Observations indicate that substantial changes in the dynamics of marine-terminating ice sheets and glaciers are tightly coupled to calving-induced changes in the terminus position. However, the calving process itself remains poorly understood and is not well parametrized in current numerical ice sheet models. In this study, we address this uncertainty by deriving plausible upper and lower limits for the maximum stable ice thickness at the calving face of marine-terminating glaciers, using two complementary models. The first model assumes that a combination of tensile and shear failure can render the ice cliff near the terminus unstable and/or enable pre-existing crevasses to intersect. A direct consequence of this model is that thick glaciers must terminate in deep water to stabilize the calving front, yielding a predicted maximum ice cliff height that increases with increasing water depth, consistent with observations culled from glaciers in West Greenland, Antarctica, Svalbard and Alaska. The second model considers an analogous lower limit derived by assuming that the ice is already fractured and fractures are lubricated by pore pressure. In this model, a floating ice tongue can only form when the ice entering the terminus region is relatively intact with few pre-existing, deeply penetrating crevasses.

Iceberg calving from all Antarctic ice shelves has never been directly measured, despite playing a crucial role in ice sheet mass balance. Rapid changes to iceberg calving naturally arise from the sporadic detachment of large tabular bergs but can also be triggered by climate forcing. Here we provide a direct empirical estimate of mass loss due to iceberg calving and melting from Antarctic ice shelves. We find that between 2005 and 2011, the total mass loss due to iceberg calving of 755 ± 24 gigatonnes per year (Gt/y) is only half the total loss due to basal melt of 1516 ± 106 Gt/y. However, we observe widespread retreat of ice shelves that are currently thinning. Net mass loss due to iceberg calving for these ice shelves (302 ± 27 Gt/y) is comparable in magnitude to net mass loss due to basal melt (312 ± 14 Gt/y). Moreover, we find that iceberg calving from these decaying ice shelves is dominated by frequent calving events, which are distinct from the less frequent detachment of isolated tabular icebergs associated with ice shelves in neutral or positive mass balance regimes. Our results suggest that thinning associated with ocean-driven increased basal melt can trigger increased iceberg calving, implying that iceberg calving may play an overlooked role in the demise of shrinking ice shelves, and is more sensitive to ocean forcing than expected from steady state calving estimates.

[1] OH and HO 2 mixing ratios and total OH reactivity were measured together with photolysis frequencies, NO x , O 3 , many VOCs, and other trace gases during the midsummer 1999 SOS campaign in Nashville, Tennessee. These measurements provided an excellent opportunity to study OH and HO 2 (collectively called HO x ), and their sources and sinks in a polluted metropolitan environment. HO x generally showed the expected diurnal evolution, with maxima around noon of up to about 0.8 pptv of OH and 80 pptv of HO 2 during sunny days. Overall, daytime observed OH and HO 2 were a factor of 1.33 and 1.56 times modeled values, within the combined 2s instrument and model uncertainties. The chain length of HO x , which is determined from the ratio of the measured total OH reactivity that cycles OH to the total HO x loss, was on average 3-8 during daytime and up to 3 during nighttime, in general agreement with expectations. However, differences occurred between observed HO x behavior and expectations from theory and models. First, HO 2 was greater than expected during daytime when NO mixing ratios were high; ozone production did not decrease as expected when NO was greater than 2 ppbv. Ozone production determined by the imbalance of the NO x photostationary state, which was almost twice that from HO 2 , also shows this dependence on NO. Second, the calculated OH production rate, which should equal the measured OH loss rate because OH is in steady state, is instead less than the measured OH loss rate by (1-2) Â 10 7 molecules cm -3 s -1 , with low statistical significance during the day and high statistical significance at night. Third, surprisingly high OH and HO 2 mixing ratios were often observed during nighttime. The nighttime OH mixing ratio and the HO 2 /OH ratio cannot be explained by known reaction mechanisms, even those involving O 3 and alkenes. Because instrument tests have failed to reveal any instrument artifacts, more exotic chemicals or chemistry, such as OH adducts or other radicals that fall apart into OH inside the instrument, are suspected.

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