The exact causes of climate change remain a mystery to science.  Many researchers link recent global warming to changes in the sun. Others remain skeptical, claiming that the sun varies only very slowly, over periods of millions of years. They say that no hard evidence exists for a solar effect on recent climate changes.

Now, new research may have provided just that evidence, with data demonstrating that solar variations have had major effects on the earth's climate as recent as 2,000 years ago. The research, conducted by a team of scientists from the Universities of Ohio, Minnesota, and Texas at Arlington, confirms that, during periods when the earth received less solar radiation, the Atlantic Ocean cooled, rainfall levels dropped, and North America experienced periods of intense drought. Some droughts lasted as long as a century.

Seven such events were detected, occurring once every 1,500 years, a period that matches the so-called "Bond Events" cycle of solar variation.

According to the study's lead author, Greg Springer, the correspondence provides "convincing evidence" of a solar effect on North American climate. "This really nails down the idea of solar influence on continental drought," said Springer.

The critical data was obtained from an 8-inch long stalagmite from a cave in West Virginia. As the stalagmite grew over a period of several thousand years, its composition precisely recorded fluctuations in the Earth's climate.

Data more recent than 2,000 years ago wasn't examined, as the authors feared the impact of Native Americans on nearby watersheds may have influenced the results. However, Springer tells DailyTech that analysis of river discharges around the area confirm that the region is still being impacted by such changes today.

The stalagmite was sampled at various points along its length, each point radiologically dated to precisely determine its age. Samples were analyzed to determine the ratio of strontium to calcium, along with levels of the isotopes carbon-13 and oxygen-18. Each of these acts as a proxy (indicator) for various climate parameters such as rainfall levels and land or sea-surface temperatures. The stalagmite therefore acts as a highly accurate record of climate variations over a period as little as a few years.

Spectral analysis of the data revealed cycles with periods corresponding to harmonics of the 1500-year Bond Events, a cycle triggered by weak solar activity which results in dramatic cooling and the occurrence of "IRD" events -- ice raft debris forming in the Atlantic.

Springer tells DailyTech that the results "certainly lends support" to global warming skeptics. However, he himself is not sure that the recent level of variance is enough to explain all the warming the Earth has undergone in the past 100 years.

Last year, scientist S. Fred Singer published the book, Unstoppable Global Warming (Every 1500 years), which tied current warming to an ongoing Bond Event.  Singer tells Daily Tech that this new study provides further evidence for a 1500-year Climate cycle, the evidence for which was first discovered in ice core data.

The research is appearing in an upcoming edition of Geophysical Research Letters.