Calculating the Social Cost of Carbon: How Integrated Assessment Models Price CO2

Every time a federal agency proposes a new regulation—whether it mandates higher fuel efficiency for passenger trucks or stricter emissions limits for natural gas power plants—it must prove that the economic benefits of the rule outweigh its costs. To do this for climate regulations, policymakers rely on a single, highly influential metric: the Social Cost of Carbon (SCC).

This number represents the monetized damage caused by emitting one additional metric ton of carbon dioxide into the atmosphere. For the average consumer, this invisible figure acts as the regulatory fulcrum determining how much industries must spend to comply with environmental rules, costs that inevitably cascade down to utility bills and vehicle prices.[2][5]

Calculating the exact economic damage of a colorless gas over the next three centuries requires a specialized class of software known as Integrated Assessment Models (IAMs). These models, which include prominent frameworks like DICE, PAGE, and FUND, act as mathematical bridges between the physical sciences and global macroeconomics. They do not merely simulate how the atmosphere warms; they simulate how that warming interacts with human systems, translating physical changes into dollar-denominated impacts on agriculture, human health, and coastal infrastructure.[3][4][7]

The architecture of an IAM operates through a rigid, four-step chain of cause and effect. First, the model projects future economic growth and the resulting greenhouse gas emissions. Second, a climate module calculates how those emissions alter atmospheric concentrations and drive up global average temperatures. Third, a "damage function" estimates the economic harm caused by that specific temperature increase. Finally, an economic discount rate is applied to translate those future damages into a present-day value.[2][4]

The U.S. government's Interagency Working Group (IWG) relies on an average of these three primary models to set federal policy. In its 2021 interim technical support document, the IWG established a central SCC estimate of $51 per metric ton of CO2, assuming a 3% discount rate. This figure serves as the baseline for regulatory impact analyses across the executive branch, providing a standardized metric so that the Department of Energy and the Environmental Protection Agency evaluate the benefits of emissions reductions using identical math.[1]

The most heavily scrutinized component of any IAM is its damage function, and the evidence supporting these functions is highly variable. Models exhibit strong predictive confidence when calculating the costs of sea-level rise and the increased energy demand for space cooling, as these rely on well-understood physical thresholds and thermodynamic relationships. However, the evidence grows significantly weaker when attempting to quantify the impact of warming on global labor productivity or the economic drag of extreme weather events, where localized variables introduce massive uncertainty.[2][6]

Agricultural damage functions highlight this uncertainty. Early iterations of the FUND model projected that moderate warming combined with increased CO2 fertilization could actually yield net economic benefits for the agricultural sectors of certain high-latitude countries. Conversely, newer econometric studies integrated into updated models suggest that crop yield collapses at higher temperature thresholds will severely outweigh any fertilization benefits. The models must aggregate these conflicting, highly localized agricultural realities into a single global damage curve.[2][7]

Despite the complexity of the climate and damage modules, the final Social Cost of Carbon is overwhelmingly dictated by a single economic variable: the discount rate. Because carbon dioxide remains in the atmosphere for centuries, the majority of the economic damages caused by today's emissions will be suffered by future generations. The discount rate determines how much society is willing to pay today to prevent a dollar of damage in the year 2100.[5][8]

The mathematical sensitivity of the SCC to the discount rate is extreme. According to the IWG's 2021 data, applying a 3% discount rate yields an SCC of $51 per ton. If that rate is lowered to 2.5%—placing a higher present value on future damages—the SCC jumps to $76 per ton. If the rate is raised to 5%, the SCC collapses to just $14 per ton. This means that a 2.5 percentage point shift in an economic assumption alters the regulatory justification for climate action by more than 400%.[8]

From a systems analysis perspective, this reliance on the discount rate transforms the SCC from a pure scientific measurement into a reflection of ethical and economic policy. A high discount rate assumes that future generations will be significantly wealthier and technologically advanced, making them better equipped to absorb climate damages. A low discount rate assumes that climate impacts could permanently degrade the baseline of economic growth, justifying aggressive, immediate capital expenditure to mitigate emissions.[5][6]

Another area of weak evidence within standard IAMs is the treatment of climate tipping points—irreversible, large-scale systemic shifts such as the collapse of the Atlantic Meridional Overturning Circulation or the rapid thawing of permafrost. Historically, models like DICE utilized smooth, continuous damage functions that struggled to account for sudden, catastrophic economic shocks. While recent updates attempt to incorporate stochastic risks, quantifying the exact probability and economic cost of a systemic planetary threshold remains highly speculative.[4][7]

IAMs must also make sweeping assumptions about human adaptation. The models generally assume that economies will dynamically adjust to changing conditions—for instance, by shifting agricultural zones or building seawalls—which mitigates the raw physical damage. However, the capital costs of these adaptations are difficult to project over a 300-year horizon. If a model overestimates the speed and efficiency of human adaptation, it will artificially depress the Social Cost of Carbon.[2][6]

Despite their inherent uncertainties, IAMs remain the only viable mechanism for federal agencies to satisfy the legal requirement for cost-benefit analysis. The courts have consistently ruled that while the models are imperfect, assigning a value of zero to carbon emissions is mathematically and scientifically indefensible. Consequently, the SCC functions as a necessary regulatory anchor, providing a structured, transparent, and legally defensible framework for evaluating the trade-offs of national energy policy.[1][5]