"Simply Put, the Earth 3C Warmer Would Be a Different Planet Than the One that We Know"

The first section of the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report, due for publication in 2007, was recently released by the US government. This section, the Working Group I (WGI), offers an overview of the science of predictions about the possible course of global warming.
 
Scientists estimate that a doubling of carbon dioxide levels would cause an increase of about 3 °C. What does it mean? 
 
Global warming of 3C refers to the average warming over the planet.  The warming over low latitude oceans would probably be about 1.5-2C, while in the polar regions it would be 5-10C.  In the past million years, thus certainly for the past 150,000 or so years that homo sapiens have existed, the Earth has never been more than 1C warmer than today.  Simply put, the Earth 3C warmer would be a different planet than the one that we know.  There would be no sea ice in the Arctic in the summer and early fall.  The ice sheets on Greenland and Antarctica would be undergoing rapid melting.  The last time that the Earth was 3C warmer was at least 3 million years ago, during the Pliocene, when sea level was 25 meters (plus or minus 10 meters), i.e., about 80 feet, higher than today.  The primary uncertainty concerns how long it would require for the ice sheets to melt and raise sea level.  In prior IPCC reports it was assumed that it required 1000 years or more for the ice sheets to respond to the global warming, so sea level rise this century might be only a half meter or so.  However, the Earth's history reveals numerous cases in which melting proceeded much more rapidly, at a rate of as much as one meter every 20 years.  Thus once ice sheets begin to respond to warming, their disintegration might proceed rapidly, out of our control.

The 2007 summary stresses that even if greenhouse gases level off now, warming will continue at about the current rate for several decades. Why?
 
If greenhouse gases leveled off now, the rate of warming would begin to slow down within a few years, but it would still increase, irregularly, warming by about 0.3C in the next 50 years and eventually by about 0.5C .  That is the amount of warming that is now "in the pipeline" due to gases already in the air.  The reason that the Earth has not yet achieved the full warming from the gases in the air is the large heat capacity of the ocean.  It requires centuries for the ocean to fully respond to any climate forcing, because the ocean is so huge.  It is about 4 km deep on the average.

Reports of Working Groups II and III, which cover the consequences of climate change and the attempts to tackle it, are due later in 2007. In your December speech to the Geophysical Union, you noted that carbon dioxide emissions are ''now surging well above" the point where damage to the planet might be limited. What did you mean? 
 
I was referring to a comparison of actual CO2 emissions with the emissions in the "alternative scenario".  The alternative scenario is defined as a path for future climate that contrasts with the business-as-usual (BAU) path that IPCC refers to in its calculations of global warming of 3C.  The alternative scenario would keep global warming less than 1C, thus within the range that the Earth has experienced during the past several hundred thousand years.  The alternative scenario requires that CO2 emissions level off during the next several years, then begin to slowly decline, and before mid-century begin to decline more rapidly.  By the end of the century CO2 emissions would need to be reduced to a level such that the atmospheric CO2 amount stabilizes.  This probably would require a reduction of emissions by 60-80%.  I was referring to the fact that for the past 10 years, rather than CO2 emissions leveling off, they have been increasing on average by 2% per year.
 
Some climate experts have expressed surprise and concern about a US government decision to release a politically sensitive report when it is still in draft form. What is your opinion on open publishing?

I think that open publishing is a good practice.
 
Do you think that this report can have a substantial political impact?
 
Prior reports have not had big impacts. Special interests have worked to discredit the reports. The public needs to be informed about the role of special interests in the climate change discussion, to realize the liabilities that will accrue if we continue to ignore climate change, and get angry at the special interests and those politicians who are in the hip pocket of special interests. 

James Hansen, thank you.

James E. Hansen is the lead climate scientist and director of the NASA Goddard Institute for Space Science.

Interview by Francesca Gilibert and Gilles Prigent