Abstract. To estimate the sea level rise (SLR) originating from changes in surface mass balance (SMB) of the Greenland ice sheet (GrIS), we present 21st century climate projections obtained with the regional climate model MAR (Modèle Atmosphérique Régional), forced by output of three CMIP5 (Coupled Model Intercomparison Project Phase 5) general circulation models (GCMs). Our results indicate that in a warmer climate, mass gain from increased winter snowfall over the GrIS does not compensate mass loss through increased meltwater run-off in summer. Despite the large spread in the projected near-surface warming, all the MAR projections show similar non-linear increase of GrIS surface melt volume because no change is projected in the general atmospheric circulation over Greenland. By coarsely estimating the GrIS SMB changes from GCM output, we show that the uncertainty from the GCM-based forcing represents about half of the projected SMB changes. In 2100, the CMIP5 ensemble mean projects a GrIS SMB decrease equivalent to a mean SLR of +4±2 cm and +9±4 cm for the RCP (Representative Concentration Pathways) 4.5 and RCP 8.5 scenarios respectively. These estimates do not consider the positive melt-elevation feedback, although sensitivity experiments using perturbed ice sheet topographies consistent with the projected SMB changes demonstrate that this is a significant feedback, and highlight the importance of coupling regional climate models to an ice sheet model. Such a coupling will allow the assessment of future response of both surface processes and ice-dynamic changes to rising temperatures, as well as their mutual feedbacks.

( 1 and Fig. 1(a)). These trends are primarily due to stricter air quality emission controls that candidate species for studying hemispheric gradients and long-term changes. 57We analyzed ten years of NMHC data collected at 44 remote global sampling sites from NOAA's 58 Global Greenhouse Gas Reference Network (GGGRN). We also include data from in-situ moni-59 toring at Summit, Greenland 8 , at Hohenpeissenberg (HPB) in Southern Germany 9 , Jungfraujoch resolved in-situ record from HPB has its minimum in 2009 ( Fig. 1 (e)), in agreement with the JFJ 78 FTIR column observations ( Fig. 1(c)). Focusing on the most recent five years (2009.5 -2014.5) 79 we find variable results in the observed rate of change; however, a consistent picture emerges 80 that shows the largest increases at NH sites (Fig. 3). Of 33 NH sites, 7 exhibit ethane growth 81 rates > 50 pmol mol -1 yr -1 , and 10 sites exhibit growth rates between 25-50 pmol -1 yr -1 (Table S1). one from JFJ ( Fig. 1(c)) 12 , and the other one from Lauder, New Zealand ( Fig. 1(d) emission increases outside of NA that currently cannot be well defined due to the sparsity of 170 observations in those regions (for instance in the middle-East, Africa, and Asia).

Sharp rises in the atmospheric abundance of ethane (C 2 H 6 ) have been detected from 2009 onwards in the Northern Hemisphere as a result of the unprecedented growth in the exploitation of shale gas and tight oil reservoirs in North America. Using time series of C 2 H 6 total columns derived from groundbased Fourier transform infrared (FTIR) observations made at five selected Network for the Detection of Atmospheric Composition Change sites, we characterize the recent C 2 H 6 evolution and determine growth rates of ∼5% yr −1 at mid-latitudes and of ∼3% yr −1 at remote sites. Results from CAM-chem simulations with the Hemispheric Transport of Air Pollutants, Phase II bottom-up inventory for anthropogenic emissions are found to greatly underestimate the current C 2 H 6 abundances. Doubling global emissions is required to reconcile the simulations and the observations prior to 2009. We further estimate that North American anthropogenic C 2 H 6 emissions have increased from 1.6 Tg yr −1 in 2008 to 2.8 Tg yr −1 in 2014, i.e. by 75% over these six years. We also completed a second simulation with new top-down emissions of C 2 H 6 from North American oil and gas activities, biofuel consumption and biomass burning, inferred from space-borne observations of methane (CH 4 ) from Greenhouse Gases Observing SATellite. In this simulation, GEOS-Chem is able to reproduce FTIR measurements at the mid-latitudinal sites, underscoring the impact of the North American oil and gas development on the current C 2 H 6 abundance. Finally we estimate that the North American oil and gas emissions of CH 4 , a major greenhouse gas, grew from 20 to 35 Tg yr −1 over the period 2008-2014, in association with the recent C 2 H 6 rise.

Atmospheric acidity is increasingly determined by carbon dioxide and organic acids1–3. Among the latter, formic acid facilitates the nucleation of cloud droplets4 and contributes to the acidity of clouds and rainwater1,5. At present, chemistry–climate models greatly underestimate the atmospheric burden of formic acid, because key processes related to its sources and sinks remain poorly understood2,6–9. Here we present atmospheric chamber experiments that show that formaldehyde is efficiently converted to gaseous formic acid via a multiphase pathway that involves its hydrated form, methanediol. In warm cloud droplets, methanediol undergoes fast outgassing but slow dehydration. Using a chemistry–climate model, we estimate that the gas-phase oxidation of methanediol produces up to four times more formic acid than all other known chemical sources combined. Our findings reconcile model predictions and measurements of formic acid abundance. The additional formic acid burden increases atmospheric acidity by reducing the pH of clouds and rainwater by up to 0.3. The diol mechanism presented here probably applies to other aldehydes and may help to explain the high atmospheric levels of other organic acids that affect aerosol growth and cloud evolution.