Are Historical Emissions Harmful? Part III

Here is a third reason why it is unclear in what sense historical emissions are harmful. This third reason has less to do with how much costs past emissions impose on future generations but rather in what way they impose such costs.

Imagine first that the amount of emissions of the present generation is completely independent of how much past generations emitted. In this case the harm of historical emissions consists in the harm they do to the environment and the burdens for humanity that follow.

Imagine second that the amount of emissions of the present generation is completely dependent on how much past generations emitted. Imagine, for example, that the present generation imposes on itself the constraint not to let CO2 concentrations exceed 550 ppm and does whatever is necessary to just reach that goal (i.e. if the past emitted much, the present emits little and if the past emitted little, the present emits much). In that case the costs of past emissions do not consist in damage to the environment (since the present generation completely counterbalances excessive emissions of the past with its own behavior) but the costs of past emissions rather consist in the burdens to the present generation of having to limit its own emissions. The "harmfulness" of past emissions consists in making the present generation forego benefits (such as flights into vacation or the money saved by building a cheaper but dirtier power plant) in order to keep concentrations below 550 ppm.

Reality will be somewhere between the first and the second scenario, i.e. the present generation's emission level will be somewhat dependent on the level of past emission levels. And therefore the costs imposed by historical emissions will also be both: Damage to the environment and sacrifices of the present generation in order to limit its own emissions in response to past excessive emissions.