Russ Steele
There is an internal U.S. Forest Service report circulating on how climate change is effecting the Tahoe National Forest and the neighboring El Dorado National Forest: A summary of current trends and probable future trends in climate and climate-driven processes in the Eldorado and Tahoe National Forests and the neighboring Sierra Nevada. This report was written by Chris Mallek, Department of Plant Sciences; University of California-Davis and Hugh Safford, Regional Ecologist, USDA Forest Service, Pacific Southwest Region. A copy of the report is here: Eldorado&TahoeNFs_ClimateChangeTrendSummary_12-15-10
The climate data in the report was derived from four weather stations with long-term meteorological records from the general area of the El Dorado and Tahoe National Forests. The stations were, Nevada City, Placerville, Truckee and Lake Spaulding. This post will focus on the Nevada City station. I will examine the other stations in future posts. From the report:
Over the last 116 years, mean annual temperatures at the Nevada City station have risen by 2.3° F (Table 1, Fig. 1), largely as the result of increases in minimum (i.e., nighttime) temperatures. According to the meteorological station record, daily maximum temperatures have actually decreased at Nevada City, which makes this a rarity in California (Table 1).
I have been investigating the Nevada City and Lake Spaulding records since they appeared in a paper by Steve LaDochy, Richard Medina, and William Pazert: Recent California climate variability: spatial and temporal patterns in temperature trends. That paper indicated there was significant warming in Nevada City, see the blob in this cropped graphic from the report. Download LaDochy
Since I only live a mile from the station, I wanted to take a look at why there was such a variation from surrounding stations. Why were minimum temperatures rising, while maximum temperatures were declining? The Grass Valley COOP did not show similar increase in the minimum temperatures.
The Nevada City weather station, COOP Number 046136, is currently at the end of Canada Ave, in Nevada City, While the Nevada City record at the Western Regional Climate Center extends back to 1893 the station has been moved multiple times, and there is a large gap in the record from 1914 to 1930.
NOAA’s National Climate Data Center records for COOP 046136 starts on 1 Jan 1931, with the station located at the Nevada City Waste Water Treatment Plant in the Deer Creek drainage. Between 1948 and 1950 the station was move into town at two different locations. In 1976 the station was move back to the Waste Water Treatment Plant on Deer Creek, where the minimum warming starts to increase, as shown in this graphic.
Data from the Western Regional Climate Center in Reno Nevada
In 1985 the station was moved to the Nevada City Water Plant on Canada Ave. According to the water plant operator the station was upgraded in 1989 with the MMTS sold state temperature measurement systems. The NCDC data does not indicate when the upgrades took place. Once the MMTS was installed the water plant operator no longer had to go out in the weather to read the thermometers in the white shelter shown in this picture, the temperatures are displayed in digital format in the operators office.
Mallek and Safford wrote in their report that the Nevada City’s station's mean annual temperature had increased by 2.3 degrees F, primarily from increased nighttime temperatures. Looking at the temperature plot above you can see the increase in temperature came when the surface station was at the Waste Water Treatment (sewer) Plant. I have no idea where the station was located at the sewer plant, but sewer plants are known to be heat islands.
Most of the mean annual 2.3F degrees increase took place at the Waste Water plant between 1976 and 1985 when the minimum temps increased by about 7 degrees, and then remained elevated after the move to the Water Plant. However, once at the Water Plant there is no significant increase in the night time minimum temperatures from 1986 to the present. It does show elevated warming over the Deer Creek and in town locations. The Water Plant location could be one of the plethora of micro-climates that we have in Nevada County. The Water Plant is surround by huge trees that block the wind and limiting exposure to the night time sky. The sensor is with 18 feet of a large tank of moving water, that produces increased humidity in the air. All of these factors could contribute to higher minimum temperatures in the current location.
Cold air collects in the Nevada city basin and flows down the Deer Creek canyon. According to the Water and Sewer Plant operators, they manage both sites, the Deer Creek drainage is often colder than the current station location, which is on a shelf of land well away from any creeks. There is no similar cold air current at Canada Ave location, which could also account for the warmer readings in it's current location.
That raises the question, in a colder location at the sewer plant what could have caused the 7.0F increase shown in the graphic above?
From 1970 to 1985 Nevada City population increased by 375 people living in 175 new housing units. This would have increased waste water processing at the sewer plant, generating more heat? I am not sure, but it seems to be a reasonable assumption, more people, more toilet flushes, and more waste water to process, thus more heat.
The increases activity at the sewer plant and the move to a warmer location could account for the majority of the 2.3 F degree increase in the minimum temperatures reported by the Forest Service. One thing is for sure, the temperature increase took place over the 9 years the weather station was at the sewer plant. Was this a signature of global warming? Why did the increases in warming stop when the station was moved to a new location? I am inclined to believe the temperature increase was caused by sighting issues, rather than a signature for global warming. On the other hand if it is, global warming has stopped at the Nevada County Water Plant.
Now on to the progressive cooling at both locations. One possible reason the daily average temperatures decreased in both locations, the Sewer Plant and Water Treatment Plant, they are both surrounded by forests that have matured and thickened over the years. The Water Plant location is totally surround by trees that block the morning and afternoon sun, and in the winter the sun is low in the sky and the tall trees on the ridge behind the plant cast long shadows across the area. As the vegetation increases in density the sun cannot warm the surrounding area affecting local temperatures, especially on less windy days.
I may not have covered all the options and readers may have some other ideas. In past posts they have contributed to the history of local weather stations, maybe they can provide more insight for warming and cooling at this station, other than anthropogenic global warming.
