2009: Defining dangerous anthropogenic interference — Michael Mann - PNAS

Few would dispute that the nations of the world should collectively do what must be done to avert elevating atmospheric greenhouse gas concentrations to the point of “dangerous anthropogenic interference” (DAI) with our climate. However, the devil, as they say, is in the details, details most recently explored by Smith et al. (1) in this issue of PNAS.
...To properly define DAI, one must take into account issues that are not only scientific, but, as I have argued elsewhere (2), economic, political, and even ethical in nature. Defining DAI begs the question, for example, “Dangerous to whom?” It amounts to the tacit adoption of some level of risk, risk that will not be shared equally among all nations and people.
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If one considers the collective impact of anthropogenic greenhouse gases alone, we have already reached 450 ppm CO2eq. It is only when the cooling due to anthropogenic aerosol production (e.g., sulfate) is taken into account (equivalent to ≈ −80 ppm CO2eq) that we appear to be safely below the 450-ppm number, at an effective ≈375 ppm CO2eq [this number is uncertain, due to the substantial uncertainty in estimates of the net impact of anthropogenic aerosols (13)]. If we were to suddenly halt the various dirty industrial and agricultural processes responsible for anthropogenic sulfate, nitrate, and other aerosols, we would suddenly find ourselves with 450 ppm CO2eq on our hands. Although cleaning our atmosphere is indisputably a desirable goal, doing so would ironically make the goal of 450 ppm CO2eq even more challenging, motivating the view that we must arguably strive to stabilize actual greenhouse gas concentrations <450 ppm CO2eq (16).