In follow up to my prior post regarding historical precipitation data for San Jose, California, I have now plotted the annual precipitation for the Northern Sierra 8 Station index maintained by the California Department of Water Resources. The data are available at the California Data Exchange Center here:
The data do not go back quite as far as the San Jose data I previously showed (1921 rather than 1875), but do give us 90 years of data covering a much broader geographical area. Again, in light of my friend’s recent concern about changing precipitation patters due to global warming (he was specifically concerned about reduced precipitation, not increased), what do the data for this large portion of the California Sierras show?
This looks rather similar to the San Jose data: a few dry years followed by a few wet years. A fair amount of variability from year to year, but overall a remarkably steady average. Note the dry years 2007 and 2008 (the last data I had for San Jose was 2007), followed by the increases in 2009 and 2010. As with the San Jose data, there is no significant trend in the Northern Sierra data, although a straight-line trend on this data is slightly more positive (more precipitation in recent years, not less).
In any event, the data for the Northern Sierra area certainly do not support a trend of decreasing precipitation due to global warming. Indeed, the data don’t seem to support anything other than: (i) significant annual variation around (ii) a stable long-term norm.