The Solar Balance
The sun is the base energy source for the Earth. What it emits is either absorbed or reflected. Observations showed some "missing energy" in this balance. Two years ago, scientists at the National Center for Atmospheric Research in Boulder, Colo., released a study claiming that inconsistencies between satellite observations of Earth's heat and measurements of ocean heating amounted to evidence of missing energy in the planet's system. Where was it going? Or, they wondered, was something wrong with the way researchers tracked energy as it was absorbed from the sun and emitted back into space? Well it was found. An international team of atmospheric scientists and oceanographers, led by Norman Loeb of NASA's Langley Research Center in Hampton, Va., and including Graeme Stephens of NASA's Jet Propulsion Laboratory in Pasadena, Calif., set out to investigate the mystery.
The Earth’s climate is a solar powered system. Globally, over the course of the year, the Earth system—land surfaces, oceans, and atmosphere—absorbs an average of about 240 watts of solar power per square meter (one watt is one joule of energy every second). The absorbed sunlight drives photosynthesis, fuels evaporation, melts snow and ice, and warms the Earth system.
The Sun doesn’t heat the Earth evenly. Because the Earth is a sphere, the Sun heats equatorial regions more than polar regions. The atmosphere and ocean work non-stop to even out solar heating imbalances through evaporation of surface water, convection, rainfall, winds, and ocean circulation. This coupled atmosphere and ocean circulation is known as Earth’s heat engine.
The climate’s heat engine must not only redistribute solar heat from the equator toward the poles, but also from the Earth’s surface and lower atmosphere back to space. Otherwise, Earth would endlessly heat up. Earth’s temperature doesn’t infinitely rise because the surface and the atmosphere are simultaneously radiating heat to space. This net flow of energy into and out of the Earth system is Earth’s energy budget.
The researchers used 10 years of data - spanning 2001 to 2010 - from NASA Langley's orbiting Clouds and the Earth's Radiant Energy System Experiment (CERES) instruments to measure changes in the net radiation balance at the top of Earth's atmosphere. The CERES data were then combined with estimates of the heat content of Earth's ocean from three independent ocean-sensor sources.
Their analysis found that the satellite and ocean measurements are, in fact, in broad agreement once observational uncertainties are factored in.
"One of the things we wanted to do was a more rigorous analysis of the uncertainties," Loeb said. "When we did that, we found the conclusion of missing energy in the system isn't really supported by the data."
"Our data show that Earth has been accumulating heat in the ocean at a rate of half a watt per square meter (10.8 square feet), with no sign of a decline," Loeb said. "This extra energy will eventually find its way back into the atmosphere and increase temperatures on Earth."
Scientists generally agree that 90 percent of the excess heat associated with increases in greenhouse gas concentrations gets stored in Earth's ocean. If released back into the atmosphere, a half-watt per square meter accumulation of heat could increase global temperatures by 0.3 or more degrees centigrade (0.54 degree Fahrenheit).
Loeb said the findings demonstrate the importance of using multiple measuring systems over time, and illustrate the need for continuous improvement in the way Earth's energy flows are measured.
For further information: http://www.jpl.nasa.gov/news/news.cfm?release=2012-029&rn=news.xml&rst=3269
Photo: JPL's Graeme Stephens