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Target Atmospheric CO2: Where Should Humanity Aim?

Author(s): Hansen JE, Sato M, Kharecha P, Beerling D, Berner R, Masson-Delmotte V, Pagani M, Raymo M, Royer DL, Zachos JC

Published: November, 2008

Publisher: The Open Atmospheric Science Journal

DOI: 10.2174/1874282300802010217

Tags: Climate Science

URL: http://benthamscience.com/open/openaccess.php?toascj/articles/V002/217TOASCJ.htm

Abstract: 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 ice-free Antarctica. Decreasing CO2 was the main cause of a cooling trend that began 50 million years ago, the planet being nearly ice-free until CO2 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 CO2 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-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.


Climate forcing by stratospheric aerosols

Author(s): Lacis A, Hansen JE, Sato M

Published: June, 1992

Publisher: Geophysical Research Letters

DOI: 10.1029/92GL01620

Tags: Stratospheric Aerosols

URL: http://www.agu.org/journals/ABS/1992/92GL01620.shtml

Abstract: We illustrate how climate forcing by stratospheric aerosols depends on aerosol properties. The climate forcing is a function of aerosol size distribution, but the size dependence can be described well by a single parameter: the area‐weighted mean radius, reff.If reff is greater than about 2 μm, the global average greenhouse effect of the aerosols exceeds the albedo effect, causing a surface heating. The aerosol climate forcing is less sensitive to other characteristics of the size distribution, the aerosol composition, and the altitude of the aerosols. Thus stratospheric aerosol forcing can be defined accurately from measurements of aerosol, extinction over a broad wavelength range.


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