Earth's Big Heat Bucket: How the Ocean Revealed the Planet's Energy Imbalance

Researchers at NASA's Jet Propulsion Laboratory and Goddard Institute for Space Studies think of the world ocean as Earth's "biggest heat bucket". Like a bucket under an overflowing sink, it is filling with the heat that rising greenhouse gases keep from escaping to space. By comparing climate simulations with millions of measurements of the ocean's heat, a team of climatologists and oceanographers produced what NASA climate scientist James Hansen called the "smoking gun" of human-caused climate change: a predicted energy imbalance that closely matched what was observed.

Why the heat hides in the ocean

A planet absorbs some radiation and emits some back to space. If the two match, its energy budget is balanced and its temperature holds steady; if they do not, it warms or cools over time, even when the change is not yet obvious. One might expect extra energy to heat the air first. But, as oceanographer Josh Willis of Caltech, working at JPL, explains, "the atmosphere, the air, really can't hold that much heat."

Two things make the difference. The atmosphere weighs a tiny fraction of what the ocean weighs, and air has a low specific heat: it takes far less energy to warm a quantity of air than the same quantity of water, as anyone who has boiled a pot knows. So excess energy may not show up as strongly warmer air. It can hide in the ocean instead.

Josh Willis, who carried out experiments at sea between New Zealand and Hawaii for his study of ocean heat storage. NASA Jet Propulsion Laboratory.

Taking the ocean's temperature

"The ocean's a big place, and it's not doing the same thing everywhere," Willis says. It warms in some places and cools in others, and what matters is the average. Measuring it everywhere was the hard part. Ships crossing between the U.S. and Europe had long dropped thermistors along their routes, so the North Atlantic was well studied. The Pacific had big gaps even along its shipping lanes, and the Southern Ocean was the worst sampled of all.

The German Atlantic Expedition of 1925 to 1927 measured the temperature, salinity and density of Atlantic surface waters from the research vessel Meteor. NOAA Photo Library.

Global coverage came from space. NASA and France's Centre National d'Etudes Spatiales launched the TOPEX/Poseidon satellite in 1992 and its successor, Jason 1, in 2001. They measure sea-surface height very accurately, and as a column of water warms it expands, raising the surface. Alongside them, the international Argo program places drifting floats that measure temperature and salinity at set depths, spaced roughly every 3 degrees (360 kilometers at the Equator); it expected to reach 3,000 floats by the end of 2006.

In 2004 Willis and two colleagues combined satellite data with roughly 1 million temperature profiles, each a set of readings from the surface down several hundred meters, in a paper in the Journal of Geophysical Research. From mid-1993 to mid-2003, they found, the heat content of the top 750 meters of the ocean rose at an average of 0.86 watts per square meter (plus or minus 0.12). That sounds small until it is spread over the area studied: about 337 trillion square meters, 93 percent of the world ocean.

The heat that matters here lies beneath the surface. Sea-surface temperature drives weather, but temperature at depth says more about the planet's energy balance, and so gives climate modelers a rare chance to test their predictions.

The model meets the measurements

Hansen, who headed NASA's Goddard Institute for Space Studies at Columbia University, had moved from studying clouds on Venus to Earth's greenhouse gases in the mid-1970s. In 1988 he testified before Congress on how different levels of greenhouse gases might affect temperatures, and over the next 17 years the observed temperatures closely followed his predictions. Seeing Willis's paper, he suggested working together, because an energy imbalance is the clearest test of whether warming comes from greenhouse gases: if they are pushing the climate, Earth must absorb more than it radiates, and the excess should appear in the ocean.

Observed (red) and modelled (black) heat content of the ocean, 1993 to 2003; the model line is the mean of five runs of the GISS ocean-atmosphere model. Graph derived from Hansen et al. 2005. NASA.

The team ran the GISS climate model five times over the years 1880 to 2003. On average, the runs gave the top 750 meters of ocean a gain of 6.0 watt-years per square meter (plus or minus 0.6) over the last decade; a watt-year is the energy one watt delivers in a year. To produce that, the model said, Earth must have been absorbing about 0.85 watts per square meter more than it radiated as of 2003 — close to what Willis had measured.

"I describe this imbalance as the smoking gun or the innate greenhouse effect," Hansen said. The greenhouse mechanism works by reducing heat radiated to space, so finding an imbalance of the expected size is "a very big, fundamental confirmation of the whole global warming problem."

Warming already in the pipeline

The ocean's huge heat capacity gives the climate thermal inertia: like a car that keeps rolling after the brakes go on, the system takes time to catch up with its energy balance. By Hansen's estimate, the imbalance would bring another 0.5 to 0.6 °C of global average surface warming, on top of the 0.7 °C between 1880 and 2000. "We're putting in the pipeline additional change that will occur over the next several decades," he said.

The same lag also buys time. Carbon dioxide would take at least several decades to stabilize, so Hansen argued for also cutting other warming agents such as soot and methane, which would help health too:

• Soot falls with more efficient use of diesel fuel and coal, which also cleans the air.
• Methane can be captured at coal mines, landfills and waste facilities.
• Flooded soil releases methane, so changing fertilizers and irrigation to reduce standing water could cut it and lower the risk of mosquito-borne disease.

What warmer water does

Hansen's team found 2005 the warmest year on record; other teams placed 1998 just ahead. The order mattered little, he said: 1998 had the help of "the El Niño of the century", and reaching the same temperature without one "just confirms the strong underlying global warming trend."

Warmer water makes some Caribbean corals expel their algae and bleach. NOAA Photo Library.

Warmer water also changes weather. No one could pin the destructive 2005 hurricane season squarely on global warming, but storms like Katrina are more likely over warmer seas. "I think that more severe storms are one impact of a warmer ocean," Hansen said.

The Ross Ice Shelf, part of a massive ice shelf in West Antarctica. If it broke up, glaciers behind it would flow faster into the ocean and raise sea level faster. NOAA Photo Library.

He was also concerned about ice shelves, which have thinned substantially in West Antarctica and Greenland. Between 1995 and 2002 parts of the Larsen Ice Shelf on the Antarctic Peninsula disintegrated, and the glaciers feeding it sped up. Ted Scambos of the National Snow and Ice Data Center agreed: warmer air did most of the damage at the Larsen, but for the large floating ice tongues off Greenland and Antarctica, "the warmth in the ocean is leading the way."

Neither paper is the last word. Measuring the ocean goes on, and models keep improving. What the researchers do know is that Earth now absorbs more energy than it emits, and that while the excess hides in the ocean, the climate has more warming still to come.

Sources

• NASA Earth Observatory, "Earth's Big Heat Bucket"; rewritten in hubnx's own words. Images from NASA's Jet Propulsion Laboratory, NASA and the NOAA Photo Library.