Out of Africa: A new paper by Christy on surface temperature issues

More weather station photos from Africa here.

These stations shown and linked above are not GHCN stations as far as I can tell, but the siting was interesting nonetheless.

This new paper by John Christy, who works with Dr. Roy Spencer on the UAH dataset, points out that Tmin seems to have a signal in Africa where Tmax does not. Land use changes and aersols that affect the boundary layer at night are theorized and possible reasons. – Anthony

Surface Temperature Variations in East Africa and Possible Causes

JOHN R. CHRISTY, WILLIAM B. NORRIS, AND RICHARD T. MCNIDER

Earth System Science Center, University of Alabama in Huntsville, Huntsville, Alabama

ABSTRACT

Surface temperatures have been observed in East Africa for more than 100 yr, but heretofore have not been subject to a rigorous climate analysis. To pursue this goal monthly averages of maximum (TMax), minimum (TMin), and mean (TMean) temperatures were obtained for Kenya and Tanzania from several sources. After the data were organized into time series for specific sites (60 in Kenya and 58 in Tanzania), the series were adjusted for break points and merged into individual gridcell squares of 1.258, 2.58, and 5.08.

Results for the most data-rich 58 cell, which includes Nairobi, Mount Kilimanjaro, and Mount Kenya, indicate that since 1905, and even recently, the trend of TMax is not significantly different from zero. However, TMin results suggest an accelerating temperature rise.

Uncertainty estimates indicate that the trend of the difference time series (TMax2 TMin) is significantly less than zero for 1946–2004, the period with the highest density of observations. This trend difference continues in the most recent period (1979–2004), in contrast with findings in recent periods for global datasets, which

generally have sparse coverage of East Africa.

The differences between TMax and TMin trends, especially recently, may reflect a response to complex changes in the boundary layer dynamics; TMax represents the significantly greater daytime vertical connection to the deep atmosphere, whereas TMin often represents only a shallow layer whose temperature is more dependent on the turbulent state than on the temperature aloft.

Because the turbulent state in the stable boundary layer is highly dependent on local land use and perhaps locally produced aerosols, the significant human development of the surface may be responsible for the rising TMin while having little impact on TMax in East Africa. This indicates that time series of TMax and TMin should

become separate variables in the study of long-term changes.

Some excerpts from the paper:

c. Possible causes for TMax and TMin differences The fact that the trends in the two temperature measurements (TMax and TMin) are likely significantly different encourages an examination of the causes for the warming of TMin and the significance of trends in TMin in the context of tracking global climate change.  Given a lack of detail on station siting and uncertainties in specifics on the boundary layer in East Africa, definitive reasons for the trends may not be available.

However, general aspects of boundary layer behavior may provide some guide for interpreting the trends.Thus, the following should be viewed as a context and hypothesis for the trend differences that deserve discussion and further attention.

…

From the conclusion:

For the 100-yr period from 1905 to 2004 in this grid cell, the trends were near zero for both TMax and TMin, but confidence in these results is low because of the relatively sparse data in the years before 1946. Beginning with 1946 and ending in 2004, near-zero trends were found for TMax. The TMin trends were more positive, and significantly so based on both measurement error and temporal sampling error. It is difficult to assess the

measurement error of these trends, but using the spread of 20 realizations in which the construction parameters

were varied, the range of 60.108C decade21 is plausible. The fact that the difference in trends in TMax and TMin continues, and in fact accelerates, in the period of 1979–2004 in East Africa may be important in interpreting the results of Vose et al. (2005).

While it is possible that East Africa difference trends are indeed different than that of the globe as provided by Vose et al. (2005), there is concern that the reduced number of stations in the 1979–2004 GHCN dataset may not be sampling many of the areas of the globe that are behaving like East Africa. Thus, it is important that the GHCN dataset be expanded to include more stations distributed around the globe.

The noticeable difference in trends of TMax and TMin implies that daytime and nighttime temperatures are responding differently to environmental factors. Changes in the surface characteristics and the boundary layer atmospheric constituents may be responsible for the relatively recent and rapid rise in TMin. There appears to be little change in East Africa’s TMax, and if TMax is a suitable proxy for climate changes affecting the deep atmosphere, there has been little impact in the past half-century.

The investigation of the surface temperature record as an indicator of human-induced climate change involves understanding the complex behavior of boundary layer processes (where surface temperatures are actually measured) and how temperatures within it are affected by the numerous changes that occur. This is an area of research open for considerable inquiry because it raises new questions concerning the types of

data indices now used to detect climate change.

At the least, the time series of both TMax and TMin should become separate variables to be studied for long-term changes.

Full paper is available here as a PDF 1.9 MB