Target atmospheric CO2: where should humanity aim?

• Main Authors: James Hansen, Makiko Sato, Pushker Kharecha, David Beerling, Valerie Masson-Delmotte, Mark Pagani, Maureen Raymo, Dana L Royer, James C Zachos
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 2008
• Subjects: Antarc*; Antarctica; Ice Sheet
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1070.1458; http://climateknowledge.org/figures/Rood_Climate_Change_AOSS480_Documents/Hansen_targetCO2.pdf

Paleoclimate data show that climate sensitivity is ~3°C for doubled CO 2 , including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6°C for doubled CO 2 for the range of climate states between glacial conditions and icefree Antarctica. Decreasing CO 2 was the main cause of a cooling trend that began 50 million years ago, large scale glaciation occurring when CO 2 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 CO 2 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-CO 2 forcings. An initial 350 ppm CO 2 target may be achievable by phasing out coal use except where CO 2 is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO 2 is not brief, there is a possibility of seeding irreversible catastrophic effects. Human activities are altering Earth's atmospheric composition. Concern about global warming due to long-lived human-made greenhouse gases (GHGs) led to the United Nations Framework Convention on Climate Change (1) with the objective of stabilizing GHGs in the atmosphere at a level preventing "dangerous anthropogenic interference with the climate system." The Intergovernmental Panel on Climate Change (IPCC, 2) and others (3) used several "reasons for concern" to estimate that global warming of more than 2-3°C may be dangerous. The European Union adopted 2°C above pre-industrial global temperature as a goal to limit human-made warming (4). Hansen et al. (5) argued for a limit of 1°C global warming (relative to 2000, 1.7°C relative to pre-industrial time), aiming to avoid practically irreversible ice sheet and species loss. This 1°C limit, with nominal climate ...