- SEA ICE AND POLAR OCEANOGRAPHY
Sea ice is an important component of the global climate, e.g. it strongly impacts atmosphere-ocean heat fluxes, greatly increases the Earth’s albedo and contributes to thermohaline forcing of the ocean. Sea ice is also a sensitive indicator of climate change: there have been well-publicised, strong reductions in Arctic summer sea ice extent in the last decade.
Existing climate models have difficulties in accurately simulating the magnitude of sea ice changes in the Arctic and in the Southern Ocean. Measuring, understanding, and predicting changes in sea ice and the polar oceans requires a broad range of expertise and is essentially interdisciplinary. In many cases, uncertainty in the underlying physics of sea ice is a limiting factor in understanding and predicting its evolution and we conduct research devoted to improving the physical realism of models of sea ice. The ocean circulation below the sea ice plays an active role in inter-annual Arctic change, especially through heat and freshwater transports into and around the Arctic. Ocean reanalyses assimilating both ocean and ice observations into models are important for both understanding and improving models and for making initialised predictions on seasonal and longer timescales.
The fast changing seasonal ice cover, especially in the Arctic, makes accurate simulation of sea ice an active research area with major implications for climate change across the northern hemisphere and for future human activities. While the rapidly changing Arctic provides many challenges, it is also viewed as an opportunity by some businesses. For example, there is an increased potential for tourism, cross-Arctic shipping and extraction of natural resources. Although there is a necessary policy debate about whether such activities should be allowed in the pristine Arctic environment, there will also be a need to predict the state of the Arctic from season to season to ensure that any allowed activities are performed safely. Our group is engaged in research to understand how predictable the sea ice is, and to provide information to policymakers and businesses about how a changed Arctic may affect activities within the region. We use state-of-the-art climate models to examine the range of possibilities for the future Arctic and collaborate with other centres producing predictions.
Much of our research, although quite specialised, is collaborative and contributes to broader-themed research projects. Our sea ice and polar oceanography research broadly falls into the following areas:
- Sea ice thermodynamics – including mushy layer theory, ice-ocean interface dynamics, consolidation of rafted ice, freezing and melting/dissolving, melt pond formation and spreading and impact on albedo and Arctic mass balance.
- Sea ice dynamics and rheology – including topographic form drag, edge jets and kinetic granular flows in the Marginal Ice Zone, evolution of the floe size distribution and its impact on the sea ice mass balance, anisotropic sea ice rheology and its representation in climate models, discrete element modelling of floe aggregate formation, and experimental determination of the relationship between sea ice rheology and friction.
- Interaction of ice with the polar ocean – including frazil ice formation and mixed layer properties, mixed layer evolution in Antarctic continental shelves, salinity-driven deep water formation, ice shelf-ocean interaction and marine ice deposition, numerical simulation of density-driven flows.
- Calibration and validation of sea ice climate models with satellite data products and other observations, sensitivity and feedback analysis, and understanding the proximate causes of sea ice changes observed in the last several decades
- Incorporation of new physics into sea ice climate models – maintaining a developmental version of the Los Alamos CICE sea ice climate model component allows us to make Arctic climate predictions and provides a route for “pull through” from fundamental process studies to climate models.
- Ocean reanalyses in the Arctic and Antarctic are being assessed against both assimilated and non-assimilated observations in order to assess the quality of different products. Energy and freshwater budgets in the Arctic are then being assessed as part of a larger project on the Earths energy and water cycles.
- Dynamics of the Southern Ocean and its response to climate change: The Southern Ocean stores vast amounts of heat, carbon, freshwater etc., and dominates the oceanic uptake of anthropogenic heat and carbon. This property gives it an oversized impact on the global climate variability. We study the dynamics of the large-scale Southern Ocean circulation (Antarctic Circumpolar current, the meridional overturning circulation), its interactions with (sub)-mesoscale eddies but also with other climate components (atmosphere, sea ice, ice shelves)
- Inter-calibration and bias correction of sea ice in climate models using recent observations allows for better constrained multi-model predictions of changing sea ice through the 21st century. The future viability of ship traffic in the Arctic is also being assessed as sea ice conditions change, with implications for economic planning and also for regional environmental and climatic impacts eg. through ship emissions.
- ICE SHEETS AND SEA LEVEL
The ice sheets are important components of the Earth system, due to their influence on climate and sea level. The two ice sheets of the modern world, which cover most of Greenland and Antarctica respectively, are both vulnerable to mass loss in a warmer climate, and this makes them the largest potential contributors to sea level rise due to anthropogenic climate change over coming centuries and millennia. The dominant processes are different in the two cases. Around the margins of the Greenland ice sheet, although the climate is cold on the annual average, there is substantial melting during the summer, which has increased in recent years. This tendency is predicted to continue in a climate of higher greenhouse gases, causing attrition and retreat of the ice sheet. To model it realistically requires attention to the albedo of the surface (its reflectiveness to sunlight), which gets darker as snow melts and then refreezes as ice, and some way of representing the relatively narrow and sloping sides of the ice sheet, where the melting mostly occurs. As the ice sheet becomes smaller, its surface will get lower, and its surface climate will become warmer still as a result (because temperature is generally higher at lower altitude). This elevation-temperature effect will be a positive feedback on mass loss. Over many millennia, the Greenland ice sheet could be virtually eliminated in a sufficiently warm climate, raising global mean sea level by about seven metres. This could be irreversible, since the regional climate of Greenland with a smaller ice sheet might be too warm for the ice sheet to regrow, even if global climate reverted to its pre-industrial condition.
In Antarctica, the climate is too cold for significant melting of the ice sheet on land, and this will remain the case even with substantial global warming. Because of that, the ice flows down to the coast, and continues as ice shelves, floating on the sea. These ice shelves fringe the continent, in some areas extending hundreds of miles from the coast, until they break up into icebergs. Some surface melting occurs on the ice shelves, but more melting occurs on the underside, where the ice is contact with the ocean. Even rather small warming of the ocean could increase this melting substantially, so that the ice shelves become thinner. Because the ice shelves act as buttresses for the ice sheet on land, thinning of the ice shelves may lead to more rapid discharge of ice from the ice sheet. This process has increased the contribution of the Antarctic to sea level rise over the last decade, and it might lead to much greater increases in coming decades and centuries, possibly stimulating in some regions a retreat of the ice sheet that could not be stopped once it had begun. Consequently the future of the Antarctic ice sheet is the largest uncertainty in sea level projections. Emissions of greenhouse gases during this century could inevitably lead to tens of metres of global mean sea level rise from Antarctica over following millennia.
To address the scientific uncertainties involved in these issues, we develop and use Earth system models that include the ice sheets, with surface snowfall and melting, and melting and freezing at the base of the ice shelves. As the ice thins, thickens and flows, the topography of the ice sheets changes, and this affects the atmospheric circulation and regional surface climate. We evaluate these models as far as possible with contemporary observations, but we have not observed ice sheet changes of the magnitude of those projected in coming centuries as a result of anthropogenic climate change. Consequently as a test and for refinement of our understanding it is also valuable to use the same models to study past climate change. During the glacial periods (ice ages), there were also large ice sheets covering most of North America and Scandinavia, causing global mean sea level to fall by over a hundred metres with respect to present. We try to simulate the growth and retreat of these ice sheets between their inception 120,000 years ago and their termination 10,000 years ago, using proxy evidence of their extent and volume for evaluation.