By Christopher D. Farrar, John M. Neil, and James F. Howle
U.S. GEOLOGICAL SURVEY
Water-Resources Investigations report 98-4217
Sacramento, California 1999
USGS scientist measuring CO2 concentration in the air in a tree
well. Wells are pits formed when snow melts away from the trunk. Carbon
dioxide can accumulate to dangerous concentrations in such pits, especially if
the winds are calm.