Target Atmospheric CO2: Where Should Humanity Aim?
James Hansen,1,2*
Makiko Sato,1,2
Pushker Kharecha,1,2
David Beerling,3
Valerie Masson-Delmotte,4
Mark Pagani,5
Maureen Raymo,6
Dana L.
Royer,7
James C.
Zachos8
Paleoclimate data show that climate...
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Target Atmospheric CO2: Where Should Humanity Aim?
James Hansen,1,2*
Makiko Sato,1,2
Pushker Kharecha,1,2
David Beerling,3
Valerie Masson-Delmotte,4
Mark Pagani,5
Maureen Raymo,6
Dana L.
Royer,7
James C.
Zachos8
Paleoclimate data show that climate sensitivity is ~3°C for doubled CO2, including only fast
feedback processes.
Equilibrium sensitivity, including slower surface albedo feedbacks, is
~6°C for doubled CO2 for the range of climate states between glacial conditions and icefree 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 425±75 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 lar
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