At the base of the Arctic food web, there are three major primary producers: small flagellates, diatoms living in open water (pelagic) and diatoms growing in sea ice (sympagic). The role of the sea ice diatoms is perceived differently across the research community. For ecologists they are central to the polar ecosystem, while those looking at global ocean scales consider them less important and have not incorporated them into their models projecting climate change feedbacks. This may reflect their minor (<10%) contribution to the total primary production in Arctic waters. However, two newly developed trophic marker approaches that can trace diatoms from sea ice and open water within the food web, consistently find a strong ice algae 'signal' in polar consumers. Even in whales, seals and polar bears, as much as 80% of their body fat reserves are from carbon originally fixed by ice algae. How is this possible? How will this change in a warming Arctic? Our project aims to answer this puzzle and to bridge the gap between the contrasting perceptions of ice algae. We propose to quantify the relative importance of ice algae vs. open water diatoms for consumers living in the high Arctic - considering different species, regions and times of the year. We will also look at material that sinks to the seabed, and is collected in sediment traps. Our first hypothesis is that the input of ice algae to Arctic food webs and to export fluxes is disproportionately higher than their contribution to total primary production. Our second hypothesis examines the mechanisms behind these energy transfers, focussing on the more subtle concept of food benefit. It is not just the total annual amount of food that matters; it also has to arrive at the right time, be accessible and be nutritious. To test these hypotheses, we have developed a method based on "Highly Branched Isoprenoids" (HBIs). These lipid molecules are specific to a series of diatom species specific either to sea ice or open water. Using the ratio of ice-versus water column-derived HBIs, we can now trace the relative roles of these energy inputs to the food web. The chemical stability of these molecules as they pass through the food web is a key advantage of this tracer method, as previously it has been very difficult to follow the fate of ice- or water column derived algae. We propose to take part in an ice drift across the Central Arctic Ocean (MOSAiC) that will give the opportunity to sample the foodweb and material from sediment traps for subsequent HBI analysis in our lab in Plymouth. We will also determine the body condition of various consumers as an integrator of net benefit derived from each food type over the season. The cruise data set will be complemented with data from other Arctic expeditions and those estimated with a second, independent diet method by our Project partners. This will give a pan-Arctic overview of the importance of ice algae to the lipid stores of key consumers. Then, simulation model outputs of future climate projection will allow scaling up to the whole Arctic Basin. First, we will work with Project partners modelling life cycles of key zooplankton species, to estimate their potential to colonise a future, more ice-free central Arctic Ocean. Second, we will use NEMO-MEDUSA - the oceanic component of the UK's Earth system model (UKESM1) - to determine whether projected increases in pelagic primary production could compensate for loss of ice algae as a food source for zooplankton. Our findings, and those of other participants in MOSAiC, will be used to initiate a "roadmap" for the incorporation of ice algae into NEMO-MEDUSA. By helping to bridge between the physical, biogeochemical and ecological functions of sea ice and requirements of large-scale modelling, we aim to improve our understanding of the changing Arctic and its provision of services to mankind.

Costs of rapid sea level rise globally to infrastructure (houses, roads, and farms, etc.) is likely to be large. A possible source of water for sea level rise is the West Antarctic Ice Sheet, and Thwaites Glacier in particular. Ice sheets and glaciers contain vast quantities of water (in the form of ice) that is continually shed to the ocean, and continually replenished by snowfall (from water that evaporates from the oceans). If the amount of ice that Thwaites Glacier loses to the ocean over the next decades is much greater than the amount it receives as snowfall, then sea level in all the world's oceans would rise, possibly as much as a meter (approximately 3 feet). In order to estimate how likely such a catastrophic scenario would be, we need to better understand the surface over which Thwaites Glacier slides. If we can better characterize that layer ("is to smooth? Is it rough? Is it soft? Is it hard?"), then computer models of Thwaites would be much improved and we can make better projections of the amount of ice that Thwaites Glacier would shed to the ocean.

That our planet is warming is undeniable. Recent increases in greenhouse gas concentrations have seen an associated warming of the atmosphere and oceans, a reduction in the total amount of snow and ice and a rise in sea level of approximately 3 mm/year. Although the precise rate of future temperature rise may be uncertain, there is little doubt that it will increase. In response to a warmer climate, large areas of the Antarctic Ice Sheet could become unstable, resulting in sudden and permanent loss of ice. Indeed for one relatively well-studied region, the Amundsen Sea Sector, this may already be underway. However, our understanding of the processes, the likelihood of collapse and the potential impact on sea-level remains poor, especially in the very different climatic regime of the Weddell Sector. This project aims to address what will happen in the near-future to a region that spans one fifth of Antarctica and the impact changes here could have on global sea-level by the end of this century. We aim to do this in three stages: We will study and understand the intricate relationships between the atmosphere, the ocean and the ice sheet in the important Weddell sector of Antarctica, which contains Filchner Ice Shelf and its catchment basins. We will determine how the atmosphere determines the ocean conditions, and how these in turn determine the melting at the base of the ice shelf. In a carefully designed field campaign we will collect data both to improve the way the models work, and also to validate their results. This first stage will yield a system of models that gives a detailed representation of the physical processes currently at work, and by using the natural variability in the system we will determine the sensitivity to change of each linked process. The next step is to force the boundaries of our modelled system with the best available estimate of the atmospheric and oceanographic properties expected over the 21st century. We will then be in a position to determine how the ocean conditions beneath the ice shelf will change, together with the rate of melting at the ice shelf base. As the melt rate changes, so will the ice shelf geometry: we will determine how the rate of ice flow from the continent responds to these changes, and its impact on sea-level rise. In the final stage we will widen the scope of the study from our large, yet still regional area, to a global context. The models to be used in the first two steps, (atmosphere, ocean and ice) are high resolution, state-of-the-art but limited-area models. We will work with our Project Partner, the Met Office Hadley Centre (MO), to incorporate our improved understanding of processes and their sensitivities within the next generation of global earth-system predictive models. Finally, we will assess the reliability of our predictions. This will be done first by ensuring consistency between the different regional models, run both within the project and by our project partners at the Alfred Wegener Institute (AWI) in Germany. We will then use a limited ensemble of runs of the new generation of MO coupled climate models to quantify the uncertainty in our predictions of the contribution of the Antarctic Ice Sheet to sea level change. The future contribution of the Antarctic Ice Sheet to sea level rise remains the least well constrained component in the budget. By bringing together from across the community leading experts in polar meteorology, oceanography, ice-ocean interaction, glaciology and model uncertainty, this project will provide the largest single improvement in the prediction of future sea level change. New observations and data are essential, but expensive. Rather than using costly commercially-available infrastructure, AWI and NERC will share the logistic burden with the project delivering excellent value as a result.

Quantitative prediction of future sea level is currently impossible because we lack an understanding of how the mass balance of the Earth's great ice sheets can be affected by climate change. Chief among the uncertainties are how changes in ocean circulation and/or temperature will influence the thickness and extent of the ice shelves and how the outflow from the ice sheet will change in response. Observations of the ocean under ice shelves are very sparse and difficult to obtain. Hence, numerical modelling has been used to provide insight into the structure and dynamics of the ocean flow in ice shelf cavities, as well as their influence on the larger scale. However, the complexities associated with this application means that models based upon hydrostatic dynamics, uniform mesh resolution and a layered structure in the vertical, may be improved upon. These complexities include the presence of a grounding line where the water column depth goes to zero under ice deep below mean sea level. The importance of this very limited region to the ice shelf above, and the associated grounded ice sheet, is massive but this is exactly the point where conventional models need to make the largest compromises in representing the real world. Also, the shape of the base of the ice shelf, and the steep change at the front between the ice and the open ocean, place important constraints on the ocean dynamics and hence they need to be represented well in a model in a similar manner to sea floor bathymetry. This, along with the representation of critical buoyancy driven processes that may be of small scale, points towards the use of non-uniform resolution in both the horizontal and vertical directions. In this project we will adapt our state-of-the-art numerical model to study the ocean circulation in the cavities beneath floating ice shelves. Unstructured and anisotropic dynamically-adaptive mesh methods in three dimensions will allow simulations with a resolution and geometric flexibility that is greater than has been possible before. Model developments will be benchmarked against earlier model results and validated on a hierarchy of test problems. Real world applications under the Filchner-Ronne and Pine Island Glacier ice shelves will be used to calibrate and validate the model against observational (including new Autosub) data. Highly timely new science will be preformed in these areas, and this project will also be an important step towards the inclusion of ice shelf cavities in global scale ocean models of the future. The final result will be an improved understanding of the physical processes occurring under ice shelves, and a powerful tool that will enable the explicit inclusion of ice shelves in global scale ocean and climate models of the future. This project fits well with NERC strategy. In particular the prediction of the future contribution of the ice sheets to sea level rise is seen as a high priority goal that cuts across the themes of Climate Systems, Earth System Science and Natural Hazards. Development of the next generation climate models is also a priority for the Climate Systems and Technologies themes.

The geological record offers an invaluable window into the different ways earth's climate can operate. The most recent large-scale changes in earth's climate, prior to modern climate change, were the Pleistocene glacial cycles, which feature growth and disintegration of large ice sheets, rapid shifts in major rain belts, and abrupt changes in ocean circulation. Changes in atmospheric CO2 concentrations, reconstructed from air bubbles in ice cores, are intimately linked with these ice age climate events. Indeed the close coupling of CO2 and temperature over glacial-interglacial cycles has become an iconic image in climate science, a poster child for the importance of CO2 in climate, and the natural template against which to compare current man-made CO2 rise. However despite the high profile of glacial-interglacial CO2 change, we still don't fully understand its cause. The leading hypotheses for glacial CO2 change involve increased CO2 uptake by the ocean during ice ages, which is vented to the atmosphere during deglaciation. However despite decades of work these hypotheses have had few direct tests, due to a lack of data on CO2 storage in the glacial ocean. One of the most glaring holes in our understanding of ice age CO2 and climate change is the behaviour of the Pacific. This basin contains half of global ocean volume, and ~30 times more CO2 than the atmosphere, and so its behaviour will have global impact. It has also recently been suggested that the North Pacific may play an active role in deglacial CO2 rise, with local deep water formation helping to release CO2 from the deep ocean to the atmosphere. If correct, this hypothesis provides a new view of Earth's climate system, with deep water able to form in each high latitude basin in the recent past, and the North Pacific potentially playing a pivotal role in deglaciation. However few data exist to test either the long-standing ideas on the Pacific's role in glacial CO2 storage, nor the more recent hypothesis that North Pacific deep water contributed to rapid deglacial CO2 rise. Given the size of the Pacific CO2 reservoir, our lack of knowledge on its behaviour is a major barrier to a full understanding of glacial-interglacial CO2 change and the climate of the ice ages. This proposal aims to transform our understanding of ice age CO2 and climate change, by investigating how the deep North Pacific stored CO2 during ice ages, and released it back to the atmosphere during deglaciations. We will use cutting-edge geochemical measurements of boron isotopes in microfossil shells (which record the behaviour of CO2 in seawater) and radiocarbon (which records how recently deep waters left the surface ocean), on recently collected samples from deep ocean sediment cores. By comparing these new records to other published data, we will be able to distinguish between different mechanisms of CO2 storage in the deep Pacific, and to test the extent of North Pacific deep water formation and CO2 release during the last deglaciation. We will also improve the techniques used to make boron isotope measurements, and add new constraints on the relationship between boron isotopes and seawater CO2 chemistry, which will help other groups using this technique to study CO2 change. To help us understand more about the mechanisms of changes in CO2 and ocean circulation, and provide synergy with scientists in other related disciplines, we will compare our data to results from earth system models, and collaborate with experts on nutrient cycling and climate dynamics. Our project will ultimately improve understanding of CO2 exchange between the ocean and the atmosphere, which is an important factor for predicting the path of future climate change.