Publications Published in PNAS

1. Abrupt tropical climate change: Past and present. Thompson, Lonnie G.; Mosley-Thompson, Ellen; Brecher, Henry; Davis, Mary; Leon, Blanca; Les, Don; Lin, Ping-Nan; Mashiotta, Tracy; Mountain, Keith.
   PNAS: 2006
   DOI: 10.1073/pnas.0603900103
   Notes
   Three lines of evidence for abrupt tropical climate change, both past and present, are presented. First, annually and decadally averaged {delta}18O and net mass-balance histories for the last 400 and 2,000 yr, respectively, demonstrate that the current warming at high elevations in the mid- to low latitudes is unprecedented for at least the last 2 millennia. Second, the continuing retreat of most mid- to low-latitude glaciers, many having persisted for thousands of years, signals a recent and abrupt change in the Earth's climate system. Finally, rooted, soft-bodied wetland plants, now exposed along the margins as the Quelccaya ice cap (Peru) retreats, have been radiocarbon dated and, when coupled with other widespread proxy evidence, provide strong evidence for an abrupt mid-Holocene climate event that marked the transition from early Holocene (pre-5,000-yr-B.P.) conditions to cooler, late Holocene (post-5,000-yr-B.P.) conditions. This abrupt event, {approx}5,200 yr ago, was widespread and spatially coherent through much of the tropics and was coincident with structural changes in several civilizations. These three lines of evidence argue that the present warming and associated glacier retreat are unprecedented in some areas for at least 5,200 yr. The ongoing global-scale, rapid retreat of mountain glaciers is not only contributing to global sea-level rise but also threatening freshwater supplies in many of the world's most populous regions.
   
   
   
2. A climate-change risk analysis for world ecosystems. Scholze, Marko; Knorr, Wolfgang; Arnell, Nigel W.; Prentice, I. Colin.
   PNAS: 2006
   DOI: 10.1073/pnas.0601816103
   Notes
   We quantify the risks of climate-induced changes in key ecosystem processes during the 21st century by forcing a dynamic global vegetation model with multiple scenarios from 16 climate models and mapping the proportions of model runs showing forest/nonforest shifts or exceedance of natural variability in wildfire frequency and freshwater supply. Our analysis does not assign probabilities to scenarios or weights to models. Instead, we consider distribution of outcomes within three sets of model runs grouped by the amount of global warming they simulate: <2{degrees}C (including simulations in which atmospheric composition is held constant, i.e., in which the only climate change is due to greenhouse gases already emitted), 2-3{degrees}C, and >3{degrees}C. High risk of forest loss is shown for Eurasia, eastern China, Canada, Central America, and Amazonia, with forest extensions into the Arctic and semiarid savannas; more frequent wildfire in Amazonia, the far north, and many semiarid regions; more runoff north of 50{degrees}N and in tropical Africa and northwestern South America; and less runoff in West Africa, Central America, southern Europe, and the eastern U.S. Substantially larger areas are affected for global warming >3{degrees}C than for <2{degrees}C; some features appear only at higher warming levels. A land carbon sink of {approx}1 Pg of C per yr is simulated for the late 20th century, but for >3{degrees}C this sink converts to a carbon source during the 21st century (implying a positive climate feedback) in 44% of cases. The risks continue increasing over the following 200 years, even with atmospheric composition held constant.
   
   
   
3. Altered soil microbial community at elevated CO2 leads to loss of soil carbon. Carney, Karen M.; Hungate, Bruce A.; Drake, Bert G.; Megonigal, J. Patrick.
   PNAS: 2007
   DOI: 10.1073/pnas.0610045104
   Notes
   Increased carbon storage in ecosystems due to elevated CO2 may help stabilize atmospheric CO2 concentrations and slow global warming. Many field studies have found that elevated CO2 leads to higher carbon assimilation by plants, and others suggest that this can lead to higher carbon storage in soils, the largest and most stable terrestrial carbon pool. Here we show that 6 years of experimental CO2 doubling reduced soil carbon in a scrub-oak ecosystem despite higher plant growth, offsetting {approx}52% of the additional carbon that had accumulated at elevated CO2 in aboveground and coarse root biomass. The decline in soil carbon was driven by changes in soil microbial composition and activity. Soils exposed to elevated CO2 had higher relative abundances of fungi and higher activities of a soil carbon-degrading enzyme, which led to more rapid rates of soil organic matter degradation than soils exposed to ambient CO2. The isotopic composition of microbial fatty acids confirmed that elevated CO2 increased microbial utilization of soil organic matter. These results show how elevated CO2, by altering soil microbial communities, can cause a potential carbon sink to become a carbon source.
   
   
   
4. Are We in the Midst of the Sixth Mass Extinction? A View from the World of Amphibians. Wake, David B.; Vredenburg, Vance T..
   PNAS: 2008
   Notes
   Many scientists argue that we are either entering or in the midst of the sixth great mass extinction. Intense human pressure, both direct and indirect, is having profound effects on natural environments. The amphibians—frogs, salamanders, and caecilians—may be the only major group currently at risk globally. A detailed worldwide assessment and subsequent updates show that one-third or more of the 6,300 species are threatened with extinction. This trend is likely to accelerate because most amphibians occur in the tropics and have small geographic ranges that make them susceptible to extinction. The increasing pressure from habitat destruction and climate change is likely to have major impacts on narrowly adapted and distributed species. We show that salamanders on tropical mountains are particularly at risk. A new and significant threat to amphibians is a virulent, emerging infectious disease, chytridiomycosis, which appears to be globally distributed, and its effects may be exacerbated by global warming. This isease, which is caused by a fungal pathogen and implicated in serious declines and extinctions of >200 species of amphibians, poses the greatest threat to biodiversity of any known disease. Our data for frogs in the Sierra Nevada of California show that the fungus is having a devastating impact on native species, already weakened by the effects of pollution and introduced predators. A general message from amphibians is that we may have little time.
   
   
   
5. Atmospheric brown clouds: Impacts on South Asian climate and hydrological cycle. Ramanathan, V.; Chung, C.; Kim, D.; Bettge, T.; Buja, L.; Kiehl, J. T.; Washington, W. M.; Fu, Q.; Sikka, D. R.; Wild, M..
   PNAS: 2005
   DOI: 10.1073/pnas.0500656102
   Notes
   South Asian emissions of fossil fuel SO2 and black carbon increased{approx}6-fold since 1930, resulting in large atmospheric concentrations of black carbon and other aerosols. This period also witnessed strong negative trends of surface solar radiation, surface evaporation, and summer monsoon rainfall. These changes over India were accompanied by an increase in atmospheric stability and a decrease in sea surface temperature gradients in the Northern Indian Ocean. We conducted an ensemble of coupled ocean-atmosphere simulations from 1930 to 2000 to understand the role of atmospheric brown clouds in the observed trends. The simulations adopt the aerosol radiative forcing from the Indian Ocean experiment observations and also account for global increases in greenhouse gases and sulfate aerosols. The simulated decreases in surface solar radiation, changes in surface and atmospheric temperatures over land and sea, and decreases in monsoon rainfall are similar to the observed trends. We also show that greenhouse gases and sulfates, by themselves, do not account for the magnitude or even the sign in many instances, of the observed trends. Thus, our simulations suggest that absorbing aerosols in atmospheric brown clouds may have played a major role in the observed regional climate and hydrological cycle changes and have masked as much as 50% of the surface warming due to the global increase in greenhouse gases. The simulations also raise the possibility that, if current trends in emissions continue, the subcontinent may experience a doubling of the drought frequency in the coming decades.
   
   
   
6. Biogeography, changing climates, and niche evolution. David B. Wakea, Elizabeth A. Hadlyc, David D. Ackerlya,.
   PNAS: 2009
   DOI: 10.1073/pnas.0911097106
   
   
7. Biography of Veerabhadran Ramanathan. Nuzzo, Regina.
   PNAS: 2005
   
   
8. Climate change threats to plant diversity in Europe. Thuiller, Wilfried; Lavorel, Sandra; Araujo, Miguel B.; Sykes, Martin T.; Prentice, I. Colin.
   PNAS: 2005
   DOI: 10.1073/pnas.0409902102
   Notes
   Climate change has already triggered species distribution shifts in many parts of the world. Increasing impacts are expected for the future, yet few studies have aimed for a general understanding of the regional basis for species vulnerability. We projected late 21st century distributions for 1,350 European plants species under seven climate change scenarios. Application of the International Union for Conservation of Nature and Natural Resources Red List criteria to our projections shows that many European plant species could become severely threatened. More than half of the species we studied could be vulnerable or threatened by 2080. Expected species loss and turnover per pixel proved to be highly variable across scenarios (27-42% and 45-63% respectively, averaged over Europe) and across regions (2.5-86% and 17-86%, averaged over scenarios). Modeled species loss and turnover were found to depend strongly on the degree of change in just two climate variables describing temperature and moisture conditions. Despite the coarse scale of the analysis, species from mountains could be seen to be disproportionably sensitive to climate change ({approx}60% species loss). The boreal region was projected to lose few species, although gaining many others from immigration. The greatest changes are expected in the transition between the Mediterranean and Euro-Siberian regions. We found that risks of extinction for European plants may be large, even in moderate scenarios of climate change and despite inter-model variability.
   
   
   
9. Climatic context and ecological implications of summer fog decline in the coast redwood region. James A. Johnstone, Todd E. Dawson.
   PNAS: 2009
   DOI: 10.1073/pnas.0915062107
   Notes
   Biogeographical, physiological, and paleoecological evidence suggests that the coast redwood [Sequoia sempervirens (D. Don) Endl.] is closely associated with the presence of summer marine fog along the Pacific coast of California. Here we present a novel record of summer fog frequency in the coast redwood region upon the basis of direct hourly measurements of cloud ceiling heights from 1951 to 2008. Our analysis shows that coastal summer fog frequency is a remarkably integrative measure of United States Pacific coastal climate, with strong statistical connections to the wind-driven upwelling system of the California Current and the broad ocean temperature pattern known as the Pacific Decadal Oscillation. By using a long-term index of daily maximum land temperatures, we infer a 33% reduction in fog frequency since the early 20th century. We present tree physiological data suggesting that coast redwood and other ecosystems along the United States west coast may be increasingly drought stressed under a summer cimate of reduced fog frequency and greater evaporative demand.
   
   
   
10. Climatic context and ecological implications of summer fog decline in the coast redwood region. James A. Johnstone, Todd E. Dawson.
    PNAS: 2009
    DOI: 10.1073/pnas.0915062107
    Notes
    Biogeographical, physiological, and paleoecological evidence suggests that the coast redwood [Sequoia sempervirens (D. Don) Endl.] is closely associated with the presence of summer marine fog along the Pacific coast of California. Here we present a novel record of summer fog frequency in the coast redwood region upon the basis of direct hourly measurements of cloud ceiling heights from 1951 to 2008. Our analysis shows that coastal summer fog frequency is a remarkably integrative measure of United States Pacific coastal climate, with strong statistical connections to the wind-driven upwelling system of the California Current and the broad ocean temperature pattern known as the Pacific Decadal Oscillation. By using a long-term index of daily maximum land temperatures, we infer a 33% reduction in fog frequency since the early 20th century. We present tree physiological data suggesting that coast redwood and other ecosystems along the United States west coast may be increasingly drought stressed under a summer c imate of reduced fog frequency and greater evaporative demand.