BOULDER, Colo., May 2 (UPI) -- The latest finding by climate scientists is that Earth's heat budget already is in the red, meaning the planet is set on a steady course for warmer average surface temperatures.
In a paper in the April 29 issue of the journal Science, James Hansen, director of the NASA Goddard Institute for Space Studies, and colleagues found Earth is absorbing considerably more energy from the sun than it is reflecting back into space. Verified by both climate modeling and observations of ocean temperatures, the authors conclude the planet can expect global warming of an average of 0.6 degrees Celsius (1.1 degrees Fahrenheit) even if carbon dioxide emissions into the atmosphere were curtailed today.
This is the classic and expected "greenhouse effect," in which more energy is trapped by greenhouse gases -- mainly CO2, but others as well -- released into the upper atmosphere.
"This energy imbalance is the 'smoking gun' that we have been looking for," Hansen said in a statement accompanying the article's release. "It shows that our estimates of the human-made and natural climate forcing agents are about right, and they are driving the Earth toward a warmer climate."
Not everyone is convinced of course.
"I do not believe this research team has made a compelling case to suggest that their computer models are sufficiently realistic to justify the implications of anthropogenic (human-caused) global warming that they make," William Kininmonth, author of the book "Climate Change: A Natural Hazard," told BBC News Online.
If the game was American football, however, the paper by Hansen and colleagues would qualify as piling on. It is only the latest of a number of papers in which researchers describe finding considerable human-caused heating already in the pipeline.
For example, in a paper published in Science last March, a group of scientists at the National Center for Atmospheric Research in Boulder found "even if greenhouse gases had been stabilized in the year 2000, we would still be committed to a warmer Earth and greater sea-level rise in the present century."
The NCAR team, which relied primarily on their climate-change simulations model or CCSM, found a slightly lower temperature increase "in the pipeline" of about 0.5 degrees C (1 degree F).
Kevin Trenberth, NCAR's head of global climate analysis, said Hansen's work is further confirmation of the warming expected over the next century.
"The number that is given by Jim Hansen could be argued to be a little bit on the high side," Trenberth told UPI's Climate.
Hansen's group found an energy imbalance of 0.85 watts-per-meter-square more energy absorbed by Earth than emitted.
"The number I use in my talks is somewhere around 0.7 (watts per meter square) or in that neighborhood," he said in his statement.
This is a lot of excess energy in the system.
"The present planetary energy imbalance is large by the standards of Earth's history," the paper said. "For example, an imbalance of 1 watt per meter square maintained for the last 10,000 years of the Holocene (post-ice-age period) is sufficient to melt ice equivalent to 1 kilometer (0.6 miles) of sea level (if there were that much ice), or raise the temperature of the ocean above the thermocline by more than 100 degrees C (190 degrees F)."
Clearly, the paper continued, "on long time scales the planet has been in energy balance to within a small fraction of 1 watt per meter square."
In other words, the fact there still are glaciers and ice caps, and oceans are not boiling, suggests the historical heat imbalance has been small.
"I think the one number that anchors all of these things, as well as anything does, is sea-level rise," Trenberth told Climate.
Over the past 10 years, observed average sea-level rise has been close to 3 millimeters (0.12 inches) per year.
The formula for sea-level rise: thermal expansion of the oceans, plus melting of glaciers, minus the amount of water stored on land. Over the past 10 years or so, however, estimates are water storage on land has increased by about 0.9 mm (0.04 inches) per year.
"Right now the numbers don't quite add up," Trenberth said. The best estimate for melting glaciers is between 0.6 and 1 millimeter per year (0.02 inches to 0.04 inches). The expansion of the ocean is 1.6 millimeters per year (0.06 inches) and inland storage is minus 0.9 millimeters.
"This means you're around 1.5 millimeters per year (of sea-level rise), whereas the observed is around 3 mm per year, so there is a little bit of mismatch that is not fully accounted for there," Trenberth said.
The Hansen paper does not resolve this discrepancy, but it does warn that a large imbalance can lead to dramatic melting over time.
"The estimated approximately 1.5 centimeters (0.6 inches) eustatic (world-wide) sea-level rise in the past decade, even if entirely (composed of) ice melt, required only 2 percent of Earth's present energy imbalance," the paper said. "Much more rapid melt is possible if ice discharge is accelerating, as some recent observations suggest, and has occurred in the past cases of sharp sea-level rise that accompanied rapid global warming.
"This portion of the planetary energy imbalance used for melting is likely to rise as the planet continues to warm, summer melt increases, and melt-water lubricates and softens the ice sheets," the paper noted.
--
Climate is a weekly series examining the evidence for and potential impact of global climate change, by veteran environmental reporter Dan Whipple. E-mail: sciencemail@upi.com
