Tipping Points- Where is the Scientific Evidence That We Are Approaching These Thresolds?

A remarkable scientific claim was made by Jim Hansen in a CBS News story. The article included the statement,

“Those human changes, he says, are driven by burning fossil fuels that pump out greenhouse gases like CO2, carbon dioxide. Hansen says his research shows that man has just 10 years to reduce greenhouse gases before global warming reaches what he calls a tipping point and becomes unstoppable. He says the White House is blocking that message.”

My question is where is the modeling support, or other theoretical support, for the claim that “man has just 10 years to reduce greenhouse gases before global warming reaches what he calls a tipping point”? While I completely support Jim Hansen’s right to make such a statement, as a climate scientist it is a requirement to provide the scientific peer reviewed reason for such a forecast.

In addition, based on whatever scientific evidence Dr. Hansen has, what specific policy action would have to be taken within the next ten years to avoid the “tipping point”? What theoretical tool has he used to produce the policy recommendations?

While I agree with Dr. Hansen that the climate system does have “tipping points” (see), the reality is, since our knowledge of the real world climate system variability and change remains limited, that we do not know if human activity moves us closer or further from them.

It is prudent to persue “no regrets” policy (i.e. “win-win”) regardless. However, if policymakers are to move beyond these policies, the scientific evidence must be based on solid peer reviewed research.

The quote by Ralph Cicerone in the same news article does not add substance to the discussion, unfortunately.

“‘Climate change is really happening,’ says Cicerone.

Asked what is causing the changes, Cicerone says it’s greenhouse gases:
‘Carbon dioxide and methane, and chlorofluorocarbons and a couple of
others, which are all the increases in their concentrations in the air
are due to human activities. It’s that simple.'”

The 2005 NRC Report from the National Academy presents a more complex message. An excerpt from the Report states,

“Policies designed to manage air pollution and land use may be associated with unintended impacts on climate. Increasing evidence of health effects makes it likely that aerosols and ozone will be the targets of stricter regulations in the future. To date, control strategies have not considered the potential climatic implications of emissions reductions. Regulations targeting black carbon emissions or ozone precursors would have combined benefits for public health and climate. However, because some aerosols have a negative radiative forcing, reducing their concentrations could actually increase radiative warming. Policies associated with land management practices could also have inadvertent effects on climate. The continued conversion of landscapes by human activity, particularly in the humid tropics, has complex and possibly important consequences for regional and global climate change as a result of changes in the surface energy budget.”

The climate system is clearly not as “simple” as expressed by Ralph Cicerone. (also see the set of postings on this Climate Science website on “Climate Change Forcings“).

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