USE OF A STOCHASTIC WEATHER GENERATOR IN THE DEVELOPMENT OF CLIMATE CHANGE SCENARIOS

Climate change scenarios with a high spatial and temporal resolution are required in the evaluation of the effects of climate change on agricultural potential and agricultural risk. Such scenarios should reproduce changes in mean weather characteristics as well as incorporate the changes in climate variability indicated by the global climate model (GCM) used. Recent work on the sensitivity of crop models and climatic extremes has clearly demonstrated that changes in variability can have more profound effects on crop yield and on the probability of extreme weather events than simple changes in the mean values. The construction of climate change scenarios based on spatial regression downscaling and on the use of a local stochastic weather generator is described. Regression downscaling translated the coarse resolution GCM grid-box predictions of climate change to site-specific values. These values were then used to perturb the parameters of the stochastic weather generator in order to simulate site-specific daily weather data. This approach permits the incorporation of changes in the mean and variability of climate in a consistent and computationally inexpensive way. The stochastic weather generator used in this study, LARS-WG, has been validated across Europe and has been shown to perform well in the simulation of different weather statistics, including those climatic extremes relevant to agriculture. The importance of downscaling and the incorporation of climate variability are demonstrated at two European sites where climate change scenarios were constructed using the UK Met. Office high resolution GCM equilibrium and transient experiments.     

Silicate, Hydrous and Carbonate Metasomatism at Lherz, France: Contemporaneous Derivatives of Silicate Melt-Harzburgite Reaction

Complex multi-stage models involving silicate, hydrous and carbonatemelts of distinct provenance have been invoked to explain themetasomatism observed in mantle rocks. In contrast, relativelysimple models requiring polybaric crystallization of alkalinesilicate melts have been proposed to explain the occurrenceof veined mantle rocks. To address the spatial and temporalrelationships between veins and wall-rocks, a sequence of drillcores was obtained from Lherz, France. In outcrop the vein (amphibole–garnetpyroxenite dyke) is spatially associated with hornblendite veinlets(lherzite), and proximal amphibole-bearing and distal apatite-bearingwall-rock peridotite. Considerable elemental and isotopic heterogeneityexists in these wall-rock peridotites, in many instances equivalentto, or greater than, that observed in mantle xenoliths fromworldwide localities. A single stage of reactive porous flowbest explains the elemental and isotopic heterogeneity in thewall-rock. In essence it is proposed that emplacement of thesilicate melt (dyke) was inextricably linked to chromatographicfractionation/reaction of derivatives which led to the coexistence,in space and time, of silicate, hydrous and carbonate melts.This model elegantly and simply describes the formation of complexmetasomatic aureoles around mantle veins and negates the need,in the case of basalt-hosted (and kimberlite-hosted) xenoliths,for complex multi-stage models involving several episodes ofmelt influx with each melt being of different provenance. KEY WORDS: mantle metasomatism; trace-element enrichment; isotopic contamination; wall-rock peridotite; Lherz peridotite