Published Tuesday, December 21, 1999, in the San Jose Mercury News


No one theory can explain all episodes of Earth warming

Special to the Mercury News

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