Description: Water vapor is the most important greenhouse gas in the atmosphere although changes in carbon dioxide constitute the “control knob” for surface temperatures. While the latter fact is well recognized, resulting in extensive spaceborne and ground based measurement programs for carbon dioxide [et~al.(1996), Chin, and Whorf, et~al.(2009), Suto, Nakajima, and Hamazaki, et~al.(2014), Cai, Yang, Zheng, Duan, and Lu], the need for an accurate characterization of the long-term changes in upper tropospheric and lower stratospheric (UTLS) water vapor has not yet resulted in sufficiently extensive long-term international measurement programs (although first steps have been taken). Here we argue for the implementation of a, long-term balloon-borne measurement program for UTLS water vapor covering the entire globe that likely will have to be sustained for hundreds of years.

Description: Key Points Liberal values in rich democracies are threatened by population growth in poor countries. The most likely repressive policy response will be barriers to immigration. Fertility reduction in high-fertility countries requires increased access to contraception.

Description: Key Points Demographic limitations involve acute aspects of public policy. Demographic controls cannot be effectively implemented under current Western values. Traditions of social freedom will have to be altered.

Description: Words are integral to thinking and communicating. Words also carry old baggage. The Anthropocene necessitates new thinking and communication at the human-nature interface. Words like progress, natural, and thresholds are pervasive in both scientific and policy discourse, but carry baggage that will likely slow understanding of the Anthropocene and appropriate adaptation. The dynamic systems thinking with emergent properties of ecology needs to replace the efficiency and growth framework of economics. Diversity and resilience are productive and less historically burdened words.

Description: This article is a review of the science goals and the activities initiated within the framework of the P ollution and its I mpacts on the S outh A merican C ryosphere ( PISAC ) initiative. Air pollution associated with biomass burning and urban emissions affects extensive areas of South America. We focus on black carbon (BC) aerosol and its impacts on air quality, water availability, and climate, with an emphasis on the Andean cryosphere. BC is one of the key short-lived climate pollutants, which is a topic of growing interest for near-term mitigation of these issues. Limited scientific evidence indicates that the Andean cryosphere has already responded to climate change with receding glaciers and snow cover, which directly affect water resources, agriculture and energy production in the Andean region of South America. Despite the paucity of systematic observations along the Andes, a few studies have detected BC on snow and glaciers in the Andes. These, in addition to existing and projected emissions and weather patterns, suggest a possible contribution of BC to the observed retreat of the Andean cryosphere. Here we provide an overview of the current understanding of these issues from scientific and policy perspectives, and propose strategic expansions to the relevant measurement infrastructure in the region.

Description: Though climate models exhibit broadly similar agreement on key long-term trends, they have significant temporal and spatial differences due to inter-model variability. Such variability should be considered when using climate models to project the future marine Arctic. Here we present multiple scenarios of 21 st -century Arctic marine access as driven by sea ice output from 10 CMIP5 models known to represent well the historical trend and climatology of Arctic sea ice. Optimal vessel transits from North America and Europe to the Bering Strait are estimated for two periods representing early-century (2011–2035) and mid-century (2036–2060) conditions under two forcing scenarios (RCP 4.5/8.5), assuming Polar Class 6 and open-water vessels with medium and no ice-breaking capability, respectively. Results illustrate that projected shipping viability of the Northern Sea Route (NSR) and Northwest Passage (NWP) depends critically on model choice. The eastern Arctic will remain the most reliably accessible marine space for trans-Arctic shipping by mid-century, while outcomes for the NWP are particularly model-dependent. Omitting three models (GFDL-CM3, MIROC-ESM-CHEM, MPI-ESM-MR), our results would indicate minimal NWP potential even for routes from North America. Furthermore, the relative importance of the NSR will diminish over time as the number of viable central Arctic routes increases gradually toward mid-century. Compared to vessel class, climate forcing plays a minor role. These findings reveal the importance of model choice in devising projections for strategic planning by governments, environmental agencies, and the global maritime industry.

Description: Despite advances in our understanding of the processes driving contemporary sea level rise, the stability of the Antarctic ice sheets and their contribution to sea level under projected future warming remains uncertain due to the influence of strong ice-climate feedbacks. Disentangling these feedbacks is key to reducing uncertainty. Here we present a series of climate system model simulations that explore the potential effects of increased West Antarctic Ice Sheet (WAIS) meltwater flux on Southern Ocean dynamics. We project future changes driven by sectors of the WAIS, delivering spatially and temporally variable meltwater flux into the Amundsen, Ross and Weddell embayments over future centuries. Focusing on the Amundsen Sea sector of the WAIS over the next 200 years, we demonstrate that the enhanced meltwater flux rapidly stratifies surface waters, resulting in a significant decrease in the rate of Antarctic Bottom Water (AABW) formation. This triggers rapid pervasive ocean warming (〉1°C) at depth due to advection from the original site(s) of meltwater input. The greatest warming predicted along sectors of the ice sheet that are highly sensitized to ocean forcing, creating a feedback loop that could enhance basal ice shelf melting and grounding line retreat. Given that we do not include the effects of rising CO 2 - predicted to further reduce AABW formation - our experiments highlight the urgent need to develop a new generation of fully-coupled ice sheet climate models, that include feedback mechanisms such as this, to reduce uncertainty in climate and sea level projections.