figure 2. Measurements of the
Sun’s energy incident on Earth
show no net increase in solar
forcing during the past 30 years,
and therefore this cannot be
responsible for warming during
that period. The data show only
small periodic amplitude variations
associated with the Sun’s 11-year
cycle. Figure by Keith Shine.
Source: TSI data from Physikalisch-
Meteorologisches Observatorium
Davos, Switzerland, adjusted down
by 4.46 W m-2 to agree with the 2008
solar minimum data from Kopp and
Lean, 2011; temperature data from the
HadCRUT4 dataset, UK | The Sun provides the primary source of energy driving Earth’s climate system, but its
variations have played very little role in the climate changes observed in recent decades.
Direct satellite measurements since the late 1970s show no net increase in the Sun’s output,
while at the same time global surface temperatures have increased For earlier periods, solar changes are less certain because they are inferred from indirect
sources — including the number of sunspots and the abundance of certain forms (isotopes) of carbon
or beryllium atoms, whose production rates in Earth’s atmosphere are influenced by variations in the
Sun. There is evidence that the 11 year solar cycle, during which the Sun’s energy output varies by roughly
0.1%, can influence ozone concentrations, temperatures, and winds in the stratosphere (the layer in the
atmosphere above the troposphere, typically from 12 to 50 km, depending on latitude and season). These
stratospheric changes may have a small effect on surface climate over the 11 year cycle. However, the
available evidence does not indicate pronounced long-term changes in the Sun’s output over the past
century, during which time human-induced increases in CO2 concentrations have been the dominant
influence on the long-term global surface temperature increase. Further evidence that current warming
is not a result of solar changes can be found in the temperature trends at different altitudes in the
atmosphere (see Question 5). |