Dr James Hansen is a top US climate scientist (Director, NASA Goddard Institute for Space Studies; member of the prestigious US National Academy of Sciences; 2007 Award for Scientific Freedom and Responsibility of the prestigious American Association for the Advancement of Science; and Adjunct Professor, Columbia University, New York, USA) (see: http://www.columbia.edu/~jeh1/ ; http://en.wikipedia.org/wiki/James_Hansen ; for 1880-present NASA GISS Global Temperature graphed data see: http://data.giss.nasa.gov/gistemp/ and http://data.giss.nasa.gov/gistemp/graphs/ ).
Dr Hansen has made the following expert statements relevant to “reduce atmospheric CO2 to 300 ppm CO2”.
(a) With 8 UK, French and US climate change scientist co-authors (2008): “Paleoclimate data show that climate sensitivity is ~3 deg-C for doubled CO2 [carbon dioxide; atmospheric CO2 280 ppm pre-industrial], 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 [parts per million], 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.” .
(b) In relation to the recent book “Climate Code Red. The case for emergency action” by David Spratt and Philip Sutton (Scribe, Melbourne, 2008): “A compelling case … we face a climate emergency.” .
(c) 2007 (Hansen, J., Mki. Sato, P. Kharecha, G. Russell, D.W. Lea, and M. Siddall, 2007: Climate change and trace gases. Phil. Trans. Royal. Soc. A, 365, 1925-1954): “Paleoclimate data show that the Earth's climate is remarkably sensitive to global forcings. Positive feedbacks predominate. This allows the entire planet to be whipsawed between climate states. One feedback, the "albedo flip" property of water substance, provides a powerful trigger mechanism. A climate forcing that "flips" the albedo of a sufficient portion of an ice sheet can spark a cataclysm. Ice sheet and ocean inertia provides only moderate delay to ice sheet disintegration and a burst of added global warming. Recent greenhouse gas (GHG) emissions place the Earth perilously close to dramatic climate change that could run out of our control, with great dangers for humans and other creatures. Carbon dioxide (CO2) is the largest human-made climate forcing, but other trace constituents are important. Only intense simultaneous efforts to slow CO2 emissions and reduce non-CO2 forcings can keep climate within or near the range of the past million years. The most important of the non-CO2 forcings is methane (CH4), as it causes the 2nd largest human-made GHG climate forcing and is the principal cause of increased tropospheric ozone (O3), which is the 3rd largest GHG forcing. Nitrous oxide (N2O) should also be a focus of climate mitigation efforts. Black carbon ("black soot") has a high global warming potential (~2000, 500, and 200 for 20, 100 and 500 years, respectively) and deserves greater attention. Some forcings are especially effective at high latitudes, so concerted efforts to reduce their emissions could still "save the Arctic", while also having major benefits for human health, agricultural productivity, and the global environment.” .
(d) 2008, in an address to the US National Press Club and a briefing to the US House Select Committee on Energy Independence & Global Warming Congressional Committee: “CEOs of fossil energy companies know what they are doing and are aware of long-term consequences of business as usual. In my opinion, these CEOs should be tried for high crimes against humanity and nature.” .
(e) Dr James Hansen et al. (2008): “Stabilization of Arctic sea ice cover requires, to first approximation, restoration of planetary energy balance. Climate models driven by known forcings yield a present planetary energy imbalance of +0.5-1 W/m2. Observed heat increase in the upper 700 m of the ocean confirms the planetary energy imbalance, but observations of the entire ocean are needed for quantification. CO2 amount must be reduced to 325-355 ppm to increase outgoing flux 0.5-1 W/m2, if other forcings are unchanged. A further imbalance reduction, and thus CO2 ~300-325 ppm, may be needed to restore sea ice to its area of 25 years ago.” .
(f) Dr James Hansen's keynote address to May 2, 2009 “350 Climate Conference” reported by U.S. Green Recovery: “Dr. James Hansen, the Director of the NASA Goddard Institute for Space Studies and a world renowned expert on global warming, gave the keynote address at the May 2, 2009 “350 Climate Conference” organized by the Climate and Society Masters of Art Program at Columbia University... He pointed out that governments around the world are under the false impression that emission levels can be set arbitrarily without fully considering what science actually demands to maintain required climatic stability. He said current focus on stabilizing carbon dioxide concentrations at 450 to 500 ppm are misguided and the safe not-to-exceed threshold is 350 ppm… To restore the planetary energy balance carbon dioxide levels should be at 325 ppm. To restore arctic sea ice to its coverage area of 25 years ago, carbon dioxide levels closer to 300 ppm would be needed. Stabilization of sea level rise may require carbon dioxide levels at or around 350 ppm, but should be reassessed frequently as more information is known on ice dynamics.” .
 J. Hansen et al, “Target atmospheric CO2 – where should humanity aim?” : http://arxiv.org/abs/0804.1126 ,
. Climate Code Red: http://www.climatecodered.net/ .
. James Hansen, Mki. Sato, P. Kharecha, G. Russell, D.W. Lea, and M. Siddall, (2007): Climate change and trace gases. Phil. Trans. Royal. Soc. A, 365, 1925-1954: http://pubs.giss.nasa.gov/abstracts/2007/Hansen_etal_2.html .
. Dr James Hansen, to the US National Press Club and a briefing to the US House Select Committee on Energy Independence & Global Warming Congressional Committee: http://www.columbia.edu/~jeh1/2008/TwentyYearsLater_20080623.pdf .
. Hansen, J., Mki. Sato, P. Kharecha, D. Beerling, R. Berner, V. Masson-Delmotte, M. Pagani, M. Raymo, D.L. Royer, and J.C. Zachos, 2008: Target atmospheric CO2: Where should humanity aim? Open Atmos. Sci. J., 2, 217-231: http://pubs.giss.nasa.gov/abstracts/2008/Hansen_etal.html (abstract) and http://pubs.giss.nasa.gov/docs/2008/2008_Hansen_etal.pdf .
. Dr James Hansen keynote address to May 2, 2009 “350 Climate Conference” reported by U.S. Green Recovery, 2009: http://usgreenrecovery.com/General/350climate.html .