Publications Published in Proceedings of the National Academy of Sciences of the United States of America

1. Additive effects of simulated climate changes, elevated CO. Zavaleta, Erika S.; Shaw, M. Rebecca; Chiariello, Nona R.; Mooney, Harold A.; Field, Christopher B..
   Proceedings of the National Academy of Sciences of the United States of America: 2003
   Notes
   Biodiversity responses to ongoing climate and atmospheric changes will affect both ecosystem processes and the delivery of ecosystem goods and services. Combined effects of co-occurring global changes on diversity, however, are poorly understood. We examined plant diversity responses in a California annual grassland to manipulations of four global environmental changes, singly and in combination: elevated CO
   
   
   
2. A new decision sciences for complex systems. Lempert, Robert J.
   Proceedings of the National Academy of Sciences of the United States of America: 2002
   Notes
   Models of complex systems can capture much useful information but can be difficult to apply to real-world decision-making because the type of information they contain is often inconsistent with that required for traditional decision analysis. New approaches, which use inductive reasoning over large ensembles of computational experiments, now make possible systematic comparison of alternative policy options using models of complex systems. This article describes Computer-Assisted Reasoning, an approach to decision-making under conditions of deep uncertainty that is ideally suited to applying complex systems to policy analysis. The article demonstrates the approach on the policy problem of global climate change, with a particular focus on the role of technology policies in a robust, adaptive strategy for greenhouse gas abatement.
   
   
   
3. Climate change hastens population extinctions. McLaughlin, J. F.; Hellmann, J. J.; Boggs, C. L.; Ehrlich, P. R..
   Proceedings of the National Academy of Sciences of the United States of America: 2002
   Notes
   Climate change is expected to alter the distribution and abundance of many species. Predictions of climate-induced population extinctions are supported by geographic range shifts that correspond to climatic warming, but few extinctions have been linked mechanistically to climate change. Here we show that extinctions of two populations of a checkerspot butterfly were hastened by increasing variability in precipitation, a phenomenon predicted by global climate models. We model checkerspot populations to show that changes in precipitation amplified population fluctuations, leading to rapid extinctions. As populations of checkerspots and other species become further isolated by habitat loss, climate change is likely to cause more extinctions, threatening both species diversity and critical ecosystem services.
   
   
   
4. Dematerialization: Variety, caution, and persistence. Ausubel, J. H.; Waggoner, P. E..
   Proceedings of the National Academy of Sciences of the United States of America: 2008
   DOI: 10.1073/pnas.0806099105
   Notes
   Dematerialization, represented by declining consumption per GDP of energy or of goods, offers some hope for rising environmental quality with development. The declining proportion of income spent on staples as affluence grows, which income elasticity < 1.0 measures, makes dematerialization widespread. Further, as learning improves efficiency of resource use, the intensity of environmental impact per production of staples often declines. We observe that combinations of low income elasticity for staples and of learning by producers cause a variety of dematerializations and declining intensities of impact, from energy use and carbon emission to food consumption and fertilizer use, globally and in countries ranging from the United States and France to China, India, Brazil, and Indonesia. Because dematerialization and intensity of impact are ratios of parameters that may be variously defined and are sometimes difficult to estimate, their fluctuations must be interpreted cautiously. Nevertheless, substantial declining intensity of impact, and especially, dematerialization persisted between 1980 and 2006.
   
   
   
5. Emissions pathways, climate change, and impacts on California. Hayhoe, K.; Cayan, D.; Field, C. B.; Frumhoff, P.C.; Maurer, E.P.; Miller, N.L.; Moser, S.C.; Schneider, S.H.; Cahill, K.N.; Cleland, E.E.; Dale, L.; Drapek, R.; Hanemann, R.M.; Kalkstein, L.S.; Lenihan, J.; Lunch, C.K.; Nielson, R.P.; Sheridan, S.C.; Verville, J.H..
   Proceedings of the National Academy of Sciences of the United States of America: 2004
   Notes
   The magnitude of future climate change depends substantially on the greenhouse gas emission pathways we choose. Here we explore the implications of the highest and lowest Intergovernmental Panel on Climate Change emissions pathways for climate change and associated impacts in California. Based on climate projections from two state-of-the-art climate models with low and medium sensitivity (Parallel Climate Model and Hadley Centre Climate Model, version 3, respectively), we find that annual temperature increases nearly double from the lower B1 to the higher A1fi emissions scenario before 2100. Three of four simulations also show greater increases in summer temperatures as compared with winter. Extreme heat and the associated impacts on a range of temperature-sensitive sectors are substantially greater under the higher emissions scenario, with some interscenario differences apparent before midcentury. By the end of the century under the B1 scenario, heatwaves and extreme heat in Los Angeles quadruple in frequency while heat-related mortality increases two to three times; alpine/subalpine forests are reduced by 50–75%; and Sierra snowpack is reduced 30–70%. Under A1fi, heatwaves in Los Angeles are six to eight times more frequent, with heat-related excess mortality increasing five to seven times; alpine/subalpine forests are reduced by 75–90%; and snowpack declines 73–90%, with cascading impacts on runoff and streamflow that, combined with projected modest declines in winter precipitation, could fundamentally disrupt California’s water rights system. Although interscenario differences in climate impacts and costs of adaptation emerge mainly in the second half of the century, they are strongly dependent on emissions from preceding decades.
   
   
   
6. Extensive regional atmospheric hydrocarbon pollution in the southwestern United States. Katzenstein, A. S.; Doezema, L. A.; Simpson, I. J.; Balke, D. R.; Rowland, F. S..
   Proceedings of the National Academy of Sciences of the United States of America: 2003
   Notes
   Light alkane hydrocarbons are present in major quantities in the near-surface atmosphere of Texas, Oklahoma, and Kansas during both autumn and spring seasons. In spring 2002, maximum mixing ratios of ethane [34 parts per 109 by volume (ppbv)], propane (20 ppbv), and n-butane (13 ppbv) were observed in north-central Texas. The elevated alkane mixing ratios are attributed to emissions from the oil and natural gas industry. Measured alkyl nitrate mixing ratios were comparable to urban smog values, indicating active photochemistry in the presence of nitrogen oxides, and therefore with abundant formation of tropospheric ozone. We estimate that 4-6 teragrams of methane are released annually within the region and represents a significant fraction of the estimated total U.S. emissions. This result suggests that total U.S. natural gas emissions may have been underestimated. Annual ethane emissions from the study region are estimated to be 0.3-0.5 teragrams.
   
   
   
7. Global atmospheric black carbon inferred from AERONET. Sato, Makiko; Hansen, James; Koch, Dorothy; Lacis, Andrew; Ruedy, Reto; Dubovik, Oleg; Holben, Brent; Chin, Mian; Novakov, Tica.
   Proceedings of the National Academy of Sciences of the United States of America: 2003
   Notes
   AERONET, a network of well calibrated sunphotometers, provides data on aerosol optical depth and absorption optical depth at >250 sites around the world. The spectral range of AERONET allows discrimination between constituents that absorb most strongly in the UV region, such as soil dust and organic carbon, and the more ubiquitously absorbing black carbon (BC). AERONET locations, primarily continental, are not representative of the global mean, but they can be used to calibrate global aerosol climatologies produced by tracer transport models. We find that the amount of BC in current climatologies must be increased by a factor of 2-4 to yield best agreement with AERONET, in the approximation in which BC is externally mixed with other aerosols. The inferred climate forcing by BC, regardless of whether it is internally or externally mixed, is ≈1 W/m², most of which is probably anthropogenic. This positive forcing (warming) by BC must substantially counterbalance cooling by anthropogenic reflective aerosols. Thus, especially if reflective aerosols such as sulfates are reduced, it is important to reduce BC to minimize global warming.
   
   
   
8. Global warming in the twenty-first century: An alternative scenario. Hansen, J.; Sato, M.; Ruedy, R.; Lacis, A.; Oinas, V..
   Proceedings of the National Academy of Sciences of the United States of America: 2000
   Notes
   A common view is that the current global warming rate will continue or accelerate, but we argue that rapid warming in recent decades has been driven mainly by non-CO2 greenhouse gases (GHGs), such as chlorofluorocarbons, CH4. and N2O, not by the products of fossil fuel burning, CO2 and aerosols, the positive and negative climate forcings of which are partially offsetting. The growth rate of non-CO2 GHGs has declined in the past decade. If sources of CH4 and O-3 precursors were reduced in the future, the change in climate forcing by non-CO2 GHGs in the next 50 years could be near zero. Combined with a reduction of black carbon emissions and plausible success in slowing CO2 emissions, this reduction of non-CO2 GHGs could lead to a decline in the rate of global warming, reducing the danger of dramatic climate change.
   
   
   
9. Nitrogen assimilation and growth of wheat under elevated carbon dioxide. Bloom, A. J.; Smart, D. R.; Nguyen, D. T.; Searles, P. S..
   Proceedings of the National Academy of Sciences of the United States of America: 2002
   Notes
   Simultaneous measurements of CO2 and O-2 fluxes from wheat (Triticum, aestivum) shoots indicated that short-term exposures to elevated CO2 concentrations diverted photosynthetic reductant from NO3- or NO2- reduction to CO2 fixation. With longer exposures to elevated CO2, wheat leaves showed a diminished capacity for NO3- photoassimilation at any CO2 concentration. Moreover, high bicarbonate levels impeded NO2- translocation into chloroplasts isolated from wheat or pea leaves. These results support the hypothesis that elevated CO2 inhibits NO3- photoassimilation. Accordingly, when wheat plants received NO3- rather than NH4+ as a nitrogen source, CO2 enhancement of shoot growth halved and CO2 inhibition of shoot protein doubled. This result will likely have major implications for the ability of wheat to use NO3- as a nitrogen source under elevated CO2-.
   
   
   
10. Plants reverse warming effect on ecosystem water balance. Zavaleta, E. S.; Thomas, B. D.; Chiariello, N. R.; Asner, G. P.; Shaw, M. R.; Field, C. B..
    Proceedings of the National Academy of Sciences of the United States of America: 2003
    Notes
    Models predict that global warming may increase aridity in water-limited ecosystems by accelerating evapotranspiration. We show that interactions between warming and the dominant biota in a grassland ecosystem produced the reverse effect. In a 2-year field experiment, simulated warming increased spring soil moisture by 5-10% under both ambient and elevated CO