The Universal Cost of Carbon applies over a different timescale than how the social cost of carbon has been calculated. Assuming that the climate impacts from fossil carbon use will persist much longer than any economic infrastructure that we use fossil fuels today to build, the UCC can be seen as a first approximation of our culpability to future generations, expressed in terms that are relevant to us.
The cost of abating our ongoing CO2 emissions has been estimated to be in the range $10–100 per ton of C (Enkvist et al. 2009). Failing that, the cost of chemically removing a ton of C as CO2 from the atmosphere using industrial chemical engineering methods has been estimated to be $2200 (Socolow et al. 2011) or $360 (Keith et al. 2018). These estimates are much lower than the ultimate cost of carbon and would be a bargain if the UCC was directly fungible with present-day mitigation costs. However, it might not in the self-interest of any single generation to pay to clean it up, since no single generation would pay the entire ultimate climate costs.
The total cleanup costs would be enormous. A return to a “safe” atmospheric CO2 concentration of 350 ppm (Hansen et al. 2005) within a few decades would require removing about 440 Gton C (using the ISAM carbon cycle/integrated assessment model at http://climatemodels.uchicago.edu/isam/ (Cao and Jain 2005)).
At a cost of $360 per ton of C (Keith et al. 2018), the total cleanup debt today is about $160 trillion, about 1.6 years of present-day GWP. The cleanup debt is accumulating at a rate of about $3.6 trillion per year, about 3.6% of GWP.
Humanity could defend some fraction of the inundating land surface using dikes. Based on a contemporary Dutch cost of about $10/m3 of dam, assuming triple dike walls 60 m high and 120 m wide at the base, we calculate a cost of about $100,000 per meter of coastline, coming to $1014 ($100 trillion, 1-year GWP) for the entire million-kilometer coastline of the world. Dividing by the amount of carbon emitted results in a cost of about $5/ton of CO2 emitted. This effort would ameliorate land loss by sea level rise, but it would obviously not change the impacts of a warmer climate. A downside is that this strategy would commit future generations to ongoing maintenance of the dikes. The almost absurd global scale of the proposal, and its relatively inexpensive price tag, is another demonstration of the massive scale of the ultimate damage from carbon energy.
This paper presents a formulation for assessing deep-time climate change in units of present-day dollars per ton of carbon. The scope of the definition for such a cost is limited by what is tractable, and the result is only valid under particular, idealized circumstances including zero growth or technology change, and human population in steady state with Earth’s carrying capacity. The UCC is unrealistic as a forecast for the future, in and of itself, but it is easy to visualize and provides a scale bar for ultimate climate damage in present-day terms.
The UCC in our formulation ranges from $10k to $750k per ton of C, with a central value about $100k per ton. The 1σ uncertainty due to geophysical parameters is about $34k, and due to geophysical plus economic, about $116k, skewing to higher values in all cases. Among geophysical parameters to the model, the climate sensitivity contributes the most to the uncertainty in the result.
The UCC is not directly comparable with the social cost of carbon, or costs of mitigation, because of fundamental differences in their assumptions and formulations, and the fact that they apply over different time scales. Assuming that the climate impacts from fossil carbon use will persist much longer than any economic infrastructure that we use fossil fuels today to build, the UCC can be seen as a first approximation of our culpability to future generations, expressed in terms that are relevant to us.