Description: Quantifying the spatial and temporal patterns of the water lost to the atmosphere through land surface evapotranspiration (ET) is essential for understanding the global hydrological cycle, but remains much uncertain. In this study, we use the Dynamic Land Ecosystem Model (DLEM) to estimate the global terrestrial ET during 2000–2009 and project its changes in response to climate change and increasing atmospheric CO 2 under two IPCC SRES scenarios (A2 and B1) during 2010–2099. Modeled results show a mean annual global terrestrial ET of about 549 (545–552) mm yr −1 during 2000–2009. Relative to the 2000s, global terrestrial ET for the 2090's would increase by 30.7 mm/year (5.6%) and 13.2 mm/year (2.4%) under the A2 and B1 scenarios, respectively. About 60% of global land area would experience increasing ET at rates of over 9.5 mm/decade over the study period under the A2 scenario. The Arctic region would have the largest ET increase (16% compared with the 2000s level) due to larger increase in temperature than other regions. Decreased ET would mainly take place in regions like central and western Asia, northern Africa, Australia, eastern South America and Greenland due to declines in soil moisture and changing rainfall patterns. Our results indicate that warming temperature and increasing precipitation would result in large increase in ET by the end of the 21 st century, while increasing atmospheric CO 2 would be responsible for decrease of ET, given the reduction of stomatal conductance under elevated CO 2 .

Description: Relative sea level rise (RSLR) has driven large increases in annual water level exceedances (duration and frequency) above minor (nuisance level) coastal flooding elevation thresholds established by the National Weather Service (NWS) at U.S. tide gauges over the last half-century. For threshold levels below 0.5 m above high tide, the rates of annual exceedances are accelerating along the U.S. East and Gulf Coasts, primarily from evolution of tidal water level distributions to higher elevations impinging on the flood threshold. These accelerations are quantified in terms of the local RSLR rate and tidal range through multiple regression analysis. Along the U.S. West Coast, annual exceedance rates are linearly increasing, complicated by sharp punctuations in RSLR anomalies during El Niño Southern Oscillation (ENSO) phases, and we account for annual exceedance variability along the U.S. West and East Coasts from ENSO forcing. Projections of annual exceedances above local NWS nuisance levels at U.S. tide gauges are estimated by shifting probability estimates of daily maximum water levels over a contemporary 5-year period following probabilistic RSLR projections of Kopp et al. (2014) for representative concentration pathways (RCP) 2.6, 4.5, and 8.5. We suggest a tipping point for coastal inundation (30 days/per year with a threshold exceedance) based on the evolution of exceedance probabilities. Under forcing associated with the local-median projections of RSLR, the majority of locations surpass the tipping point over the next several decades regardless of specific RCP.

Description: VALUE is an open European network to validate and compare downscaling methods for climate change research. VALUE aims to foster collaboration and knowledge exchange between climatologists, impact modellers, statisticians and stakeholders to establish an interdisciplinary downscaling community. A key deliverable of VALUE is the development of a systematic validation framework to enable the assessment and comparison of both dynamical and statistical downscaling methods. In this paper, we present the key ingredients of this framework. VALUE’s main approach to validation is user focussed: starting from a specific user problem, a validation tree guides the selection of relevant validation indices and performance measures. Several experiments have been designed to isolate specific points in the downscaling procedure where problems may occur: what is the isolated downscaling skill? How do statistical and dynamical methods compare? How do methods perform at different spatial scales? Do methods fail in representing regional climate change? How is the overall representation of regional climate, including errors inherited from global climate models? The framework will be the basis for a comprehensive community-open downscaling intercomparison study, but is intended also to provide general guidance for other validation studies.

Description: Key Points Statement on the importance of climate change politics

Description: Earth's equilibrium climate sensitivity (ECS) and forcing of Earth's climate system over the industrial era have been re-examined in two new assessments: the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC), and a study by A. Otto and others ( Nature Geosci ., 2013). The ranges of these quantities given in these assessments and also in the Fourth (2007) IPCC Assessment are analyzed here within the framework of a planetary energy balance model, taking into account the observed increase in global mean surface temperature over the instrumental record together with best estimates of the rate of increase of planetary heat content. This analysis shows systematic differences among the several assessments and apparent inconsistencies within individual assessments. Importantly, the likely range of ECS to doubled CO 2 given in AR5, 1.5 to 4.5 K/(3.7 W m -2 ) exceeds the range inferred from the assessed likely range of forcing, 1.2 to 2.9 K/(3.7 W m −2 ), where 3.7 W m −2 denotes the forcing for doubled CO 2 . Such differences underscore the need to identify their causes and reduce the underlying uncertainties. Explanations might involve underestimated negative aerosol forcing, overestimated total forcing, overestimated climate sensitivity, poorly constrained ocean heating, limitations of the energy balance model, or a combination of effects. Summary: Recent assessments of Earth's climate sensitivity and forcings over the industrial period, taking into account the observed increase in global mean surface temperature and rate of increase of planetary heat content, exhibit differences and apparent inconsistencies.

Description: Key Points Onset of the Anthropocene Redefining the Holocene

Description: The expected urbanization of the planet in the coming century coupled with aging infrastructure in developed regions, increasing complexity of man-made systems, and pressing climate change impacts have created opportunities for reassessing the role of infrastructure and technologies in cities and how they contribute to greenhouse gas (GHG) emissions. Modern urbanization is predicated on complex, increasingly coupled infrastructure systems, and energy use continues to be largely met from fossil fuels. Until energy infrastructures evolve away from carbon-based fuels, GHG emissions are critically tied to the urbanization process. Further complicating the challenge of decoupling urban growth from GHG emissions are lock-in effects and interdependencies. This paper synthesizes state-of-the-art thinking for transportation, fuels, buildings, water, electricity, and waste systems and finds that GHG emissions assessments tend to view these systems as static and isolated from social and institutional systems. Despite significant understanding of methods and technologies for reducing infrastructure-related GHG emissions, physical, institutional, and cultural constraints continue to work against us, pointing to knowledge gaps that must be addressed. This paper identifies three challenge themes to improve our understanding of the role of infrastructure and technologies in urbanization processes and position these increasingly complex systems for low-carbon growth. The challenges emphasize how we reimagine the role of infrastructure in the future and how people, institutions, and ecological systems interface with infrastructure.

Description: Population growth, dietary changes and increasing biofuel use are placing unprecedented pressure on the global food system. While this demand likely cannot be met by expanding agricultural lands, much of the world's cropland can attain higher crop yields. Therefore, it is important to examine whether increasing crop productivity to the maximum attainable yield (i.e. yield gap closure) alone can substantially improve food security at global and national scales. Here we show that closing yield gaps through conventional technological development (i.e. fertilizers and irrigation) can potentially meet future global demand if diets are moderated and crop-based biofuel production is limited. In particular, we find that increasing dietary demand will be largely to blame should crop production fall short of demand. In converting projected diets to a globally adequate diet (3000 kcal/cap/day; 20% animal kcal) under current agrofuel use, we find that ~1.8 to ~2.6 billion additional people can be fed in 2030 and ~2.1 to ~3.1 billion additional people in 2050, depending on the extent to which yields can improve in those time periods. Therefore, the simple combination of yield gap closure and moderating diets offers promise for feeding the world's population but only if long-term sustainability is the focus.

Description: In the past decade there has been a massive growth in the horizontal drilling and hydraulic fracturing of shale gas and tight oil reservoirs to exploit formerly inaccessible or unprofitable energy resources in rock formations with low permeability. In North America, these unconventional domestic sources of natural gas and oil provide an opportunity to achieve energy self-sufficiency and to reduce greenhouse gas emissions when displacing coal as a source of energy in power plants. However, fugitive methane emissions in the production process may counter the benefit over coal with respect to climate change and therefore need to be well quantified. Here we demonstrate that positive methane anomalies associated with the oil and gas industries can be detected from space and that corresponding regional emissions can be constrained using satellite observations. Based on a mass-balance approach, we estimate that methane emissions for two of the fastest growing production regions in the United States, the Bakken and Eagle Ford formations, have increased by 990 ± 650 ktCH 4 yr − 1 and 530 ± 330 ktCH 4 yr − 1 between the periods 2006–2008 and 2009–2011. Relative to the respective increases in oil and gas production, these emission estimates correspond to leakages of 10.1 ± 7.3 % and 9.1 ± 6.2 % in terms of energy content, calling immediate climate benefit into question and indicating that current inventories likely underestimate fugitive emissions from Bakken and Eagle Ford.