Excerpts from James Hansen’s paper of 15 April 2013

Warwick Rowell

Making Things Clearer:
Exaggeration, Jumping the Gun, and The Venus Syndrome

(My emphasis)

CO2, the dominant climate forcing on the long run, will stay in the climate system for millennia. The magnitude of the eventual climate response to increasing CO2 depends especially on climate sensitivity. .. Our best evaluation of climate sensitivity comes from Earth’s paleoclimate history, via comparisons of periods with differing climate forcings.

(present) climate sensitivity is in the upper half of the range that has usually been estimated. Furthermore, slow feedbacks (not included in these estimates), such as change of ice sheet size and methane emissions, make the sensitivity still higher.

In 2005 (AGU meeting) I noted that we needed to get on a different global emissions path, with decreasing emissions, within 10 years — not because dramatic climate change would occur in 10 years, but because otherwise we will build into the climate system future changes that will be out of our control.

Climate effects are occurring already and are generally consistent with expectations. I was recently at a meeting that included many of the top researchers in climate change. There was universal agreement about the urgency of the climate crisis.

Certainty of our predicament follows from basic considerations including: (1) the huge inertia and thus slow response of key parts of the climate system, especially the ocean and ice sheets, and improving observations by Argo floats and gravity satellites that confirm trends and the existence of further change in the pipeline, (2) the long lifetime of any ocean warming that is allowed to occur, (3) the millennial time scale that fossil fuel CO2 will stay in the climate system, (4) paleoclimate confirmation of the magnitude of the eventual climate response to large CO2 increase.

These scientists, people who know what they are talking about, were not concerned about jumping the gun, but rather about whether the race might already be over.

If we burn all the fossil fuels it is certain that sea level would eventually rise by tens of meters. The only argument is how soon the rise of several meters needed to destroy habitability of all coastal cities would occur.

it is not an exaggeration to suggest, based on best available scientific evidence, that burning all fossil fuels could result in the planet being not only ice-free but human-free.

if we continue business-as-usual fossil fuel burning, is likely to cause an extended phase of extreme climate chaos.

As ice sheets begin to shed ice more and more rapidly, our climate simulations indicate that a point will be reached when the high latitude ocean surface cools while low latitude surfaces are warming. An increased temperature gradient, i.e., larger temperature contrast between low and high latitudes, will drive more powerful storms.