Characterizing weather extremes from the past to add context to future impacts
"Atmospheric rivers" are large-scale extreme weather systems that are making headlines more frequently. When viewed in satellite images, they appear just as described - like rivers in the sky. Though they are often reported in places like California, these weather systems have the potential to bring high heat and dump disastrous amounts of precipitation on areas throughout the mid and high latitudes.
A team of researchers, including UConn Department of Earth Sciences associate professor Clay Tabor and Ph.D. student Joseph Schnaubelt, looked at how atmospheric rivers impacted the Greenland Ice Sheet in the past to get a better understanding of how these weather systems may enhance melting in the Arctic as the climate continues to warm. Their results are published in AGU Advances.
An important question that paleoclimate scientists like Schnaubelt and Tabor are trying to answer is how the Arctic will respond to climate change, and for this they focused deep into the past on a time called the Last Interglacial, between 130,000 and 115,000 years ago.
"Earth goes through glacial cycles, and the Last Interglacial was the last time the Arctic was warmer than present day," says Schnaubelt. "We know that that's the direction we're headed toward, and we wanted to see how atmospheric rivers impacted the Greenland Ice Sheet."
Tabor explains that atmospheric rivers can have an impact in different ways. On one hand, they can lead to increased accumulation of the ice sheet, where they bring massive amounts of snow, but they can also bring more heat to the region, leading to enhanced rainfall and melting of the ice sheets. These nuances are important to tease out because melting ice sheets contribute to sea level rise.
For this study, the researchers analyzed data from a simulation from the National Center for Atmospheric Research (NCAR) that spans the entire Last Interglacial, and lets the researchers characterize how these storms responded to different variables, namely the Earth's orbital change. Earth's tilt as well as the shape of its orbit around the sun can have a significant impact on global climate, especially temperatures.
"We found that during times when the orbital configuration made the Arctic the warmest, we had more summertime storms impacting the ice sheet," says Schnaubelt. "That's trouble for the future, because we know that the Arctic's getting warmer, we could expect there to be more summertime storms, and when this happened, we saw more ice sheet melt."
Schnaubelt says another key finding is that elevation is an important factor in how atmospheric rivers impact the ice sheet. At lower elevations, precipitation tends to fall as rain, but at higher elevations, storms lift upwards where they cool and precipitation falls as snow. As the ice sheet melts and experiences reductions in elevation, this could lead to enhanced melting from atmospheric rivers.
"The simulation we used is great because it had fully interactive coupling between the ice sheet, ocean, and atmosphere," says Schnaubelt. "For analyzing the storms, we used two different algorithms, also from different collaborators. We call them feature detection algorithms, but we have this big data set, and we need something that can trace out these storms for us."
The simulation allowed the researchers to look at individual storms every six hours, which is rarely reported as most paleoclimate studies look at annual or seasonally averaged data.
"We're getting at the weather driving the climatological changes. I think this helps a lot with connecting the research to future climate change. When we talk about one degree of warming, it gets undersold sometimes because people say that's not a big deal," says Tabor. "Probably, by itself, one degree changes little, but we're seeing changes in the distribution of extremes. These atmospheric rivers are extreme weather phenomenon. Being able to capture that in the past is important to put the future in context and the impacts on people's lives."
Schnaubelt adds that the one degree of warming is why paleoclimate research is so important. While average global temperature may have only risen by one or one and half degrees, some places like the Arctic experienced warming closer to three to five degrees and the result was Earth saw the sea level rise between six and nine meters (20 to 30 feet). Seemingly small increases in global temperature can have a big impact and we know this by looking to the past.
This scenario is not an exact one to one comparison to today, says Tabor, largely due to difference in orbital effects between now and the Last Interglacial, however there are many similarities.
"I think the concern is that there's maybe more similarities than we'd like to admit with a small amount of warming, and how that can affect sea level and global patterns," says Tabor.
The researchers saw the largest melt signal due to atmospheric rivers early in the Last Interglacial when the Arctic was at its warmest, but the timing for the ice sheet minimum lagged when they saw the most loss of the ice sheet, says Schnaubelt. This emphasizes a key aspect of ice sheets, in that they respond to signals over very long periods of time.
"The ice sheets get some of these melt signals early on, and they continue to melt, even as the climate begins to head back toward a cooler state," says Tabor. "In simulations of future climate change, we see similar situations where even if we stop emitting CO2, the ice sheets are still out of equilibrium. It'll take thousands of years for the ice sheets to respond to that signal."
Understanding these nuances and trends requires more studies like this, which was made possible due to an immense community resource in the NCAR simulation project, which Tabor says speaks to the importance of federally funded research.
"It's important to continue to support these projects that maybe initially seem like a high upfront cost and effort, but there continues to be a lot of important science coming out of it, including this study."
