Geoscience Reference
In-Depth Information
atmosphere. Satellite observations have shown that the solar constant—the
total amount of solar energy l ux received by the earth—is not actually
“constant.” Rather, it varies on timescales of days to years, apparently related
to the number of sunspots. Climatologists (and economists) have noted an
eleven-year sunspot cycle that has been correlated with such climate-related
phenomena as agricultural production in the United States. During periods
of high sunspot activity, the value of the solar constant increases, triggering a
chain reaction in the atmosphere and ocean that ultimately leads to changes
in precipitation and temperature patterns in dif erent regions of the earth.
Scientists are now exploring the details of changing solar output and asso-
ciated ef ects on the earth's climate system. h ey use climate models to show
the ef ects of small energy changes received in the upper atmosphere. h ese
changes can be amplii ed through positive feedbacks in the planet's climate
system. Climate modeler Gerald Meehl and his colleagues at the National
Center for Atmospheric Research in Boulder, Colorado, have proposed two
mechanisms for this amplii cation. In the “top-down” mechanism, increased
sunspot activity leads to an increase in the amount of ultraviolet light hitting
the stratosphere—enhancing the formation of ozone, absorbing more ultravi-
olet light, and thereby heating the stratosphere. h e amount of heating in the
stratosphere is not uniform across the globe but varies with latitude. h e tem-
perature dif erences lead to an increase in winds and tropical precipitation.
In the second, “bottom-up,” mechanism, greater sunspot numbers lead to
increased absorption of solar energy by the surface ocean in the subtropics
and so increased evaporation of seawater. h e trade winds become stronger,
and the evaporated moisture carried by the trade winds increases precipita-
tion in the western Pacii c and subtropical zone. h ese stronger winds blow-
ing from east to west across the Pacii c increase the upwelling of cooler water
in the eastern equatorial Pacii c, lowering sea surface temperatures, similar
to conditions during a La Niña event.
Although the mechanisms linking sunspots and climate are still being
investigated, these climate models are important steps in understanding
why precipitation patterns in the American West and elsewhere appear to be
closely tied to sunspot cycles.
Radiocarbon and Past Solar Activity
In an attempt to understand past changes in solar output in deeper geologi-
cal time, Earth scientists have devised ingenious proxy methods for solar
