David Pollard's work is taking him back in time five million years, not only to learn about the past but also what could be coming in the future.
Pollard, a senior scientist in Penn State's Earth and Environmental Systems Institute, is developing computer programs that can model the behavior of ice sheets in the past, and from that gain insight into what upcoming generations could see.
"Understanding how ice sheets have responded in the past can help to predict how they will respond to future warming and contribute to sea level rise and flooding," Pollard said.
The computer model Pollard created simulates how the Antarctic ice sheet has advanced and retreated over the last 5 million years. He notes that one sector, the West Antarctic Ice Sheet, rests on bedrock far below sea level and is particularly vulnerable to warming ocean temperatures. The model code describes not only the current characteristics of the ice sheet – such as the velocity of ice flow and temperatures of the ice – but also simulates how the ice extent and thickness change over time.
To assess the accuracy of the model, Pollard, along with Robert DeConto, a professor at the University of Massachusetts, compared its results to actual geological data from the multinational ANDRILL project gathered by drilling into sediments below floating ice fringing West Antarctica. The natural variations of the ice sheet simulated by the model corresponded well with the stacked layers in the sediment cores over the last 5 million years, allowing scientists to expand the relevance of the data beyond the local drill site to the entire region of West Antarctica.
Now, Pollard is focusing the model on a smaller region of the West Antarctic coast that includes Pine Island Glacier, an area where the ice has begun to dramatically thin and flow faster in recent decades. By zooming in on a small region, the spacing between model grid points can be finer, improving the details of the simulations. Other researchers at Penn State and elsewhere are also paying close attention to Pine Island and nearby glacier).
Pollard's model includes the warming effects of both ocean and atmospheric temperatures. Ocean temperatures affect the ice via floating ice shelves — tongues of ice that extend out over the ocean beyond where the grounded ice becomes afloat.
"Ice shelves hold back the grounded ice like a flying buttress on a cathedral. If the ocean melts them from below, buttressing is reduced and ice flows more rapidly out of the interior," Pollard said.
He said that for the next several decades, ocean warming will likely remain more important than atmospheric warming when it comes to ice loss in this region. That's because for now surface temperatures there rarely reach the melting point, so there is little melting of surface ice or snow. But that could change in the next century or two. With business-as-usual greenhouse gas emissions, surface melting could play a bigger role. On longer time scales of centuries to a millennium, Pollard's model, like previous studies, suggests there is a potential for self-accelerating runaway retreat as more and more grounded ice in the West Antarctic interior is lost to the ocean.
These changes are already contributing to sea level rise, and are expected to accelerate in the future, increasing the chances of flooding in some parts of the world.