Studies of tree rings in the ancient fir forests of the Pacific Northwest suggests the earth cooled down four times in the last 860 years during periods when sunspots almost disappeared from the surface of the sun.
The studies were done of the annual rings of Douglas firs in Oregon, Washington and Vancouver Island in Canada. While the ages of the trees ranged from 152 years to 1,268 years, five of the trees covered a time span of almost 900 years when four "Little Ice Ages" took place on the earth's surface.
Measuring the radioactive carbon-14 changes in the wood of the tree rings, Drs. Minze Stuiver and Paul D. Quay of the University of Washington said in Science magazine that they found the carbon-14 levels much higher than normal during periods that the earth cooled down. When carbon-14 levels are high, sunspot activity is almost always low.
Sunspots show up as dark circles on the sun's surface and are often associated with tremendous bursts of energy that occasionally trigger flares of gas and heat millions of miles from the sun into space. Many scientists have long believed that periodic changes in the sunspot cycles trigger changes in the earth's climate.
Looking back through three rings all the way to the year 1010, Stuiver and Quay found the carbon-14 contents of the tree rings rose sharply during four periods of cooling on the earth's surface. The periods fell in the years 1010 to 1050, 1280 to 1340, 1420 to 1530 and 1645 to 1715.
The last period overlapped what natural historians call the "Little Ice Age" of the 17th century, which was marked by excessively cold winters in most of Western Europe and in the colonies of New England.
It also partially coincided with a period known as the "Maunder Minimum," during which scientists noticed almost a complete disappearance of sunspots from the solar surface. At the time, scientists had just begun to keep track of sunspots and record them for posterity.
Stuiver and Quay said their study suggests sunspots were absent during the periods of minumum activity for an average of 79 years each time The longest period of "absent" sunspot activity was 118 years, the two scientists said corresponding to the period from 1282 to 1342 when it was unusally cold over most of northern Europe.