Changes
in the sun's intensity: Changes occurring within (or inside)
the sun can affect the intensity of the sunlight that reaches the Earth's
surface. The intensity of the sunlight can cause either warming (for
stronger solar intensity) or cooling (for weaker solar intensity). According
to NASA
research, reduced
solar activity from the 1400s to the 1700s was likely a key factor in the “Little
Ice Age” which resulted in a slight cooling of North America, Europe
and probably other areas around the globe. (See additional discussion under
The Last 2,000 Years.)
Volcanic eruptions: Volcanoes can affect the climate
because they can emit aerosols and carbon dioxide into the atmosphere.
Aerosol emissions: Volcanic aerosols tend to block
sunlight and contribute to short term cooling. Aerosols do not produce
long-term change because they leave the atmosphere not long after they
are emitted. According to the United
States Geological Survey (USGS), the eruption of the Tambora Volcano in Indonesia in 1815 lowered
global temperatures by as much as 5ºF and historical accounts in New
England describe 1816 as “the year without a summer.”
Carbon
dioxide emissions: Volcanoes also emit carbon dioxide (CO2),
a greenhouse gas, which has a warming effect. For about two-thirds
of the
last 400 million years, geologic evidence suggests CO2 levels
and temperatures were considerably higher than present. One theory is
that volcanic eruptions from rapid sea floor spreading elevated CO2 concentrations,
enhancing the greenhouse effect and raising temperatures. However, the
evidence for this theory is not conclusive and there are
alternative
explanations for historic CO2 levels (NRC,
2005). While volcanoes may
have raised pre-historic CO2 levels
and temperatures, according to the USGS
Volcano Hazards Program,
human activities now emit 150 times as much CO2 as
volcanoes (whose emissions are relatively modest compared to some
earlier times).