Principal Investigator

CMI Director

At a Glance

Pacala’s lab focused on three areas in 2019: 1) They continued their work on the terrestrial biosphere and carbon cycle, and on the role of land use change in carbon mitigation. They completed a series of studies that improve the effects of drought on the carbon cycle in climate models, particularly in the tropics. Also, Pacala spent a large amount of his time completing the National Academy of Sciences report on negative emissions technologies. He chaired the effort and co-wrote the chapters on negative emissions from land use change. 2) They continued their work on the possibility that tropical forests may be spontaneously switching to vine-dominance, which would cause them to lose 95% of their carbon to the atmosphere. This is a possible new tipping point. They expect to have an answer by the 2020 CMI meeting. 3) They started a large project on net-zero-emitting infrastructure for the U.S., which is described below. Although organized initially by the CMI, it is now a collaborative effort involving the CMI, the Andlinger Center, the Princeton Environmental Institute, the Woodrow Wilson School and Princeton University’s central administration, as well as collaborators from the Environmental Defense Fund, The Nature Conservancy, the Natural Resources Defense Council, Exxon, and bp.


Research Highlight

To meet the 2 ̊C target of the Paris Agreement, global greenhouse gas emissions would have to decline from their current value of over 50 billion metric tons of carbon dioxide (CO2) equivalent per year (Global Carbon Project, 2018), to net-zero sometime between the middle and end of the current century. (IPCC 1.5 ̊ Special Report, 2018). Emissions in developed countries would have to drop even faster, and reach net-zero approximately at mid-century (IPCC, 2018). Would it be possible for the U.S. to build an energy system with zero net emissions in one billion seconds (just over 31 years)? Is the technology even available at a cost that the economy could bear?

The answers to these questions have changed dramatically over the past 10 years because of unprecedented breakthroughs in energy technology. The cost of solar electricity fell during the last decade by 76%, wind electricity by 69%, and lithium ion battery packs by 85% (Figure 1.1). Over the same decade, hydrofracking and horizontal drilling made natural gas inexpensive and abundant in the U.S., while carbon capture and storage technology both matured and came down in price. As a result, it would now be possible for the U.S. to build a non-emitting energy system at a cost only marginally higher than consumers pay for energy today. The system would be largely electric, with electricity from a roughly 50:50 mix of renewables and natural gas with carbon capture and storage (plus existing sources such as hydro and perhaps nuclear electricity), and with electric light- and medium-duty transport. Such a system would probably use more gas than is consumed today.

The U.S. is better positioned for this energy system than any other nation, because of its abundant wind and solar resources, abundant gas, well-characterized reservoirs for storage, and land for the negative emissions required to offset difficult-to-mitigate sources. The new CMI infrastructure project emerged to determine what it would take to build such a system, as well as alternatives. (“Infrastructure” is broadly defined here to include all plant, equipment, and services associated with energy resource extraction, conversion, transmission and distribution, and utilization.) The Pacala group proposes to describe qualitatively and quantitatively the engineering/industrial activities and financial flows required to decarbonize the U.S. economy, that is, to achieve net-zero greenhouse gas emissions, by mid-century.

Figure 1.1. Cost of renewable electricity and lithium ion batteries plummeted over the last 10 years. (Graph produced by J. Jenkins).

Deep decarbonization scenarios for the U.S. have been proposed by many others. In most cases, the objective is to minimize total cost, using a mix of top-down and bottom-up analysis. Capital and operating costs (and associated learning curves) are considered at varying levels of detail, and costs tend to be reported as aggregated and amortized values. But there is little attention to constraints related to rates of deployment. The lack of transparency limits the extent to which such exercises can inform actionable mitigation plans that identify spend-and-build schedules needed to achieve decarbonization targets.

In this project, bottom-up analysis will quantify the cost-and-build schedules for plausible mixes of investments across the energy system that deliver net-zero greenhouse gas emissions by mid-century. The group will emphasize expert engineering judgement. Analysis will be region-by-region, sector-by-sector, and major-project-by-major-project. Only technologies that have a reasonable basis for commercial-cost estimation today will be included, i.e., ones that arguably have already been demonstrated at industrial scales. The analysis will transparently report asset turnover rates and costs and time associated with pre-investment activities (feasibility studies, environmental impact assessments, community acceptance, and permitting). The analysis will include cost and schedule variances likely to be experienced for development of new natural resource capacity or construction of major projects, including balance-of- plant and supporting infrastructures.

Alternative decarbonization plans, each emphasizing a different technological pathway, will be articulated. For example, one plan might emphasize a balanced portfolio of low-carbon energy supply technologies and energy-use efficiency improvements. Another might emphasize aggressive energy efficiency improvements. Other plans might emphasize variable renewable electricity generation, nuclear energy, or fossil fuels with CO2 capture and storage.

A cost minimization will not be performed but a detailed and comprehensive energy accounting model will be used to ensure consistency across sectors and energy forms. Resource requirements over time – investment capital, workforce, major raw materials, manufactured equipment and bulk materials – will be quantified and contrasted with those for business-as-usual energy infrastructure development. For the near-term (2021-2025), annual capital commitments for each plan will be quantified. Capital commitments and resource requirements beyond 2025 will be estimated in five to 10-year tranches.



Anderson, C.M., R.S. Defries, R. Litterman, P.A. Matson, D.C. Nepstad, S.W. Pacala, W.H. Schlesinger, M.R. Shaw, P. Smith, C. Weber, and C.B. Field, 2019. Maximize natural climate solutions—and decarbonize the economy. Science, 363(6430): 933-934.

Martinez Cano, I., H.C. Muller-Landau, S.J. Wright, S.A. Bohlman, and S.W. Pacala, 2019. Tropical tree height and crown allometries for the Barro Colorado Natural Monument, Panama: a comparison of alternative hierarchical models incorporating interspecific variation in relation to life history traits. Biogeosciences, 16: 847-862.

Muller-Landau, H. C. and S.W. Pacala. What determines the abundance of lianas and vines? In A. Dobson, R. Holt, and D. Tilman, Eds., for the volume Unsolved Problems in Ecology, in press.

Weng, E., R. Dybzinski, C.E. Farrior, and S.W. Pacala, 2019. Competition alters predicted forest carbon cycle responses to nitrogen availability and elevated CO2: simulations using an explicitly competitive, game-theoretic vegetation demographic model. Biogeosciences Discussions,; in review Biogeosciences.

Zeppel, M., W.R.L. Anderegg, H. Adams, P. Hudson, A. Cook, R. Rumman, D. Eamus, D. Tissue, and S.W. Pacala, 2019. Embolism recovery strategies among species influenced by biogeographic origin and nocturnal stomatal conductance. Ecology and Evolution, in press.