Hansen’s summary of his 2008 publication.

This is Hansen’s summary (1 page) with conclusions followed by his Questions and Answers (2 pages).

Target Atmospheric CO2: Where Should Humanity Aim?Humanity must find a path to reduced atmospheric carbon dioxide, to less than the amount in the air today, if climate disasters are to be averted, according to a study to be published in Open Atmospheric Science Journal by a group of ten scientists from the United States, the United Kingdom and France1. They argue that such a path is feasible, but requires a prompt moratorium on new coal use that does not capture CO2 and phase-out of existing coal emissions by 2030.

“There is a bright side to this conclusion” according to James Hansen, the lead author on the study, “by following a path that leads to a lower CO2 amount we can alleviate a number of problems that had begun to seem inevitable, such as increased storm intensities, expanded desertification, loss of coral reefs, and loss of mountain glaciers that supply fresh water to hundreds of millions of people.”

Atmospheric carbon dioxide is already 385 parts per million (ppm) and it is increasing by about 2 ppm each year as a result of the burning of fossil fuels (coal, oil, and gas), with a smaller contribution from burning of forests. The authors use evidence of how the Earth responded to past changes of CO2 and on-going climate changes to show that atmospheric CO2 has already entered the dangerous zone.

The authors suggest that global policies should have an initial target for atmospheric CO2 of 350 ppm. They note that the optimum CO2 level for maintaining a planet similar to that on which civilization developed is likely to be less than 350 ppm, but a 350 ppm target already reveals that dramatic policy changes are needed urgently. By the time such fundamental changes are achieved, knowledge will exist to help fine-tune the target CO2.

 The conclusion that humanity must aim for a CO2 amount less than the current amount is a dramatic change from most previous studies, which suggested that the dangerous level of CO2 was likely to be 450 ppm or higher. The change is caused by realization that ‘slow’ feedback processes, such as ice melt and release of greenhouse gases by the soil and ocean in a warming climate, can occur on the time scale of decades and centuries. This realization stems from both improving data on the Earth’s climate history and ongoing observations of change, especially in the polar regions.

The authors conclude that “humanity today, collectively, must face the uncomfortable fact that industrial civilization itself has become the principal driver of global climate.” Specifically, they say that humanity “must begin now to move toward the era beyond fossil fuels”, and “the most difficult task, phase-out over the next 20-25 years of coal use that does not capture CO2, is Herculean, yet feasible when compared with the efforts that went into World War II. The stakes, for all life on the planet, surpass those of any previous crisis. The greatest danger is continued ignorance and denial, which could make tragic consequences unavoidable.”