Quotes by James Hansen

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 ... If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects.

Two degrees Celsius is guaranteed disaster... It is equivalent to the early Pliocene epoch [between 5.5 and 2.5 million years ago] when the sea level was 25m higher. What we don't know is how long it takes ice sheets to disintegrate, but we know we'd be starting a process which then is going to be out of control. Because the way it works – the planet is out of energy balance, most of the additional energy is going into the ocean, which melts the ice shelves, which then allows the ice sheets to discharge ice more rapidly – if you want to stop that and you've pushed it up to two degrees, then you've got to cool off the ocean. Well that's going to take hundreds of years. So you would have a situation which can't be fixed except with some geo-engineering, which is a pretty awful inheritance to leave for our children....

We've reached a point where it's clear we can't burn all the coal or unconventional fossil fuels [such as oil from tar sands, deepwater drilling and sources revealed by melting ice]. We've got to phase them out. The large pools of oil and gas that are readily available to Russia, Saudi Arabia and the Middle Eastern countries is enough to get us well over 450ppm1.

• 1. In 2008 Hansen published a paper with some ten co-authors, "Target Atmospheric CO2: Where Should Humanity Aim?" which redefined our understanding of what constitutes dangerous climate change. It concluded that we need to reduce the levels of carbon dioxide in the atmosphere from 390 parts per million (ppm) to below 350

If we burn even half of Earth's remaining fossil fuels we will destroy the planet as humanity knows it. The added emissions of heat-trapping carbon dioxide will set our Earth irreversibly onto a course toward an ice-free state, a course that will initiate a chain reaction of irreversible and catastrophic climate changes.

The concentration of CO2 in our atmosphere now stands at 387 parts per million, the highest level in 600,000 years and more than 100 ppm higher than the amount at the dawn of the Industrial Revolution. Burning just the oil and gas sitting in known fields will drive atmospheric CO2 well over 400 ppm and ignite a devil's cauldron of melted icecaps, bubbling permafrost, and combustible forests from which there will be no turning back. But if we cut off the largest source of carbon dioxide, coal, we have a chance to bring CO2 back to 350 ppm and still lower through agricultural and forestry practices that increase carbon storage in trees and soil.