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Understanding the changing hydrologic regime and storage requirements in the Upper Colorado River basin
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The changing hydrologic regime of the Upper Colorado River Basin presents a daunting challenge for water resources management. A major source of concern is that of ascertaining the nature of runoff variability and re- calibrating the systemic management and planning based on a more reliable envelope of water supply variations to meet societal needs. In this letter, we examine the long-term variability and change in the Upper Colorado annual runoff volume—quantified as shifts in the mean, interannual variability, and persistence—in a recent tree-ring based reconstruction extending back to 762AD. A simple model for reservoir storage requirement shows sensitivity to the changing hydrologic regime, with episodes of abrupt shifts toward significantly higher storage requirements, often not readily evident in runoff statistics. The results also suggest that benchmarking of climate models for regional water resources assessment should focus on the runoff statistics that are most relevant for storage requirement computations.
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Projected increase in continental runoff due to plant responses to increasing carbon dioxide
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In addition to influencing climatic conditions directly through radiative forcing, increasing carbon dioxide concentration in- fluences the climate system through its effects on plant physi- ology1. Plant stomata generally open less widely under increased carbon dioxide concentration2, which reduces transpiration1,3–6 and thus leaves more water at the land surface7. This driver of change in the climate system, which we term ‘physiological for- cing’, has been detected in observational records of increasing average continental runoff over the twentieth century8. Here we use an ensemble of experiments with a global climate model that includes a vegetation component to assess the contribution of physiological forcing to future changes in continental runoff, in the context of uncertainties in future precipitation. We find that the physiological effect of doubled carbon dioxide concentrations on plant transpiration increases simulated global mean runoff by 6 per cent relative to pre-industrial levels; an increase that is com- parable to that simulated in response to radiatively forced climate change (11 6 6 per cent). Assessments of the effect of increasing carbon dioxide concentrations on the hydrological cycle that only consider radiative forcing9–11 will therefore tend to underestimate future increases in runoff and overestimate decreases. This sug- gests that freshwater resources may be less limited than previously assumed under scenarios of future global warming, although there is still an increased risk of drought. Moreover, our results high- light that the practice of assessing the climate-forcing potential of all greenhouse gases in terms of their radiative forcing potential relative to carbon dioxide does not accurately reflect the relative effects of different greenhouse gases on freshwater resources.
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The velocity of climate change
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The ranges of plants and animals are moving in response to recent changes in climate1. As temperatures rise, ecosystems with ‘nowhere to go’, such as mountains, are considered to be more threatened2,3. However, species survival may depend as much on keeping pace with moving climates as the climate’s ultimate per- sistence4,5. Here we present a new index of the velocity of temper- ature change (km yr21), derived from spatial gradients (6C km21) and multimodel ensemble forecasts of rates of temperature increase (6C yr21) in the twenty-first century. This index represents the instantaneous local velocity along Earth’s surface needed to maintain constant temperatures, and has a global mean of 0.42 km yr21 (A1B emission scenario). Owing to topographic effects, the velocity of temperature change is lowest in mountainous biomes such as tropical and subtropical coniferous forests (0.08kmyr21), temperate coniferous forest, and montane grass- lands. Velocities are highest in flooded grasslands (1.26 km yr21), mangroves and deserts. High velocities suggest that the climates of only 8% of global protected areas have residence times exceeding 100 years. Small protected areas exacerbate the problem in Mediterranean-type and temperate coniferous forest biomes. Large protected areas may mitigate the problem in desert biomes. These results indicate management strategies for minimizing biodiversity loss from climate change. Montane landscapes may effectively shelter many species into the next century. Elsewhere, reduced emissions, a much expanded network of protected areas6, or efforts to increase species movement may be necessary7.
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Climate change and the ecologist
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The evidence for rapid climate change now seems overwhelming. Global temperatures are predicted to rise by up to 4 °C by 2100, with associated alterations in precipitation patterns. Assessing the consequences for biodiversity, and how they might be mitigated, is a Grand Challenge in ecology.
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SPECIAL REPORT:CLIMATE OF OPPORTUNITY
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Awareness about climate change is at an all-time high. Will this surge of attention translate into more jobs for climate scientists?
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Aerosols heat up
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Solid particles suspended in the atmosphere have long played second fiddle to greenhouse gases as agents of climate change. A study of atmospheric heating over the Indian Ocean could provoke a rethink.
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Scientific reticence and sea level rise
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I suggest that ‘scientific reticence’, in some cases, hinders communication with the public about dangers of global warming. If I am right, it is important that policy-makers recognize the potential influence of this phenomenon.
Scientific reticence may be a consequence of the scientific method. Success in science depends on objective skepticism. Caution, if not reticence, has its merits. However, in a case such as ice sheet instability and sea level rise, there is a danger in excessive caution. We may rue reticence, if it serves to lock in future disasters.Barber (1961) describes a ‘resistance by scientists to scientific discovery’, with a scholarly discussion of several sources of cultural resistance.There are aspects of the phenomenon that Barber discusses in the ‘scientific reticence’ that I describe, but additional factors come into play in the case of global climate change and sea level rise.
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Projections of Future Drought in the Continental United States and Mexico
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Using the Palmer drought severity index, the ability of 19 state-of-the-art climate models to reproduce ob- served statistics of drought over North America is examined. It is found that correction of substantial biases in the models’ surface air temperature and precipitation fields is necessary. However, even after a bias correction, there are significant differences in the models’ ability to reproduce observations. Using metrics based on the ability to reproduce observed temporal and spatial patterns of drought, the relationship between model per- formance in simulating present-day drought characteristics and their differences in projections of future drought changes is investigated. It is found that all models project increases in future drought frequency and severity. However, using the metrics presented here to increase confidence in the multimodel projection is complicated by a correlation between models’ drought metric skill and climate sensitivity. The effect of this sampling error can be removed by changing how the projection is presented, from a projection based on a specific time interval to a projection based on a specified temperature change. This modified class of projections has reduced intermodel uncertainty and could be suitable for a wide range of climate change impacts projections.
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THE SPATIAL AND TEMPORAL VARIABILITY OF RAIN-ON-SNOW
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Snow melt during rainfall causes large-scale flooding and avalanching. These rain-on- snow events are most well-documented in the coastal mountain ranges of western North America. To determine what role they play in interior mountains, we analyzed flood frequencies in the Columbia River basin and modeled rain-on-snow potential from daily temperature and precipitation data. Applying the model with geographically distributed weather data allowed maps of rain-on-snow potential at 2km spatial resolution to be generated for characteristic climate years of 1982 (cold and wet), 1988 (warm and dry), and 1989 (average). It was found that rain-on-snow events are more likely during cool, wet years (such as 1982). A greater number of events and more widespread distribution of events occur during this type of climate. The cool temperatures allow low-elevation snow to accumulate and frequent storms bring the possibility of mid-winter rain. Warm, dry years (1988) are less likely to experience rain-on-snow events. There is little low-elevation snow at these times and only occasional precipitation. During all years, areas most susceptible to rain-on-snow are those where topography allows incursion of relatively warm, moist marine air that flows from the Pacific Ocean into the Columbia Plateau and up the Snake River Valley. These areas include the Cascade mountains; northern Idaho, northeastern Washington, and northwestern Montana where valleys open into the Columbia plateau; the Blue Mountains of northeastern Oregon; and western Wyoming and central Idaho adjacent to the Snake River.
KEYWORDS: snow, avalanches, rain-on-snow, floods
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RAIN-ON-SNOW EVENTS IN THE WESTERN UNITED STATES
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Severity of rain on snow depends on a number of factors, and an overall decrease in these events appears to be driven, in part, by changes in El Niño–Southern Oscillation.
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