Climate change pushes precipitation — and ability to predict it — to its limits

Climate change pushes precipitation — and ability to predict it — to its limits

By Sarah Anderson, Dec. 7, 2021 –

Primed by a drought that has lasted longer than the 1930s Dust Bowl, wildfires scorched over 5 million acres of land in the western United States this year. The water level of Lake Powell is dipping dangerously low amid the severe dry spell, threatening the hydroelectric power it generates.

On the other side of the country, a record-breaking 2020 hurricane season produced so many tropical storms that the list of 21 names for them had to be supplemented with Greek alphabet letters for just the second time in history. Hurricane Laura alone caused 42 deaths and almost $20 billion in flood damage.

Climate change is pushing precipitation to both extremes. The elevated annual mean temperature in the United States is accompanied by higher levels of rain and snow in the eastern, southeastern and midwestern regions of the country and decreases in precipitation in the western and southwestern areas, according to the National Oceanic and Atmospheric Administration (NOAA).

“The basic result is that we (will) see more floods and more droughts,” said Richard Alley, a climate scientist and professor of geosciences at Pennsylvania State University, during the annual 2021 Comer Climate Conference, held virtually this year. “You get more intense events in a warmer world.”

As the annual mean temperature increases across the United States, precipitation concentrates in certain regions. (Image source: NOAA)


Climate change has heightened the need to predict and prepare for extreme precipitation events, but it has also impaired the ability to do so. According to NOAA, the United States’ average seasonal precipitation skill — the accuracy in forecasting the amount of precipitation over an upcoming season in a specific region — has declined in recent years.

Precipitation skill varies for different areas, in part because naturally occurring cycles in ocean temperature have a well-documented effect on precipitation trends. El Niño, a climate phenomenon resulting from a warming of the central and eastern Pacific Ocean that occurs every three to seven years on average, has a consistent impact on rainfall in the places along its path each time it strikes.

Seasonal precipitation skill is “actually pretty good in places that are affected by El Niño because we can predict El Niño, and El Niño statistically affects weather patterns,” said David Battisti, a professor of atmospheric sciences at the University of Washington.

The U.S. seasonal precipitation skill has deteriorated since 2013. (Image source: NOAA)

Elsewhere, however, technology has struggled to keep up with new variables as the climate evolves. The impact of climate change on precipitation — and precipitation skill — can be largely traced to the ocean, which stores approximately 93% of excess heat driven by emissions from fossil fuel use, according to the Intergovernmental Panel on Climate Change.

“If you heat the planet, you get more evaporation from the ocean, just like if you heat a pan on your stove, you start to get steam coming off of it,” Battisti said. The additional water vapor in the air means when conditions are right for rain, it rains harder.

As the relationship between rising ocean temperatures and precipitation became clear, the U.S. National Weather Service realized it needed to incorporate ocean processes into its forecasts, said Joellen Russell, a climate scientist and oceanographer at the University of Arizona, at the Comer Climate Conference. However, rather than developing a new climate model that integrated ocean from the start, the ocean element was tacked onto the existing atmosphere component.

“They coupled it, but it wasn’t really designed to be coupled,” Russell said, resulting in a weather simulation that cannot account for the full network of interactions between ocean and atmosphere. It more closely resembles a layer of red painted over a layer of blue than a pointillist blending of red and blue dots that appears purple.

The accuracy of U.S. precipitation prediction tools is also limited by the lack of information on ocean conditions due to shortcomings in operational oceanography, Russell said. For example, orbital satellites often sample from confined and redundant areas, collecting spotty real-time data to feed into the model.

While some scientists focus on how to gather and use current data to further improve seasonal precipitation skill, other researchers are looking to the past to predict the long-term impacts of climate change on precipitation in specific regions. Dylan Parmenter, a Ph.D. student in the Department of Earth and Environmental Sciences at the University of Minnesota, presented his work studying the chemical composition of stalagmites to reconstruct rainfall patterns in the Amazon at the Comer Climate Conference.

Dylan Parmenter collects a cross section of stalagmite from a cave, which shows tree ring-like features corresponding to different time points. (Images courtesy of Dylan Parmenter)

Stalagmites form when water vapor carried from the ocean combines with the soil at a cave site and drips onto the floor of the cave. In this way, “stalagmites are kind of like fossilized precipitation,” Parmenter said. The oxygen atoms in the water vapor exist as two versions — one heavier and one lighter. If it rains as the water vapor cloud travels, the heavier oxygen atoms fall to the ground with the rain before reaching the cave. By measuring the ratio of the two types of oxygen in a sample of stalagmite, Parmenter can estimate the precipitation conditions when the stalagmite was created. To determine the age of the sample, he measures to what extent uranium atoms in the stalagmite have decayed, expelling some of their subatomic particles.

These ancient rainfall records can help scientists understand the effect of past climate patterns on precipitation, providing a useful reference point when predicting the impact of modern climate change.

“If we want to know how precipitation in a certain region is going to change from climate change, there’s nothing to compare that to,” Parmenter said. “Our work is going back and saying: With these natural climate change processes in the past, how did it react?”

While Russell acknowledges there is no time in history perfectly representative of the current climate conditions, the fundamental insight gained from research like Parmenter’s will be critical in informing responses to climate change.

“We are in the undiscovered country of the future, as Shakespeare put it. So, no, it’s not going to be exactly the same as any previous (time),” she said. “But being able to look at how these mechanisms have worked in the past can help us accelerate the learning that is going to be required to do the preventing, the mitigating and the adapting.”

Featured photo at top: A satellite image captures Hurricane Laura’s landfall. (Image source: NOAA)

Sarah Anderson is a health, environment and science reporter at Medill and a Ph.D. chemist.  Follow her on Twitter @seanderson63.

Melting tropical glaciers threaten freshwater sources while sea ice melt accelerates rising oceans

Melting tropical glaciers threaten freshwater sources while sea ice melt accelerates rising oceans

By Sarah Anderson, Nov. 16, 2021

“Tropical glacier” —  the term sounds like an oxymoron and, due to climate change, it might become one.

These bodies of ice nestle in the mountain ranges of tropical regions, providing a major source of freshwater and tourism revenue. But studies predict most tropical glaciers will disappear within the next 10 years taking critical water resources with them.

“It’s hugely important to understand the rate at which these glaciers are melting,” said Alice Doughty, a lecturer of Earth and climate sciences at the University of Maine.

Doughty and Meredith Kelly, a professor of Earth sciences at Dartmouth College, are developing a model to investigate tropical glacier melt in places such as the Rwenzori Mountains in Uganda and the Sierra Nevada del Cocuy in Colombia. They presented their findings at the virtual 2021 Comer Climate Conference, an annual event usually held in southwestern Wisconsin.

Glaciers sand down the rock beneath them as they melt, acting “sort of like bulldozers,” Kelly said. The debris piles up, depositing a series of ridge-like features called moraines at the glacier’s retreating boundaries. Kelly examines satellite images of a glacier site to identify moraines the glacier left behind, then analyzes samples of rock to determine when the moraines were created. By measuring a type of beryllium atom that accumulates in the rock as it is exposed to Earth’s atmosphere, Kelly can approximate how long ago the rock was freed from the ice’s hold to form the moraine. Collectively, this information allows her to generate a map of the size and shape of the glacier at a specific time in the past.

Moraines in the Sierra Nevada del Cocuy photographed by satellites (left- map and data courtesy of Jordan Herbert, M.S. student at Dartmouth College) and on the ground (right- image courtesy of Gordon Bromley, professor at the University of Maine).


Doughty then works to develop a computer model to simulate how climate variables interact to produce the glacier. She tries to “grow the glacier,” adjusting temperature, precipitation and other inputs until the glacier output matches Kelly’s map.

Data from any nearby weather stations provide a useful starting point; observing whether current climate conditions yield the modern glacier helps her evaluate the model. When the simulation is optimized, Kelly and Doughty will be able to use it to predict the effect of climate change on glacial melt.

“Once we calibrate the model, we can just as easily make things warmer,” Doughty said. “And so we can have estimates like in the Rwenzori, one degree of warming and those glaciers are gone.”

Other scientists are interested in developing similar models for the melting of sea ice, “a really big part of the climate system,” said Ed Brook, a professor of Earth, ocean and atmospheric sciences at Oregon State University.

Sea ice helps insulate the ocean from heat and gases in the atmosphere and contributes to sea level rise when it melts, but it doesn’t leave behind the same physical record as glaciers. While seasonal sea ice melting can be tracked using IP25, an organic molecule produced by algae that grow along the receding ice edge, the presence of permanent sea ice in the past has remained elusive.

“We don’t have a very good method for reconstructing how much sea ice there was at any particular time,” Brook said.

Frank Pavia is analyzing chunks of Arctic Ocean floor gathered in the mid-1990s, but he has journeyed to other ice-covered seas to collect similar samples. (Image courtesy of Frank Pavia)

Frank Pavia, a postdoctoral researcher in geological and planetary sciences at the California Institute of Technology, presented his research at the Comer Climate Conference, exploring a new way to monitor this more stable sea ice cover. His method relies on interplanetary dust particles, the solar system’s version of dust that rains down on Earth from outer space.

Interplanetary dust particles deposit a light type of helium atom onto the sea floor. If the surface of the ocean is blocked by ice, however, the particles (and the helium) can’t enter the water. Pavia is examining whether the amount of helium in the ocean floor can be used as a measure of sea ice cover. To account for any differences in helium levels due to changes in how fast atoms settle to the sea floor, he also measures a special thorium atom that is produced inside the ocean and sinks to the bottom at a constant rate, regardless of ice cover.

To test his method, Pavia acquired samples of the floor of the Arctic Ocean from the Last Glacial Maximum, one period of well-characterized sea ice cover in the Arctic. The age of the samples had been previously determined by measuring a radioactive form of carbon in the shells of tiny marine organisms that indicates when they were alive.

When sea ice cover was thick, Pavia detected high amounts of thorium but low levels of helium, demonstrating that while atoms were efficiently burying in the sea floor, helium couldn’t access the ocean due to the sea ice. When there was no sea ice cover, he measured similar signals for both thorium and helium, revealing that the helium atoms were successfully deposited into the ice-free ocean. Pavia is also interested in seeing if the period of melting in between gives a surge in helium, corresponding to an influx of interplanetary dust particles that accumulated on top of the ice over time.

While accurate measurements will require that the dust particles are evenly distributed in the ocean floor rather than concentrated in specific pockets, the approach “has a lot of promise,” Brook said. The prospect of detecting nonseasonal sea ice melting — an event that leaves very few fingerprints — via a helium spike could be a major advantage of the method, he said.

“It’s potentially very important, because if the pulse of particles was the signature of melting a bunch of permanent ice, then you would have a sign of that process, which would be very hard to see other ways,” Brook said.

After further validation, Pavia plans to use his method to reconstruct poorly understood sea ice patterns during past periods of warming in the Arctic. Like Kelly, he aims to provide a map that other researchers can use to test and refine models that simulate sea ice melting as climate change progresses.

“The hope is to help improve the projections of sea ice coverage into the future in the Arctic,” Pavia said.

Featured photo at top: Meredith Kelly and Alice Doughty study the pace of melting of tropical glaciers like this one in Uganda’s Rwenzori Mountains. (Image courtesy of Alice Doughty)

Sarah Anderson is a health, environment and science reporter at Medill and a Ph.D. chemist.  Follow her on Twitter @seanderson63.

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