Why is such a small increase such a big deal?


Climate change scientists may be willing to let the solar physicists split hairs, however, at least for now. “Whether the Sun’s output has increased by 0.05 percent per decade over the past two solar cycles makes little difference to climate,” says Dr. Gerald North, first Study Scientist for NASA’s Tropical Rainfall Measuring Mission, and a climate modeler with Texas A&M. “With up to 95 percent confidence we can say that changes in solar output of 0.1 percent over the 11-year solar cycle produces a maximum increase in temperature of two hundredths of a degree (Kelvin), hardly significant in itself.”

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Photograph of Sunset

Climate modeler Jim Hansen, of NASA’s Goddard Institute for Space Studies, agrees that the climate impact of .05 percent per decade would be practically non-existent if it were only maintained for one decade. “If such a small change were followed by no further change or a decrease, it’s not important,” he says. “But if that rate of change were maintained for a century, it would be a change of 0.5 percent, which would be very important. A half of a percent change in solar output could raise temperatures, eventually, about three-quarters of a degree Celsius, which, coincidentally, roughly equals the observed warming in the past century,” says Hansen. The apparent coincidence is no smoking gun, however. Because of their great heat storage capacity, the Earth’s oceans would buffer any increase in the Sun’s output for a long time. “Nevertheless, the potential is there for the Sun to be a significant player in the climate game, at least over the long term,” says Hansen, “which is why we need to keep studying the issue.”

The controversy caused by the uncertainty over the accuracy and reliability of various sensors underscores the need for overlapping observations that can be used for cross comparison. In pursuit of that objective, NASA launched its Solar Radiation and Climate Experiment (SORCE) in January 2003. Using a suite of four radiometers that will measure more characteristics of the Sun’s output than have ever been observed before, the mission will add its solar observations to the ones currently being made by SOHO-VIRGO and ACRIM3. “Previous sensors would experience drifts in measurements caused by environmental influences on the sensor—such as a change in temperature of the whole spacecraft [and not just the sensor itself] when it was exposed to the Sun,” explains Robert Cahalan, SORCE Project Scientist. SORCE will use a new measurement approach to filter out this background noise; it will collect observations in short, regular pulses rather than continuously—not giving background noise a chance to interfere with the signal. It will also be the first mission to measure irradiance at the individual wavelengths that account for 95 percent of the total solar energy, rather than as one lump sum.

Until more observations are collected, however, we are left with controversy and the dependence of difficult environmental decisions on our imperfect understanding of the Sun and its influence on climate. The question of whether there is an overall upward trend in the Sun’s output over the last two decades becomes more than a scientific debate when it steps out from the pages of research journals and into the world where individuals and societies are facing difficult decisions about how to respond to climate change.

When asked how he felt about the possibility that his results might be used as justification for not doing anything to reduce greenhouse gas emissions, Willson said, “It would be just as wrong to take this one result and use it as a justification for doing nothing as it is wrong to force costly and difficult changes for greenhouse gas reductions per the Kyoto Accords, whose justification using the Intergovernmental Panel on Climate Change reports was more political science than real science.”

The potential for the findings to be used such a way is something Lean has considered. “The fact that some people could use Willson’s results as an excuse to do nothing about greenhouse gas emissions is one reason we felt we needed to look at the data ourselves,” says Lean. “Since so much is riding on whether current climate change is natural or human-driven, it’s important that people hear that many in the scientific community don’t believe there is any significant long-term increase in solar output during the last 20 years.”

  • Willson, R.C., and A.V. Mordvinov. (2003) Secular total solar irradiance trend during solar cycles 21–23. Geophysical Research Letters, 30 (5), 1199.
  • Frohlich, C., and J. Lean. (2002) Solar irradiance variability and climate. Astronomische Nachrichten 323: 203–212.

Although small changes in total solar irradiance may be insignifcant in the short term, if the sun continues to change it could impact the Earth’s climate, perhaps amplifying or counteracting global warming. Because of this, the scientific debate over solar variability has spilled over into politics. (Photograph copyright Joe Klein, SkyChasers.net)