A new NASA study tries to lay to rest the skepticism about climate change, especially vocal this year on the GOP presidential campaign trail. It finds, like other major scientific research, that greenhouse gases generated by human activities -- not changes in solar activity -- are the primary cause of global warming.

NASA researchers updated calculations of the Earth's energy imbalance, which is the difference between the amount of solar energy absorbed by the Earth's surface and the amount returned to space as heat. They found that despite unusually low solar activity between 2005 and 2010, the planet continued to absorb more energy (half a watt more per square meter) than it returned to space during that time period.

The Hansen-led study, published in the December issue of Atmospheric Chemistry and Physics, said the magnitude of the Earth's energy imbalance is fundamental to climate science. If the imbalance is positive and more energy enters the system than exits, the Earth warms. If the imbalance is negative, the planet cools.

The researchers concluded that the 0.58 watts per square meter imbalance implies that carbon dioxide levels need to be reduced to about 350 parts per million to restore the energy budget to equilibrium. They say the most recent measurements put CO2 levels at 392 parts per million and those concentrations are expected to keep rising.

Scientists have been refining calculations of the Earth's energy imbalance for years, but NASA researchers say their newest estimate is an improvement because they had access to better measurements of ocean temperature.