Dr. Anthony Walker

Dr. Anthony P. Walker, Senior Scientist and Leader of the Ecosystem Processes Group; Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee

Research Interests: The impact of increased atmospheric carbon dioxide (CO₂) on terrestrial ecosystems, from photosynthesis through longer term ecosystem scales, and how these ecosystem responses interact with the Earth System as a whole. Bringing together data, quantitative computer models, and scientific experts from diverse fields to provide robust understanding of how complex ecosystems cycle carbon, water, and nutrients.

Research Highlights: During the last 650,000 years, the Earth experienced seven cycles of glacial advance and retreat, which is to say seven periods of cooling and warming. Then, about 11,700 years ago, at the end of the last ice age, the Earth entered an interglacial period, and the modern climate era began. Most of these climatic fluctuations have been attributed to small variations in Earth’s orbit that changed the amount of solar energy reaching our planet.

This is not what’s happening in the current era of climate change. According to information from NASA’s Global Climate Change website, Earth’s current warming trend is very likely to be the result of humanity’s industrial activity, which has increased levels of atmospheric carbon dioxide (CO₂), a heat-trapping greenhouse gas, 47 percent (from 280 parts per million to 414 parts per million) during the past 150 years.

This increase in atmospheric CO₂ has exacerbated the "greenhouse effect," the planetary warming that results when the atmosphere traps the heat radiating from Earth into space. In turn, this enhanced greenhouse effect has been associated with a range of climatological impacts observed in the United States, such as longer growing seasons, more severe storms with increased precipitation, loss of sea ice, and more frequent and intense drought and heat waves. The effects of climate change are expected to continue throughout this century and beyond. However, the severity of its effects over the next few decades will largely depend on the amount of global greenhouse gasses emissions and how Earth’s natural systems respond.

The study of that response is the focus of Walker and his colleagues in the Oak Ridge National Laboratory’s Ecosystem Processes Group. The Group’s work aims to develop a more thorough understanding of the spatial and temporal dynamics of Earth’s vital and changing ecosystems through ecosystem-scale manipulation experiments, observations, and integrated modeling. This ambitious approach has resulted in some of the most detailed studies of ecosystem responses to increasing atmospheric CO₂, increasing temperature, shifting rainfall patterns, and other environmental changes to date.

Walker’s most recent work has investigated the use of terrestrial ecosystem models integrated with datasets across spatial scales — from leaves, to ecosystems, to entire landscapes — to gain a systems-level understanding of ecosystem response to global environmental change.

The datasets Walker and his colleagues use come from a variety of sources, including NASA’s Oak Ridge National Laboratory Distributed Active Archive Center (ORNL DAAC), which was established in 1993 by an interagency agreement between NASA and the Department of Energy (DOE). ORNL DAAC manages, archives, and distributes data, tools, and resources in NASA’s Earth Observing System Data and Information System (EOSDIS) collection pertaining to terrestrial biogeochemistry, ecology, and environmental processes — information critical to understanding change within Earth's biological, geological, and chemical components.

Among the ORNL DAAC datasets used by Walker are Level-2 Daily Solar-Induced Fluorescence 1995-2003, satellite-derived chlorophyll estimates in terrestrial ecosystems; Harmonized Global Land Use for Years 1500 -2100 (Version 1), a single consistent, spatially gridded set of land-use change scenarios for studies of human impacts on the past, present, and future Earth system; and Daymet, daily gridded estimates of seven weather parameters on a 1 km x 1 km gridded surface over continental North America, Hawaii, and Puerto Rico.

In 2017 Walker published a study in the journal New Phytologist that evaluated four plant trait scaling hypotheses on their ability to predict global distributions of terrestrial photosynthesis rates and how they affected measurements of global gross primary production in the Sheffield Dynamic Global Vegetation Model (SDGVM), which is used to investigate the response of vegetation to environmental change on a global scale.

While conducting their research, Walker and his colleagues compared the SDGVM’s predictions of photosynthetic activity with the Solar Induced Fluorescence (SIF) data, which provides a measurement of the light emitted by chlorophyll molecules in leaves during photosynthesis. Developed by Joanna Joiner, an atmospheric physicist in the Laboratory for Atmospheric Chemistry and Dynamics at NASA’s Goddard Space Flight Center, SIF data is derived from the Global Ozone Monitoring Experiment (GOME) instrument on the European Space Agency's European Remote-Sensing 2 (ERS-2) satellite. It is generally regarded as a proxy for photosynthetic activity of plants in terrestrial ecosystems and used in assessments of global carbon fixation (i.e., plants’ use of CO₂) or gross primary production (GPP), which scientists use to monitor ecosystem health, and the carbon cycle on a global scale.

Although assessing photosynthetic activity in agriculture wasn't the focus of this study, the SIF data indicated higher levels of photosynthetic activity throughout Earth’s agricultural areas than that predicted by the model. This was a significant conclusion, as it suggests that photosynthesis in the planet’s agricultural areas may be higher than previously thought and suggested by GPP-predicting models like the SDGVM.

In another ongoing study, Walker and his colleagues at ORNL, the Pacific Northwest National Laboratory (PNNL), NASA’s Jet Propulsion Laboratory, the USDA Forest Service, National Center for Atmospheric Research, the Lawrence Berkeley National Laboratory (LBNL), and the University of Puerto Rico are using the latest Daymet data in Puerto Rico. Daymet’s high-resolution meteorological data are being used as inputs in tests of the Functionally Assembled Terrestrial Ecosystem Simulator (FATES) model, a numerical terrestrial ecosystem model that simulates and predicts growth, death, and regeneration of plants and subsequent tree-size distributions. ORNL DAAC Staff Scientist Michele Thornton lead development efforts for these data going back to 1950 specifically for the project, as that was the time when deforestation and land use in Puerto Rico was at its peak.

Walker also uses the ORNL DAAC’s Harmonized Global Land Use data set in global carbon cycle model simulations, which are used in a variety of ways, including as an input in the Global Carbon Project’s annual carbon budgets, which are developed by researchers around the world. Produced by the Harmonized Global Land team on an annual basis, these data provide a single consistent, spatially gridded set of land-use change scenarios for studies of human impacts on the past, present, and future Earth system, which makes them valuable for estimating and representing land use change in the Ecosystem Processes Group’s simulations and for calculating fluctuations in CO₂ absorption in the wake of land use change.

In another, more recent, study published in New Phytologist (2021), Walker and experts from diverse disciplines in the atmospheric and environmental sciences collaborated to evaluate the broad, multidisciplinary evidence that increasing levels of atmospheric CO₂ have led to corresponding increases in plant growth, vegetation biomass, and soil organic matter in terrestrial ecosystems since pre‐industrial times, thereby creating a carbon sink.

Confirming this response is important, as a substantial global terrestrial carbon sink would function as a feedback mechanism slowing the rate of CO₂ increase and therefore climate change. However, ecosystem responses to CO₂ are complex and often become even more so as concurrent changes in other agents of global change are considered, which can make the evidence for a CO₂-driven terrestrial carbon sink difficult to detect.

To determine whether terrestrial ecosystems are absorbing more atmospheric CO₂, Walker and his fellow researchers gathered and analyzed a range of data streams, including Moderate Resolution Imaging Spectroradiometer (MODIS)- derived measurements of vegetation cover and greenness, data from Free-Air CO₂ Enrichment (FACE) experiments, tree-ring study data, and forest-inventory analysis data, and so on, from ORNL DAAC and other sources. Following their analysis, the researchers reported a high degree of uncertainty in all the various data streams such that no single data stream was sufficient to provide a definitive answer to the existence of a global carbon sink on its own. However, they also noted that, when taken together, the results of the study do suggest that, historically, increased atmospheric CO₂ has played a role in land ecosystem carbon uptake since the industrial revolution and that this effect will likely decline into the future.

“We currently are not able to quantify this effect with any certainty,” said Walker. “I think further integrated studies with diverse contributors, data streams, and theory encoded in models of various complexity will help us to better quantify this global effect.”

Whenever those future studies take place, it’s likely that Walker and his colleagues in ORNL's Ecosystem Processes Group will rely on the terrestrial biogeochemistry, ecological, and environmental process data from NASA and ORNL DAAC as they work toward their findings.

Representative data products used or created: Available through ORNL DAAC:

• L2 Daily Solar-Induced Fluorescence (SIF) from ERS-2 GOME, 1995-2003, doi:10.3334/ORNLDAAC/1758
• Harmonized Global Land Use for Years 1500 -2100 (Version 1), doi:10.3334/ORNLDAAC/1248
• Daymet: Annual Climate Summaries on a 1-km Grid for North America (Version 4), doi:10.3334/ORNLDAAC/1852
• A Global Data Set of Leaf Photosynthetic Rates, Leaf N and P, and Specific Leaf Area, doi:10.3334/ORNLDAAC/1224
• BOREAS HYD-04 Areal Snow Course Survey Data, doi:10.3334/ORNLDAAC/267

Other data products used:

• Functionally Assembled Terrestrial Ecosystem Simulator (DOE)

Read about the Research: Walker, A.P., Kauwe, M.G.D., Bastos, A., Belmecheri, S., Georgiou, K., Keeling, R.F., et al. (2021). Integrating the evidence for a terrestrial carbon sink caused by increasing atmospheric CO2. New Phytologist, 229(5): 2413-2415. doi:10.1111/nph.16866

Walker, A.P., Johnson, A.L., Rogers, A., Anderson, J., Bridges, R.A., Fisher, R.A., et al. (2021). Multi-hypothesis comparison of Farquhar and Collatz photosynthesis models reveals the unexpected influence of empirical assumptions at leaf and global scales. Global Change Biology, 27(4): 804–822. doi:10.1111/gcb.15366

Walker, A.P., Quaife, T., van Bodegom, P.M., De Kauwe, M.G., Keenan, T.F., Joiner, J., et al. (2017). The impact of alternative trait-scaling hypotheses for the maximum photosynthetic carboxylation rate (Vcmax) on global gross primary production. New Phytologist, 215(4): 1370–1386. doi:10.1111/nph.14623

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