Some global warming may be natural, researchers say
As much as one-third of the climate warming since 1975 may come from natural variability in weather patterns, particularly the El Nino effect, say UW scientists, rather than from the “greenhouse effect.”
A study led by Atmospheric Sciences Professor John Wallace looked at almost a century of Northern Hemisphere temperature records. Between 1975 and 1990 there was a 0.3 degree Celsius increase in the temperature of the atmosphere. The study found that natural changes in atmosphere circulation appear to have caused a third of that warming.
For almost the entire century (1900-1990), the circulation patterns may account for nearly half the Northern Hemisphere’s month to month variations.
The study highlights uncertainties in global warming assessments by the United Nations and others reported in recent news accounts. “While human activity may well also have contributed to the overall warming, we think that a significant part of the change is due to natural causes that have nothing to do with greenhouse warming,” Wallace says.