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We review twentieth century and projected twenty-first century changes in climatic and hydrologic conditions in the northeastern United States and the implications of these changes for forest ecosystems. Climate warming and increases in precipitation and associated changes in snow and hydrologic regimes have been observed over the last century, with the most pronounced changes occurring since 1970. Trends in specific climatic and hydrologic variables differ in their responses spatially (e.g., coastal vs. inland) and temporally (e.g., spring vs. summer). Trends can differ depending on the period of record analyzed, hinting at the role of detach)-scale climatic variation that is superimposed over the longer-term trend. Model predictions indicate that continued increases in temperature and precipitation across the northeastern United States can be expected over the next century. Ongoing increases in growing season length (earlier spring and later autumn) will most likely increase evapotranspiration and frequency of drought. In turn, an increase in the frequency of drought will likely increase the risk of fire and negatively impact forest productivity, maple syrup production, and the intensity of autumn foliage coloration. Climate and hydrologic changes could have profound effects on forest structure, composition, and ecological functioning in response to the changes discussed here and as described in related articles in this issue of the Journal. Nous avons passe en revue les changements dans les conditions climatiques et hydrologiques survenues pendant le 20e siecle et projetees au cours du 21e siecle dans le nord-est des Etats-Unis ainsi que les repercussions de ces changements sur les ecosystemes forestiers. Un rechauffement du climat et une augmentation des precipitations ainsi que des changements dans les regimes nivologique et hydrologique ont ete observes pendant le dernier siecle et les changements les plus prononces sont survenus depuis 1970. Les tendances de variables climatiques et hydrologiques specifiques different dans f espace (p. ex. zone cotiere vs interieur des terres) et dans le temps (p. ex. printemps vs ete) dans leur reponse. Les tendances different selon la periode couverte par les donnees qui sont analysees, un indice du role de la variation du climat a une echelle decennale qui se superpose a la tendance a long terme. Les predictions du modele indiquent qu'on peut s'attendre a des augmentations continues de la temperature et des precipitations partout dans la region au cours du prochain siecle. L'allongement en cours de la saison de croissance (printemps qui arrive plus tot et automne qui arrive plus tard) augmentera l'evapotranspiration et la frequence des secheresses. Des secheresses plus frequentes vont probablement augmenter les risques d'incendie et avoir un impact negatif sur la productivite de la foret, la production de sirop d'erable et l'intensite de la coloration automnale du feuillage. Les changements climatiques et hydrologiques pourraient avoir de serieux effets sur la structure, la composition et le fonctionnement ecologique de la foret en reaction aux hangements discutes dans cet article et decrits dans d'autres articles relies au meme sujet et publies dans ce numero. [Traduit par la Redaction], Introduction and overview of climate and hydrology in the region Introduction At a regional scale, interpretations of the effects of recent climate change and predictions of the potential effects of [...]

Projections of changes in climate extremes are critical to assessing the potential impacts of climate change on human and natural systems. Modeling advances now provide the opportunity of utilizing global general circulation models (GCMs) for projections of extreme temperature and precipitation indicators. We analyze historical and future simulations of ten such indicators as derived from an ensemble of 9 GCMs contributing to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC-AR4), under a range of emissions scenarios. Our focus is on the consensus from the GCM ensemble, in terms of direction and significance of the changes, at the global average and geographical scale. The climate extremes described by the ten indices range from heat-wave frequency to frost-day occurrence, from dry-spell length to heavy rainfall amounts. Historical trends generally agree with previous observational studies, providing a basic sense of reliability for the GCM simulations. Individual model projections for the 21st century across the three scenarios examined are in agreement in showing greater temperature extremes consistent with a warmer climate. For any specific temperature index, minor differences appear in the spatial distribution of the changes across models and across scenarios, while substantial differences appear in the relative magnitude of the trends under different emissions rates. Depictions of a wetter world and greater precipitation intensity emerge unequivocally in the global averages of most of the precipitation indices. However, consensus and significance are less strong when regional patterns are considered. This analysis provides a first overview of projected changes in climate extremes from the 1PCC-AR4 model ensemble, and has significant implications with regard to climate projections for impact assessments.

An IPCC founder and tenacious advocate for climate action. An IPCC founder and tenacious advocate for climate action., Author(s): Katharine Hayhoe, Donald Wuebbles Author Affiliations: John Houghton (1931-2020) John Houghton (right) at the opening of a weather centre in 1985. [see PDF for image] Credit: PA Archive/PA John [...]

Methane (C[H.sub.4]) is the most abundant organic trace gas in the atmosphere. In the distant past, variations in natural sources o methane were responsible for trends in atmospheric methane levels recorded in ice cores. Since the 1700s, rapidly growing human activities, particularly in the areas of agriculture, fossil fuel use, and waste disposal, have more than doubled methane emissions. Atmospheric methane concentrations have increased by a factor of 2-3 in response to this increase, and continue to rise. These increasing concentrations have raised concern due to their potential effects on atmospheric chemistry and climate Methane is important to both tropospheric and stratospheric chemistry, significantly affecting levels of ozone, water vapor, the hydroxyl radical, and numerous other compounds. In addition, methane is currently the second most important greenhouse gas emitted from human activities. On a per molecule basis, it is much more effective a greenhouse gas than additional C[O.sub.2]. In this review, we examine past trends in the concentration of methane in the atmosphere, the sources and sinks that determine it growth rate, and the factors that will affect its growth rate in the future. We also present current understanding of the effects of methane on atmospheric chemistry, and examine the direct and indirect impacts of atmospheric methane on climate. Keywords: methane; greenhouse gases; atmospheric chemistry; global change