Guest post by Bob Tisdale
SkepticalScience recently published a post titled Mercury rising: Greater L.A. to heat up an average 4 to 5 degrees by mid-century. It’s a cross post of a UCLA press release with the same title. It struck me odd, because we recently showed that Western North American land surface temperatures have declined in recent years. Refer to Figure 6 in the post IPCC Models vs Observations – Land Surface Temperature Anomalies for the Last 30 Years on a Regional Basis.
So, does a 4 to 5 deg F rise in Greater Los Angeles land surface temperature anomalies by 2050 sound realistic? That equates to a rise of 2.5 to 2.8 deg C. Considering that, based on a preliminary look at the data, greater Los Angeles land surface temperature anomalies have been cooling for the 3-plus decades, it seems to be a real stretch of the imagination.
The GHCN-CAMS land surface temperature dataset comes in a number of resolutions, including 0.5 degree latitude and longitude. Lucky for us, it’s available through the KNMI Climate Explorer. And that means we can capture data for some reasonably small geographical areas. The GHCN-CAMS land surface temperature dataset was presented in the Fan and Dool (2007) paper A global monthly land surface air temperature analysis for 1948-present. It will allow us to get an idea of what Greater Los Angeles surface temperatures have been doing since 1948.
The UCLA press release states (my bold face):
Some of the smallest changes predicted, yet still nearing a 4-degree increase, are in Oxnard (3.68 degrees), Venice (3.70), Santa Barbara (3.73), Santa Monica (3.74), San Pedro (3.78), Torrance (3.80), Long Beach (3.82) and Santa Ana (3.85). Among the highest predicted increases are Wrightwood (5.37), Big Bear Lake (5.23), Palm Springs (5.15), Palmdale (4.92), Lancaster (4.87), Bakersfield (4.48) and Santa Clarita (4.44). Table 2 in the study calls out 27 distinct locations, such as downtown Los Angeles (3.92), San Fernando (4.19), Woodland Hills (4.26), Eagle Rock (3.98), Pasadena (4.05), Pomona (4.09), Glendale (3.99) and Riverside (4.23).
So they’ve made predictions for an area larger than the City or County of Los Angeles. For the sake of discussion, let’s say it represents the area bordered by the coordinates of 33N-35.5N, 121W-117W. See Figure 1. Those coordinates fit with the 0.5 degree grids. And we’ll call that dataset Santa Barbara-Ventura-Los Angeles-Orange Counties.
Figure 2 presents a time-series graph of the Santa Barbara-Ventura-Los Angeles-Orange Counties land surface temperature anomalies since January 1948. The data has a linear trend of 0.177 deg C/decade. In order for the land surface temperatures for that dataset to rise 2.5 deg C by 2050, the linear trend of the data has to change drastically to about 0.667 deg C/decade from June 2012 through December 2050.
In Figure 3, I’ve smoothed the Santa Barbara-Ventura-Los Angeles-Orange Counties land surface temperature anomalies with a 13-month running-average filter to reduce some of the variability. What caught my eye was the shift in 1976 that coincides with the Pacific Climate Shift. Curiously, it appears the dataset has been cooling since that shift.
Let’s take a look at the linear trends before and after the 1976 Pacific Climate Shift. We’ll switch back to the “raw” data. Before the climate shift, January 1948 to December 1975, the Santa Barbara-Ventura-Los Angeles-Orange Counties land surface temperature anomalies rose at a rate of only 0.108 deg C/decade, and after, from January 1977 to May 2012, they’ve cooled at a rate of -0.082 deg C/decade.
In Figure 5, I’ve added the projection of about 2.5 deg warming by 2050 to the graph to show how unrealistic that projection looks, especially when we consider that surface temperatures for the Santa Barbara-Ventura-Los Angeles-Orange Counties data have been dropping for 3+ decades.
Maybe I looked at too large an area. Let’s take a look at the data for the coordinates of 33.5N-34.5N, 118.5W-117.5W. See Figure 6 for the location. We call that dataset “Los Angeles Plus.”
As shown in Figure 7, the pre-1976 warming rate for the “Los Angeles Plus” land surface temperature anomalies is greater than the larger dataset, at about 0.33 deg C/decade. But the post-1976 trend is still negative at -0.074 deg C/decade.
One last try: Let’s decrease the area of the data again, Figure 8, this time looking at the land surface temperature anomalies for the coordinates of 34N-34.5N, 119W-118W. That captures Malibu, the Valley and much of the City of Los Angeles.
Doesn’t help. As shown in Figure 9, the trend after the 1976 Pacific Climate Shift for the Malibu-The Valley-Los Angeles land surface temperatures is negative at -0.086 deg C/decade.
Based on this quick look at land surface temperature data for the Greater Los Angeles area, the Hall et al (2012) study referred to in the UCLA press release appears to have no basis in reality.
The GHCN-CAMS land surface temperature data presented in this post is available through the KNMI Climate Explorer.