Publications Published in Proceedings of the National Academy of Sciences

1. A cold phase of the East Pacific triggers new phytoplankton blooms in San Francisco Bay. Cloern, James E.; Jassby, Alan D.; Thompson, Janet K.; Hieb, Kathryn A..
   Proceedings of the National Academy of Sciences: 2007
   DOI: 10.1073/pnas.0706151104
   Notes
   Ecological observations sustained over decades often reveal abrupt changes in biological communities that signal altered ecosystem states. We report a large shift in the biological communities of San Francisco Bay, first detected as increasing phytoplankton biomass and occurrences of new seasonal blooms that began in 1999. This phytoplankton increase is paradoxical because it occurred in an era of decreasing wastewater nutrient inputs and reduced nitrogen and phosphorus concentrations, contrary to the guiding paradigm that algal biomass in estuaries increases in proportion to nutrient inputs from their watersheds. Coincidental changes included sharp declines in the abundance of bivalve mollusks, the key phytoplankton consumers in this estuary, and record high abundances of several bivalve predators: Bay shrimp, English sole, and Dungeness crab. The phytoplankton increase is consistent with a trophic cascade resulting from heightened predation on bivalves and suppression of their filtration control on phytoplankton growth. These community changes in San Francisco Bay across three trophic levels followed a state change in the California Current System characterized by increased upwelling intensity, amplified primary production, and strengthened southerly flows. These diagnostic features of the East Pacific "cold phase" lead to strong recruitment and immigration of juvenile flatfish and crustaceans into estuaries where they feed and develop. This study, built from three decades of observation, reveals a previously unrecognized mechanism of oceanestuary connectivity. Interdecadal oceanic regime changes can propagate into estuaries, altering their community structure and efficiency of transforming land-derived nutrients into algal biomass.
   
   
   
2. Ambient temperature predicts sex ratios and male longevity. Catalano, Ralph; Bruckner, Tim; Smith, Kirk R..
   Proceedings of the National Academy of Sciences: 2008
   DOI: 10.1073/pnas.0710711104
   Notes
   The theory that natural selection has conserved mechanisms by which women subjected to environmental stressors abort frail male fetuses implies that climate change may affect sex ratio at birth and male longevity. Using time series methods, we find that cold ambient temperatures during gestation predict lower secondary sex ratios and longer life span of males in annual birth cohorts composed of Danes, Finns, Norwegians, and Swedes born between 1878 (earliest year with complete life tables) and 1914 (last birth cohort for which male life span can be estimated). We conclude that ambient temperature affects the characteristics of human populations by influencing who survives gestation, a heretofore unrecognized effect of climate on humanity.
   
   
   
3. Changes in climate and land use have a larger direct impact than rising CO2 on global river runoff trends. Piao, Shilong; Friedlingstein, Pierre; Ciais, Philippe; de Noblet-Ducoudre, Nathalie; Labat, David; Zaehle, Sonke.
   Proceedings of the National Academy of Sciences: 2007
   DOI: 10.1073/pnas.0707213104
   Notes
   The significant worldwide increase in observed river runoff has been tentatively attributed to the stomatal "antitranspirant" response of plants to rising atmospheric CO2 [Gedney N, Cox PM, Betts RA, Boucher O, Huntingford C, Stott PA (2006) Nature 439: 835838]. However, CO2 also is a plant fertilizer. When allowing for the increase in foliage area that results from increasing atmospheric CO2 levels in a global vegetation model, we find a decrease in global runoff from 1901 to 1999. This finding highlights the importance of vegetation structure feedback on the water balance of the land surface. Therefore, the elevated atmospheric CO2 concentration does not explain the estimated increase in global runoff over the last century. In contrast, we find that changes in mean climate, as well as its variability, do contribute to the global runoff increase. Using historic land-use data, we show that land-use change plays an additional important role in controlling regional runoff values, particularly in the tropics. Land-use change has been strongest in tropical regions, and its contribution is substantially larger than that of climate change. On average, land-use change has increased global runoff by 0.08 mm/year2 and accounts for {approx}50% of the reconstructed global runoff trend over the last century. Therefore, we emphasize the importance of land-cover change in forecasting future freshwater availability and climate.
   
   
   
4. Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks. Canadell, Josep G.; Le Quere, Corinne; Raupach, Michael R.; Field, Christopher B.; Buitenhuis, Erik T.; Ciais, Philippe; Conway, Thomas J.; Gillett, Nathan P.; Houghton, R. A.; Marland, Gregg.
   Proceedings of the National Academy of Sciences: 2007
   DOI: 10.1073/pnas.0702737104
   Notes
   The growth rate of atmospheric carbon dioxide (CO2), the largest human contributor to human-induced climate change, is increasing rapidly. Three processes contribute to this rapid increase. Two of these processes concern emissions. Recent growth of the world economy combined with an increase in its carbon intensity have led to rapid growth in fossil fuel CO2 emissions since 2000: comparing the 1990s with 20002006, the emissions growth rate increased from 1.3% to 3.3% y1. The third process is indicated by increasing evidence (P = 0.89) for a long-term (50-year) increase in the airborne fraction (AF) of CO2 emissions, implying a decline in the efficiency of CO2 sinks on land and oceans in absorbing anthropogenic emissions. Since 2000, the contributions of these three factors to the increase in the atmospheric CO2 growth rate have been {approx}65 {+/-} 16% from increasing global economic activity, 17 {+/-} 6% from the increasing carbon intensity of the global economy, and 18 {+/-} 15% from the increase in AF. An increasing AF is consistent with results of climatecarbon cycle models, but the magnitude of the observed signal appears larger than that estimated by models. All of these changes characterize a carbon cycle that is generating stronger-than-expected and sooner-than-expected climate forcing.
   
   
   
5. Development at the wildland - urban interface and the mitigation of forest-fire risk. Spyratos, Vassilis; Bourgeron, Patrick S.; Ghil, Michael.
   Proceedings of the National Academy of Sciences: 2007
   DOI: 10.1073/pnas.0704488104
   Notes
   This work addresses the impacts of development at the wildlandurban interface on forest fires that spread to human habitats. Catastrophic fires in the western United States and elsewhere make these impacts a matter of urgency for decision makers, scientists, and the general public. Using a simple fire-spread model, along with housing and vegetation data, we show that fire size probability distributions can be strongly modified by the density and flammability of houses. We highlight a sharp transition zone in the parameter space of vegetation flammability and house density. Many actual fire landscapes in the United States appear to have spreading properties close to this transition. Thus, the density and flammability of buildings should be taken into account when assessing fire risk at the wildlandurban interface. Moreover, our results highlight ways for regulation at this interface to help mitigate fire risk.
   
   
   
6. Discovery and measurement of an isotopically distinct source of sulfate in Earth’s atmosphere. Dominguez, Gerardo; Jackson, Terri; Brothers, Lauren; Barnett, Burton; Nguyen, Bryan; Thiemens, Mark H..
   Proceedings of the National Academy of Sciences: 2008
   DOI: 10.1073/pnas.0805255105
   Notes
   Sulfate (SO4) and its precursors are significant components of the atmosphere, with both natural and anthropogenic sources. Recently, our triple-isotope (16O, 17O, 18O) measurements of atmospheric sulfate have provided specific insights into the oxidation pathways leading to sulfate, with important implications for models of the sulfur cycle and global climate change. Using similar isotopic measurements of aerosol sulfate in a polluted marine boundary layer (MBL) and primary sulfate (p-SO4) sampled directly from a ship stack, we quantify the amount of p-SO4 found in the atmosphere from ships. We find that ships contribute between 10% and 44% of the non-sea-salt sulfate found in fine [diameter (D) < 1.5 m) particulate matter in coastal Southern California. These fractions are surprising, given that p-SO4 constitutes 2–7% of total sulfur emissions from combustion sources [Seinfed JH, Pandis SN (2006) Atmospheric Chemistry and Physics (Wiley–Interscience, New York)]. Our findings also suggest that the interact n of SO2 from ship emissions with coarse hydrated sea salt particles may lead to the rapid removal of SO2 in the MBL. When combined with the longer residence time of p-SO4 emissions in the MBL, these findings suggest that the importance of p-SO4 emissions in marine environments may be underappreciated in global chemical models. Given the expected increase of international shipping in the years to come, these findings have clear implications for public health, air quality, international maritime law, and atmospheric chemistry.
   
   
   
7. Identification of human-induced changes in atmospheric moisture content. Santer, B. D.; Mears, C.; Wentz, F. J.; Taylor, K. E.; Gleckler, P. J.; Wigley, T. M. L.; Barnett, T. P.; Boyle, J. S..
   Proceedings of the National Academy of Sciences: 2007
   DOI: 10.1073/pnas.0702872104
   Notes
   Data from the satellite-based Special Sensor Microwave Imager (SSM/I) show that the total atmospheric moisture content over oceans has increased by 0.41 kg/m2 per decade since 1988. Results from current climate models indicate that water vapor increases of this magnitude cannot be explained by climate noise alone. In a formal detection and attribution analysis using the pooled results from 22 different climate models, the simulated "fingerprint" pattern of anthropogenically caused changes in water vapor is identifiable with high statistical confidence in the SSM/I data. Experiments in which forcing factors are varied individually suggest that this fingerprint "match" is primarily due to human-caused increases in greenhouse gases and not to solar forcing or recovery from the eruption of Mount Pinatubo. Our findings provide preliminary evidence of an emerging anthropogenic signal in the moisture content of earth's atmosphere.
   
   
   
8. Impact of an extreme climatic event on community assembly. Thibault, Katherine M.; Brown, James H..
   Proceedings of the National Academy of Sciences: 2008
   DOI: 10.1073/pnas.0712282105
   Notes
   Extreme climatic events are predicted to increase in frequency and magnitude, but their ecological impacts are poorly understood. Such events are large, infrequent, stochastic perturbations that can change the outcome of entrained ecological processes. Here we show how an extreme flood event affected a desert rodent community that has been monitored for 30 years. The flood (i) caused catastrophic, species-specific mortality; (ii) eliminated the incumbency advantage of previously dominant species; (iii) reset long-term population and community trends; (iv) interacted with competitive and metapopulation dynamics; and (v) resulted in rapid, wholesale reorganization of the community. This and a previous extreme rainfall event were punctuational perturbationsthey caused large, rapid population- and community-level changes that were superimposed on a background of more gradual trends driven by climate and vegetation change. Captured by chance through long-term monitoring, the impacts of such large, infrequent events provide unique insights into the processes that structure ecological communities.
   
   
   
9. Indicators of 21st century socioclimatic exposure. Diffenbaugh, Noah S.; Giorgi, Filippo; Raymond, Leigh; Bi, Xunqiang.
   Proceedings of the National Academy of Sciences: 2007
   DOI: 10.1073/pnas.0706680105
   Notes
   Policies that attempt to curb greenhouse gas emissions, allocate emissions rights, or distribute compensation to those most damaged by climate change must explicitly incorporate the international heterogeneity of the climate change threat. To capture the distinct susceptibilities associated with lack of infrastructure, potential property loss, and gross human exposure, we develop an integration of climate change projections and poverty, wealth, and population metrics. Our analysis shows that most nations of the world are threatened by the interaction of regional climatic changes with one or more relevant socioeconomic factors. Nations that have the highest levels of poverty, wealth, and population face greater relative exposure in those dimensions. However, for each of those socioeconomic indicators, spatial heterogeneity in projected climate change determines the overall international pattern of socioclimatic exposure. Our synthesis provides a critical missing piece to the climate change debate and should facilitate the formulation of climate policies that account for international variations in the threat of climate change across a range of socioeconomic dimensions.
   
   
   
10. Influence of ocean winds on the pelagic ecosystem in upwelling regions. Rykaczewski, Ryan R.; Checkley, David M., Jr..
    Proceedings of the National Academy of Sciences: 2008
    DOI: 10.1073/pnas.0711777105
    Notes
    Upwelling of nutrient-rich, subsurface water sustains high productivity in the ocean's eastern boundary currents. These ecosystems support a rate of fish harvest nearly 100 times the global mean and account for >20% of the world's marine fish catch. Environmental variability is thought to be the major cause of the decadal-scale biomass fluctuations characteristic of fish populations in these regions, but the mechanisms relating atmospheric physics to fish production remain unexplained. Two atmospheric conditions induce different types of upwelling in these ecosystems: coastal, alongshore wind stress, resulting in rapid upwelling (with high vertical velocity, w); and wind-stress curl, resulting in slower upwelling (low w). We show that the level of wind-stress curl has increased and that production of Pacific sardine (Sardinops sagax) varies with wind-stress curl over the past six decades. The extent of isopycnal shoaling, nutricline depth, and chlorophyll concentration in the upper ocean also correlate positively with wind-stress curl. The size structure of plankton assemblages is related to the rate of wind-forced upwelling, and sardine feed efficiently on small plankters generated by slow upwelling. Upwelling rate is a fundamental determinant of the biological structure and production in coastal pelagic ecosystems, and future changes in the magnitude and spatial gradient of wind stress may have important and differing effects on these ecosystems. Understanding of the biological mechanisms relating fisheries production to environmental variability is essential for wise management of marine resources under a changing climate.