Half the heat believed to have built up on Earth in recent years can't be accounted for.
Scientists at the National Center for Atmospheric Research (NCAR) say that there aren't enough satellite sensors, ocean floats, and other instruments to track this missing heat, which they believe may be building up in the deep oceans.
"The heat will come back to haunt us sooner or later," says NCAR scientist Kevin Trenberth.
"The reprieve we've had from warming temperatures in the last few years will not continue. It is critical to track the build-up of energy in our climate system so we can understand what is happening and predict our future climate."
Until 2003, the measured heat increase was consistent with computer model expectations. But a new set of ocean monitors since then has shown a steady decrease in the rate of oceanic heating - while the satellite-measured imbalance between incoming and outgoing energy continues to grow.
Some trapped solar energy goes towards melting glaciers and sea ice; some warms the planet's surface. But the oceans absorb about 90 percent, and in recent years the increase in heat measured in the top 3,000 feet of the ocean has stalled.
The team estimates that, based on satellite data, the amount of energy build-up appears to be about 1.0 watts per square meter or higher, while ocean instruments indicate a build-up of about 0.5 watts per square meter.
That means about half the heat is unaccounted for.
The scientists suggest that last year's rapid onset of El Niño may be one way in which the solar energy has reappeared.
A percentage could be illusory, the result of imprecise measurements by satellites and surface sensors or incorrect processing of data, they say.
But much may be in the ocean. Some increase can be detected between depths of 3,000 and 6,500 feet, but more heat may be deeper still, beyond the reach of ocean sensors.
The team calls for more ocean sensors, more systematic data analysis and new approaches to calibrating satellite instruments to help resolve the mystery.