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Q2:   The Sun


  • Can global warming be attributed to the Sun?


Answer:


Records of sunspot activity date back hundreds of years.  These records are based on observations, not instrumentally acquired data.  In the late 1970s, satellites became a source of instrumentally recording solar output, but have not yet had the opportunity to meaningfully contribute to long term trend analysis due to their limited time scale (1).

Based on the data, it is believed that the Sun’s change in radiative forcing (~ 0.09 W/m2) since the start of industrial era (about 1750) is much less than that of greenhouse gases (~ 2.6 W/m2) (2).

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Figure 1:  Various contributors to climate change and their associated radiative forcings in 2005 relative to the start of industrial era (about 1750) (2)


The Sun’s role in the global warming process will become clearer as more time passes, as longer data records will be available for study.  The IPCC has revised its estimate of the radiative forcing attributable to solar irradiation since the Third Assessment Report (TAR);  the most recent IPCC report shows a reduced role for solar irradiation as a factor in recently observed warming compared to its earlier TAR3.

A recent study by Lockwood & Fröhlich (3) asserts that the Earth’s surface air temperature does not respond to changes in the solar cycle, and that the trend of increasing global average temperature is in fact in direct opposite to a decrease in instrumentally recorded solar irradiance and sunspot activity (see Figure 2).

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Figure 2:  Sunspot activity, solar irradiance and global mean surface air temperature from 1975 to 2005.  R is the international sunspot number as compiled by the World Data Centre for the Sunspot Index.  TSI (Wm-2) is the total solar irradiance as compiled by the World Radiation Centre.  T (°C) is the global mean surface air temperature departure from the 1951-1980 mean as compiled by the Goddard Space Flight Center.  (Source: after Lockwood & Fröhlich 2007)