NRL Collaborates on Research to Improve Understanding of Climate Change

1/14/2011 - 1-11r
Contact: Donna McKinney, (202) 767-2541

The Naval Research Laboratory's Judith Lean is a co-author in new research that will help scientists better understand how the Sun's radiation interacts with the climate system. Scientists have a continuous, 32-year record of the Sun's energy output, which they call total solar irradiance (TSI). This new research improves the accuracy of this record. Solar radiation is the primary energy input to Earth's climate, which scientific consensus indicates has been warming since the Industrial Revolution. Greg Kopp, from the University of Colorado, Laboratory for Atmospheric and Space Physics, is the lead author of the study.

Shown by the green line are fluctuations in the Sun's total irradiance estimated from an NRL model that combines the two primary influences of bright faculae and dark by sunspots, features that are evident in the solar image, made by the Solar and Heliospheric Observatory. The relative proportions of the facular brightening and sunspot darkening are determined via regression with direct observations, shown by the green symbols, made by the Total Irradiance Monitor (TIM) on the Solar Radiation and Climate Experiment (SORCE). The TIM, shown in the upper left, employs a new radiometric design and laboratory calibrations to measure with unprecedented accuracy and precision the Sun's energy input to Earth. The close agreement between the measurements and model provides confidence in reconstructions of past solar irradiance variations prior to the space-based measurements.

NRL's Lean, who works in the Space Science Division, specializes in the effects of the sun on climate and space weather and in modeling how the Sun's energy output varies on multiple time scales. She said, "Scientists estimating Earth's climate sensitivities need accurate and stable solar irradiance records to know exactly how much warming to attribute to changes in the Sun's output, versus anthropogenic or other natural forcings." Kopp, the lead author, said, "Improved accuracies and stabilities in the long-term total solar irradiance record mean improved estimates of the Sun's influence on Earth's climate."

The research, which is now available on the Geophysical Research Letters Papers in Press site, supports a lower TSI value than scientists had previously accepted. This new value has been reported by the LASP-built Total Irradiance Monitor (TIM) instrument on the NASA Solar Radiation and Climate Experiment (SORCE) mission; validation tests at a new calibration facility at LASP support the lower TSI value. With the ground-based calibration facility, scientists can validate their instruments against a reference standard calibrated by the National Institute of Standards and Technology (NIST) under conditions similar to on-orbit. Before the development of the calibration facility, scientists would often get different measurements from different solar irradiance instruments, based on variations in their calibration. Without the calibration facility, scientists had to rely on overlapping measurements that let them to intercalibrate between instruments in order to maintain a long-term record of the Sun's output through time.

"The calibration facility indicates that the TIM is producing the most accurate total solar irradiance results to date, providing a baseline value that allows us to make the entire 32-year record more accurate," Kopp said. "This baseline value will also help ensure that we can maintain this important climate data record for years into the future, reducing the risks from a potential gap in spacecraft measurements."

"We are eager to see how this lower irradiance value affects global climate models, which use various parameters to reproduce current climate: incoming solar radiation is a decisive factor," Lean said. "An improved and extended solar data record will make it easier for us to understand how fluctuations in the sun's energy output over time affect temperatures, and how Earth's climate responds to radiative forcing."

Lean's model, which is now adjusted to the new lower absolute TSI values, reproduces with high fidelity the TSI variations that TIM observes and indicates that solar irradiance levels during the recent prolonged solar minimum period were likely comparable to levels in past solar minima. Using this model, Lean estimates that solar variability produces about 0.1°C global warming during the 11-year solar cycle, but is not the main cause of global warming in the past three decades.

Geophysical Research Letters Papers in Press site:

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