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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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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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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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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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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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Using Tree Rings to Predict the Response of Tree Growth to Climate Change in the Continental United States during the Twenty-First Century
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In the early 1900s, tree-ring scientists began analyzing the relative widths of annual growth rings preserved in the cross sections of trees to infer past climate variations. Now, many ring-width index (RWI) chronologies, each representing a specific site and species, are archived online within the International Tree-Ring Data Bank (ITRDB). Comparing annual tree-ring- width data from 1097 sites in the continental United States to climate data, the authors quantitatively evaluated how trees at each site have historically re- sponded to interannual climate variations. For each site, they developed a climate-driven statistical growth equation that uses regional climate variables to model RWI values. The authors applied these growth models to predict how tree growth will respond to twenty-first-century climate change, considering four climate projections. Although caution should be taken when extrapolating past relationships with climate into the future, the authors observed several clear and interesting patterns in the growth projections that seem likely if warming continues. Most notably, the models project that productivity of dominant tree species in the southwestern United States will decrease substantially during this century, especially in warmer and drier areas. In the northwest, nonlinear growth relationships with temperature may lead to warming-induced declines in growth for many trees that historically responded positively to warmer tem- peratures. This work takes advantage of the unmatched temporal length and spatial breath of annual growth data available within the ITRDB and exem- plifies the potential of this ever-growing archive of tree-ring data to serve in meta-analyses of large-scale forest ecology.
KEYWORDS: Tree rings; Climate change; Forests; United States
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LATITUDINAL GRADIENTS OF BIODIVERSITY: Pattern, Process, Scale, and Synthesis
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The latitudinal gradient of decreasing richness from tropical to extra- tropical areas is ecology’s longest recognized pattern. Nonetheless, notable exceptions to the general pattern exist, and it is well recognized that patterns may be dependent on characteristics of spatial scale and taxonomic hierarchy. We conducted an exten- sive survey of the literature and provide a synthetic assessment of the degree to which variation in patterns (positive linear, negative linear, modal, or nonsignificant) is a consequence of characteristics of scale (extent or focus) or taxon. In addition, we considered latitudinal gradients with respect to generic and familial richness, as well as species evenness and diversity. We provide a classification of the over 30 hypotheses advanced to account for the latitudinal gradient, and we discuss seven hypotheses with most promise for advancing ecological, biogeographic, and evolutionary understanding. We conclude with a forward-looking synthesis and list of fertile areas for future research.
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Impact of deforestation in the Amazon basin on cloud climatology
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Shallow clouds are prone to appear over deforested surfaces whereas deep clouds, much less frequent than shallow clouds, favor forested surfaces. Simultaneous atmospheric soundings at forest and pasture sites during the Rondonian Boundary Layer Experiment (RBLE-3) elucidate the physical mechanisms responsible for the observed correlation between clouds and land cover. We demonstrate that the atmospheric boundary layer over the forested areas is more unstable and characterized by larger values of the convective available potential energy (CAPE) due to greater humidity than that which is found over the deforested area. The shallow convection over the deforested areas is relatively more active than the deep convection over the forested areas. This greater activity results from a stronger lifting mechanism caused by mesoscale circulations driven by deforestation-induced heterogeneities in land cover.
climate land-cover heterogeneity mesoscale circulations
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The emergence of land change science for global environmental change and sustainability
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Land change science has emerged as a fundamental component of global environmental change and sustainability research. This interdisciplinary field seeks to understand the dynamics of land cover and land use as a coupled human–environment system to ad- dress theory, concepts, models, and applications relevant to environmental and societal problems, including the intersection of the two. The major components and advances in land change are addressed: observation and monitoring; understanding the coupled system—causes, impacts, and consequences; modeling; and synthesis issues. The six articles of the special feature are introduced and situated within these components of study.
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The role of terrestrial plants in limiting atmospheric CO2 decline over the past 24 million years
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Environmental conditions during the past 24 million years are thought to have been favourable for enhanced rates of atmospheric carbon dioxide drawdown by silicate chemical weathering1–7. Proxy records indicate, however, that the Earth’s atmospheric carbon dioxide concentrations did not fall below about 200–250 parts per million during this period8. The stabilization of atmospheric carbon dioxide concentrations near this minimum value suggests that strong negative feedback mechanisms inhibited further drawdown of atmospheric carbon dioxide by high rates of global silicate rock weathering. Here we investigate one possible negative feedback mechanism, occurring under relatively low carbon dioxide concentrations and in warm climates, that is related to terrestrial plant productivity and its role in the decomposition of silicate minerals9–11. We use simulations of terrestrial and geochemical carbon cycles and available experimental evidence to show that vegetation activity in upland regions of active orogens was severely limited by near-starvation of carbon dioxide in combination with global warmth over this period. These conditions diminished biotic-driven silicate rock weathering and thereby attenuated an important long-term carbon dioxide sink. Although our modelling results are semi-quantitative and do not capture the full range of biogeochemical feedbacks that could influence the climate, our analysis indicates that the dynamic equilibrium between plants, climate and the geosphere probably buffered the minimum atmospheric carbon dioxide concentrations over the past 24 million years.
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