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.
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.
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.
The first piece of evidence that the warming over the past few decades isn’t part of a natural cycle is how fast the change is happening. The biggest temperature swings our planet has experienced in the past million years are the ice ages. Based on a combination of paleoclimate data and models, scientists estimate that when ice ages have ended in the past, it has taken about 5,000 years for the planet to warm between 4 and 7 degrees Celsius. The warming of the past century—0.7 degrees Celsius—is roughly eight times faster than the ice-age-recovery warming on average.
The second reason that scientists think the current warming is not from natural influences is that, over the past century, scientists from all over the world have been collecting data on natural factors that influence climate—things like changes in the Sun’s brightness, major volcanic eruptions, and cycles such as El Niño and the Pacific Decadal Oscillation. These observations have failed to show any long-term changes that could fully account for the recent, rapid warming of Earth’s temperature.
Finally, scientists know that carbon dioxide is a greenhouse gas and that it is released into the air when coal and other fossil fuels burn. Paleoclimate data show that atmospheric carbon dioxide levels are higher than they have been in the past 800,000 years. There is no plausible explanation for why such high levels of carbon dioxide would not cause the planet to warm.
- Hegerl, G. C., Zwiers, F. W., Braconnot, P., Gillett, N. P., Luo, Y., Orsini, J. A., Nicholls, N., et al. (2007). Chapter 9: Understanding and attributing climate change. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. [Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B. , Tignor, M., and Miller, H.L. (eds.)] Cambridge and New York: Cambridge University Press.
- Jansen, E., Overpeck, J., Briffa, K.R. , Duplessy, J.-C , Joos, F., Masson-Delmotte, V., Olgao, D., et al. (2007). Chapter 6: Paleoclimate. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. [Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B. , Tignor, M., and Miller, H.L. (eds.)] Cambridge and New York: Cambridge University Press.
- Lean, J. L., & Rind, D. H. (2008). How natural and anthropogenic influences alter global and regional surface temperatures: 1889 to 2006. Geophysical Research Letters, 35(18).
- Lockwood, M., & Fröhlich, C. (2008). Recent oppositely directed trends in solar climate forcings and the global mean surface air temperature. II. Different reconstructions of the total solar irradiance variation and dependence on response time scale. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 464(2094), 1367-1385.
- Lüthi, D., Le Floch, M., Bereiter, B., Blunier, T., Barnola, J., Siegenthaler, U., Raynaud, D., et al. (2008). High-resolution carbon dioxide concentration record 650,000–800,000 years before present. Nature, 453(7193), 379-382. [Download 800,000-Year CO2 Data]
- Steele, L. P., Krummel, P. B., & Langenfelds, R. L. (2007). Atmospheric CO2 concentrations from sites in the CSIRO Atmospheric Research GASLAB air sampling network (August 2007 version). In Trends: A Compendium of Data on Global Change, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, US Department of Energy, Oak Ridge, TN, USA.