Where did the heat go?
It might be in the depths of the ocean. The world’s seas absorb more than 90 percent of the extra energy that greenhouse gases trap on earth, yet the ocean is rarely included in global warming estimates, which are typically based on measurements of air temperature. A recent study in the journal Geophysical Research Letters found that deep ocean waters below 2,300 feet have heated up since 2000, even as the temperature of surface seawater remained stable. “Warming over the last decade has been hidden below the ocean surface,” said Richard Allan, a climate scientist at Britain’s University of Reading. If you take the oceans into account, he says, “global warming has actually not slowed down.” If oceans are indeed the reason for the pause, the extra heat would eventually rise to the surface — causing a sudden new warming trend. Some climate scientists, however, think that much of the heat is missing because it never made it into Earth’s climate system in the first place.

Why would that happen?
Possibly because the sun hasn’t been shining as brightly. Over an average of 11 years, the sun’s energy output rises and falls, subtly influencing Earth’s climate. The last solar maximum occurred in 2000; since then, a prolonged solar minimum has kept the sun dimmer than usual. Kevin Trenberth of the National Center for Atmospheric Research says that lower solar radiation could account for up to 15 percent of the missing heat. Another theory is that about 30 percent of the missing heat is due to an influx of sunlight-blocking particles into the stratosphere — vast quantities of pollution from coal-burning China and several mid-sized volcanic eruptions, including on Montserrat in the Caribbean and in Papua New Guinea. These particles work in the opposite way to greenhouse gases, reflecting solar radiation away from the planet.