Target atmospheric CO2: Where should humanity aim?

TL;DR

This paper discusses the historical and scientific basis for setting atmospheric CO2 targets, emphasizing the need to reduce CO2 levels to 350 ppm to avoid catastrophic climate impacts and preserve a planet similar to the one on which civilization developed.

Contribution

It provides a synthesis of paleoclimate data and current climate science to recommend a specific CO2 reduction target of 350 ppm for climate stability.

Findings

Climate sensitivity is ~3°C for doubled CO2 with fast feedbacks.

Equilibrium sensitivity is ~6°C including slower feedbacks.

CO2 levels exceeded 450 ppm during past glaciation cycles.

Abstract

Paleoclimate data show that climate sensitivity is ~3 deg-C for doubled CO2, including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6 deg-C for doubled CO2 for the range of climate states between glacial conditions and ice-free Antarctica. Decreasing CO2 was the main cause of a cooling trend that began 50 million years ago, large scale glaciation occurring when CO2 fell to 450 +/- 100 ppm, a level that will be exceeded within decades, barring prompt policy changes. If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm. The largest uncertainty in the target arises from possible changes of non-CO2 forcings. An initial 350 ppm CO2 target may be achievable by phasing out coal use except where CO2 is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects.