Questions in the ‘Climate: Sun-Earth’ Category

Has the Sun been more active in recent decades, and could it be responsible for some global warming?

By Holli Riebeek June 24, 2010

Scientists are still debating whether or not the Sun’s activity increased during the latter half of the 20th century, but even the highest estimates of activity can’t account for the warming observed since about 1950.

Studies do show that solar variability has significantly influenced past climate changes. For example, a decrease in solar activity is thought to have triggered the Northern Hemisphere’s Little Ice Age between approximately 1650 and 1850, when temperatures dipped low enough that rivers that don’t freeze in today’s human-warmed climate froze over.

Scientists use substitutes (proxies) like records of sun spots, which have been kept since Galileo’s time, or carbon in tree rings to estimate the amount of energy the Sun has sent to Earth. Though not perfect, these estimates give a rough approximation of how much the Sun’s activity has varied over time. Scientists are still debating over how reliable proxies are in determining the Sun’s past activity, but current estimates indicate that the Sun is probably now as active as or more active than it has ever been during the past 8,000 years.

Records of sunspots dating back to 1610 provide an indication of energy output from the Sun. In general, more sunspots (blue peaks) mean more intense solar activity and more energy received by the Earth. On average, there are more sunspots now than during the Maunder Minimum 350 years ago, but the increase in solar activity alone is not sufficient to explain the temperature increases that have occurred since 1950. (Graph by Robert Simmon, based on data from Hoyt and Schatten, 1997.)

A shorter, but more detailed record comes from NASA satellites, which have been recording the Sun’s activity from space since 1978. The measurements, however, come from six different satellites, each with its own bias. It is difficult to combine the measurements from these satellites into a single 25-year-plus record to get a trend of solar activity. Different scientific teams have attempted to create a continuous record from the satellite data. Each long-term record shows the rise and fall of two 11-year sunspot cycles, but they differ from one another in the average trend over the full period. When stitched together one way, the satellites seemed to record a slight increase in solar activity, but in other analyses, solar activity remained constant.

Regardless, even when scientists assume that solar activity is increasing based on proxy data and the satellite record, they can’t account for all of the warming observed at the end of the twentieth century. Climate models can only reproduce the warming observed since 1950 when a rise in greenhouse gases is built into the system.

References
Foucal, P., Frölich, C., Spruit, H., and Wigley, T. (2006). Variations in solar luminosity and their effect on the Earth’s climate. Nature, 443, 161-166.

Fröhlich, C. (2007). Solar Irradiance Variability Since 1978. In Solar Variability and Planetary Climates (pp. 53-65).

Hoyt, D. V., and Schatten K. H. (1997). Group Sunspot Numbers: A New Solar Activity Reconstruction. Solar Physics, 179 (1), 189-219.

Lean, J. L. (2009). Cycles and trends in solar irradiance and climate. Wiley Interdisciplinary Reviews: Climate Change, 1(1), 111-122.

Lindsey, R. (2003). Under a Variable Sun. NASA’s Earth Observatory. Accessed June 20, 2010.

Muscheler, R., Joos, F., Müller, S.A., Snowball, I. (2005). Climate: how unusual is today’s solar activity? Nature, 436, E3-E4.

Rind, D., Shindell, D., Perlwitz, Ju., Lerner, P., Lonergan, P., Lean, J., and McLinden, C. (2004). The relative importance of solar and anthropogenic forcing of climate change between the Maunder Minimum and the present. Journal of Climate, 17, 906-929.

Solanki, S.K., Ususkin, I.G., Kromer, B., Schössler, M., Beer, J. (2004). Unusual activity of the Sun during recent decades compared to the previous 11,000 years. Nature, 431, 1084-1087.

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