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Scientists at the Department of Energy’s 91���� used their knowledge of complex ecosystem processes, energy systems, human dynamics, computational science and Earth-scale modeling to inform the nation’s latest National Climate Assessment, which draws attention to vulnerabilities and resilience opportunities in every region of the country.

The fifth assessment, or NCA5, was with an emphasis on assessing risk for citizens while giving decision-makers a framework to help value and protect resources. The assessment is overseen by the U.S. Global Change Research Program, which synthesizes findings from thousands of studies to advance understanding of the evolving Earth system and to identify climate resilience strategies. The report has been issued about twice a decade since 2000, as mandated by federal law. NCA5 was developed by about 500 authors and 250 other contributors across 14 federal agencies.

For the first time ever, NCA5 includes a chapter highlighting economic impacts and opportunities associated with climate action. The assessment also includes a new chapter on social systems and justice, which provides insights into how people understand, experience and respond in different ways to climate change. This chapter, as well as NCA5’s 10 region-specific chapters, describes how some communities experience disproportionate impacts from climate change, according to a White House statement. The report provides a new web-based portal, the , which lets individuals explore climate projections in their own state or county.

The report presents a new risk-based framing that identifies those things people value that are most at risk due to climate change, said ORNL’s Peter Thornton, lead author of NCA5’s and technical contributor to the .

“Anyone reading the report will be able to find something that is directly relevant to their experience,” said Thornton, who leads the Earth Systems Science section and is director of the Climate Change Science Institute at ORNL. “In the land chapter when we talk about goods and services, we’re referring to things like having a solid foundation for your house, or building a street and expecting that if it’s engineered well it will still be there in 10 years, or land on which you can have woods or a garden growing. People have always assumed that they can build their house and it will be fine,” he said. “But with climate change that is not necessarily true in all cases.”

Land has always tended to be resilient, Thornton said, with regrowth after wildfires or windstorms. “But what if the harvest is too intense or frequent, or the fire is too big, too hot or too often? If there’s too much flooding you can lose your topsoil or your seed bank or have landslides. You can lose what allows the trees to regrow. We are pushing the limits of that every year. We need to be able to quantify repeated disturbances and recognize that the land’s resilience may not always be there. We may reach a tipping point into a much less desirable system.”

Mitigation options dependent on values, choices

The land chapter also addresses value and choices moving into the future. As climate change advances, we may find those choices narrowing, Thornton said. Sea level rise could require massive sea wall investments for people to continue using coastal property as desired. Farmers may lose choices about what to plant as climate zones and rainfall patterns shift.

There is hope built into the report, Thornton said, as scientists outline mitigation options that can slow down environmental change. “The approach in our chapter was to strike a balance and to help people recognize what they have and to reflect on that value and not take our resources for granted.”

Thornton’s research informed the topic of how land ecosystems provide value by absorbing carbon from the atmosphere. “It’s a well-studied topic and maybe one of the better quantified benefits that ecosystems provide to people,” he said. “But it’s invisible. It can blend into the background, but it has real value as trees grow every year and lock away the carbon we emit through fossil fuel combustion. That system is not one we can take for granted.”

For ORNL’s John Field, a contributing author to the land chapter, one key takeaway was just how large of a land footprint will be needed for efforts to decarbonize the economy and mitigate climate change. “Decarbonization will require a highly visible build-out of wind, solar and electricity transmission, but the biggest land use changes may be to reforestation or biomass production,” Field said.

Before participating in the NCA5, “I had not fully appreciated how much wind and solar power expansion will be required to transform our energy system, and how widely that will likely be dispersed across the country. Our chapter also highlighted some of the challenges in understanding likely agricultural land-use futures. Most decarbonization and mitigation scenarios assume continued productivity increases and a contracting total U.S. cropland area, though other lines of evidence call these assumptions into question,” said Field, a scientist in ORNL’s Bioresource Science and Engineering group. Field’s research focuses on how to sustainably scale up bioenergy systems and other land-based mitigation measures.

The events described in the land chapter are directly threatening the nation’s energy systems, said Rebecca Efroymson, a contributing author for the NCA5’s and a Distinguished Scientist in the Biodiversity and Ecosystem Health group at ORNL. “Energy systems are threatened by extreme temperatures and precipitation, sea level rise, more intense storms, droughts and wildfires,” she said. “Impacts vary among regions and will vary over time.” The chapter also outlines technologies, policies and markets that can alter susceptibility to climate change.

Accelerated climate change challenges energy resilience

“We are developing solutions at the same time we continue to identify the problems,” Efroymson said. “Fuels like oil and natural gas have a large presence on coastlines, and biofuels that are dependent on crop productivity can be affected by changes in climate and extreme events. Power systems can be similarly affected, including wind energy sources that are growing offshore, hydropower that is susceptible to drought, and power transmission and distribution lines that are vulnerable to wildfire.”

“We are developing energy resilience solutions, but the scope of the climate problem is expanding. It’s a moving target. Planning for future conditions is key,” Efroymson said.

The energy chapter, for the first time in the NCA’s history, calls attention to energy and environmental injustices and to the overburdened populations that bear them, including energy access, affordability and the siting of sometimes-polluting energy facilities. ORNL is contributing to research on energy justice, recognizing increasing interconnectivity between energy and health, water, food and transportation.

ORNL’s Forrest Hoffman is a contributing author to the NCA5’s , which incorporates recent advances in computational science, observational data, and Earth-scale modeling and simulation to understand and quantify the drivers of climate change, how the Earth system responds to the drivers, and changes in Earth system processes that underpin the many facets of climate change and extreme events. Our understanding of these changes is supported by observations, modeling and climate change attribution.

“ORNL's history of work on terrestrial carbon cycle processes has long contributed to our understanding of Earth system processes and provided constraints on land carbon sinks,” said Hoffman, Distinguished Scientist and leader of the Computational Earth Sciences group at ORNL. “This understanding and related research on how those processes may change as climate continues to change contributed directly to the report.”

Model benchmarking informs findings across NCA5

The science underpinning the chapter and other sections of NCA5 is supported by DOE’s extensive computational resources, including the exascale capabilities at the , a DOE Office of Science user facility at ORNL that houses the Frontier system, the world’s first exascale supercomputer and also its fastest.

In the latest assessment, “One of the most interesting findings is that we now have a reliable statistical framework for assessing the degree to which extreme events are more likely or more intense because of climate change,” Hoffman said. “In my section of the chapter, we reiterated the fact that we cannot fully close the global carbon cycle due to uncertainties in processes and emissions.”

Hoffman said ORNL’s work on the International Land Model Benchmarking System, or ILAMB, which broadly compares international climate models with observational datasets, provided the basis for many of the findings across the NCA5 report. ILAMB was created with support from the U.S. Department of Energy Office of Science’s Biological and Environmental Research program to help evaluate the fidelity of increasingly complex land carbon cycle models.

ILAMB “continues to contribute to constraining model results and informing additional model development. These models then do a better job of providing useful information about the effects of climate change,” Hoffman said.

ORNL’s wide-ranging ecosystem research projects also provide observational data that are intrinsic to confirming Earth system models, including the ecosystem manipulation experiments , or SPRUCE, the Oak Ridge Free-Air Carbon Dioxide Enrichment, or FACE, as well as the , or NGEE Arctic, project.

ORNL’s Christa Brelsford used her expertise in human dynamics as a contributing author to the NCA5’s . The most critical message in the chapter is the extent to which interactions across multiple systems and sectors — climatic, environmental, engineered and social — interact to produce risks, hazards and opportunities for productive climate action, Brelsford said.

The authors took a “complex systems approach to understanding cities, climate change, and urban characteristics, with people who are experts in governance, community-engaged processes and actionable problem solving,” said Brelsford, research scientist in ORNL’s Critical Infrastructure Resilience group. “We brought these fields together in a way that is intellectually coherent, consistent, and demonstrates where the state of the art is in these fields — individually and collectively — and how they can be combined to produce insights that are useful for understanding and addressing the climate crisis.”

Human, natural system interactions create risks, opportunities

Brelsford said that “sometimes the magnitude of the climate crisis feels insurmountable. “What can one person, or even one national lab, do to avert the worst consequences of climate change? Our chapter shows that concrete, actionable opportunities to avoid the biggest human consequences are easier to find when we work collaboratively across researchers, stakeholders and frontline communities. Interconnections between human and natural systems create both unexpected risks and opportunities for positive solutions.”

This complexity can be burdensome to frontline communities, Brelsford added. “The interactions of multiple forms of vulnerability — from structural and economic conditions to innate characteristics such as age, race and disability status — with the multiple systems, geographic scales, and both climate and non-climate risks and hazards shape people’s ability to respond to climate change,” she said. 

Brelsford praised her NCA5 colleagues for a very positive experience that both supported knowledge development and ensured that “authors from diverse intellectual backgrounds without prior collaborative relationships worked together productively, efficiently and equitably.”  

For ORNL’s David McCollum, who was a review editor for the complex systems chapter, the NCA5 “points clearly to the importance of co-producing knowledge with those individuals and institutions on the consumer side of scientific research. The very inclusion of the chapter in the report was new in and of its own right. Previous national climate assessments have shied away from addressing multi-sector issues, particularly from a social systems perspective. Our chapter was quite seminal in that sense.”

A key takeaway from the latest assessment, McCollum said, “is that the world is in a state of continual change, and there exists great uncertainty on where we are ultimately heading. We scientists, therefore, need to bring multiple forms of evidence to bear on our explorations of the future — quantitative and qualitative, modeling and non-modeling.”

Bringing big team science to the task

ORNL’s culture of bringing together large, multi-disciplinary teams to solve big challenges was a good fit as the scientists worked with colleagues from across the nation on such a large undertaking, the researchers noted.

“Scientists across a number of ORNL’s diverse directorates possess deep expertise in climate change topics: physical climate science, impacts and adaptation and mitigation/decarbonization. This is reflected in ORNL contributors to NCA5, who hail from the biological and environmental systems sciences, energy science and technology fields, computational science, and national security science,” said McCollum, Senior Scientist in the lab’s Mobility and Energy Transitions Analysis group.

What remains unknown in the future is how global policies and individual choices may affect how the climate evolves and whether we can build resilience in the face of change, the scientists noted.

“When we look into the future, we know that Earth systems will change, and we’ve vastly improved our predictions around those systems to inform decision-making,” Thornton said. “But what we don’t know is how policies around the world will change. The biggest source of uncertainty is about what choices we, as a society, will make.”

UT-Battelle manages ORNL for DOE’s Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit . — Stephanie Seay