Climate archivists: Researchers mine the past for clues to the future
August 11, 2015
To better understand the present – and more accurately predict the future – you must understand the past.
Such is the inspiration for those in the Nelson Institute Center for Climatic Research (CCR) who study paleoclimatology and paleoecology. By combining powerful climate models with data from ancient clues such as fossil pollen, lake sediments and tree rings, these historical ecologists work to reconstruct past climates and ecosystems.
Their findings can help us understand the causes of 21st century climate change – which may result in new and strange climates very different from any experienced today – and the effects on plant and animal communities.
“I really think about this as a tool for aiding climate adaptation efforts,” says Jack Williams, director of the Center for Climatic Research and a professor of geography. “But there’s also the broader question of understanding how the climate system works. The geologic record has been incredibly valuable for that.”
Williams says the pre-industrial past offers a baseline for analyzing the impacts of human activities like carbon emissions and deforestation.
“You can’t understand how humans are affecting the climate system without having an understanding of the natural processes at play,” he continues.
For example, that we now know how greenhouse gases drive up global temperatures is, in a very significant way, “founded on our understanding of past climates and the drivers of past climate variations,” he says.
Williams is one of many CCR scientists who lead the way in this field, building on the legacy of researchers like Reid Bryson, the center’s founder and a pioneer of modern climatology, and John Kutzbach, an emeritus professor of atmospheric and oceanic sciences and environmental studies.
“We live in these little slices of time and our science is funded in little slices of time, so I use the past to get perspective – not necessarily as an analog for the future, but as a wider range of perspectives for thinking about the future,” says Sara Hotchkiss, an associate professor of botany and CCR faculty affiliate.
Hotchkiss studies how rare climatic events and slow changes shape the ecological history of landscapes. “Ecological systems have legacies built into them,” she says. “I look for events beyond our experience and look at how systems respond.”
Hotchkiss conducts research in Wisconsin as well as the Hawaiian islands, which she says provide a model system for studying climate change. “The nature of the problems they have are the same as global problems; they’re just felt more intensely and sooner because it’s a little place,” she says.
Throughout history, she explains, Polynesian cultures have experienced some of humanity’s greatest successes and failures of living within limits. For example, Hotchkiss is part of an interdisciplinary team studying how Hawaiian agricultural practices changed as the state developed and intensified dryland agriculture, and how vulnerability to drought may have ultimately put society at risk.
“It has real implications for the world, including how cultures respond to sensing their limits,” she says. “People are concerned about the response of ecosystems to climatic change. That’s where our resilience lies culturally, as well as in terms of natural systems.”
On the island of Maui, Hotchkiss is studying the climate sensitivity of the forest’s upper limit – a critical ecosystem boundary. In Hawaii the upper forest line is mainly controlled by water availability, so it is sensitive to drought and climate-driven shifts in trade winds.
In collaboration with UW-Madison graduate Shelley Crausbay and the U.S. Geological Survey, Hotchkiss has placed weather stations across the face of mountain slopes in Maui to study how climate and forest composition vary across the mountain gradient. At the same time, she collected lake sediment cores at, above and below the forest line, analyzing fossil pollen from the cores to see how forest boundaries have shifted with past climate changes.
Early results suggest that the upper forest limit is likely to shift downward in the future, which Hotchkiss says is bad news for several endangered native bird species. The forest-dwelling birds may be caught in a climate squeeze as mosquitoes, which carry temperature-sensitive bird diseases, move up the forest slope.
Range of possibility
Because her research has practical applications, Hotchkiss engages with state and federal officials and land managers on most of her projects.
“We’re able to do what they can’t afford to do – extensive monitoring and broad studies of sensitivity,” she says. The findings are directly applicable to adaptation efforts.
“Managers have a really hard time with climate change predictions because they’re so broad in scale. It doesn’t help enough locally,” she adds. “I’m trying to work at the scale at which we manage land and at which we live on land.”
In Wisconsin, Hotchkiss is studying how differences in landscape affect responsiveness to a changing climate – a useful perspective for the Wisconsin Department of Natural Resources, with which Hotchkiss frequently collaborates. “They want us to investigate the sensitivities of natural communities to climate change,” she explains.
The range of variation and rare events in climate, such as extremes in flooding, drought, heat or cold, must be considered in modeling future climate and land management scenarios, but perspective is lacking in terms of ecosystem response – a void Hotchkiss is helping to fill.
“We’re using the past to define the range of possibility and to think about the sensitivity of individual species in case we move beyond that range,” she continues. “What can you learn about a local area that can help you know which areas are likely to be more or less sensitive to climate changes, and to what kinds of changes? That’s where I mine the past.”
For instance, as climate variability increases in Wisconsin, the state is likely to see more dry periods, even as net precipitation increases. Looking to the past for context, Hotchkiss is studying how increased frequency or intensity of drought could impact fire-sensitive landscapes and the use of fire as a land management tool. She’s also looking into potential effects on the ecology of lakes and peatlands, which hold the biggest pools of stored carbon in the region.
Ice age ecologist
Much of Jack Williams’ research also has direct implications for conservation biologists and land managers.
“When the world is changing to a condition that may be very different than what we’re used to and what our management process is based on, that’s a fundamental challenge for practitioners and for ecologists,” he explains.
“The past gives us actual data about species’ responses to climate change,” he says.
His work focuses on the environmental changes of the last 20,000 years. “This is the last big period of climate change, when you’ve gone from an ice age – a glacial era – to the Holocene interglacial period,” he explains.
This time period serves as a model system for understanding how ecosystems respond to large or rapid climate change, periods of drought and increases in carbon dioxide (the greenhouse gas most associated with global warming). “These are all things that happened over this time period that are similar in magnitude to what’s happening now,” Williams says.
Using networks of pollen data, Williams has helped build databases that enable him and other scientists to examine environmental changes not only at a specific location, but across the continent, mapping how a species’ range has shifted because of climate change. Through his findings, he has advanced the concept of “no-analog” communities, or communities of species remixed into combinations not seen today.
“The records show very clearly that species were not all heading at the same rate, in the same direction, as climates changed in the past,” he says. “As a result, we had this reshuffling of species into new communities.”
From past to present
Williams believes no-analog communities formed in response to no-analog climates – mixtures of climatic conditions that happened in the past but don’t happen today. A similar situation could soon emerge, he says, exacerbated by rapid climate change, invasive species and changing land use patterns.
“All these things are creating a novel world,” he says. “Just as some of the late glacial climates were outside the bounds of what we see today, and as we saw species reshuffling in response to these past climates, we may expect a similar response in the 21st century.”
For example, some arctic and alpine climates – those at the coldest end of the spectrum of today’s climates – are at risk of being lost in the 21st century, he says, placing species that are endemic or uniquely adapted to these climates at a heightened risk of extinction.
But how do you prepare for environmental scenarios in a future climate very different from the present? Again, the past comes into play, using the geological record as a testing ground.
By asking climate models to predict past species distributions during past periods of no-analog climates, Williams explains, researchers can assess a model’s robustness and predictability.
One of Williams’ colleagues, Zhengyu Liu, has undertaken a major effort to improve the predictive power of climate models.
Liu, a past director of CCR and a professor of atmospheric and oceanic sciences and environmental studies, is leading a team of scientists producing a state-of-the-art continuous simulation of the past 21,000 years of global climate change. Eventually, the simulation will run through the present and extend 2,000 years into the future.
This National Science Foundation-funded project explores a new paradigm of model-data comparison, coupling the detailed results of the continuous simulation with physical evidence of past climate conditions, such as from fossils and the Greenland and Antarctic ice cores. Matches between the simulated past climate and actual past data help to validate and refine the model and improve its credibility in predicting future climates.
“If the model can reproduce the past with sufficient and credible value, then the future prediction might be true,” Liu explains. “When you have the best model and the best data to verify a model, then you sync it and use the model to make predictions.”
Interdisciplinary collaborations like this are commonplace at the Center for Climatic Research.
“What’s great about CCR is that everyone is working on an important different piece; it’s a nexus,” says Williams, explaining that each member of the diverse research team brings a different expertise.
Hotchkiss agrees. “My lab does some of the simplest kinds of reconstructing of climate ourselves, but it’s critical for me to interact with climate modelers and the people who really understand the physics of climate to make sure I’m not diving into a complicated ecosystem response based on a spurious notion,” she says. “CCR is one of the only places in the world where you can do that, and they’ve been doing it for long enough that it’s part of the culture.”