New Study Finds World Temperatures Approaching Ancient Levels
A new study led by James Hansen of NASA's Goddard Institute for Space Studies, a part of The Earth Institute, finds that the world's temperature is reaching a level that has not been seen in thousands of years.
The study was published in the September 26, 2006 issue of the Proceedings of the National Academy of Sciences (PNAS).
After examining global temperature records from the last century, Hansen and his colleagues conclude that, because of a rapid warming trend over the past 30 years, the Earth is now in danger of surpassing the warmest levels seen during the current interglacial period, which has lasted nearly 12,000 years.
The scientists also concluded that these data showed the Earth has been warming at the remarkably rapid rate of approximately 0.36° Fahrenheit (0.2° Celsius) per decade for the past 30 years.
"This evidence implies that we are getting close to dangerous levels of human-made pollution," said Hansen. "In recent decades, human-made greenhouse gases (GHGs) have become the largest climate change factor. GHGs trap heat in the Earth's atmosphere and warm the surface. Some GHGs, which include water vapor, carbon dioxide, methane, nitrous oxide and ozone, occur naturally, while others are due to human activities.
The study notes that the world's warming is greatest at high latitudes of the Northern Hemisphere, and it is larger over land than over ocean areas. The extra warming at high latitudes is because of positive feedback effects on ice and snow. As the Earth warms, snow and ice melt, uncovering darker surfaces that absorb more sunlight and further increase warming.
Warming is also seen to be less over oceans than over land because the deep oceans can absorb large amounts of heat and, because they are so big, they take longer to warm. In addition, the Western Pacific Ocean, which is a major source of heat for the rest of the world's oceans as well as the atmosphere, has warmed significantly over the past century. At the same time, the Eastern Pacific Ocean has not warmed as much, mainly because cold water rises from the deep ocean in the Eastern Pacific, keeping the surface waters cooler.
Hansen and his colleagues suggest that the increased temperature difference between the Western and Eastern Pacific could lead to the development of strong El Niño conditions, such as those of 1983 and 1998. An El Niño occurs when the warm surface waters in the West Pacific move eastward toward South America, in the process altering weather patterns around the world.
Perhaps the most striking result suggested by the study is that the warming in recent decades has brought global temperature to a level within about one degree Celsius (1.8° F) of the maximum temperature of the past million years, which they suggest is a sensible upper limit for additional global warming. "If further global warming reaches 2 or 3 degrees Celsius, we will likely see changes that make Earth a different planet than the one we know The last time it was that warm was in the middle Pliocene, about three million years ago, when sea level was estimated to have been about 25 meters [80 feet] higher than today."
Citation: Hansen J et al. (2006), Global temperature change, Proceedings of the National Academy of Sciences 103(39):14288-14299.