Read here. As with most major climatic changes, solar influences are the major driver, not trace gases, such as CO2. Using Sr/Ca ratios and δ13C data obtained from stalagmite of Buckeye Creek Cave in West Virginia, USA, researchers reviewed 7,000 years of N. American hydro-climate. Researchers discovered that drought conditions followed the solar irradiance cycles of 200 and 500 years, approximately, in addition to even longer solar oscillations.
"The five researchers conclude by stating that their findings "corroborate works indicating that millennial-scale solar-forcing is responsible for droughts and ecosystem changes in central and eastern North America and that their high-resolution time series "provide much stronger evidence in favor of solar-forcing of North American drought by yielding unambiguous spectral analysis results."