Evidence shows, he said, that change can take place very fast. At one point nearly 15,000 years ago, when the last Ice Age was ending, the climate warmed 5 degrees Celsius in no more than 10 years. Some processes in the climate system may operate like an off-on switch, Severinghaus suggested. A questioner asked the panel what has happened in the science of the climate since 1995, when the International Panel on Climate Change brought out its last report. Severinghaus replied that the possibility of discontinuous change has become more prominent, and the IPCC's next report is likely to be more cautious in projecting future temperature increases as smooth curves.
Concentrations can move rapidly from one continent to another.
Mark Thiemens of UC San Diego observed that aerosols emitted in India can turn up in California within four days. No one yet knows what these aerosols are composed of, he added, or what effects they may have on health. Susan Trumbore of UC Irvine, reporting on the natural sinks that store carbon, noted that those on land have inherent limits of their capacity. Land management alone cannot stop the rise in carbon dioxide concentrations in the atmosphere, she said. That can be accomplished only by reducing emissions from burning fossil fuel.