Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center U.S. HCN Temperature Trends: A brief overview

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center Some Topics The U.S. HCN Datasets What is climate data homogenization? Some examples Analysis of U.S. temperature trends Representativeness of temperature trends versus temperature means

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center U.S. HCN Version 1 Originally released in stations Addresses –Time of observation bias (Karl et al. 1986; Vose et al. 2003) –Station History Changes (Karl and Williams 1987) – MMTS instrument change (Quayle et al. 1991) –Urbanization (Karl et al. 1988)

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center U.S. HCN Version stations Addresses –Time of observation bias (Karl et al. 1986; Vose et al. 2003) –Station History and Undocumented Changes (Menne and Williams, Journal of Climate, in review)

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center The Time of Observation Bias

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center The Time of Observation Bias 1950s 1960s 1980s s 1990s Since the 1950s, observation times have shifted from afternoon/ evening to morning Hour of observation histograms at bottom of each U.S. decadal map (Figure courtesy of Xioamoa Lin, University of Nebraska)

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center Impact of Time of Observation Changes Average year by year difference over the conterminous United States between the Time of Observation Bias (TOB)-adjusted data and the unadjusted (raw) data.

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center Homogenization

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center σ (°C) Simulated temperature series with random shifts caused by station moves/site changes Series in red treated as the target in subsequent figures All shifts are considered to be undocumented True “climate” trend in all simulated series is zero (Annual Averages)

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center Case 7 unadjusted Target series and differences with neighbors before adjustment for undocumented shifts

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center Target series and differences with neighbors following adjustment by pairwise algorithm

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center σ (°C) Simulated temperature series following adjustment by pairwise algorithm Original Target Series in Red Adjusted Target Series in Green Adjusted Neighbor Series in Black (Annual Averages)

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center An Example: Reno, Nevada From: /2008/01/10/how-not-to-measure- temperature-part-46-renos-ushcn- station/ /2008/01/10/how-not-to-measure- temperature-part-46-renos-ushcn- station/

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center Reno, Nevada (a)Mean annual TOB and fully adjusted (TOB+Pairwise) minimum temperatures at Reno, Nevada (b)Difference between minimum temperatures at Reno and the mean from its 10 nearest neighbors. Move to Airport Likely urban warming ASOS Equip. Moves

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center U.S. HCN Version 2 Temperature Trends (1895 to 2006) UnadjustedAdjusted Minimum Temperature Adjusted °C dec -1 Unadjusted Maximum Temperature

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center Impact of Documented and Undocumented Changes Year by year differences averaged over the conterminous United States between the fully adjusted (TOB+pairwise homogenization algorithm) HCN V2 data and the TOB-only adjusted data.

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center What about siting?

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center SurfaceStations.Org Stations in red have a classification >= 3 Station in black have a classification of 1 or 2

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center U.S. HCN Version 2 Temperature Trends (1895 to 2006) All StationsRating 3, 4 or 5 Minimum Temperature °C dec -1 Maximum Temperature All StationsRating 3, 4 or 5

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center U.S. HCN Version 2 Temperature Trends (1895 to 2006)

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center Impact of Siting on Trends SeasonMaximum TemperatureMinimum Temperature All StationsRatings 3, 4 & 5All StationsRatings 3, 4 & 5 Annual D-J-F M-A-M J-J-A S-O-N U.S. annual and seasonal temperature trends (°C dec -1 ) 1895 to 2006

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center Trends Versus Means Here is a not-so-well maintained or well sited USHCN station at Marysville

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center Can station measurements be unrepresentative, and the (adjusted) trend be representative? °C Difference between Annual Minimum Temperature at Marysville, CA and Surrounding Stations Documented and undocumented shifts identified by pairwise algorithm (using monthly values) indicated by vertical bars Raw data in red Adjusted data in green

Visit with Anthony Watts April 23, NOAA’s National Climatic Data Center Adjusted Minimum Temperature Series at Marysville, CA