Many cities located in valleys with limited ventilation experience serious air pollution problems. The ventilation of an urban valley can be limited not only by orographic barriers, but also by urban heat islandinduced circulations and/or the capping effect of temperature inversions. Furthermore, land-use/-cover changes caused by urbanization alter the dynamics of temperature inversions and urban heat islands, thereby affecting air quality in an urban valley. By means of idealized numerical simulations, it is shown that in a mountain valley subject to temperature inversions urbanization can have an important influence on air quality through effects on the inversion breakup. Depending on the urban area fraction in the simulations, the breakup time changes, the cross-valley wind system can evolve from a confined to an open system during the daytime, the slope winds can be reversed by the interplay between the urban heat island and the temperature inversion, and the breakup pattern can migrate from one dominated by the growth of the convective boundary layer to one also involving the removal of mass from the valley floor by the upslope winds. The analysis suggests that the influence of urbanization on the air quality of an urban valley may lead to contrasting and possibly counterintuitive effects when considering temperature inversions. More urban land does not necessarily imply worse air quality, even when considering that the amount of pollutants emitted grows with increased urbanization.
Colombia, one of the world's most species‐rich nations, is currently undergoing a profound social transition: the end of a decades‐long conflict with the Revolutionary Armed Forces of Colombia, known as FARC. The peace agreement process will likely transform the country's physical and socioeconomic landscapes at a time when humans are altering Earth's atmosphere and climate in unprecedented ways. We discuss ways in which these transformative events will act in combination to shape the ecological and environmental future of Colombia. We also highlight the risks of creating perverse development incentives in these critical times, along with the potential benefits – for the country and the world – if Colombia can navigate through the peace process in a way that protects its own environment and ecosystems.
Urban valleys can experience serious air pollution problems as a combined result of their limited ventilation and the high emission of pollutants from the urban areas. Idealized simulations were analyzed to elucidate the breakup of an inversion layer in urban valleys subject to a strong low-level temperature inversion and topographic effects on surface heating such as topographic shading, as well as the associated air pollution transport mechanisms. The results indicate that the presence and evolution in time of the inversion layer and its interplay with an urban heat island within the valley strongly influence the venting of pollutants out of urban valleys. Three mechanisms of air pollution transport were identified. These are transport by upslope winds, transport by an urban heat island–induced circulation, and transport within a closed slope-flow circulation below an inversion layer.
Abstract. Many natural and social phenomena depend on river flow regimes
that are being altered by global change. Understanding the mechanisms behind
such alterations is crucial for predicting river flow regimes in a changing
environment. Here we introduce a novel physical interpretation of the scaling
properties of river flows and show that it leads to a parsimonious
characterization of the flow regime of any river basin. This allows river basins to be classified as regulated or unregulated, and to identify a critical
threshold between these states. We applied this framework to the Amazon river
basin and found both states among its main tributaries. Then we introduce the
“forest reservoir” hypothesis to describe the natural capacity of river
basins to regulate river flows through land–atmosphere interactions (mainly
precipitation recycling) that depend strongly on the presence of forests. A
critical implication is that forest loss can force the Amazonian river basins
from regulated to unregulated states. Our results provide theoretical and
applied foundations for predicting hydrological impacts of global change,
including the detection of early-warning signals for critical transitions in
river basins.
Abstract. Some large-scale components of the Earth's climate system
have been identified as policy-relevant “tipping elements”, meaning that
anthropogenic forcing and perturbations may push them across a tipping point
threshold, with potential global scale impact on ecosystems and concomitant
environmental and social phenomena. A pronounced change in the amplitude
and/or frequency of the El Niño–Southern Oscillation (ENSO) is among
such tipping elements. Here, we use the Planet Simulator (PlaSim), an Earth
system model of intermediate complexity, to investigate the potential impact
on global climate and terrestrial ecosystems of shifting the current dynamics
of the ENSO into a permanent El Niño. When forced with sea surface
temperature (SST) derived from observations, the PlaSim model yields a
realistic representation of large-scale climatological patterns, including
realistic estimates of the global energy and water balances, and gross
primary productivity (GPP). In a permanent El Niño state, we found
significant differences in the global distribution of water and energy
fluxes, and associated impacts on GPP, indicating that vegetation production
decreases in the tropics, whereas it increases in temperate regions. We
identify regions in which these El Niño-induced changes are consistent
with potential state transitions in global terrestrial ecosystems, including
potential greening of western North America, dieback of the Amazon
rainforest, and further aridification of south-eastern Africa and Australia.
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