Reply to Caldeira and Myhrvold: Radiative forcing is a useful, accepted metric to compare climate influence of alternative energy choices

Caldeira and Myhrvold (1) argue that the temperature change metric proposed in their recent paper (2) would be more useful to policymakers than the cumulative radiative forcing metric used by Alvarez et al. (3). We believe both metrics are useful, although the simplicity, transparency, and relatively low uncertainty of the cumulative radiative forcing metric makes it particularly useful in policy formulation.

Emissions of greenhouse gases lead to an increase in their atmospheric concentrations and radiative forcing, which in turn perturbs many aspects of climate including global atmospheric and oceanic temperatures, precipitation, and sea level. The changing climate affects a diverse array of human and natural systems, but the diversity of impacts of concern and associated metrics make it difficult to find a single impact metric, e.g., temperature, that is universally relevant in policy considerations. The Intergovernmental Panel on Climate Change (IPCC)’s Fourth Assessment Report discusses this challenge, concluding that global warming potentials (GWPs, a metric based on time-integrated radiative forcing) “remain the recommended metric to compare future climate impacts of emissions of long-lived climate gases” (ref. 4, p 211).

Myhrvold and Caldeira’s (2) paper proposed a useful but complex metric—changes in global mean temperature. To convert radiative forcing to global temperature changes, the authors must assume time constants, sensitivities, and feedbacks in the climate system, all of which introduce uncertainty to the results. For example, the IPCC reports a range spanning more than a factor of 2 (2.0–4.5 °C) in the likely value of the climate sensitivity, i.e., the global temperature response to a doubling of CO₂ concentrations (5). On a practical level, each added element of complexity makes the implications of a metric more uncertain and more difficult to interpret.

The cumulative radiative forcing approach in Alvarez et al. (3) extended the GWP metric used by the IPCC in a straightforward and transparent manner to enable consideration of the time-dependent climate influence of fuel-technology choices being made now, without prejudging which impacts on climate are important. This approach added an important additional dimension, time, to the considerations of policymakers when confronted with a problem like climate change that has both short-term and long-term implications. The approach in Alvarez et al. can also be used to identify the emissions rate below which a fuel-technology switch reduces cumulative radiative forcing. For example, if well-to-wheels methane emissions were kept below 1.6% of total natural gas produced (approximately half of the current amount estimated by the Environmental Protection Agency and the literature), then converting a fleet of gasoline cars to compressed natural gas would immediately reduce cumulative radiative forcing and continue to do so indefinitely.

In the end, both approaches (and likely others) are needed to fully judge the trade-offs that policymakers need to make. The technology warming potentials described by Alvarez et al. can inform the myriad of policy and corporate investment decisions being made every day, thus increasing the likelihood that those decisions put the global economy on a trajectory that yields climate benefits in both the short and the long term.