Land cover changes (LCCs) play an important role in the climate system. Research over recent decades highlights the impacts of these changes on atmospheric temperature, humidity, cloud cover, circulation, and precipitation. These impacts range from the local-and regional-scale to sub-continental and global-scale. It has been found that the impacts of regional-scale LCC in one area may also be manifested in other parts of the world as a climatic teleconnection. In light of these findings, this article provides an overview and synthesis of some of the most notable types of LCC and their impacts on climate. These LCC types include agriculture, deforestation and afforestation, desertification, and urbanization. In addition, this article provides a discussion on challenges to, and future research directions in, assessing the climatic impacts of LCC.

[1] This paper documents various unresolved issues in using surface temperature trends as a metric for assessing global and regional climate change. A series of examples ranging from errors caused by temperature measurements at a monitoring station to the undocumented biases in the regionally and globally averaged time series are provided. The issues are poorly understood or documented and relate to micrometeorological impacts due to warm bias in nighttime minimum temperatures, poor siting of the instrumentation, effect of winds as well as surface atmospheric water vapor content on temperature trends, the quantification of uncertainties in the homogenization of surface temperature data, and the influence of land use/land cover (LULC) change on surface temperature trends. Because of the issues presented in this paper related to the analysis of multidecadal surface temperature we recommend that greater, more complete documentation and quantification of these issues be required for all observation stations that are intended to be used in such assessments. This is necessary for confidence in the actual observations of surface temperature variability and long-term trends.Citation: Pielke, R. A., Sr., et al. (2007), Unresolved issues with the assessment of multidecadal global land surface temperature trends,

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Abstract. A 3-day period of strong, synoptic-scale stagnation, in which daytime boundary-layer winds were light and variable over the region, occurred in mid July of the 1995 Southern Oxidants Study centered on Nashville, Tennessee. Profiler winds showed light and variable flow throughout the mixed layer during the daytime, but at night in the layer between 100 and 2000 m AGL (which had been occupied by the daytime mixed layer) the winds accelerated to 5-10 m s-1 as a result of nocturnal decoupling from surface friction, which producect inertial oscillations. In the present study, we investigate the effects of these wind changes on the buildup and transport of ozone (03). The primary measurement system used in this study was an airborne differential absorption lidar (DIAL) system that profiled 03 in the boundary layer as the airplane flew along. Vertical cross sections showed that 03 concentrations exceeding 120 ppb extended up to nearly 2 km AGL, but that the 03 hardly moved at all horizontally, instead forming a dome of pollution over or near the city. The analysis concentrates on four meteorological processes that determine the 3-D spatial distribution of 03 and the interaction between urban and rural pollution: (1) daytime buildup of 03 over the urban area, (2) the extent of the drift of pollution cloud during the day as it formed, which controls peak 03 concentrations, (3) nighttime transport by the accelerated winds above the surface, and (4) vertical mixing of pollution layers the next day. Other consequences of very light-wind conditions were intra-regional differences in daytime mixed-layer depth over distances of 50 km or less, and indications of an urban heat-island circulation. IntroductionOver relatively simple topography, such as that found near Nashville, Tennessee, the overriding control on the transport and dispersion of atmospheric pollutants is the large-scale (usually synoptic) wind speed and direction. Occasionally, the larger-scale flow becomes weak, indicating that synoptic controls on flow and transport are weak (except for the large-scale subsidence accompanying these conditions). Two implications of such stagnation are (1) that pollution emissions are confined to a much smaller volume than when stronger flow is present, often leading to the highest pollutant concentrations in a season, and (2) that smaller-mesoscale influences, such as surface heating differences, can express themselves as locally generated flows or variations in mixing properties.Because stagnation periods routinely produce the highest pollution concentrations, it is important to understand the key meteorological processes affecting the distribution and magnitude of the ozone buildup. We identify four processes that control the spread of the urban plume. The first three represent a sequence occurring through different stages of the diurnal heating cycle, and the fourth, advection, represents a daytime control that proved to be significant even under very light wind conditions. The processes are as follows: (1)

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The techniques of nonlinear analysis are used to examine the behavior of the stable nocturnal boundary layer (SNBL) when it is subjected to changes in incoming radiation or in surface characteristics. A single‐column model and nonlinear bifurcation techniques are used to demonstrate that any atmospheric forcing, such as weak radiative forcing from greenhouse gases or cloud cover, can trigger a potentially significant positive feedback. Multiple solutions occur in some parameter spaces. This analysis shows that any forcing that decreases the stability, whether by increasing greenhouse gases or surface heat capacity, can cause large increases in surface temperature as the SNBL shifts from a weak turbulent regime, which allows the surface to cool, to a turbulent regime, which mixes warm air from aloft. Positive feedback may be a key factor in interpreting the long‐term observed nocturnal warming trend in the SNBL.

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 (T Max ), minimum (T Min ), and mean (T Mean ) 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 T Max is not significantly different from zero. However, T Min results suggest an accelerating temperature rise.Uncertainty estimates indicate that the trend of the difference time series (T Max 2 T Min ) 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 , in contrast with findings in recent periods for global datasets, which generally have sparse coverage of East Africa.The differences between T Max and T Min trends, especially recently, may reflect a response to complex changes in the boundary layer dynamics; T Max represents the significantly greater daytime vertical connection to the deep atmosphere, whereas T Min 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 T Min while having little impact on T Max in East Africa. This indicates that time series of T Max and T Min should become separate variables in the study of long-term changes.