For example, Swetnam has been able to determine the role of climate in controlling fire activity in the Southwest by sampling over huge areas. He looked at the fire scars in tree rings and found remarkable patterns involving synchronous fire years, and the patterns were the same from one mountain range to another. Two or three times per decade fires occurred during the same year in mountain after mountain range. This pattern of fire synchrony occurring on such a broad scale must be driven by climate. The tree rings tell a story in terms of seasonal precipitation, and with the calibrated model, scientists can estimate precipitation going back hundreds of years, and in some cases, thousands of years.
Interesting patterns emerge when the fire history and climate history data are put together.
La Ni a pattern is just one. The reverse typically occurs during a La Ni a year when the winter is drier than usual, which is what happened this last year. The fire histories are locked into the cycle of this pattern. With an El Ni o it is wet, and the grass builds up and trees produce more leaves that fall to the ground. Two or three years later if there is a La Ni a event forest fires are more severe due to the abundance of fuel.