An open access publication of the American Academy of Arts & Sciences
Winter 2013

A Trillion Tons

Hal Harvey, Franklin M. Orr, Jr., and Clara Vondrich

There is a consensus among scientists that stark dangers await in a world where the global mean temperature rises by more than about 2 degrees Celsius. That threshold corresponds to a collective human carbon emissions “budget” of around a trillion tons, of which half has been spent. This paper uses a new simulation model to look at strategies to stay within that budget, specifically assessing the impact of improvements in energy efficiency, aggressive deployment of renewables, and energy technology innovation. The simulations examine the timing of investments, turnover of capital stock, and the effect of learning on costs, among other factors. The results indicate that efficiency, renewables, and technology innovation are all required to keep humanity within the trillion-ton budget. Even so, these measures are not by themselves sufficient: changes in land use and a price on carbon emissions are also needed.

HAL HARVEY is CEO of Energy Innovation: Policy and Technology LLC and Senior Fellow for Energy and the Environment at the Paulson Institute at the University of Chicago. He is the founder of the ClimateWorks Foundation, where he previously served as CEO. He is former Environment Program Director at The William and Flora Hewlett Foundation and founder and past President of the Energy Foundation. An energy engineer, he has worked on energy policy in a dozen countries.

FRANKLIN M. ORR, JR., is the Keleen and Carlton Beal Professor in the Department of Energy Resources Engineering and Director of the Precourt Institute for Energy at Stanford University. He is also a Senior Fellow at the Woods Institute for the Environment. His research focuses on understanding the physical mechanisms that control displacement performance in gas injection processes for oil recovery and storage of greenhouse gases. His work has appeared in the Journal of Fluid MechanicsEnergy Conversion and Management, and the International Journal of Greenhouse Gas Control, among other publications.

CLARA VONDRICH is Director of Leadership Initiatives at the ClimateWorks Foundation. She is an attorney with a background in international arbitration and energy policy. Previously, she served as Counsel to the President's Commission on the BP Deepwater Horizon Oil Spill, focusing on ecological restoration and environmental justice. She also helped prepare amicus briefs in support of state plaintiffs in the landmark U.S. Supreme Court case American Electric Power v. Connecticut, which sought to hold major electric utilities liable for their contribution to climate change.

How much carbon can humans safely emit into the atmosphere? Climate scientists argue that a 2 degree Celsius (about 4 degree Fahrenheit) increase in global mean temperature is a threshold above which the probability of highly adverse consequences grows significantly. Such an increase would correspond to roughly a trillion tons of total human-caused carbon emissions over time.1 If one trillion tons is humanity’s carbon budget, how much have we used so far? How fast will we emit the remainder under current trends? And what can we do to make sure that we don’t bust the budget?

To consider the relative contributions of different variables, ClimateWorks, a foundation that supports public policies that mitigate climate change, and its partners at Climate Interactive developed the system-dynamics computer model En-ROADS (Energy–Rapid Overview and Decision-Support simulator). En-ROADS is a global model that assesses how changes in energy supply and demand might affect emissions and, in turn, climate outcomes.2 It is designed to rapidly assess the impact of various policy scenarios on cumulative emissions by manipulating variables as diverse as global GDP, energy efficiency, innovation, carbon price, and fuel mix.

.  .  .


  • 1Myles Allen et al., “Warming Caused by Cumulative Carbon Emissions Towards the Trillionth Tonne,” Nature 458 (April 30, 2009): 1163–1166. Other carbon budgets have been proposed, depending on the “acceptable risk” stipulated for crossing the 2 degree Celsius threshold. For instance, Malte Meinshausen and colleagues propose a carbon budget of roughly 830 billion tons to assure a 50 percent chance of staying under 2 degrees Celsius. See Meinshausen et al., “Greenhouse-Gas Emission Targets for Limiting Global Warming to 2° C,” Nature 458 (April 30, 2009): 1158–1162. Despite these differences, the “need in principle for a cumulative limit near or below one trillion tonnes is generally accepted”; see Myles Allen et al., “The Case for Mandatory Sequestration,” Nature Geoscience 2 (2009): 813–814.
  • 2En-roads was created by Climate Interactive, Ventana Systems, MIT Sloan School of Management, and the ClimateWorks Foundation global research team. It is designed to complement, not supplant, other more disaggregated models addressing similar questions, such as those in the Energy Modeling Forum’s EMF-22 suite. En-ROADS relies on other models and Energy Information Administration (EIA) projections for testing and data. It is based on the Ph.D. dissertations of John Sterman, Professor in the MIT Sloan School of Management, and Tom Fiddaman of Ventana Systems. We wish to distinguish En-ROADS from the MIT Emissions Prediction and Policy Analysis model (EPPA), another simulation related to climate and energy. EPPA was developed as part of MIT’s Joint Program on the Science and Policy of Global Change. En-ROADS is a system-dynamics (high-order nonlinear differential equation) simulation; a more detailed description of En-ROADS methodology and assumptions is available at
To read this essay or subscribe to Dædalus, visit the Dædalus access page
Access now