26 September 2006 Human-made greenhouse gases are dominant climate change factor Washington -- A new study by NASA climatologists finds that the world's temperature is reaching a level that has not been seen in thousands of years. The study concludes that, because of a rapid warming trend over 30 years, Earth is now reaching and passing through the warmest temperatures in the current interglacial period, which has lasted nearly 12,000 years. This warming is forcing a migration of plant and animal species toward the poles, according to a September 25 press release from the NASA Goddard Space Flight Center in Maryland. The work appears in the current issue of the Proceedings of the National Academy of Sciences, authored by James Hansen of NASA's Goddard Institute for Space Studies in New York, and colleagues from Columbia University, Sigma Space Partners in Maryland and the University of California at Santa Barbara (UCSB). The study examines worldwide instrumental temperature measurements during the past century. These data reveal that Earth has been warming at the rapid rate of about 0.2 degree Celsius per decade for 30 years. This observed warming is similar to the warming rate predicted in the 1980s in initial global climate model simulations with changing levels of greenhouse gases. "This evidence implies that we are getting close to dangerous levels of human-made [anthropogenic] pollution," Hansen said. In recent decades, anthropogenic greenhouse gases, such as carbon dioxide and methane, have become the dominant climate change factor. WORLD WARMING The study states that the world's warming is greatest at high latitudes of the Northern Hemisphere, and larger over land than over ocean areas. The enhanced warming at high latitudes is attributed to effects of ice and snow. As Earth warms, snow and ice melt, uncovering darker surfaces that absorb more sunlight and increase warming, a process called a positive feedback. Warming is less evident over ocean than over land because of the great heat capacity of the deep-mixing ocean, which causes warming to occur more slowly there. Hansen and colleagues in New York collaborated with David Lea and Martin Medina-Elizade of UCSB to obtain comparisons of recent temperatures with the history of the Earth over the past million years. The California researchers obtained a record of tropical ocean surface temperatures from the magnesium content in the shells of microscopic sea-surface animals, as recorded in ocean sediments. PACIFIC AND INDIAN OCEANS One finding from this collaboration is that the western equatorial Pacific and Indian oceans are now as warm as, or warmer than, at any prior time in the Holocene -- the relatively warm period that has existed for nearly 12,000 years, since the end of the last major ice age. The western Pacific and Indian oceans are important because, as the researchers show, temperature change there is indicative of global temperature change. "The western Pacific is important for another reason, too,” Lea said. “It is a major source of heat for the world’s oceans and for the global atmosphere." In contrast to the western Pacific, the eastern Pacific Ocean has not shown an equal magnitude of warming. The researchers explain the lesser warming in the east Pacific Ocean, near South America, by the cooling effect of upwelling -- rising of deeper colder water to shallower depths. The deep ocean layers have not yet been affected much by anthropogenic warming. Hansen and his collaborators suggest the increased temperature difference between the western and eastern Pacific may increase the likelihood of strong El Niños, such as those of 1983 and 1998. An El Niño is an event that typically occurs every several years when the warm surface waters in the west Pacific slosh eastward toward South America, altering weather patterns around the world. MOST IMPORTANT RESULT The most important result the researchers found is that the warming in recent decades has brought global temperature to a level within about 1 degree Celsius of the maximum temperature of the past million years. “That means that further global warming of 1 degree Celsius defines a critical level,” Hansen said. “If warming is kept less than that, effects of global warming may be relatively manageable. During the warmest interglacial periods the Earth was reasonably similar to today.” If further global warming reaches 2 degrees or 3 degrees Celsius, he added, changes are likely “that will make Earth a different planet than the one we know. The last time it was that warm was in the middle Pliocene, about 3 million years ago,” when sea level was an estimated 25 meters higher than today. Global warming already is beginning to have noticeable effects. Plants and animals can survive only within certain climatic zones, so with the warming of recent decades many are starting to migrate toward the Earth’s poles. A study that appeared in the journal Nature in 2003 found that 1,700 plant, animal and insect species moved toward the poles at an average rate of six kilometers per decade in the last half of the 20th century. STRESS ON WILDLIFE That migration rate is not fast enough to keep up with the current rate of movement of temperature zones, which shifted about 40 kilometers per decade in the period 1975-2005. "Rapid movement of climatic zones is going to be another stress on wildlife,” Hansen said. “It adds to the stress of habitat loss due to human developments,” he added. “If we do not slow down the rate of global warming, many species are likely to become extinct. In effect, we are pushing them off the planet." More information about the warming planet and climate change is available on the NASA Web site. The full text of the press release is available on the NASA Web site. (Distributed by the Bureau of International Information Programs, U.S. Department of State. Web site: http://usinfo.state.gov)NASA Study Finds World Reaching Warmest Levels in 12,000 Years
