Dr. James Hansen first brought global warming to the attention of the public during testimony before a U.S. Senate committee on June 23, 1988. He warned that the year’s record temperatures were a product of human activity and that curbing fossil fuel use was necessary for the continued well-being of Earth’s ecosystems and civilization. However, society’s continued use of carbon-based fuels has stressed many earth systems to a critical point beyond which irreversible change will or has already occurred.
On June 23, 2008, twenty years after his landmark testimony, Dr. Hansen reiterated his warnings before the U.S. Congress. He cited several examples of earth systems reaching or nearing a tipping point. A tipping level (point) is a level at which “no additional forcing is required for large climate change and impacts.” (Hansen, 2008) According to Hansen, a “point of no return” is reached when unstoppable irreversible (on a practical time scale) occurs. The disintegration of the Greenland ice cap is an example. Some, like Josefino Comiso of the NASA Goddard Space Flight Center, feel that the tipping point for perennial Arctic sea ice has already passed (National Geographic, 2007). David Barber, of the University of Manitoba is projecting that the North Pole will be ice free for the first time in history. Hansen believes to “preserve creation, the planet on which civilization developed” a target of less than 350 parts per million of carbon dioxide (from its current concentration of 385 parts per million) is required (Hansen, 2008).
Audio (and PowerPoint slides) of Dr. Hansen’s June 23, 2008 presentation before the National Press Club, his statement before Congress and more are available at the Worldwatch Institute.
The Physical Environment textbook links: Climate Change: Global Warming; Greenhouse Effect and Global Warming; Feedbacks
Graph from Dr. Hansen’s PowerPoint presentation.