Reasonable emissions of greenhouse gases: efficient abatement for a stock pollutant.

• Author: Chang, Howard F.
• Position: Response to article by David Hunter and James Salzman in this issue, p. 1741

In their contribution to this Symposium, David Hunter and James Salzman note that two critical hurdles for a plaintiff in climate change litigation are the questions of what duty the defendant owes the plaintiff and whether the defendant has breached this duty. (1) I think that Hunter and Salzman are right to focus on the question of what emissions would be reasonable to expect from the defendant, as this element of the plaintiffs case raises some of the most difficult issues for a court to resolve. Unreasonable conduct is an explicit element of a negligence claim. (2) Similarly, to succeed with a nuisance claim, the plaintiff must show "unreasonable interference" with either private use and enjoyment of the plaintiffs land or a right common to the general public. (3) As Hunter and Salzman note, the primary approach to determining whether the defendant's conduct is reasonable is to weigh the costs and benefits of the alternative courses of action, as required by the Learned Hand formula applied to the precautions taken by a defendant in a negligence case. (4) Furthermore, the determination of the reasonable level of emissions is necessary not only to rule on the question of liability but also to determine the appropriate remedy in the event the defendant is held liable. Whether the court awards damages for the harm caused by unreasonable emissions or grants injunctive relief, the court must first determine what emissions are reasonable.

This Commentary will discuss some of the complex problems with which a court would have to grapple in order to determine whether a defendant's emissions of greenhouse gases (GHGs) are unreasonable. For example, Hunter and Salzman state that trends in technology that lower the costs of preventing climate change are making findings of liability more likely over time. (5) This issue is more complicated than it might appear at first blush, however, because the implications of this progress in technology for the liability of any given defendant are ambiguous.

Consider the effect on a defendant of an unexpected innovation that now makes it less costly to abate carbon dioxide emissions. If this innovation lowers the marginal costs of abatement for the defendant in question, then this fact alone makes it more likely for us to deem the defendant's failure to increase abatement to be unreasonable than would be the case in the absence of that innovation. This result follows because the level of abatement that is optimal from the standpoint of economic efficiency would be higher, where the marginal cost of abatement equals the marginal social benefit. (6) With a lower marginal cost at the prior level of abatement, a polluter must now abate more to drive its marginal cost of abatement up until this marginal cost equals the marginal benefit.

Suppose, however, that this innovation reduces the marginal costs of abatement by others by much more than it reduces the defendant's own costs. Then it may be reasonable to expect others to abate instead. (7) Abatement by others would reduce total expected emissions, which in turn would reduce the harm expected to flow from the defendant's marginal emissions, as long as the marginal environmental cost of GHG...