A March 2006 paper in the Journal of Climate by K.E. Runnalls and T.R. Oke entitled “A Technique to Detect Microclimatic Inhomogeneities in Historical Records of Screen-Level Air Temperature” further demonstrates significant problems with the accurate quantification of multi-decadal land surface temperature trends (and thanks to Dev Niyogi and Paolo M. for alerting me to this paper).
The abstract reads,
“A new method to detect errors or biases in screen-level air temperature records at standard climate stations is developed and applied. It differs from other methods by being able to detect microclimatic inhomogeneities in time series. Such effects, often quite subtle, are due to alterations in the immediate environment of the station such as changes of vegetation, development (buildings, paving), irrigation, cropping, and even in the maintenance of the site and its instruments. In essence, the technique recognizes two facts: differences of thermal microclimate are enhanced at night, and taking the ratio of the nocturnal cooling at a pair of neighboring stations nullifies thermal changes that occur at larger-than-microclimatic scales. Such ratios are shown to be relatively insensitive to weather conditions. After transforming the time series using Hurst rescaling, which identifies long-term persistence in geophysical phenomena, cooling ratio records show distinct discontinuities, which, when compared against detailed station metadata records, are found to correspond to even minor changes in the station environment. Effects detected by this method are shown to escape detection by current generally accepted techniques. The existence of these microclimatic effects are a source of uncertainty in long-term temperature records, which is in addition to those presently recognized such as local and mesoscale urban development, deforestation, and irrigation.”
In the conclusions, it is stated,
“Gradual changes in the immediate environment over time, such as vegetation growth, or encroachment by built features such as paths, roads, runways, fences, parking lots, and buildings into the vicinity of the instrument site typically lead to trends in the cooling ratio series. Distinct régime transitions can be caused by seemingly minor instrument relocations (such as from one side of the airport to another, or even within the same instrument enclosure) or due to vegetation clearance. This contradicts the view that only substantial station moves, involving significant changes in elevationand/or exposure are detectable in temperature data. It is not surprising that small station moves, even without changes of elevation or exposure, are capable of introducing inhomogeneities into the record,because there are often several confounding changes occurring at the same time. For example, a stationmove often coincides with screens being repainted, cleaned, or replaced, new instruments installed, and observers being reinstructed about their practices. Further, it is common for the new instrument site to bewithout grass for a few years, and there are many indications of muddy conditions around the instruments until grass is both planted and properly maintained. These factors, combined with subtle changes in the immediate surroundings (such as moving away from a parking lot or building), appear to be a significant causeof inhomogeneities in temperature records As isolated occurrences, activities such as painting, cleaning, or releveling screens or instruments do not frequentlycause significant changes to cooling régimes.”
“We suggest these effects are possibly underappreciated by many agencies responsible for maintaining the qualityof climatological records. Whether such small thermal effects amount to a significant concern largely dependsupon whether by their nature they are biased. That is, ifthe majority of the anomalies tend toward net warmingor net cooling. If they do, even tenths of a degree in onedirection take on real significance in the global climate change debate. Intuition, experience, and review of classic microclimatic case studies (e.g., Geiger 1965)suggests to us that the net impact of the most commonchanges (compaction due to trampling, increased paving,tree growth, removal or soiling of snow cover, construction of buildings and introduction of irrigation)lead to alteration of nocturnal controls on the surface heat balance (thermal admittance, sky view factor androughness and shelter) in ways that reduce nocturnal cooling and consequently increase the minimum temperature.Removal of trees and desiccation will act in the opposite direction. Are the environments of climatestations preferentially modified during the inexorableprocess of development in a way that leads to net thermalimpacts? We suspect they are, but the question deserves attention and objective analysis.”
“This study suggests that it might be beneficial to reexamine stations that passed previous homogeneity analyses and to consider the implications of the concerns raised by the work here for the large databases ofair temperature data that are assumed to be homogeneous and unbiased.”
The CCSP Report “Temperature Trends in the Lower Atmosphere: Steps for Understanding and Reconciling Differences“, despite my urgings to the contrary (see) , chose to inadequately investigate the robustness of the land surface temperature data as a metric to use for accurate multi-decadal surface temperature trends. The Journal of Climate paper further shows that significant problems exist with the robustness of this data set with respect to the assessment of mutli-decadal evaluations of global warming variability and trends.