rocks dating back millenia, clouds have a light footprint. There are no cloud fossils.
So if scientists want to understand what clouds were like before the industrial revolution — before humans started pumping greenhouse gases and pollution into the sky in gargantuan quantities — they have to examine historical observations: weather logs, nautical records, and even art and literature. However, with such a blurry picture of the past, it’s harder to see into the future.
Clouds can be too complicated for computers
Observations of clouds are then fed into climate models. But computer models also struggle to understand clouds. “The big question for climate models is, what are the combinations going to be going forward?” Collis said.
There are two general approaches to clouds in climate models: top-down and bottom-up. Top-down simulations can model the whole planet and apply forcings, like different concentrations of carbon dioxide, and seeing what happens over time, zooming into different regions.
Other simulations start at the microscopic level of droplets and aerosols and then scale up. The problem is that clouds lie right in between these two approaches — too small and ephemeral to be captured in most global climate simulations and too complicated for computers to assemble from their constituent parts. So clouds tend to be represented in an oversimplified way in computer models.
“We have to understand what’s happening on these tiny, tiny scales that you need a microscope to see, all the way to the scale of the entire planet,” said Pendergrass. “All of those things are relevant to the problem. So trying to make a computer model that does that is not computationally feasible to do in any kind of direct way.”
Despite the challenges, scientists are making progress and filling in uncertainties about the future of the planet.
For instance, researchers last year published a new estimate for boundaries of climate sensitivity for the first time in decades. Climate sensitivity refers to how much the planet is expected to warm in response to a doubling of carbon dioxide concentrations in the atmosphere compared to pre-industrial levels. It’s a critical metric used to refine models of climate change. A better understanding of clouds and their feedback into the climate system was a big reason why they were able to narrow their predictions.
But scientists don’t have decades to come up with their next round of refinements, and the current pace of advances in the field is excruciatingly slow. “We’re going to see a substantial amount of global warming before we can model the clouds scaled globally,” Pendergrass said.
So in the meantime, scientists are painstakingly piecing together records from the past, observations from the present, and models of the future to get a sharper picture of the cloudy skies.
Author: Umair Irfan