Observed changes in top-of-the-atmosphere radiation and upper-ocean heating...

In an article published in Nature Geosciences we present an analysis of the heat entering our planet since 2000, measured at the top of the atmosphere by satellite instruments and down to a depth of 1800m within the world's oceans by automated profiling floats.

Global annual average temperature departures from 1951-1980 baseline for two datasets (HadCRUT3 and GISSTEMP)

Despite an apparent break in the warming trend of the global ocean surface since the warm El Niño year of 1998 (see Figure, right), our analysis (lead by Dr Norman Loeb of the NASA Langley research centre) suggests that heat has continuing to build up at the rate of 0.5 Watts for each square metre of the Earth's surface - that is equivalent to the total heat produced by over 250 billion kiloWatt electric heaters distributed over the globe (assuming a global area of 510 million km2). This is important since it suggests that while surface warming appears to be absent over the last decade, energy continued to build up below the ocean surface, at a rate that is consistent with the effects of increased greenhouse gas concentrations in the atmosphere.

Global climate change results from an imbalance between the amount of sunlight absorbed by Earth and the thermal radiation emitted back to space. Elevated concentrations of greenhouse gases, due primarily to the burning of fossil fuels, have reduced the efficiency at which Earth can cool to space through thermal radiative emission, resulting in a positive energy imbalance. Once this excess energy enters Earth's climate, small amounts are used heating up the atmosphere and the land as well as melting ice, but the bulk enters the oceans which have the capacity to store vast quantities of energy. Therefore fluctuations in radiative energy entering at the top of Earth's atmosphere (see previous post) must vary in unison with changes in ocean heating rate.

The effects of El Niño and La Niña - the climatic phenomena affecting normal weather patterns across the tropical Pacific - are enough to disturb this steady build up of heat. Yet scientists had previously puzzled over additional "missing energy" that was detected entering the planet but which appeared not to arrive in the oceans. Our analysis suggest that changes to the way sea temperatures are measured, improvements in the satellite data products and substantial statistical margins of error are enough to account for this discrepancy. Furthermore, they confirm that energy has indeed been accumulating in Earth's climate since 2000 and that much of this "excess energy" has been continuing to heat the sub-surface ocean.

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Richard P. Allan
Location: Department of Meteorology (2U15)


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