Stratocumulus clouds are rather boring. They’re not as elegant as cirrus clouds (those high horsetail wisps) or as majestic as cumulonimbus clouds (big, scary thunderheads). But stratocumulus clouds, which hover low and create vast decks of cloud cover, have a supreme value in our world: Their white tops reflect lots of solar radiation back into space.
But Earth’s portfolio of clouds in 2019 could potentially be altered by extreme climate change. Those stratocumulus cloud decks could vanish, intensifying global warming.
That’s the conclusion of a study published in the journal Nature Geoscience, based on a computer model that provides a new warning that climate change could deliver surprises on top of the existing and clearly predictable consequences.
The lead researcher, Tapio Schneider, a climate scientist at Caltech, hypothesized that very high levels of atmospheric carbon dioxide could suppress the formation of stratocumulus cloud decks. He and his colleagues modeled the formation of such clouds and, after two years of computer calculations, concluded that the steady rise in atmospheric CO2 could trigger a sudden spike in temperature associated with disappearing stratocumulus clouds.
The effect appeared intense if CO2 reached 1,200 parts per million — three times the current level, which is already much higher than the preindustrial level of carbon dioxide. If CO2 reached 1,300 parts per million, the new report states, the global atmospheric temperature would rise 8 degrees Celsius above whatever warming had already been produced from greenhouse gases. “It’s a dramatic effect,” Schneider said. The stratocumulus cloud decks “break up altogether,” he said.
“Once the stratocumulus decks have broken up, they only re-form once CO2 concentrations drop substantially below the level at which the instability first occurred,” the study said.
Kerry Emanuel, a professor of atmospheric science at MIT, said of the study: “It provides a plausible, but not yet proven, route by which you could have a tipping point in the climate.”
Climate scientists have long been confounded by clouds. A cloud can amplify global warming, or it can limit it, depending on what kind of cloud it is, and its size, location, thickness, duration, etc. But clouds are hard to pin down in a computer model. They are remarkably insubstantial elements of the natural world. If you could bring all the clouds and water vapor in the atmosphere to the surface, it would form a liquid layer less than an inch deep, Schneider said, and clouds alone would create a layer no deeper than a coat of paint.
“You need to predict what small fraction of that water vapor will condense into clouds,” Schneider said.
There is no easy way to test whether clouds would really behave this way. What’s certain is that a spike of 8 degrees C, in addition to warming already baked in from greenhouse gas emissions, would presumably be catastrophic, not only for human civilization but for countless species and ecosystems jolted by the rapid climate change.