Large-scale use of wind power can alter local and global climate by extracting kinetic energy and altering turbulent transport in the atmospheric boundary layer. We report climate-model simulations that address the possible climatic impacts of wind power at regional to global scales by using two general circulation models and several parameterizations of the interaction of wind turbines with the boundary layer. We find that very large amounts of wind power can produce nonnegligible climatic change at continental scales. Although large-scale effects are observed, wind power has a negligible effect on global-mean surface temperature, and it would deliver enormous global benefits by reducing emissions of CO 2 and air pollutants. Our results may enable a comparison between the climate impacts due to wind power and the reduction in climatic impacts achieved by the substitution of wind for fossil fuels. G lobal wind-power capacity is growing by Ϸ8 GW⅐yr Ϫ1 , making wind the fastest growing nonfossil source of primary energy (1). The cost of electricity from wind power is now Ϸ40 dollars per MW⅐h Ϫ1 at the best sites, and costs are declining swiftly (2). Wind power could play a substantial role in global energy supply when CO 2 emissions are strongly constrained to limit anthropogenic climatic change. Although the local environmental and aesthetic impacts of wind power have been explored, there has been little assessment of the climatic impacts of wind turbines.Wind power is a renewable resource, but the rate of its renewal is finite and, in some respects, comparatively small. The yearly average horizontal flux of kinetic energy at the Ϸ100-m hub heights of large wind turbines can be Ͼ1 kW⅐m Ϫ2 . These large power fluxes enable the economic extraction of wind power, but an array of wind turbines cannot extract this power arbitrarily because turbines interfere with their neighbors by slowing local winds. Most of the kinetic energy that drives wind turbines originates with the generation of available potential energy at planetary scales, which fuels winds throughout the atmosphere. Within the atmospheric boundary layer, turbulent mixing transports momentum downward to the surface and converts kinetic energy to heat by means of viscous (frictional) dissipation. The downward flux of kinetic energy averages Ϸ1.5 W⅐m Ϫ2 over the global land surface (3). Ultimately, this small downward flux of kinetic energy limits the power that can be extracted by wind-turbine arrays (4).Although the generation and dissipation of kinetic energy is a minor (Ϸ0.3%) component of global energy fluxes, the winds mediate much larger energy fluxes by transporting heat and moisture. Therefore, alteration of kinetic energy fluxes can have much greater climatic effects than alteration of radiative fluxes by an equal magnitude (3, 5). MethodsWe explored the climatic impact of wind turbines by altering surface drag coefficients in a suite of numerical experiments using two different general circulation models, one of which was developed at the National...

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Mass human starvation is currently likely if global agricultural production is dramatically reduced for several years following a global catastrophe: e.g. super volcanic eruption, asteroid or comet impact, nuclear winter, abrupt climate change, super weed, extirpating crop pathogen, super bacterium, or super crop pest. This study summarizes the severity and probabilities of such scenarios, and provides an order of magnitude technical analysis comparing caloric requirements of all humans for five years with conversion of existing vegetation and fossil fuels to edible food. Here we present mechanisms for global-scale conversion including: natural gas-digesting bacteria, extracting food from leaves, and conversion of fiber by enzymes, mushroom or bacteria growth, or a two-step process involving partial decomposition of fiber by fungi and/or bacteria and feeding them to animals such as beetles, ruminants (cattle, sheep, etc), rats and chickens. We perform an analysis to determine the ramp rates for each option and the results show that careful planning and global cooperation could maintain humanity and the bulk of biodiversity.

Many global catastrophic risks threaten major disruption to global food supplies, including nuclear wars, volcanic eruptions, asteroid and comet impacts, and plant disease outbreaks. This paper discusses options for increasing the resilience of food supplies to these risks. In contrast with local catastrophes, global food supply catastrophes cannot be addressed via food aid from external locations. Three options for food supply resilience are identified: food stockpiles, agriculture, and foods produced from alternative (non-sunlight) energy sources including biomass and fossil fuels. Each of these three options has certain advantages and disadvantages. Stockpiles are versatile but expensive. Agriculture is efficient but less viable in certain catastrophe scenarios. Alternative foods are inexpensive pre-catastrophe but need to be scaled up post-catastrophe and may face issues of social acceptability. The optimal portfolio of food options will typically include some of each, and will additionally vary by location as regions vary in population and access to food input resources. Furthermore, if the catastrophe shuts down transportation, then resilience requires local self-sufficiency in food. Food supply resilience requires not just the food itself, but also the accompanying systems of food production and distribution. Overall, increasing food supply resilience can play an important role in global catastrophic risk reduction. However, it is unwise to attempt maximizing food supply resilience, because doing so comes at the expense of other important objectives, including catastrophe prevention. Taking all these issues into account, the paper proposes a research agenda for analysis of specific food supply resilience decisions.

In order to augment the efficiency and distillate yield in the concentrator-coupled hemispherical basin solar still, a phase change material (PCM) was added. Two modes of operation have been studied experimentally, (1) single-slope solar still without the PCM effect, and (2) single-slope solar still with the PCM effect. The temperature of water (Tw), temperature of PCM (TPCM), air temperature (Tair), inner cover temperature (Tic) and outer cover temperature (Toc) were measured. Experimental results indicate that the effect of thermal storage in the concentrator-coupled hemispherical basin solar still increases the productivity by 26%. It was concluded that the productivity greatly increased due to the still integrated with PCM.

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