What was done
Working in a cienega (a wet, marshy area where groundwater bubbles to the surface) -- located at approximately 31.3°N, 109.3°W in the drainage of Black Draw Wash/Rio de San Bernardino of southeastern Arizona (USA) and northeastern Sonora (Mexico) -- Brunelle et al. collected sediments during the summers of 2004 and 2005 from the incised channel wall of the Rio de San Bernardino arroyo and the cienega surface of the San Bernardino National Wildlife Refuge, from which samples were taken, as they describe it, "for charcoal analysis to reconstruct fire history," as well as pollen data to infer something about climate.

What was learned
The U.S. and Mexican researchers say "preliminary pollen data show taxa that reflect winter-dominated precipitation [which implies summer drought] correspond to times of greater fire activity," and that the results from the fire reconstruction "show an increase in fire activity coincident with the onset of ENSO, and an increase in fire frequency during the Medieval Climate Anomaly." In fact, they write that during this latter period, from approximately AD 900 to 1260, "background charcoal reaches the highest level of the entire record and fire peaks are frequent," after which they say "the end of the MCA shows a decline in both background charcoal and fire frequency, likely associated with the end of the MCA-related drought in western North America (Cook et al., 2004)."

What it means
With respect to the future -- based on their findings -- Brunelle et al. speculate that if the region of their study warms, "the role of fire in the desert grasslands is likely to change," such that "warming and the continuation of ENSO variability will likely increase fire frequency (similar to the MCA) while extreme warming and the shift to a persistent El Nino climate would likely lead to the absence of fires, similar to >5000 cal yr BP [italics added]." Consequently, it would appear that (1) the region of their study is not yet as warm as it was during the MCA, and (2) if the region's temperature ever were to significantly eclipse that of the MCA, wildfires there could well drop to a barely noticeable level.

Reference
Cook, E.R., Woodhouse, C., Eakin, C.M., Meko, D.M. and Stahle, D.W. 2004. Long-term aridity changes in the western United States. Science 306: 1015-1018.