What role does the Sun play in climate change?
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As a massive nuclear fusion reactor a million times larger than Earth, the Sun is responsible for almost all the energy reaching our planet. Prior to the late 1950s, solar variations were the single largest influence on warming, explaining much of the global temperature increase since the start of the century. Over the past 30 years, however, the solar influence appears to be minimal, compared to the continued buildup of greenhouse gases. As noted in the IPCC’s 2007 assessment, virtually all scientific studies indicate that anthropogenic forcings1 (human-produced emissions) have overwhelmed any natural forcings, such as solar radiation.
If the Sun were responsible for the current warming trend, the troposphere and the stratosphere would both be warming. The stratosphere, however, has actually cooled over the past few decades, as predicted by climate scientists. This is because more of the energy moving from the Earth’s surface back into space is being held by greenhouse gases in the troposphere, leaving less heat for the stratosphere to absorb.
Some scientists have hypothesized that cosmic rays inhibit cloud formation and thus are at the root of increasing global temperatures. This hypothesis is based in part on the fact that fewer cosmic rays reach Earth when solar activity is high. However, any such connection would not explain the last 30 years of warming, because cosmic rays are not well correlated with global temperature over this period.
For all of these reasons, it is highly unlikely that the Sun is responsible for the observed modern warming since the 1970s.
1A climate forcing is anything that can change the climate. Natural forcings include changes in solar output, orbital changes, and emissions from volcanoes. Anthropogenic (or human-produced) forcings include carbon and other greenhouse gas emissions, particulate emissions, and land-cover changes.
More information: Meehl, et al. 2004. Combinations of Natural and Anthropogenic Forcings in Twentieth-Century Climate. Journal of Climate. Volume 17:3721-3727
Lean, J. 2004. Solar Irradiance Reconstruction. IGBP PAGES/World Data Center for Paleoclimatology
Data Contribution Series # 2004-035. NOAA/NGDC Paleoclimatology Program, Boulder CO, USA.
Lockwood and Frohlich. 2007. Recent oppositely directed trends in solar climate forcings and the global mean surface air temperature. Proceedings of the Royal Society