Event-by-Event Fluctuations in Ultrarelativistic Heavy-Ion Collisions

Motivated by forthcoming experiments at RHIC and LHC, we study event-by-event fluctuations in ultrarelativistic heavy-ion collisions in participant nucleon as well as thermal models. The calculated physical observables, including multiplicity, kaon to pion ratios, and transverse momenta agree well with recent NA49 data at the SPS, and indicate that such studies do not yet reveal the presence of new physics. Finally, we present a simple model of how a first order phase transition can be signaled by very large fluctuations.Comment: final version, 4 pages, to appear in Phys. Lett.

Non-linear retreat of Jakobshavn Isbræ since the Little Ice Age controlled by geometry

Rapid acceleration and retreat of Greenland's marine-terminating glaciers during the last two decades have initiated questions on the trigger and processes governing observed changes. Destabilization of these glaciers coincides with atmosphere and ocean warming, which broadly has been used to explain the rapid changes. To assess the relative role of external forcing versus fjord geometry, we investigate the retreat of Jakobshavn Isbræ in West Greenland, where margin positions exist since the Little Ice Age maximum in 1850. We use a one-dimensional ice flow model and isolate geometric effects on the retreat using a linear increase in external forcing. We find that the observed retreat of 43 km from 1850 until 2014 can only be simulated when multiple forcing parameters – such as hydrofracturing, submarine melt and frontal buttressing by sea ice – are changed simultaneously. Surface mass balance, in contrast, has a negligible effect. While changing external forcing initiates retreat, fjord geometry controls the retreat pattern. Basal and lateral topography govern shifts from temporary stabilization to rapid retreat, resulting in a highly non-linear glacier response. For example, we simulate a disintegration of a 15 km long floating tongue within one model year, which dislodges the grounding line onto the next pinning point. The retreat pattern loses complexity and becomes linear when we artificially straighten the glacier walls and bed, confirming the topographic controls. For real complex fjord systems such as Jakobshavn Isbræ, geometric pinning points predetermine grounding line stabilization and may therefore be used as a proxy for moraine build-up. Also, we find that after decades of stability and with constant external forcing, grounding lines may retreat rapidly without any trigger. This means that past changes may precondition marine-terminating glaciers to reach tipping-points, and that retreat can occur without additional climate warming. Present-day changes and future projections can therefore not be viewed in isolation of historic retreat.submittedVersio

Deglaciation of the Norwegian fjords

Melting glaciers and ice sheets are perhaps the most visible signs of a warming cli- mate. Glaciers are retreating on every continent, ice sheets are shedding icebergs into a warming ocean at an accelerating rate, and the atmosphere melts more ice as tempera- tures rise. Despite these lines of evidence, and a growing scientific and public attention to melting ice; we are still not able to present robust numbers of future sea level rise. While the overall picture is one of retreat and meltdown, complex patterns arise when we zoom in within a certain region. The geologic record testaments both collapse and periods of growth, and provides important clues to future mass loss. However, these records show that variable responses were present within regions exposed to the same climate forcing. This is also found in Greenland and Antarctica today, where observations reveal that neighbouring glaciers respond differently to the same climate warming. Caution is therefore needed when explaining the responses of these glaciers. The instrumental record helps us to improve process understanding, yet is unable to assess changes over time scales longer than a few decades. The geologic record provides a longer perspective, but is not able to resolve short-lived variations. This time scale issue is critical because we need to understand both the short-term and long- term response to improve understanding of glacier and ice sheet dynamics and their sensitivity to climate change. In this thesis, we use a suite of numerical model tools combined with geological data to assess how external forcing triggers and drives short- and long-term change. Equally important, we study how site-specific factors such as topography can prevent, delay, dampen, amplify, and override the ambient forcing. We assess theoretical cases as well as past and present glaciers in Norway and Greenland, with the goal to answer the overarching question: how do glaciers and ice sheets respond to climate change? We move from the sensitivity of a Norwegian ice cap to Holocene climate change in Paper I, via the impact of fjord geometry on grounding line stability in Paper II, to the abrupt retreat of the nearby Hardangerfjorden glacier during the Younger Dryas cold period in Paper III. We continue with the most active glacier in Greenland in Paper IV, and finish with a comprehensive study of the triggers and drivers of the deglaciation of the Norwegian fjords in Paper V. For marine-terminating glaciers, we find that grounding line dynamics and ice- ocean interactions are fundamental over time scales up to a century or two. Beyond this time frame, changes to the surface mass balance are likely to drive widespread, multi-centennial to millennial scale deglaciation. Based on the results presented in this thesis, we also suggest that topography is a factor that cannot be ignored. Once triggered, the response to climate change is to a large degree controlled by the under- lying bed topography of ice caps (Paper I), and by the fjord bathymetry and width of marine-terminating glaciers (Papers II–V). The implications are that continued intense studies of warming seas around Green- land and Antarctica are justified, but also that assessments of atmospheric-induced melt will be important to estimate long-term sea level change. The striking impact of topo- graphic factors found in this thesis also shows a potential to use geometry to predict future evolution and estimate past retreat and advance

Simulating the climatic response of Hardangerjøkulen in southern Norway since the Little Ice Age

Glacier and ice cap volume changes currently amount to half of the total contribution from the cryosphere to sea-level rise. Ice caps and their outlet glaciers are dynamically different from the Greenland and Antarctic ice sheets, and respond considerably faster to climate change. We use a shallow-ice version of the Ice Sheet System Model (ISSM) to model the dynamics and evolution of the maritime-continental Hardangerjøkulen ice cap We force the ice flow model with a dynamically calibrated mass balance history based on moraine evidence from the Little Ice Age maximum in 1750, as well as later outlet glacier front positions from moraines and direct measurements. Glaciological mass balance measurements force the model from the 1960s onwards, and we validate the model using a surface digital elevation model from 1995 and aerial photographs. The model successfully reproduces most of the LIA extent of the ice cap. Outlet glaciers retreat too far in the model for the early 1900s, while observed ice extent after 1960 is accurately represented. This coincides with the period where direct mass balance data is used as forcing, indicating its key role. A comparison with a digital elevation model from 1995 reveals a very good agreement of surface topography, except for a too thick eastern ice cap. We find a non-linear relationship between mass balance perturbations and ice volume response, where Hardangerjøkulen is more sensitive to negative than positive mass balance changes. We discuss these findings in light of reconstructed past changes and future predictions

Simulating the evolution of Hardangerjøkulen ice cap in southern Norway since the mid-Holocene and its sensitivity to climate change

Understanding of long-term dynamics of glaciers and ice caps is vital to assess their recent and future changes, yet few long-term reconstructions using ice flow models exist. Here we present simulations of the maritime Hardangerjøkulen ice cap in Norway from the mid-Holocene through the Little Ice Age (LIA) to the present day, using a numerical ice flow model combined with glacier and climate reconstructions. In our simulation, under a linear climate forcing, we find that Hardangerjøkulen grows from ice-free conditions in the mid-Holocene to its maximum extent during the LIA in a nonlinear, spatially asynchronous fashion. During its fastest stage of growth (2300–1300 BP), the ice cap triples its volume in less than 1000 years. The modeled ice cap extent and outlet glacier length changes from the LIA until today agree well with available observations. Volume and area for Hardangerjøkulen and several of its outlet glaciers vary out-of-phase for several centuries during the Holocene. This volume–area disequilibrium varies in time and from one outlet glacier to the next, illustrating that linear relations between ice extent, volume and glacier proxy records, as generally used in paleoclimatic reconstructions, have only limited validity. We also show that the present-day ice cap is highly sensitive to surface mass balance changes and that the effect of the ice cap hypsometry on the mass balance–altitude feedback is essential to this sensitivity. A mass balance shift by +0.5 m w.e. relative to the mass balance from the last decades almost doubles ice volume, while a decrease of 0.2 m w.e. or more induces a strong mass balance–altitude feedback and makes Hardangerjøkulen disappear entirely. Furthermore, once disappeared, an additional +0.1 m w.e. relative to the present mass balance is needed to regrow the ice cap to its present-day extent. We expect that other ice caps with comparable geometry in, for example, Norway, Iceland, Patagonia and peripheral Greenland may behave similarly, making them particularly vulnerable to climate change

Impact of Fjord Geometry on Grounding Line Stability

Recent and past retreat of marine-terminating glaciers are broadly consistent with observed ocean warming, yet responses vary significantly within regions experiencing similar ocean conditions. We assess how fjord geometry modulates glacier response to a regional ocean warming on decadal to millennial time scales, by using an idealized, numerical model of fast-flowing glaciers including a crevasse-depth calving criterion. Our simulations show that, given identical climate forcing, grounding line responses can differ by tens of kilometers due to variations in channel width. We identify fjord mouths and embayments as vulnerable geometries, showing that glaciers in these fjords are prone to rapid, irreversible retreat, independent of the presence of a fjord sill. This irreversible retreat has relevance for the potential future recovery of marine ice sheets, if the current anthropogenic warming is reduced, or reversed, as well as for the response of marine ice sheets to past climate states; including the warm Bølling-Allerød interstadial, the Younger Dryas cold reversal and the Little Ice Age.publishedVersio

Impact of Fjord Geometry on Grounding Line Stability

Recent and past retreat of marine-terminating glaciers are broadly consistent with observed ocean warming, yet responses vary significantly within regions experiencing similar ocean conditions. We assess how fjord geometry modulates glacier response to a regional ocean warming on decadal to millennial time scales, by using an idealized, numerical model of fast-flowing glaciers including a crevasse-depth calving criterion. Our simulations show that, given identical climate forcing, grounding line responses can differ by tens of kilometers due to variations in channel width. We identify fjord mouths and embayments as vulnerable geometries, showing that glaciers in these fjords are prone to rapid, irreversible retreat, independent of the presence of a fjord sill. This irreversible retreat has relevance for the potential future recovery of marine ice sheets, if the current anthropogenic warming is reduced, or reversed, as well as for the response of marine ice sheets to past climate states; including the warm Bølling-Allerød interstadial, the Younger Dryas cold reversal and the Little Ice Age

Simulating the evolution of Hardangerjøkulen ice cap in southern Norway since the mid-Holocene and its sensitivity to climate change

Understanding of long-term dynamics of glaciers and ice caps is vital to assess their recent and future changes, yet few long-term reconstructions using ice flow models exist. Here we present simulations of the maritime Hardangerjøkulen ice cap in Norway from the mid-Holocene through the Little Ice Age (LIA) to the present day, using a numerical ice flow model combined with glacier and climate reconstructions. In our simulation, under a linear climate forcing, we find that Hardangerjøkulen grows from ice-free conditions in the mid-Holocene to its maximum extent during the LIA in a nonlinear, spatially asynchronous fashion. During its fastest stage of growth (2300–1300 BP), the ice cap triples its volume in less than 1000 years. The modeled ice cap extent and outlet glacier length changes from the LIA until today agree well with available observations. Volume and area for Hardangerjøkulen and several of its outlet glaciers vary out-of-phase for several centuries during the Holocene. This volume–area disequilibrium varies in time and from one outlet glacier to the next, illustrating that linear relations between ice extent, volume and glacier proxy records, as generally used in paleoclimatic reconstructions, have only limited validity. We also show that the present-day ice cap is highly sensitive to surface mass balance changes and that the effect of the ice cap hypsometry on the mass balance–altitude feedback is essential to this sensitivity. A mass balance shift by +0.5 m w.e. relative to the mass balance from the last decades almost doubles ice volume, while a decrease of 0.2 m w.e. or more induces a strong mass balance–altitude feedback and makes Hardangerjøkulen disappear entirely. Furthermore, once disappeared, an additional +0.1 m w.e. relative to the present mass balance is needed to regrow the ice cap to its present-day extent. We expect that other ice caps with comparable geometry in, for example, Norway, Iceland, Patagonia and peripheral Greenland may behave similarly, making them particularly vulnerable to climate change

Petermann ice shelf may not recover after a future breakup.

Floating ice shelves buttress inland ice and curtail grounded-ice discharge. Climate warming causes melting and ultimately breakup of ice shelves, which could escalate ocean-bound ice discharge and thereby sea-level rise. Should ice shelves collapse, it is unclear whether they could recover, even if we meet the goals of the Paris Agreement. Here, we use a numerical ice-sheet model to determine if Petermann Ice Shelf in northwest Greenland can recover from a future breakup. Our experiments suggest that post-breakup recovery of confined ice shelves like Petermann's is unlikely, unless iceberg calving is greatly reduced. Ice discharge from Petermann Glacier also remains up to 40% higher than today, even if the ocean cools below present-day temperatures. If this behaviour is not unique for Petermann, continued near-future ocean warming may push the ice shelves protecting Earth's polar ice sheets into a new retreated high-discharge state which may be exceedingly difficult to recover from