Uncertainties in Solar Measurements
Despite all that scientists have learned about solar irradiance over the past few decades, they are still a long way from forecasting changes in the solar cycles or incorporating these changes into climate models. One of their biggest obstacles has been technology. Because even the smallest shifts in solar energy can affect climate drastically, measurements of solar radiation have to be extremely precise. Instruments in use today still are subject to a great deal of uncertainty.
 

Solar Radiation and Climate Experiment (SORCE)
Introduction
Earth’s Energy Balance
Solar Variability
The Sun and Global Warming
Uncertainties in Solar Measurements

The SORCE Satellite
Total Irradiance Monitor (TIM)
Spectral Irradiance Monitor (SIM)
Solar Stellar Comparison Experiment (SOLSTICE)
Extreme Ultraviolet Photometer System (XPS)

Related Articles
SOLSTICE
Watching the Sun
ACRIMSAT
Sunspots and the Solar Max
Clouds and Radiation
Why isn’t Earth Hot as an Oven?

Related Datasets
Reflected Solar Radiation
Outgoing Heat Radiation

The various sensors agree closely in the timing and amplitude of rapid daily variations due to the passage of individual sunspot groups. The sensors also agree in the amplitude of the 11-year cycle, but disagree significantly in the decadal average level of the TSI—up to 6 watts per square meter. This difference is larger than the total variation in solar irradiance in the past 500 years, so a more accurate assessment is needed to study the Sun’s impact on climate change. An upcoming NASA research satellite, the Solar Radiation and Climate Experiment (SORCE), will carry instruments designed to do just that. (Graph adapted from C. Frölich of the World Radiation Center in Davos Switzerland)

The total change in TSI over the 11-year cycle is believed to be 0.1 percent of the Sun’s total energy on a yearly average. Individual sunspot events are very accurately reproduced in independent TSI measurements, so that the relative accuracy on weekly and 11-year time scales is sufficient to characterize such changes. However, the most accurate estimates of the long-term average TSI are uncertain by several times the amplitude of the 11 year cycle. This large uncertainty in absolute calibration of the instruments means that any possible trend from one 11 year cycle to the next, the most important change for global warming, is not known accurately enough to even decide whether the trend is positive, negative, or zero. With such data, scientists have a good approximation of the 11 year cycle, but no real insight into more subtle changes that may occur over many decades and centuries.

Even larger uncertainties exist for measurements of the amount of solar radiation that is absorbed by the Earth’s atmosphere, ocean, and land. As of now, researchers know that the atmosphere absorbs between 20 and 25 percent of the TSI and that the land absorbs 45 to 50 percent. With solar radiation, a 5 percent difference is huge. A difference of even 1 percent would completely throw off climate models of global warming and scientist’s understanding of convection (warm, upward moving air currents) in the atmosphere.

The other big problem scientists face is too little data. Even in instances when solar energy measurements are accurate, researchers often don’t have enough information with which to draw conclusions. Building models to forecast long term trends, in particular, requires a tremendous amount of past data on those trends. At this time, scientists only have roughly twenty years of satellite data on the Sun —an equivalent of just two 11-year cycles. Most of the data researchers do have on the Sun are for TSI. Relatively very little data have been gathered on the spectral changes in the Sun. Scientists haven’t determined with precision how the fluctuations in the Sun’s output of visible wavelengths differ from near infrared or from ultraviolet. The dearth of spectral data presents another serious obstacle for climate modelers since distinct wavelengths are absorbed by different components of the Earth’s climate system, which react differently with one another as their energy levels change.

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