Environment & Energy

[font face=Times, Serif][font size=5]Target Atmospheric CO[font size="1"]₂[/font]: Where Should Humanity Aim?[/font]

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[font size=2]Abstract: Paleoclimate data show that climate sensitivity is ~3°C for doubled CO[font size="1"]₂[/font], including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6°C for doubled CO[font size="1"]₂[/font] for the range of climate states between glacial conditions and ice-free Antarctica. Decreasing CO[font size="1"]₂[/font] was the main cause of a cooling trend that began 50 million years ago, the planet being nearly ice-free until CO[font size="1"]₂[/font] fell to 450 ± 100 ppm; barring prompt policy changes, that critical level will be passed, in the opposite direction, within decades. 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 CO[font size="1"]₂[/font] will need to be reduced from its current 385 ppm to at most 350 ppm, but likely less than that. The largest uncertainty in the target arises from possible changes of non-CO[font size="1"]₂[/font] forcings. An initial 350 ppm CO[font size="1"]₂[/font] target may be achievable by phasing out coal use except where CO[font size="1"]₂[/font] is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO[font size="1"]₂[/font] is not brief, there is a possibility of seeding irreversible catastrophic effects.[/font]

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[font size=4]5. SUMMARY[/font]

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Humanity’s task of moderating human-caused global climate change is urgent. Ocean and ice sheet inertias provide a buffer delaying full response by centuries, but there is a danger that human-made forcings could drive the climate system beyond tipping points such that change proceeds out of our control. The time available to reduce the human-made forcing is uncertain, because models of the global system and critical components such as ice sheets are inadequate. However, climate response time is surely less than the atmospheric lifetime of the human-caused perturbation of CO[font size="1"]₂[/font]. Thus remaining fossil fuel reserves should not be exploited without a plan for retrieval and disposal of resulting atmospheric CO[font size="1"]₂[/font].

Paleoclimate evidence and ongoing global changes imply that today’s CO[font size="1"]₂[/font], about 385 ppm, is already too high to maintain the climate to which humanity, wildlife, and the rest of the biosphere are adapted. Realization that we must reduce the current CO[font size="1"]₂[/font] amount has a bright side: effects that had begun to seem inevitable, including impacts of ocean acidification, loss of fresh water supplies, and shifting of climatic zones, may be averted by the necessity of finding an energy course beyond fossil fuels sooner than would otherwise have occurred.

We suggest an initial objective of reducing atmospheric CO[font size="1"]₂[/font] to 350 ppm, with the target to be adjusted as scientific understanding and empirical evidence of climate effects accumulate. Although a case already could be made that the eventual target probably needs to be lower, the 350 ppm target is sufficient to qualitatively change the discussion and drive fundamental changes in energy policy. Limited opportunities for reduction of non-CO[font size="1"]₂[/font] human-caused forcings are important to pursue but do not alter the initial 350 ppm CO[font size="1"]₂[/font] target. This target must be pursued on a timescale of decades, as paleoclimate and ongoing changes, and the ocean response time, suggest that it would be foolhardy to allow CO[font size="1"]₂[/font] to stay in the dangerous zone for centuries.

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