In recent decades, changes that human activities have wrought in Earth's life support system have worried many people. The human population has doubled in the past 40 years and is projected to increase by the same amount again in the next 40. The expansion of infrastructure and agriculture necessitated by this population growth has quickened the pace of land transformation and degradation. We estimate that humans have modified >50% of Earth's land surface. The current rate of land transformation, particularly of agricultural land, is unsustainable. We need a lively public discussion of the problems resulting from population pressures and the resulting land degradation.
This paper describes the detennination of sheet erosion rates by using dendrogeomorphological methods on exposed tree roots, Two sites on the northern slope of the Guadarrama Mountains, Central Spain, were studied: a popular trail in a Scots pine forest (Senda Schmidt, Valsain) growing on granites and gneisses, and an open holm-oak forest on granitic slopes (Monterrubio), These sites were selected because they showed high denudation morphologies due to accelerated soil-erosion processes caused by human influence (trampling by continuous trekking and overgrazing), resulting in exposed roots, The method applied is based on the morphological pattern of roots, defined by the growth-ring series of the sampled roots, In order to confinn the validity of the criteria used and to make the estimations of erosion more accurate, several anatomical indicators of exposed and non exposed Pin us sylvestris roots were characterized, The study entailed a statistical analysis of exposure time and erosion depth. The influence of environmental factors affecting the variation in velocity of the erosion processes was also examined. With a significance level of 95%, the mean erosion rates were in the range of 1.7 2.6 mm/year (29 44 Mw/year) on Senda Sclnnidt over the last 101 years, and 1.1 1.8 mm/year (19 31 t/halyear) in Monterrubio over the last 42 years. Using a multifactor analysis of variance, we observed a change in the erosion rates as a fimction of position on the path along Senda Schmidt. In Monterrubio, however, we reached no significant conclusions, apart from an inverse relationship between erosion and slope gradient that was difficult to interpret.Climate conditions in Senda Sclnnidt and the accuracy of dating Scots pine indicate that the evaluation on P sylvestris roots is fairly reliable, which is not the case for oak roots. Although this paper is based on the application of an existing method, its novelty lies in being the first attempt in Spain to estimate 'accelerated' sheet erosion rates (due to recreational activities and overgrazing) using dendrogeomorphological techniques, supplemented by anatomical indicators for P sylvestris.
Mountain glaciers respond directly to changes in precipitation and temperature, thus their margin extent is a high-sensitivity climate proxy. Here, we present a robust 10Be chronology for the glacier maximum areal extent of central Spain paleoglaciers dated at 26.1 ± 1.3 ka BP. These glaciers reached their maximum extent several thousand years earlier than those from central Europe due to the increased precipitation within a cold period between 25 to 29 ka BP, as confirmed by a local speleothem record. These paleoclimate conditions impacted the maximum extent of mountain glaciers along the western and central Mediterranean region. The cause and timing of the enhanced precipitation implies a southward shift of the North Atlantic Polar Front followed by storm tracks in response to changes in insolation via orbital parameters modulation. Thus, these mountain paleoglaciers from the Mediterranean region record an ocean-continent climate interaction triggered by external forcing.
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