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Extinction rates under nonrandom patterns of habitat loss. Seabloom, Eric W.; Dobson, Andy P.; Stoms, David M..
PNAS:
2002
DOI: 10.1073/pnas.162064899
Notes
Most models that examine the effects of habitat conversion on species extinctions assume that habitat conversion occurs at random. This assumption allows predictions about extinction rates based on the species-area relationship. We show that the spatially aggregated nature of habitat conversion introduces a significant bias that may lead species-loss rates to exceed those predicted by species-area curves. Correlations between human activity and major compositional gradients, or species richness, also alter predicted species extinction rates. We illustrate the consequences of nonrandom patterns of habitat conversion by using a data set that combines the distribution of native vascular plants with human activity patterns in California.
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Fine-scale processes regulate the response of extreme events to global climate change. Diffenbaugh, Noah S.; Pal, Jeremy S.; Trapp, Robert J.; Giorgi, Filippo.
PNAS:
2005
DOI: 10.1073/pnas.0506042102
Notes
We find that extreme temperature and precipitation events are likely to respond substantially to anthropogenically enhanced greenhouse forcing and that fine-scale climate system modifiers are likely to play a critical role in the net response. At present, such events impact a wide variety of natural and human systems, and future changes in their frequency and/or magnitude could have dramatic ecological, economic, and sociological consequences. Our results indicate that fine-scale snow albedo effects influence the response of both hot and cold events and that peak increases in extreme hot events are amplified by surface moisture feedbacks. Likewise, we find that extreme precipitation is enhanced on the lee side of rain shadows and over coastal areas dominated by convective precipitation. We project substantial, spatially heterogeneous increases in both hot and wet events over the contiguous United States by the end of the next century, suggesting that consideration of fine-scale processes is critical for accurate assessment of local- and regional-scale vulnerability to climate change.
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Geography and macroeconomics: New data and new findings. Nordhaus, William D..
PNAS:
2006
DOI: 10.1073/pnas.0509842103
Notes
The linkage between economic activity and geography is obvious: Populations cluster mainly on coasts and rarely on ice sheets. Past studies of the relationships between economic activity and geography have been hampered by limited spatial data on economic activity. The present study introduces data on global economic activity, the G-Econ database, which measures economic activity for all large countries, measured at a 1{degrees} latitude by 1{degrees} longitude scale. The methodologies for the study are described. Three applications of the data are investigated. First, the puzzling "climate-output reversal" is detected, whereby the relationship between temperature and output is negative when measured on a per capita basis and strongly positive on a per area basis. Second, the database allows better resolution of the impact of geographic attributes on African poverty, finding geography is an important source of income differences relative to high-income regions. Finally, we use the G-Econ data to provide estimates of the economic impact of greenhouse warming, with larger estimates of warming damages than past studies.
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Global and regional drivers of accelerating CO2 emissions. Raupach, Michael R.; Marland, Gregg; Ciais, Philippe; Le Quere, Corinne; Canadell, Josep G.; Klepper, Gernot; Field, Christopher B..
PNAS:
2007
DOI: 10.1073/pnas.0700609104
Notes
Edited by William C. Clark, Harvard University, Cambridge, MA, and approved April 17, 2007 (received for review January 23, 2007)CO2 emissions from fossil-fuel burning and industrial processes have been accelerating at a global scale, with their growth rate increasing from 1.1% y-1 for 1990-1999 to >3% y-1 for 2000-2004. The emissions growth rate since 2000 was greater than for the most fossil-fuel intensive of the Intergovernmental Panel on Climate Change emissions scenarios developed in the late 1990s. Global emissions growth since 2000 was driven by a cessation or reversal of earlier declining trends in the energy intensity of gross domestic product (GDP) (energy/GDP) and the carbon intensity of energy (emissions/energy), coupled with continuing increases in population and per-capita GDP. Nearly constant or slightly increasing trends in the carbon intensity of energy have been recently observed in both developed and developing regions. No region is decarbonizing its energy supply. The growth rate in emissions is strongest in rapidly developing economies, particularly China. Together, the developing and least-developed economies (forming 80% of the world's population) accounted for 73% of global emissions growth in 2004 but only 41% of global emissions and only 23% of global cumulative emissions since the mid-18th century. The results have implications for global equity.
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Global sea level linked to global temperature. Martin Vermeera, Stefan Rahmstorf.
PNAS:
2009
DOI: 10.1073/pnas.0907765106
Notes
We propose a simple relationship linking global sea-level variations on time scales of decades to centuries to global mean temperature. This relationship is tested on synthetic data from a global climate model for the past millennium and the next century. When applied to observed data of sea level and temperature for 1880–2000, and taking into account known anthropogenic hydrologic contributions to sea level, the correlation is >0.99, explaining 98% of the variance. For future global temperature scenarios of the Intergovernmental Panel on Climate Change’s Fourth Assessment Report, the relationship projects a sea-level rise ranging from 75 to 190 cm for the period 1990–2100.
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Global temperature change. Hansen, James; Sato, Makiko; Ruedy, Reto; Lo, Ken; Lea, David W.; Medina-Elizade, Martin.
PNAS:
2006
DOI: 10.1073/pnas.0606291103
Notes
Global surface temperature has increased {approx}0.2{degrees}C per decade in the past 30 years, similar to the warming rate predicted in the 1980s in initial global climate model simulations with transient greenhouse gas changes. Warming is larger in the Western Equatorial Pacific than in the Eastern Equatorial Pacific over the past century, and we suggest that the increased West-East temperature gradient may have increased the likelihood of strong El Ninos, such as those of 1983 and 1998. Comparison of measured sea surface temperatures in the Western Pacific with paleoclimate data suggests that this critical ocean region, and probably the planet as a whole, is approximately as warm now as at the Holocene maximum and within {approx}1{degrees}C of the maximum temperature of the past million years. We conclude that global warming of more than {approx}1{degrees}C, relative to 2000, will constitute "dangerous" climate change as judged from likely effects on sea level and extermination of species.
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Greenhouse gas growth rates. Hansen, James; Sato, Makiko.
PNAS:
2004
DOI: 10.1073/pnas.0406982101
Notes
We posit that feasible reversal of the growth of atmospheric CH4 and other trace gases would provide a vital contribution toward averting dangerous anthropogenic interference with global climate. Such trace gas reductions may allow stabilization of atmospheric CO2 at an achievable level of anthropogenic CO2 emissions, even if the added global warming constituting dangerous anthropogenic interference is as small as 1{degrees}C. A 1{degrees}C limit on global warming, with canonical climate sensitivity, requires peak CO2 {approx} 440 ppm if further non-CO2 forcing is +0.5 W/m2, but peak CO2 {approx} 520 ppm if further non-CO2 forcing is -0.5 W/m2. The practical result is that a decline of non-CO2 forcings allows climate forcing to be stabilized with a significantly higher transient level of CO2 emissions. Increased "natural" emissions of CO2, N2O, and CH4 are expected in response to global warming. These emissions, an indirect effect of all climate forcings, are small compared with human-made climate forcing and occur on a time scale of a few centuries, but they tend to aggravate the task of stabilizing atmospheric composition.
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Human-modified temperatures induce species changes: Joint attribution. Root, Terry L.; MacMynowski, Dena P.; Mastrandrea, Michael D.; Schneider, Stephen H..
PNAS:
2005
DOI: 10.1073/pnas.0502286102
Notes
Average global surface-air temperature is increasing. Contention exists over relative contributions by natural and anthropogenic forcings. Ecological studies attribute plant and animal changes to observed warming. Until now, temperature-species connections have not been statistically attributed directly to anthropogenic climatic change. Using modeled climatic variables and observed species data, which are independent of thermometer records and paleoclimatic proxies, we demonstrate statistically significant "joint attribution," a two-step linkage: human activities contribute significantly to temperature changes and human-changed temperatures are associated with discernible changes in plant and animal traits. Additionally, our analyses provide independent testing of grid-box-scale temperature projections from a general circulation model (HadCM3).
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In-situ measurements of the mixing state and optical properties of soot with implications for radiative forcing estimates. Ryan C. Moffet, Kimberly A. Prather.
PNAS:
2009
DOI: 10.1073/pnas.0900040106
Notes
Our ability to predict how global temperatures will change in the future is currently limited by the large uncertainties associated with aerosols. Soot aerosols represent a major research focus as they influence climate by absorbing incoming solar radiation resulting in a highly uncertain warming effect. The uncertainty stems from the fact that the actual amount soot warms our atmosphere strongly depends on the manner and degree in which it is mixed with other species, a property referred to as mixing state. In global models and inferences from atmospheric heating measurements, soot radiative forcing estimates currently differ by a factor of 6, ranging between 0.2–1.2 W/m2, making soot second only to CO2 in terms of global warming potential. This article reports coupled in situ measurements of the size-resolved mixing state, optical properties, and aging timescales for soot particles. Fresh fractal soot particles dominate the measured absorption during peak traffic periods (6–9 AM local time). Immediately after sunrise, soot particles begin to age by developing a coating of secondary species including sulfate, ammonium, organics, nitrate, and water. Based on these direct measurements, the core-shell arrangement results in a maximum absorption enhancement of 1.6 over fresh soot. These atmospheric observations help explainthe larger values for soot forcing measured by others and will be used to obtain closure in optical property measurements to reduce one of the largest remaining uncertainties in climate change.
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Integrated model shows that atmospheric brown clouds and greenhouse gases have reduced rice harvests in India. Auffhammer, Maximilian; Ramanathan, V.; Vincent, Jeffrey R..
PNAS:
2006
DOI: 10.1073/pnas.0609584104
Notes
Contributed by V. Ramanathan, October 31, 2006 (sent for review August 9, 2006)Previous studies have found that atmospheric brown clouds partially offset the warming effects of greenhouse gases. This finding suggests a tradeoff between the impacts of reducing emissions of aerosols and greenhouse gases. Results from a statistical model of historical rice harvests in India, coupled with regional climate scenarios from a parallel climate model, indicate that joint reductions in brown clouds and greenhouse gases would in fact have complementary, positive impacts on harvests. The results also imply that adverse climate changes due to brown clouds and greenhouse gases contributed to the slowdown in harvest growth that occurred during the past two decades.
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