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Atmospheric science

Sun's energy output

Accurately estimating the Sun's energy output is vital for attributing climate change correctly. Improved measurements have now been obtained from a space-based instrument.

Credit: © von Au

The amount of solar radiation striking Earth is slightly less than previously recognized, a study suggests.

Using data gathered by a sensor on a satellite launched in 2003, solar scientists Greg Kopp of the Laboratory for Atmospheric and Space Physics in Boulder, Colorado and Judith Lean of the Naval Research Laboratory in Washington DC, estimate that total solar irradiance — or TSI, the amount of energy entering the top of Earth's atmosphere — is 1,360.8 watts per square metre1. That's about 0.34 per cent below the TSI value established in the 1990s and substantially below the error bars of the previous estimate. The new TSI value doesn't reflect a change in the Sun's radiation output; instead, the researchers report, the more accurate estimate comes courtesy of an instrument specifically designed to limit the stray light being scattered into cavities where sensors are located — a major source of error in earlier generations of space-based instruments, Kopp notes.

Besides providing improved estimates of Earth's energy balance, the new results should help scientists better identify and track any long-term trends in solar irradiance.


  1. Kopp, G. & Lean, J. L. A new, lower value of total solar irradiance: Evidence and climate significance. Geophys. Res. Lett. 38, L01706 (2011).

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Perkins, S. Sun's energy output. Nature Clim Change (2011).

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