Consequences of More Extreme Precipitation Regimes for Terrestrial Ecosystems

摘要

Amplification of the hydrological cycle as a consequence of global warming is forecast to lead to more extreme intra-annual precipitation regimes characterized by larger rainfall events and longer intervals between events. We present a conceptual framework, based on past investigations and ecological theory, for predicting the consequences of this underappreciated aspect of climate change. We consider a broad range of terrestrial ecosystems that vary in their overall water balance. More extreme rainfall regimes are expected to increase the duration and severity of soil water stress in mesic ecosystems as intervals between rainfall events increase. In contrast, xeric ecosystems may exhibit the opposite response to extreme events. Larger but less frequent rainfall events may result in proportional reductions in evaporative losses in xeric systems, and thus may lead to greater soil water availability. Hydric (wetland) ecosystems are predicted to experience reduced periods of anoxia in response to prolonged intervals between rainfall events. Understanding these contingent effects of ecosystem water balance is necessary for predicting how more extreme precipitation regimes will modify ecosystem processes and alter interactions with related global change drivers.nnHuman activities have caused dramatic and unprecedented changes in the global chemical and physical environment, including well-documented increases in atmospheric carbon dioxide (CO2) concentration and mean annual temperature (Karl and Knight 1998, New et al. 2001, IPCC 2007). If greenhouse gas emissions continue to increase at present rates, atmospheric CO2 concentrations will more than double preindustrial levels during the current century, and general circulation models (GCMs) predict additional increases in mean global temperature of between 1.1 and 6.4 degrees Celsius (IPCC 2007). Alterations in patterns of global atmospheric circulation and hydrologic processes are predicted to modify mean annual precipitation and to increase the inter-and intra-annual variability of precipitation (Easterling et al. 2000, Schär et al. 2004, Seneviratne et al. 2006, IPCC 2007). The combined effects of increased atmospheric CO2, elevated global temperatures, and altered precipitation regimes represent a rapid and unprecedented change to the fundamental drivers of chemical and biological processes within ecosystems (Amundson and Jenny 1997). The complexity and pace of these global anthropogenic changes pose a major challenge for ecosystem scientists and managers (NRC 2001), particularly given their potential impact on the provisioning of ecosystem services (Bennett et al. 2005).nnAmplification of the hydrological cycle, a consequence of global warming, has been expressed in the form of increased cloudiness, latent heat fluxes, and more frequent climate extremes (Huntington 2006, IPCC 2007). Key predictions of hydrological amplification are an increased risk of drought and heat waves (recently exemplified by the extremely dry and hot summer of 2003 in Europe; Ciais et al. 2005, Reichstein et al. 2007) and an increased probability of intense precipitation events and flooding. The complexity, interactions, and scope of global-scale atmospheric processes have made potential changes in precipitation patterns difficult to predict, compared with the more consistent projections for increased atmospheric CO2 and temperature. Thus, although most GCMs predict a modest increase in rainfall at the global scale, they often disagree on the magnitude and even the direction of change at regional and especially local scales (IPCC 2007, Zhang et al. 2007). In contrast, projections have been consistent for intensified intra-annual precipitation regimes (through larger individual precipitation events) with longer intervening dry periods than at present (Easterling et al. 2000, IPCC 2007). Less frequent but more intense precipitation events may increase the severity of within-season drought, significantly alter evapotranspiration, and generate greater runoff (Fay et al. 2003, MacCracken et al. 2003). These intra-annual modifications to the hydrological cycle are distinct from the better-known alterations in interannual precipitation variability associated with large-scale climate dynamics (e.g., the El Niño Southern and Pacific Decadal oscillations), although both intra-and interannual changes lie along a continuum of altered temporal patterns in hydrology. Our focus here is on increased intra-annual variability in precipitation (i.e., more extreme rainfall regimes), a more subtle but chronic and pervasive change in the way that precipitation is delivered to terrestrial ecosystems.nnThere is growing evidence at global, regional, and local scales that intra-annual precipitation regimes have already become more extreme. For example, global precipitation records show an average increase of only 9 millimeters (mm) of precipitation over land areas (excluding Antarctica) during the 20th century (figure 1). Regionally, however, these records show an increased frequency of wet days in portions of North America, Europe, and Southern Africa; an increased frequency and duration of dry periods in European-African, Australian, Mediterranean, and Asian monsoon regions; and an increased proportion of total precipitation originating from the largest precipitation events in several regions (figure 1; New et al. 2001, Groisman et al. 2005). Elevated temperatures have been associated with a 10% increase in annual precipitation in the contiguous United States over the past century; this increase is expressed primarily as an intensification of the largest precipitation events, particularly in the summer (Karl and Knight 1998). Thus, the link between higher temperatures and more extreme precipitation regimes has solid theoretical underpinnings and model validation (Karl and Trenberth 2003), as well as emerging empirical support from global climate data sets (Karl et al. 1995, Kunkel et al. 1999, Groisman et al. 2005).