What was done
The authors developed what they call "the first complete high-resolution [17-year] climate proxy for the southwest [United States] in the form of δ¹⁸O variations in a speleothem covering the entire Holocene," which they derived from a 14-cm-long stalagmite found in Pink Panther Cave in the Guadalupe Mountains of New Mexico.

What was learned
Spectral analysis performed on the raw δ¹⁸O data revealed significant peaks that the researchers say "closely match previously reported periodicities in the ¹⁴C content of the atmosphere, which have been attributed to periodicities in the solar cycle (Stuiver and Braziunas, 1993)." More specifically, they say that cross spectral analysis of the Δ¹⁴C and δ¹⁸O data confirms that the two records have matching periodicities at 1533 years (the Bond cycle), 444 years, 170 years, 146 years, and 88 years (the Gleissberg cycle). In addition, they report that periods of increased solar radiation correlate with periods of decreased rainfall in the southwestern United States (via changes in the North American monsoon), and that this behavior is just the opposite of what is observed with the Asian monsoon. These observations thus lead them to suggest that the proposed solar link to Holocene climate operates "through changes in the Walker circulation and the Pacific Decadal Oscillation and El Niņo-Southern Oscillation systems of the tropical Pacific Ocean."

What it means
Since the warming of the 20th century appears to represent the most recent rising phase of the Bond cycle, which in its previous rising phase produced the Medieval Warm Period (see Bond et al., 2001), and since we could still be imbedded in that rising temperature phase, which could well continue for some time to come, there is a reasonable probability that the desert southwest of the United States could experience an intensification of aridity in the not too distant future, and that wetter conditions could be expected in the monsoon regions of Asia, even without any further increases in atmospheric greenhouse gases, which realization makes planning for these contingencies a much higher priority - and ever so more logical - than fooling ourselves into thinking we can prevent such climate changes by cutting back on the burning of fossil fuels.

References
Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G. 2001. Persistent solar influence on North Atlantic climate during the Holocene. Science 294: 2130-2136.

Stuiver, M. and Braziunas, T.F. 1993. Sun, ocean climate and atmospheric ¹⁴CO2: An evaluation of causal and spectral relationships. The Holocene 3: 289-305.