There’s more to climate change than warmer air
As greenhouse gases accumulate in the atmosphere, satellite instruments show a growing imbalance between energy entering the atmosphere from the Sun and energy leaving from Earth’s surface. This imbalance is the source of long-term global warming.But tracking the growing amount of heat on Earth is far more complicated than measuring temperatures at the planet’s surface. The oceans absorb about 90 percent of the solar energy that is trapped by greenhouse gases. Additional amounts of heat go toward melting glaciers and sea ice, as well as warming the land and parts of the atmosphere. Only a tiny fraction warms the air at the planet’s surface.Satellite measurements indicate that the amount of greenhouse-trapped solar energy has risen over recent years while the increase in heat measured in the top 3,000 feet of the ocean has stalled. Although it is difficult to quantify the amount of solar energy with precision, Trenberth and Fasullo estimate 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 total amount of heat is unaccounted for.A percentage of the missing heat could be illusory, the result of imprecise measurements by satellites and surface sensors or incorrect processing of data from those sensors, the authors say. 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, even as the satellite-measured imbalance between incoming and outgoing energy continues to grow.Some of the missing heat appears to be going into the observed melting of ice sheets in Greenland and Antarctica, as well as Arctic sea ice, the authors say.Much of the missing heat may be in the ocean. Some heat increase can be detected between depths of 3,000 and 6,500 feet (about 1,000 to 2,000 meters), but more heat may be deeper still beyond the reach of ocean sensors.Trenberth and Fasullo call for additional ocean sensors, along with more systematic data analysis and new approaches to calibrating satellite instruments, to help resolve the mystery. The Argo profiling floats that researchers began deploying in 2000 to measure ocean temperatures, for example, are separated by about 185 miles (300 kilometers) and take readings only about once every 10 days from a depth of about 6,500 feet (2,000 meters) up to the surface. Plans are underway to have a subset of these floats go to greater depths."Global warming at its heart is driven by an imbalance of energy: more solar energy is entering the atmosphere than leaving it," Fasullo says. "Our concern is that we aren’t able to entirely monitor or understand the imbalance. This reveals a glaring hole in our ability to observe the build-up of heat in our climate system."