The sky is the limit
Does the Social Cost of Carbon Matter? Evidence from US Policy
Robert Hahn & Robert Ritz
Journal of Legal Studies, January 2015, Pages 229-248
Abstract:
We evaluate a recent US initiative to include the social cost of carbon (SCC) in regulatory decisions. To our knowledge, this paper provides the first systematic analysis of the extent to which applying the SCC has affected national policy. We examine all economically significant federal regulations since 2008 and obtain an unexpected result: putting a value on changes in carbon dioxide emissions does not generally affect the ranking of the preferred policy compared with the status quo. Overall, we find little evidence that using the SCC has mattered for the choice of policy in the United States. This is true even for policies explicitly aimed at reducing carbon dioxide emissions. We offer some possible explanations for the patterns observed in the data.
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Federal Crop Insurance and the Disincentive to Adapt to Extreme Heat
Francis Annan & Wolfram Schlenker
American Economic Review, May 2015, Pages 262-266
Abstract:
Despite significant progress in average yields, the sensitivity of corn and soybean yields to extreme heat has remained relatively constant over time. We combine county-level corn and soybeans yields in the United States from 1989-2013 with the fraction of the planting area that is insured under the federal crop insurance program, which expanded greatly over this time period as premium subsidies increased from 20 percent to 60 percent. Insured corn and soybeans are significantly more sensitive to extreme heat than uninsured crops. Insured farmers do not have the incentive to engage in costly adaptation as insurance compensates them for potential losses.
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Incorporating Climate Uncertainty into Estimates of Climate Change Impacts
Marshall Burke et al.
Review of Economics and Statistics, May 2015, Pages 461-471
Abstract:
Quantitative estimates of the impacts of climate change on economic outcomes are important for public policy. We show that the vast majority of estimates fail to account for well-established uncertainty in future temperature and rainfall changes, leading to potentially misleading projections. We reexamine seven well-cited studies and show that accounting for climate uncertainty leads to a much larger range of projected climate impacts and a greater likelihood of worst-case outcomes, an important policy parameter. Incorporating climate uncertainty into future economic impact assessments will be critical for providing the best possible information on potential impacts.
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Temperature and Human Capital in the Short- and Long-Run
Joshua Graff Zivin, Solomon Hsiang & Matthew Neidell
NBER Working Paper, May 2015
Abstract:
We provide the first estimates of the potential impact of climate change on human capital, focusing on the impacts from both short-run weather and long-run climate. Exploiting the longitudinal structure of the NLSY79 and random fluctuations in weather across interviews, we identify the effect of temperature in models with child-specific fixed effects. We find that short-run changes in temperature lead to statistically significant decreases in cognitive performance on math (but not reading) beyond 26C (78.8F). In contrast, our long-run analysis, which relies upon long-difference and rich cross-sectional models, reveals no statistically significant relationship between climate and human capital. This finding is consistent with the notion that adaptation, particularly compensatory behavior, plays a significant role in limiting the long run impacts from short run weather shocks.
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The Effects of Temperature on Political Violence: Global Evidence at the Subnational Level
Alexander Bollfrass & Andrew Shaver
PLoS ONE, May 2015
Abstract:
A number of studies have demonstrated an empirical relationship between higher ambient temperatures and substate violence, which have been extrapolated to make predictions about the security implications of climate change. This literature rests on the untested assumption that the mechanism behind the temperature-conflict link is that disruption of agricultural production provokes local violence. Using a subnational-level dataset, this paper demonstrates that the relationship: (1) obtains globally, (2) exists at the substate level — provinces that experience positive temperature deviations see increased conflict; and (3) occurs even in regions without significant agricultural production. Diminished local farm output resulting from elevated temperatures is unlikely to account for the entire increase in substate violence. The findings encourage future research to identify additional mechanisms, including the possibility that a substantial portion of the variation is brought about by the well-documented direct effects of temperature on individuals' propensity for violence or through macroeconomic mechanisms such as food price shocks.
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Climatic Effects on Planning Behavior
Yong Liu, Vassilis Kostakos & Hongxiu Li
PLoS ONE, May 2015
Abstract:
What mechanism links climate change and social change? Palaeoanthropological analysis of human remains suggests that abrupt climate change is linked to societal restructuring, but it has been challenging to reliably identify the exact mechanisms underlying this relationship. Here we identify one potential mechanism that can link climate to behavior change, and underpins many of the reported findings on social restructuring. Specifically, we show that daily weather is linked to human planning behavior, and this effect is moderated by climate. Our results demonstrate that as weather gets colder, humans increase their planning in cold regions and decrease planning in warm regions. Since planning has previously been linked to group efficiency, cooperation, and societal organization, our work suggests planning is one mechanism that can link climate change to societal restructuring.
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Future population exposure to US heat extremes
Bryan Jones et al.
Nature Climate Change, forthcoming
Abstract:
Extreme heat events are likely to become more frequent in the coming decades owing to climate change. Exposure to extreme heat depends not only on changing climate, but also on changes in the size and spatial distribution of the human population. Here we provide a new projection of population exposure to extreme heat for the continental United States that takes into account both of these factors. Using projections from a suite of regional climate models driven by global climate models and forced with the SRES A2 scenario and a spatially explicit population projection consistent with the socioeconomic assumptions of that scenario, we project changes in exposure into the latter half of the twenty-first century. We find that US population exposure to extreme heat increases four- to sixfold over observed levels in the late twentieth century, and that changes in population are as important as changes in climate in driving this outcome. Aggregate population growth, as well as redistribution of the population across larger US regions, strongly affects outcomes whereas smaller-scale spatial patterns of population change have smaller effects. The relative importance of population and climate as drivers of exposure varies across regions of the country.
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Unabated global mean sea-level rise over the satellite altimeter era
Christopher Watson et al.
Nature Climate Change, May 2015, Pages 565–568
Abstract:
The rate of global mean sea-level (GMSL) rise has been suggested to be lower for the past decade compared with the preceding decade as a result of natural variability, with an average rate of rise since 1993 of +3.2 ± 0.4 mm yr−1. However, satellite-based GMSL estimates do not include an allowance for potential instrumental drifts (bias drift). Here, we report improved bias drift estimates for individual altimeter missions from a refined estimation approach that incorporates new Global Positioning System (GPS) estimates of vertical land movement (VLM). In contrast to previous results, we identify significant non-zero systematic drifts that are satellite-specific, most notably affecting the first 6 years of the GMSL record. Applying the bias drift corrections has two implications. First, the GMSL rate (1993 to mid-2014) is systematically reduced to between +2.6 ± 0.4 mm yr−1 and +2.9 ± 0.4 mm yr−1, depending on the choice of VLM applied. These rates are in closer agreement with the rate derived from the sum of the observed contributions, GMSL estimated from a comprehensive network of tide gauges with GPS-based VLM applied and reprocessed ERS-2/Envisat altimetry. Second, in contrast to the previously reported slowing in the rate during the past two decades, our corrected GMSL data set indicates an acceleration in sea-level rise (independent of the VLM used), which is of opposite sign to previous estimates and comparable to the accelerated loss of ice from Greenland and to recent projections, and larger than the twentieth-century acceleration.
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Declining vulnerability to river floods and the global benefits of adaptation
Brenden Jongman et al.
Proceedings of the National Academy of Sciences, 5 May 2015, Pages E2271–E2280
Abstract:
The global impacts of river floods are substantial and rising. Effective adaptation to the increasing risks requires an in-depth understanding of the physical and socioeconomic drivers of risk. Whereas the modeling of flood hazard and exposure has improved greatly, compelling evidence on spatiotemporal patterns in vulnerability of societies around the world is still lacking. Due to this knowledge gap, the effects of vulnerability on global flood risk are not fully understood, and future projections of fatalities and losses available today are based on simplistic assumptions or do not include vulnerability. We show for the first time (to our knowledge) that trends and fluctuations in vulnerability to river floods around the world can be estimated by dynamic high-resolution modeling of flood hazard and exposure. We find that rising per-capita income coincided with a global decline in vulnerability between 1980 and 2010, which is reflected in decreasing mortality and losses as a share of the people and gross domestic product exposed to inundation. The results also demonstrate that vulnerability levels in low- and high-income countries have been converging, due to a relatively strong trend of vulnerability reduction in developing countries. Finally, we present projections of flood losses and fatalities under 100 individual scenario and model combinations, and three possible global vulnerability scenarios. The projections emphasize that materialized flood risk largely results from human behavior and that future risk increases can be largely contained using effective disaster risk reduction strategies.
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The social cost of atmospheric release
Drew Shindell
Climatic Change, May 2015, Pages 313-326
Abstract:
I present a multi-impact economic valuation framework called the Social Cost of Atmospheric Release (SCAR) that extends the Social Cost of Carbon (SCC) used previously for carbon dioxide (CO2) to a broader range of pollutants and impacts. Values consistently incorporate health impacts of air quality along with climate damages. The latter include damages associated with aerosol-induced hydrologic cycle changes that lead to net climate benefits when reducing cooling aerosols. Evaluating a 1 % reduction in current global emissions, benefits with a high discount rate are greatest for reductions of co-emitted products of incomplete combustion (PIC), followed by sulfur dioxide (SO2), nitrogen oxides (NOx) and then CO2, ammonia and methane. With a low discount rate, benefits are greatest for PIC, with CO2 and SO2 next, followed by NOx and methane. These results suggest that efforts to mitigate atmosphere-related environmental damages should target a broad set of emissions including CO2, methane and aerosol/ozone precursors. Illustrative calculations indicate environmental damages are $330-970 billion yr−1 for current US electricity generation (~14–34¢ per kWh for coal, ~4–18¢ for gas) and $3.80 (−1.80/+2.10) per gallon of gasoline ($4.80 (−3.10/+3.50) per gallon for diesel). These results suggest that total atmosphere-related environmental damages plus generation costs are much greater for coal-fired power than other types of electricity generation, and that damages associated with gasoline vehicles substantially exceed those for electric vehicles.
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Carol McAusland & Nouri Najjar
Environmental and Resource Economics, May 2015, Pages 37-70
Abstract:
We analyze whether a carbon consumption tax is logistically feasible. We consider a carbon footprint tax (CFT), which would be modeled after a credit-method value added tax. The basis for the tax would be a product's carbon footprint, which includes all of the emissions released during production of the good and its inputs as well as any greenhouse gases latent in the product. Our analysis suggests that a pure CFT, requiring the calculation of the carbon footprint of every individual product, may be prohibitively costly. However a hybrid CFT seems economically feasible. The hybrid CFT would give firms the option to either calculate the carbon footprint of their outputs — and have their products taxed based on those footprints — or use product-class specific default carbon footprints as the tax basis, thereby saving on calculation costs. Because the CFT would be levied on all goods consumed domestically, the CFT would keep domestic firms on an even footing with those producing in countries without active climate policy, protecting competitiveness and reducing leakage.
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How to interpret expert judgment assessments of 21st century sea-level rise
Hylke de Vries & Roderik van de Wal
Climatic Change, May 2015, Pages 87-100
Abstract:
In a recent paper Bamber and Aspinall (Nat Clim Change 3:424–427, 2013) (BA13) investigated the sea-level rise that may result from the Greenland and Antarctic ice sheets during the 21st century. Using data from an expert judgment elicitation, they obtained a final high-end (P95) value of +84 cm integrated sea-level change from the ice sheets for the 2010–2100 period. However, one key message was left largely undiscussed: The experts had strongly diverging opinions about the ice-sheet contributions to sea-level rise. We argue that such (lack of) consensus should form an essential and integral part of the subsequent analysis of the data. By employing a method that keeps the level of consensus included, and that is also more robust to outliers and less dependent on the choice of the underlying distributions, we obtain on the basis of the same data a considerably lower high-end estimate for the ice-sheet contribution, +53 cm (+38-77 cm interquartile range of "expert consensus"). The method compares favourably with another recent study on expert judgement derived sea-level rise by Horton et al. (Q Sci Rev 84:1–6, 2014). Furthermore we show that the BA13 results are sensitive to a number of assumptions, such as the shape and minimum of the underlying distribution that were not part of the expert elicitation itself. Our analysis therefore demonstrates that one should be careful in considering high-end sea-level rise estimates as being well-determined and fixed numbers.
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A Link between the Hiatus in Global Warming and North American Drought
Thomas Delworth et al.
Journal of Climate, May 2015, Pages 3834–3845
Abstract:
Portions of western North America have experienced prolonged drought over the last decade. This drought has occurred at the same time as the global warming hiatus—a decadal period with little increase in global mean surface temperature. Climate models and observational analyses are used to clarify the dual role of recent tropical Pacific changes in driving both the global warming hiatus and North American drought. When observed tropical Pacific wind stress anomalies are inserted into coupled models, the simulations produce persistent negative sea surface temperature anomalies in the eastern tropical Pacific, a hiatus in global warming, and drought over North America driven by SST-induced atmospheric circulation anomalies. In the simulations herein the tropical wind anomalies account for 92% of the simulated North American drought during the recent decade, with 8% from anthropogenic radiative forcing changes. This suggests that anthropogenic radiative forcing is not the dominant driver of the current drought, unless the wind changes themselves are driven by anthropogenic radiative forcing. The anomalous tropical winds could also originate from coupled interactions in the tropical Pacific or from forcing outside the tropical Pacific. The model experiments suggest that if the tropical winds were to return to climatological conditions, then the recent tendency toward North American drought would diminish. Alternatively, if the anomalous tropical winds were to persist, then the impact on North American drought would continue; however, the impact of the enhanced Pacific easterlies on global temperature diminishes after a decade or two due to a surface reemergence of warmer water that was initially subducted into the ocean interior.
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The carbon implications of declining household scale economies
Anthony Underwood & Sammy Zahran
Ecological Economics, August 2015, Pages 182–190
Abstract:
In the United States, average household size decreased significantly over the past half century. From 1950 to 2010, the number of households increased 72% faster than population size. In this paper we consider how this drift toward more and smaller households, occurring alongside rising affluence, undermines efforts to curb carbon dioxide (CO2) emissions by eroding household scale economies of consumption and associated CO2 emissions. To estimate the household scaling of CO2 emissions, we link consumer expenditure data to an economic input–output life-cycle assessment model. We find that the CO2 scaling benefits of cohabitation are compellingly large, with the carbon footprint of a representative person cohabiting with others being 23% less, on average, than if that same person lived alone. Additionally, we find that household scale economies: 1) decrease in income, reflecting the rise in the percentage of household expenditures devoted to more rival goods and services; and 2) increase intuitively in household size, reflecting the direct expenditure sharing benefits of cohabitation. The combined downward pressure on scale economies from declining household size and rising incomes, typifying the trajectory of developing societies toward more and smaller households and rising affluence, places significant upward pressure on CO2 emissions globally.
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Thomas Lontzek et al.
Nature Climate Change, May 2015, Pages 441–444
Abstract:
Perhaps the most 'dangerous' aspect of future climate change is the possibility that human activities will push parts of the climate system past tipping points, leading to irreversible impacts. The likelihood of such large-scale singular events is expected to increase with global warming, but is fundamentally uncertain. A key question is how should the uncertainty surrounding tipping events affect climate policy? We address this using a stochastic integrated assessment model, based on the widely used deterministic DICE model. The temperature-dependent likelihood of tipping is calibrated using expert opinions, which we find to be internally consistent. The irreversible impacts of tipping events are assumed to accumulate steadily over time (rather than instantaneously), consistent with scientific understanding. Even with conservative assumptions about the rate and impacts of a stochastic tipping event, today's optimal carbon tax is increased by ~50%. For a plausibly rapid, high-impact tipping event, today's optimal carbon tax is increased by >200%. The additional carbon tax to delay climate tipping grows at only about half the rate of the baseline carbon tax. This implies that the effective discount rate for the costs of stochastic climate tipping is much lower than the discount rate for deterministic climate damages. Our results support recent suggestions that the costs of carbon emission used to inform policy are being underestimated, and that uncertain future climate damages should be discounted at a low rate.
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Climate policy in hard times: Are the pessimists right?
Aya Kachi, Thomas Bernauer & Robert Gampfer
Ecological Economics, June 2015, Pages 227–241
Abstract:
Conventional wisdom holds that the state of the economy has a strong impact on citizens' appetite for environmental policies, including climate policy. Assuming median voter preferences prevail, periods of economic prosperity are likely to be conducive, and economic downturns are likely to be detrimental to ambitious climate policy. Using original surveys in the United States and Germany, we engage in a critical re-assessment of this claim. The results show that, for the most part, individuals' perceptions of their own economic situations have no significant effect on their policy support. Negative perceptions of the national economic outlook reduce support for climate policy in the US, but not in Germany. However, the magnitude of this national economy effect in the US is small. On the other hand, individuals' climate risk perceptions consistently have a statistically significant and large effect across various model specifications, and interestingly, this pattern holds for the US, whose government is among the less ambitious in global climate policy, as well as Germany, which is among the frontrunners. Our study indicates that the state of the economy may not trump climate risk considerations as conventional wisdom claims.
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Fossil fuels in a trillion tonne world
Vivian Scott et al.
Nature Climate Change, May 2015, Pages 419–423
Abstract:
The useful energy services and energy density value of fossil carbon fuels could be retained for longer timescales into the future if their combustion is balanced by CO2 recapture and storage. We assess the global balance between fossil carbon supply and the sufficiency (size) and capability (technology, security) of candidate carbon stores. A hierarchy of value for extraction-to-storage pairings is proposed, which is augmented by classification of CO2 containment as temporary (<1,000 yr) or permanent (>100,000 yr). Using temporary stores is inefficient and defers an intergenerational problem. Permanent storage capacity is adequate to technically match current fossil fuel reserves. However, rates of storage creation cannot balance current and expected rates of fossil fuel extraction and CO2 consequences. Extraction of conventional natural gas is uniquely holistic because it creates the capacity to re-inject an equivalent tonnage of carbon for storage into the same reservoir and can re-use gas-extraction infrastructure for storage. By contrast, balancing the extraction of coal, oil, biomass and unconventional fossil fuels requires the engineering and validation of additional carbon storage. Such storage is, so far, unproven in sufficiency.
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Victor Vilarrasa & Jesus Carrera
Proceedings of the National Academy of Sciences, 12 May 2015, Pages 5938–5943
Abstract:
Zoback and Gorelick [(2012) Proc Natl Acad Sci USA 109(26):10164–10168] have claimed that geologic carbon storage in deep saline formations is very likely to trigger large induced seismicity, which may damage the caprock and ruin the objective of keeping CO2 stored deep underground. We argue that felt induced earthquakes due to geologic CO2 storage are unlikely because (i) sedimentary formations, which are softer than the crystalline basement, are rarely critically stressed; (ii) the least stable situation occurs at the beginning of injection, which makes it easy to control; (iii) CO2 dissolution into brine may help in reducing overpressure; and (iv) CO2 will not flow across the caprock because of capillarity, but brine will, which will reduce overpressure further. The latter two mechanisms ensure that overpressures caused by CO2 injection will dissipate in a moderate time after injection stops, hindering the occurrence of postinjection induced seismicity. Furthermore, even if microseismicity were induced, CO2 leakage through fault reactivation would be unlikely because the high clay content of caprocks ensures a reduced permeability and increased entry pressure along the localized deformation zone. For these reasons, we contend that properly sited and managed geologic carbon storage in deep saline formations remains a safe option to mitigate anthropogenic climate change.
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Effect of warming temperatures on US wheat yields
Jesse Tack, Andrew Barkley & Lawton Lanier Nalley
Proceedings of the National Academy of Sciences, forthcoming
Abstract:
Climate change is expected to increase future temperatures, potentially resulting in reduced crop production in many key production regions. Research quantifying the complex relationship between weather variables and wheat yields is rapidly growing, and recent advances have used a variety of model specifications that differ in how temperature data are included in the statistical yield equation. A unique data set that combines Kansas wheat variety field trial outcomes for 1985–2013 with location-specific weather data is used to analyze the effect of weather on wheat yield using regression analysis. Our results indicate that the effect of temperature exposure varies across the September−May growing season. The largest drivers of yield loss are freezing temperatures in the Fall and extreme heat events in the Spring. We also find that the overall effect of warming on yields is negative, even after accounting for the benefits of reduced exposure to freezing temperatures. Our analysis indicates that there exists a tradeoff between average (mean) yield and ability to resist extreme heat across varieties. More-recently released varieties are less able to resist heat than older lines. Our results also indicate that warming effects would be partially offset by increased rainfall in the Spring. Finally, we find that the method used to construct measures of temperature exposure matters for both the predictive performance of the regression model and the forecasted warming impacts on yields.
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Trade-off between intensity and frequency of global tropical cyclones
Nam-Young Kang & James Elsner
Nature Climate Change, forthcoming
Abstract:
Global tropical cyclone climate has been investigated with indicators of frequency, intensity and activity. However, a full understanding of global warming's influence on tropical cyclone climate remains elusive because of the incomplete nature of these indicators. Here we form a complete three-dimensional variability space of tropical cyclone climate where the variabilities are continuously linked and find that global ocean warmth best explains the out-of-phase relationship between intensity and frequency of global tropical cyclones. In a year with greater ocean warmth, the tropical troposphere is capped by higher pressure anomaly in the middle and upper troposphere even with higher moist static energy anomaly in the lower troposphere, which is thought to inhibit overall tropical cyclone occurrences but lead to greater intensities. A statistical consequence is the trade-off between intensity and frequency. We calculate an average increase in global tropical cyclone intensity of 1.3 m s−1 over the past 30 years of ocean warming occurring at the expense of 6.1 tropical cyclones worldwide.
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Carbon Dioxide Emission Standards for U.S. Power Plants: An Efficiency Analysis Perspective
Benjamin Hampf & Kenneth Løvold Rødseth
Energy Economics, forthcoming
Abstract:
On June 25, 2013, President Obama announced his plan to introduce carbon dioxide emission standards for electricity generation. This paper proposes an efficiency analysis approach that addresses which emission rates (and standards) would be feasible if the existing generating units adopt best practices. A new efficiency measure is introduced and further decomposed to identify different sources' contributions to emission rate improvements. Estimating two Data Envelopment Analysis (DEA) models - the well-known joint production model and the new materials balance model - on a dataset consisting of 160 bituminous-fired generating units, we find that the average generating unit's electricity-to-carbon dioxide ratio is 15.3 percent below the corresponding best-practice ratio. Further examinations reveal that this discrepancy can largely be attributed to non-discretionary factors and not to managerial inefficiency. Moreover, even if the best practice ratios could be implemented, the generating units would not be able to comply with the EPA's recently proposed carbon dioxide standard.
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Dynamic thinning of glaciers on the Southern Antarctic Peninsula
B. Wouters et al.
Science, 22 May 2015, Pages 899-903
Abstract:
Growing evidence has demonstrated the importance of ice shelf buttressing on the inland grounded ice, especially if it is resting on bedrock below sea level. Much of the Southern Antarctic Peninsula satisfies this condition and also possesses a bed slope that deepens inland. Such ice sheet geometry is potentially unstable. We use satellite altimetry and gravity observations to show that a major portion of the region has, since 2009, destabilized. Ice mass loss of the marine-terminating glaciers has rapidly accelerated from close to balance in the 2000s to a sustained rate of –56 ± 8 gigatons per year, constituting a major fraction of Antarctica's contribution to rising sea level. The widespread, simultaneous nature of the acceleration, in the absence of a persistent atmospheric forcing, points to an oceanic driving mechanism.
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The impact of climate extremes and irrigation on US crop yields
T.J. Troy, C. Kipgen & I. Pal
Environmental Research Letters, May 2015
Abstract:
Climate variability and extremes are expected to increase due to climate change; this may have significant negative impacts for agricultural production. Previous work has primarily focused on the impact of mean growing season temperature and precipitation on rainfed crop yields with little work on irrigated crop yields or climate extremes and their timing. County-level crop yields and daily precipitation and temperature data are pooled to quantify the impact of climate variability and extremes on four major staple crops in the United States. Conditional density plots are used to graphically explore the relationship between climate extremes and crop yields, thereby avoiding assumptions about linearity or underlying probability distributions. Non-linear and threshold-type relationships exist between yields and both precipitation and temperature climate indices; irrigation significantly reduces the impact of all climate indices. In some cases, this occurs by shifting the threshold, such that a more extreme weather event is necessary to negatively impact yields. In other cases, irrigation essentially decouples the crop yields from climate. This work demonstrates that irrigation may be a beneficial adaptation mechanism to changes in climate extremes in coming decades.
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Seepage: Climate change denial and its effect on the scientific community
Stephan Lewandowsky et al.
Global Environmental Change, July 2015, Pages 1–13
Abstract:
Vested interests and political agents have long opposed political or regulatory action in response to climate change by appealing to scientific uncertainty. Here we examine the effect of such contrarian talking points on the scientific community itself. We show that although scientists are trained in dealing with uncertainty, there are several psychological reasons why scientists may nevertheless be susceptible to uncertainty-based argumentation, even when scientists recognize those arguments as false and are actively rebutting them. Specifically, we show that prolonged stereotype threat, pluralistic ignorance, and a form of projection (the third-person effect) may cause scientists to take positions that they would be less likely to take in the absence of outspoken public opposition. We illustrate the consequences of seepage from public debate into the scientific process with a case study involving the interpretation of temperature trends from the last 15 years. We offer ways in which the scientific community can detect and avoid such inadvertent seepage.
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Anthropogenic contribution to global occurrence of heavy-precipitation and high-temperature extremes
E.M. Fischer & R. Knutti
Nature Climate Change, May 2015, Pages 560–564
Abstract:
Climate change includes not only changes in mean climate but also in weather extremes. For a few prominent heatwaves and heavy precipitation events a human contribution to their occurrence has been demonstrated. Here we apply a similar framework but estimate what fraction of all globally occurring heavy precipitation and hot extremes is attributable to warming. We show that at the present-day warming of 0.85 °C about 18% of the moderate daily precipitation extremes over land are attributable to the observed temperature increase since pre-industrial times, which in turn primarily results from human influence. For 2 °C of warming the fraction of precipitation extremes attributable to human influence rises to about 40%. Likewise, today about 75% of the moderate daily hot extremes over land are attributable to warming. It is the most rare and extreme events for which the largest fraction is anthropogenic, and that contribution increases nonlinearly with further warming. The approach introduced here is robust owing to its global perspective, less sensitive to model biases than alternative methods and informative for mitigation policy, and thereby complementary to single-event attribution. Combined with information on vulnerability and exposure, it serves as a scientific basis for assessment of global risk from extreme weather, the discussion of mitigation targets, and liability considerations.
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Recent reversal in loss of global terrestrial biomass
Yi Liu et al.
Nature Climate Change, May 2015, Pages 470–474
Abstract:
Vegetation change plays a critical role in the Earth's carbon (C) budget and its associated radiative forcing in response to anthropogenic and natural climate change. Existing global estimates of aboveground biomass carbon (ABC) based on field survey data provide brief snapshots that are mainly limited to forest ecosystems. Here we use an entirely new remote sensing approach to derive global ABC estimates for both forest and non-forest biomes during the past two decades from satellite passive microwave observations. We estimate a global average ABC of 362 PgC over the period 1998–2002, of which 65% is in forests and 17% in savannahs. Over the period 1993–2012, an estimated −0.07 PgC yr−1 ABC was lost globally, mostly resulting from the loss of tropical forests (−0.26 PgC yr−1) and net gains in mixed forests over boreal and temperate regions (+0.13 PgC yr−1) and tropical savannahs and shrublands (+0.05 PgC yr−1). Interannual ABC patterns are greatly influenced by the strong response of water-limited ecosystems to rainfall variability, particularly savannahs. From 2003 onwards, forest in Russia and China expanded and tropical deforestation declined. Increased ABC associated with wetter conditions in the savannahs of northern Australia and southern Africa reversed global ABC loss, leading to an overall gain, consistent with trends in the global carbon sink reported in recent studies.
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Quantifying the net contribution of the historical Amazonian deforestation to climate change
Jean-François Exbrayat & Mathew Williams
Geophysical Research Letters, 28 April 2015, Pages 2968–2976
Abstract:
Recent large-scale carbon (C) emissions from deforestation have been estimated by combining remotely-sensed land use change information with satellite-based aboveground biomass (AGB) data. However, these estimates are constrained to the satellite era while regions such as the Amazon basin have been heavily impacted by deforestation before this period. Assessing the net contribution of past tropical deforestation to the growth in atmospheric CO2 is therefore challenging. We address this lack of data by constructing two maps of potential AGB with a machine learning algorithm trained on the relationship between AGB and climate and topography in intact forest landscapes of the Amazon basin. Reconstructions converge to a current deficit of 11.5–12% in AGB or a net loss of ~7–8 Pg C of AGB in the Amazon basin compared to current estimates. This represents a net contribution of ~1.8 ppm of atmospheric CO2 or 1.5% of the historical growth.
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Amplified Arctic warming by phytoplankton under greenhouse warming
Jong-Yeon Park et al.
Proceedings of the National Academy of Sciences, 12 May 2015, Pages 5921–5926
Abstract:
Phytoplankton have attracted increasing attention in climate science due to their impacts on climate systems. A new generation of climate models can now provide estimates of future climate change, considering the biological feedbacks through the development of the coupled physical–ecosystem model. Here we present the geophysical impact of phytoplankton, which is often overlooked in future climate projections. A suite of future warming experiments using a fully coupled ocean−atmosphere model that interacts with a marine ecosystem model reveals that the future phytoplankton change influenced by greenhouse warming can amplify Arctic surface warming considerably. The warming-induced sea ice melting and the corresponding increase in shortwave radiation penetrating into the ocean both result in a longer phytoplankton growing season in the Arctic. In turn, the increase in Arctic phytoplankton warms the ocean surface layer through direct biological heating, triggering additional positive feedbacks in the Arctic, and consequently intensifying the Arctic warming further. Our results establish the presence of marine phytoplankton as an important potential driver of the future Arctic climate changes.
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Shuwei Dai et al.
International Journal of Climatology, forthcoming
Abstract:
Since late 1970s, climate warming has been widely recognized. In the Midwest, farmers cannot rely on the normal calendar anymore, and it has become critically necessary to evaluate the most recent climate trends relative to growing season in order to conduct adaptation efforts for agriculture. Based on the homogenized historical monthly temperature and precipitation records during the period of 1980–2013 from 302 observing stations in the 12 Midwestern US states, we investigate the climate trends on four timescales: monthly, early growing season, late growing season, and the entire growing season. The climate metrics include maximum temperature, minimum temperature, average temperature, diurnal temperature range, and precipitation. Nonparametric Sen's Slope together with the nonparametric Mann–Kendall test is used to estimate the decadal trend and to detect the statistical significance. The results show that growing season average temperature has increased at a rate of 0.15 °C decade−1 over the Midwest United States. Within the growing season, minimum temperature is increasing faster in the early growing season, especially in June, while maximum temperature is increasing faster in the late growing season, especially in September. Spatially, statistically significant (p ≤ 0.05) growing season warming is more focused in the southern part of the region in the early growing season but in the northern part of the region in the late growing season. Over the Midwest, dominant trends in diurnal temperature range are decreasing during most months, with the exception of September. The majority of the locations show increasing trends in growing season precipitation, yet few are statistically significant. Furthermore, precipitation has been increasing in the early growing season but decreasing in the late growing season. This within-season reversing trend in precipitation is found in 8 of 12 Corn Belt states: Illinois, Iowa, Michigan, Minnesota, Missouri, Nebraska, North Dakota, and Wisconsin.
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Natural and Forced North Atlantic Hurricane Potential Intensity Change in CMIP5 Models
Mingfang Ting et al.
Journal of Climate, May 2015, Pages 3926–3942
Abstract:
Possible future changes of North Atlantic hurricane intensity and the attribution of past hurricane intensity changes in the historical period are investigated using phase 5 of the Climate Model Intercomparison Project (CMIP5), multimodel, multiensemble simulations. For this purpose, the potential intensity (PI), the theoretical upper limit of the tropical cyclone intensity given the large-scale environment, is used. The CMIP5 models indicate that the PI change as a function of sea surface temperature (SST) variations associated with the Atlantic multidecadal variability (AMV) is more effective than that associated with climate change. Thus, relatively small changes in SST due to natural multidecadal variability can lead to large changes in PI, and the model-simulated multidecadal PI change during the historical period has been largely dominated by AMV. That said, the multimodel mean PI for the Atlantic main development region shows a significant increase toward the end of the twenty-first century under both the RCP4.5 and RCP8.5 emission scenarios. This is because of enhanced surface warming, which would place the North Atlantic PI largely above the historical mean by the mid-twenty-first century, based on CMIP5 model projection. The authors further attribute the historical PI changes to aerosols and greenhouse gas (GHG) forcing using CMIP5 historical single-forcing simulations. The model simulations indicate that aerosol forcing has been more effective in causing PI changes than the corresponding GHG forcing; the decrease in PI due to aerosols and increase due to GHG largely cancel each other. Thus, PI increases in the recent 30 years appears to be dominated by multidecadal natural variability associated with the positive phase of the AMV.
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Biodiversity influences plant productivity through niche–efficiency
Jingjing Liang et al.
Proceedings of the National Academy of Sciences, 5 May 2015, Pages 5738–5743
Abstract:
The loss of biodiversity is threatening ecosystem productivity and services worldwide, spurring efforts to quantify its effects on the functioning of natural ecosystems. Previous research has focused on the positive role of biodiversity on resource acquisition (i.e., niche complementarity), but a lack of study on resource utilization efficiency, a link between resource and productivity, has rendered it difficult to quantify the biodiversity–ecosystem functioning relationship. Here we demonstrate that biodiversity loss reduces plant productivity, other things held constant, through theory, empirical evidence, and simulations under gradually relaxed assumptions. We developed a theoretical model named niche–efficiency to integrate niche complementarity and a heretofore-ignored mechanism of diminishing marginal productivity in quantifying the effects of biodiversity loss on plant productivity. Based on niche–efficiency, we created a relative productivity metric and a productivity impact index (PII) to assist in biological conservation and resource management. Relative productivity provides a standardized measure of the influence of biodiversity on individual productivity, and PII is a functionally based taxonomic index to assess individual species' inherent value in maintaining current ecosystem productivity. Empirical evidence from the Alaska boreal forest suggests that every 1% reduction in overall plant diversity could render an average of 0.23% decline in individual tree productivity. Out of the 283 plant species of the region, we found that large woody plants generally have greater PII values than other species. This theoretical model would facilitate the integration of biological conservation in the international campaign against several pressing global issues involving energy use, climate change, and poverty.