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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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Adapting to a Changing Climate in the Southeast
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Whether it’s change to native terrestrial habitats or sea level rise and impacts to vital coastal wetlands and marshes, we are only beginning to understand what is happening across the country, what is likely to occur in the years ahead, and how our agency will act. Indeed, of the 128 national wildlife refuges in the Southeast more than half are located along the coast. The number of days per year with peak temperatures over 90F is expected to rise significantly. By the end of this century, projections indicate much of North Carolina will have 90F plus days for one-third of the year, up from less than 30 days in that temperature zone in the 1960s and 1970s. Arkansas will see 90F days for up to 150 days a year, and NorthFlorida for nearly 6 months a year.
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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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