The vast Antarctic ice sheet is held together through a precarious balancing act.

It loses mass whenever ice melts or breaks off into the ocean. And it restores some of that mass when snow falls and builds up on the surface of the ice.

But as the climate warms, the Antarctic ice sheet is falling out of balance.

It's losing ice faster than it can replace, raising global sea levels in the process. The problem has compelled scientists to study a broad range of factors that could affect mass balance on the Antarctic ice sheet.

Increasingly, research suggests that enormous "atmospheric rivers" may have an important — but sometimes overlooked — influence.

Atmospheric rivers are huge bands of moisture-laden air rippling through the sky. These bands often originate in the tropics or subtropics and then they stream across the globe, carrying water vapor with them. They often dump buckets of heavy rain or snow wherever they make landfall.

More snow might sound like a boon for the shrinking Antarctic ice sheet. But it's often more complicated than that. Atmospheric rivers can cause heavy snowfall in some cases — but at other times, they can also spur rapid melting.

Scientists are working to determine which effect is stronger — and whether that might change as the Earth grows warmer.

"That's still an unanswered question in my opinion," said Meredith Fish, a climate scientist and postdoctoral associate at Rutgers University. "What are the differences in storm characteristics that lead to one impact versus the other?"

A new study, published in the journal Geophysical Research Letters, illustrates the complicated influence of atmospheric rivers in Antarctica.

Using satellite data collected by NASA's ICESat-2, the study links atmospheric river events to unusually heavy snowfall in West Antarctica in the Southern Hemisphere's winter of 2019.

The satellite data allowed the scientists to observe increases or decreases in the height of the ice sheet — a way of measuring how much snow has accumulated over time. The data suggests an abnormal increase in height across West Antarctica between May and August 2019, one of the coldest times of the year in Antarctica.

The researchers found that these increases coincided with a number of atmospheric river events in West Antarctica.

The study demonstrates that atmospheric rivers can cause snow to quickly accumulate in a short amount of time. But they also can have the opposite effect — especially during warmer months. The same study found that around 90% of summer atmospheric river events between 2019 and 2020 were associated with surface melting in West Antarctica.

Clouds are likely the culprit. The same clouds that cause snow to fall onto the ice sheet can also trap heat underneath them, temporarily warming the surface of the ice sheet.

"It is a really kind of interesting and contrasting process that is occurring," said Susheel Adusumilli, a doctoral student at the University of California, San Diego's Scripps Institution of Oceanography and the study's lead author.

Other recent studies have documented the same kinds of conflicting effects.

A 2014 paper linked atmospheric rivers to heavy snow events in East Antarctica, similar to the recent events in West Antarctica. On the other hand, a 2019 study found that atmospheric rivers are also linked to major melting events in West Antarctica.

And it's not just the ice sheet that hangs in the balance. Atmospheric rivers may also affect Antarctic sea ice.

A study published last year in Science Advances found that atmospheric rivers could be linked to at least two major "polynyas" in the Southern Ocean — these are large holes that open up in the sea ice cover when the ice melts.

Scientists are still trying to figure out exactly when atmospheric rivers cause snow to build up versus when they cause ice to melt away.

Air temperatures probably have a lot to do with it — if it's warmer outside, then ice melt is more likely. But atmospheric rivers are known to increase melting even during the winter sometimes, according to Fish, the Rutgers University scientist, who also co-authored the new study.

"Figuring out if there's anything else besides just temperature that drives that difference is still a good question to answer," she said.

More warming, more rivers, more melting?

These questions are growing more important as the climate warms.

For one thing, warmer air temperatures may increase the likelihood of severe melting events across the board. For another, models suggest that climate change may cause an increase in atmospheric rivers in Antarctica.

Some research suggests that may already be happening.

One study, published last year in Geophysical Research Letters, found that atmospheric rivers in the Southern Hemisphere have been shifting poleward over the last few decades. That shift could be linked to strengthening wind patterns around Antarctica, a phenomenon that many scientists believe is driven at least in part by climate change.

If atmospheric rivers hit Antarctica more frequently in the future, it's important for scientists to understand how they may affect the ice sheet.

Antarctica is already losing more than 200 billion tons of ice each year. Most of that comes from glaciers at the edge of the ice sheet breaking apart and crumbling into the ocean. But scientists suspect that surface melting may cause greater losses in the future as the ice sheet continues to warm.

For now, scientists don't think that atmospheric rivers are actually causing that much mass loss in Antarctica. Even when they cause melting events, temperatures typically cool back down relatively quickly, Adusumilli said. That means most of the meltwater freezes back onto the ice sheet before it can run off into the ocean.

But in a warmer future, that could change.

And that's not the only reason to worry. Meltwater can cause problems on the ice sheet, even when it's not pouring into the sea.

When liquid water forms into pools, it can sometimes seep down into cracks on the ice sheet. When it refreezes, it expands and causes those cracks to widen. It's a process known as "hydrofracturing" — and in extreme cases, it can cause large chunks of ice to break apart and collapse into the sea.

If atmospheric rivers could increase the likelihood of these events in the future, they're worth paying attention to, Adusumilli said.

"That is the big unknown, how the future of surface melt will interact with the ice sheet," he said. "I don't think surface melt on its own will be a major driver of mass loss. But as it starts cracking the ice, the downstream impacts of this melt could be very huge, and it's one of the biggest avenues for current research."

Reprinted from E&E News with permission from POLITICO, LLC. Copyright 2021. E&E News provides essential news for energy and environment professionals.