Geoscience Reference
In-Depth Information
activity. One of these methods is the amount of radiocarbon produced in the
earth's atmosphere, which is closely related to the output of energetic par-
ticles from the sun (the solar wind). Periods of low solar activity correspond
to high radiocarbon production in the earth's atmosphere and vice versa.
Radiocarbon (carbon-14) combines with oxygen to form carbon dioxide
and is taken up by plants and trees during photosynthesis. Years with higher
levels of radiocarbon production will thereby be recorded in the growth
rings of the trees. Geochemists measure the minute amounts of carbon-14 in
tree rings and compare this radiocarbon age to the actual (calendar year) age
of the tree rings, obtained by counting the annual rings.
h e tree rings used in these studies are those of the long-lived bristle-
cone pines in the White Mountains of eastern California, with living trees
that are 4,000 years old and fossilized trees extending the record back to
11,000 years before the present. h ese studies have shown that, in addition
to a 200-year cycle in solar activity, there are solar cycles with periods of 55,
90, 400, 1,000, and 2,500 years. One key i nding is that these periodicities
are seen in a number of climate records in the West, including extreme l oods
and droughts, suggesting that solar activity somehow af ects when and where
precipitation falls by inl uencing atmospheric circulation.
Another cycle that climate scientists have detected has a 1,500-year peri-
odicity. For decades, geologists have noted that glaciers throughout the
Holocene advanced and retreated every 1,500 years or so. More recently, this
1,500-year cycle has also been documented in oceanic sediment cores in the
form of discrete layers of debris transported to the North Atlantic by icebergs
(known as “ice-rat ed debris”). In the eastern Pacii c, changes in sea surface
temperatures from the Santa Barbara Basin similarly exhibit a 1,500-year
cycle. h us, this cycle appears to correlate with solar activity, but details of
how it inl uences climate are still being investigated.
terrestrial causes of climate change:
volcanic eruptions
Volcanic eruptions—violent natural events in their own right—have been
linked with past climate variations. Although eruptions occur relatively fre-
quently, they are not cyclical. Over timescales of millions of years, changes in
the number of volcanic events on land and undersea (where volcanism occurs
at mid-ocean ridges and during the formation of oceanic plateaus) eventually
