Published Tuesday, December 21, 1999, in the San Jose Mercury News
No one theory can explain all episodes of Earth warming
BY JAN NULL
Q. Although almost everyone believes in the current global
warming and its linkage to greenhouse emissions, there have been episodes of warming in
the past without such an obvious connection. Around the end of the first millennium,
Greenland was warm enough to be settled by Scandinavian farmers, and
grapes grew in Maine (or Nova Scotia, depending on where ``Vinland'' was).
Was this a worldwide warming, or was it confined to the North Atlantic? And do we know
what caused it? The ``Little Ice Age'' that followed the warming has been linked to
sunspots. Don Luttrell - San Jose
A. The warm period you speak of occurred around 1000 to 1300 AD and is
referred to as the Medieval Warm Period, or the Medieval Climatic Optimum. From all the
evidence I have seen, the Medieval Warming Period was not a true global event, but was
confined to regions around the Atlantic Basin.
As far as the ``Little Ice Age,'' one hypothesis, as you suggest, has to do with sunspots.
However, other climatologists attribute it to volcanic eruptions and massive
ash clouds limiting the energy from the sun warming Earth.
Like the current global warming, there are almost as many theories as researchers.
Q. Why is the rain year July 1 to June 30? Is it only in this area,
or nationwide? Worldwide? Seems it would make more sense, and easier to remember and
comprehend, if measured by calendar year! William Dixon - San Jose
A. The use of a July 1 to June 30 rainfall season is common only among
meteorologists in the Western United States. This is because the natural summer
``drought'' we experience every year makes it a logical place to separate seasons.
In other parts of the country, where a significant amount of their annual precipitation
falls in the form of summertime thunderstorms, meteorologists simply use the January
through December calendar year.
To further confuse the issue, hydrologists, who study surface and groundwater, use a
``water year'' that begins Oct. 1 and ends Sept. 30. Since most of the West gets very
little rain from July through September, data for the water year closely matches our
rainfall season.
Q. This fall I've heard and seen a few people refer to the winter
of 1998-99 as a La Niņa winter. As I recall, El Niņo conditions were with us into
January. In late, very late, January or early February, the conditions changed to La
Niņa, as I recall. Am I correct? Joe Parks - Sunnyvale
A. Afraid not. El Niņo conditions ended in the Pacific during the
late spring and summer of 1998. By early fall, the equatorial Pacific had cooler than
average waters -- the condition we call La Niņa. The water temperatures, and other La
Niņa indicators, have remained cool since that time with little variation.
A review of last season in the Bay Area showed near normal rainfall, with above normal
rain in Northern California and the Pacific Northwest, and below normal in Southern
California. This is similar to what we have seen in past La Niņas, and is what the
long-range climate models are indicating.
However, it is unclear from past events, what difference the fact that this is a second
consecutive La Niņa year might make in this season's rainfall totals. And, we must also
keep in mind that La Niņa is only one of a number of atmospheric processes that could
influence this winter's weather.
Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send him questions c/o Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone and fax them at (510) 657-2246 or e-mail them to weathercorner@ggweather.com.