“Changing Behavior in the Diurnal Range of Surface Air Temperatures over Mexico” By Englehart And Douglas 2005

There is a 2005 paper that provides additional evidence on the role of landscape change on the surface temperature record. It is

Englehart, P. J., and A. V. Douglas, 2005: Changing behavior in the diurnal range of surface air temperatures over Mexico, Geophys. Res. Lett., 32, L01701, doi:10.1029/2004GL021139.

The abstract reads 

“The diurnal range in surface temperatures (DTR = maximum – minimum temperature) has been widely used as one indicator of potential climate change. On hemispheric space scales DTR trends over about the last half-century tend to be decreasing. This paper analyzes regional scale trends in DTR for Mexico (1940-2001). Our principal finding is that in recent decades (post-1970) DTR trends over Mexico are positive as maximum temperatures are warming at a significantly higher rate than minimum temperatures. Regional land use and land cover changes (LCCs) are identified as potential forcing mechanisms responsible for at least part of the observed DTR behavior.”

The conclusion includes the text

“Here, our contention is that Mexico’s positive DTR trends at least partly reflect interactions between land use and climate. There is ample support for the view that land cover changes (LCCs) can significantly affect regional climate [e.g., Couzin, 1999; Henderson-Sellers et al., 1993; Pielke et al., 1998]. Clearly, Mexico’s land cover has and continues to undergo profound changes [Secretarı´a de Dessarrollo Social (SEDESOL), 1994]; these reflect the complex interplay of demographic pressures, socioeconomic conditions, and development policies. The net result often is overexploitation of natural resources in processes typically referred to as land degradation or desertification [Landa et al., 1997]. Figure 4 provides a few readily available indicators related to the processes of LCC in Mexico. Although subject to considerable uncertainty, they help underscore the extent and severity of Mexico’s LCCs.

Bonan [1997, 2001] presents modeling and observational analyses of LCCs in the US as a forcing mechanism for regional climate change. He reports that conversion of forest to cropland or natural grassland to cropland, produces a growing season cooling effect that is reflected in lower values of Tmax and reduced DTR. Increased albedo associated with LCCs is cited as a primary causal factor. Based on Mexico’s land use trends – for example, annual deforestation rates estimated at 1-4% [SEDESOL, 1994] and widespread overutilization of grasslands as well as extensive soil erosion (Figure 4) – it seems likely that regional-scale albedos are increasing. Yet the observed Tmax and DTR trends run counter to the idea of increased albedo promoting a cooling effect. Nonetheless, our interpretation of these circumstances is that LCCs in Mexico are responsible for some part of the observed trends. We believe that in contrast to the temperate latitude US case, LCCs for Mexico tend to favor reduced evaporative cooling and increased sensible relative to latent heat flux, and so promote warmer Tmax and increased DTR. Various modeling studies, DeFries et al. [2002] among others, support this view. Moreover, the interpretation is broadly consistent with the evidence of progressive land degradation. For example, severe overgrazing reduces overall vegetative cover and decreases transpiration. Overgrazing leads to increased surface runoff and erosion and reduced available soil moisture for evaporation. With bare ground increasing soil temperatures rise with greater sensible heating expected. Balling et al. [1998] invoke similar reasoning to account for observed differences in Tmax and DTR between northwest Mexico and adjacent areas in southern Arizona. It is unlikely that this simple picture – land use changes modifying the components of the surface energy budget – can completely account for the observed DTR trends. For example, the spring season in northwest Mexico normally is dry with much of the region’s vegetation dormant. On this basis, it is difficult to envision LCCs strongly influencing moisture levels in the near surface atmosphere. Yet the post-1970 DTR trend estimates (Table 1) for the spring season are actually higher than for the moist summer season.

The predominant LCC in Mexico may well involve land conversion for human settlement. Extensification, that is, increasing area devoted to human settlement, likely occurs across the spectrum of place sizes from metropolitan areas to small villages. In a case study of four villages in rural southwest Mexico, Landa et al. [1997] estimate annual land conversion rates for settlement at 10-40%, whereas rates for deforestation and agriculture land uses were generally <10%. Given that more than 90% of the stations used in constructing the temperature data set are located in settlements with population <5000, we suspect that extensification processes are reflected in surface temperatures in a manner analogous to that suggested for overgrazing. That is, rather than reflecting a ‘‘classic” urban heat island signature – strong trends in Tmin with little or no change in Tmax – extensification in this context tends to reduce vegetative cover and hence be related to higher Tmax and DTR.”

The issue of the role of landscape change on the multi-decadal assessment of the surface temperature trends, which is one of the issues we raised in our paper

Pielke Sr., R.A., C. Davey, D. Niyogi, S. Fall, J. Steinweg-Woods, K. Hubbard, X. Lin, M. Cai, Y.-K. Lim, H. Li, J. Nielsen-Gammon, K. Gallo, R. Hale, R. Mahmood, S. Foster, R.T. McNider, and P. Blanken, 2007: Unresolved issues with the assessment of multi-decadal global land surface temperature trends. J. Geophys. Res., 112, D24S08, doi:10.1029/2006JD008229,

remains a topic that the National Climate Data Center and other such groups continue to essentially ignore.


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