(excerpt from "The Time Bomb", originally printed in Scientific American 290, no. 3, 68-77, February 2004)
The Time Bomb (PDF)
Dr. James E. Hansen
The goal of the United Nations Framework Convention on Climate Change, produced in Rio De Janeiro in 1992, is to stabilize atmospheric composition to "prevent dangerous anthropogenic interference with the climate system" and achieve that in ways that do not disrupt the global economy.
The United States was the first developed country to sign the convention, which has since been ratified by practically all countries.
Defining the level of warming that constitutes "dangerous anthropogenic interference" (DAI) is thus a crucial but difficult part of the global warming problem.
The United Nations established an Intergovernmental Panel on Climate Change (IPCC) with responsibility for analysis of global warming. IPCC has defined climate forcing scenarios,used these for simulation of 21st century climate, and estimated the impact of temperature and precipitation changes on agriculture, natural ecosystems, wildlife and other matters.
Significant effects are found, but even with warming of several degrees there are winners and losers. IPCC estimates sea level change as large as several tens of centimeters in 100 year, if global warming reaches several degrees Celsius.
Their calculated sea level change is due mainly to thermal expansion of ocean water, with little change in ice sheet volume.
These moderate climate effects, even with rapidly increasing greenhouse gases, leave the impression that we are not close to DAI.
The IPCC analysis also abets the emphasis on adaptation to climate change, as opposed to mitigation, in recent international discussions.
Adaptation is required, to be sure, because climate change is already underway.
However I will argue that we are much closer to DAI than is generally realized, and thus the emphasis should be on mitigation.