A new NASA study has confirmed that it's greenhouse gases - not changes in solar activity - that are the main cause of global warming.
The study involves an updated calculation of the difference between the amount of solar energy absorbed by Earth's surface and the amount returned to space as heat - and show that, despite unusually low solar activity between 2005 and 2010, the planet continued to absorb more energy than it returned to space.
Total solar irradiance - the amount of energy that hits each square meter of the Earth's atmosphere - falls by about a tenth of a percent during cyclical lulls in solar activity caused by shifts in the sun's magnetic field.
Usually, solar minimums occur about every eleven years and last a year or so, but the most recent minimum was the longest recorded, at about three years.
And Hansen's team has concluded that the Earth absorbed over half a watt more solar energy per square meter than it emitted throughout the six-year study period. This imbalance is more than twice as much as the fall in incoming solar energy between maximum and minimum solar activity.
"The fact that we still see a positive imbalance despite the prolonged solar minimum isn't a surprise, given what we've learned about the climate system, but it's worth noting because this provides unequivocal evidence that the sun is not the dominant driver of global warming," says James Hansen, director of NASA's Goddard Institute for Space Studies.
According to the team's calculations, the imbalance implies that carbon dioxide levels need to be reduced to about 350 parts per million to restore the energy budget to equilibrium. They're currently 392 parts per million, and scientists expect the figure to continue to rise in the future.
The improvements in the calculations stem from better measurements of ocean temperature, made by free-floating instruments that directly monitor the temperature, pressure and salinity of the upper ocean to a depth of 6,560 feet.
The network of instruments, known collectively as Argo, has grown dramatically in recent years.
And the information collected by Argo, along with other ground-based and satellite data, shows the upper ocean has absorbed 71 percent of the excess energy and the Southern Ocean 12 percent. The abyssal zone of the ocean, between about 9,800 and 20,000 feet below the surface, absorbed five percent, while ice absorbed eight percent and land four percent.
The updated calculation has important implications for climate modeling, suggesting that most climate models overestimate how readily heat mixes deeply into the ocean. They may also be significantly underestimating the cooling effect of small airborne particles called aerosols.
"Climate models simulate observed changes in global temperatures quite accurately, so if the models mix heat into the deep ocean too aggressively, it follows that they underestimate the magnitude of the aerosol cooling effect," says Hansen.