Thursday, 08 December 2011 10:28
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| Image courtesy Alaska Dispatch. |
Many scientists say this small rise in the planet's annual temperature is only the beginning of a multi-decade sizzle that could ultimately lead to loss of summer sea ice, rising ocean levels, thawing permafrost, disrupted ecosystems -- plus more violent storms, droughts and heat waves.
So what's behind the simmer? The industrial activities of modern life? Searing solar rays? Drunken wobbling by the globe on its axis?
The impact of human activity
New calculations by two Swiss climate scientists say we can blame 74 percent of the warming over the past 60 years on human activities -- with the release of massive quantities of greenhouse gases like carbon dioxide as the main driver.
Natural variability can account for less than one-quarter of the rise in global temperatures since 1950, according to the study, published last weekend in the journal Nature Geoscience.
"We find that since the mid-twentieth century, greenhouse gases contributed 0.85 degrees Celsius of warming ... about half of which was off-set by the cooling effects of aerosols," wrote Markus Huber and Reto Knutti, with the Institute for Atmospheric and Climate Science in Zurich.
"It is thus extremely likely ... that the greenhouse gas induced warming since the mid-twentieth century was larger than the observed rise in global average temperatures, and extremely likely that anthropogenic forcings were by far the dominant cause of warming."
Examining the earth's heat balance
The notion that CO2 trickling from car exhaust pipes, power plants and other human sources might be warming up the planet isn't exactly breaking news. But Huber and Knutti conjured the latest calculations by creating a new method for analyzing the Earth's heat balance -- the amount of energy entering the air in minus the amount of energy slipping away.
Modeling climate requires crunching thousands of numbers through millions of calculations on supercomputers, but the two Swiss researchers streamlined the process.
"Previous attempts to disentangle anthropogenic and natural warming used a statistically complex technique called optimal fingerprinting to compare observed patterns of surface air temperature over time with the modeled climate response to greenhouse gases, solar radiation and aerosols from volcanoes and other sources," explained Scientific American writer Quirin Schiermeier in this story about the breakthrough.
Knutti and Huber "utilized a much simpler model of Earth's total energy budget and ran the model many thousands of times," he wrote. "These included global values for incoming shortwave radiation from the Sun, solar energy leaving Earth, heat absorbed by the oceans and climate-feedback effects (such as reduced snow cover, which amplifies warming by exposing darker surfaces that absorb more heat)."
Add it all up, and the rise in greenhouse gases released by human activity trumps all other factors. Without cooling by aerosols -- dust and other particles in the air -- the Earth would have warmed even more than it already has, they concluded.
And don't blame the Sun, they said.
"Changes in solar radiation -- a hypothesis for global warming proffered by many climate skeptics -- contributed no more than around 0.07 degree C to the recent warming," Schiermeier wrote in the Scientific American article.