Greenhouse gases emitted by human activities help trap more heat in the Earth's atmosphere, warming the planet. But a new analysis warns that scientists don't fully understand where all that heat is going.
They can't explain where about half the heat that has built up on Earth in recent years has gone, warn Kevin Trenberth and John Fasullo, researchers at the National Center for Atmospheric Research.
That inability to balance the Earth's "energy budget" will make it harder to weigh the merits of policies to fight climate change and determine which natural events are driven by warming, the pair say in a "Perspectives" essay published in the latest issue of the journal Science.
Researchers are able to track overall changes in the amount of energy coming in and out of the Earth's atmosphere using satellites. They know that the amount of energy coming to Earth outweighs the amount leaving -- and the leftover energy is driving global warming.
But the problem, according to Trenberth and Fasullo, is that researchers have trouble pinpointing which parts of the Earth are storing heat.
The planet's oceans absorb about 90 percent of incoming energy in the form of heat, but measurements collected between 2004 and 2008 show that the rate at which oceans are absorbing heat is slowing, even as emissions of heat-trapping gases have risen.
A mystery that could 'haunt' the planet
"If the extra energy has not gone into the ocean, where has it gone?" the new analysis asks.
Trenberth and Fasullo say they expect some of the energy may be stored in the deep ocean, where scientists aren't measuring.
Some of the accounting problem may also be from errors in measurements collected by satellites and other instruments.
The researchers say some of the "missing" heat has likely helped drive recent record declines in Arctic sea ice and rapidly accelerating melting of ice sheets in Greenland and Antarctica.
Accounting for that missing heat is important for several reasons, the scientists say.
They note that some geoengineering proposals designed to cool the planet by injecting reflective particles in the upper atmosphere are based on the idea that scientists have "understanding and control of the energy flow," and can provide a detailed accounting of how energy moves through Earth and its atmosphere.
Closing Earth's "energy budget" gap may also be necessary to answer more fundamental questions, such as whether events like the December 2009 cold snaps in parts of the United States and Europe were caused by natural weather patterns or prompted by changes in clouds or pollution.
"The heat will come back to haunt us sooner or later," Trenberth said. "The reprieve we've had from warming temperatures in the last few years will not continue. It is critical to track the buildup of energy in our climate system so we can understand what is happening and predict our climate future."