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climate simulations Another key area into which the project is


putting a great deal of effort is the addition of more realistic processes. Looking at the development history of climate models, there has not only been an increase in resolution, but also more of the physical processes occurring in the climate system are included. Te earliest models simply considered the atmosphere, while the ocean was just a fixed temperature and surface. Now we include chemical reactions that occur in the atmosphere and affect cloud formation and ozone formation.


In future models, we’re looking at including both many of the chemical species in the atmosphere and any equations for how those species interact. Climate models will also include the behaviour of the biosphere; plants and plankton have a big influence on the carbon cycle and if we want to understand our own impact on that cycle, our models need to represent the natural carbon cycle. Science proceeds through the interplay


between theory and observation, and finding climate-quality observations, which means


having a good spatial coverage and very long time length, is incredibly difficult. As a community, we are actually quite concerned that there’s going to be a significant gap in satellite coverage as many of them are about to go offline. Tere may be a lowering in the amount of information and data we receive until replacement satellites are deployed, but there aren’t many of them waiting in the wings. Unfortunately, this is one of those issues that we will have to deal with when and if it happens.


Donald J. Wuebbles, The Harry E. Preble Professor of Atmospheric Sciences at the University of Illinois, US


level of detail they provide. Instead of focusing on the atmosphere and oceans, these models now include representations of all the different processes that affect the climate system, such as atmospheric chemistry and ice interactions. Te models don’t always do this successfully, and we will certainly need to improve them further during the next decade, but they are definitely off to a good start. Current models that have completed runs for the next international assessment are grouped together under CMIP5, the Coupled Model Intercomparison Project Phase 5. We have been analysing around 30 models and while they have, on a general level, improved since the 2007 United Nations Intergovernmental Panel on Climate Change (IPCC) report, the basic findings are pretty similar. Te lack of some of the major changes within


W


the models that we had hoped to see comes down to the need to get everything completed in time for the current assessment. It is always a little disappointing that we aren’t able to take all of the major steps forward, but time constraints have meant that proposed changes weren’t made or that models weren’t run at as high a resolution as we would have liked. Nevertheless, we are making progress and getting closer to fully representing Earth systems. As we are doing so, the basic message of the concerns about climate change remains the same – this is one of the biggest issues confronting humanity this century. Speaking for here in the US, the models


generally do a great job. Te representations of the Earth do indeed look like the Earth, the temperature changes are about right and the average precipitation changes are fairly accurate. Where the models fall down, however,


42 SCIENTIFIC COMPUTING WORLD


hat used to be termed ‘climate models’ are now being viewed as Earth system models, because of the


is in dealing with extremes. Most models, for example, fail to predict enough change in extreme precipitation. Tat is a little worrisome, but our focus now is on identifying those models that do offer these predictions, as well as all the other climate aspects. We expect to find four or five models that fall into this category, but most – especially when getting beyond two sigma – aren’t there yet. Of course, the climate community is still trying to determine how to align the models. In some cases, the issue can be as simple as


the fact that the model isn’t treating radiative transfer accurately (the absorption, reemission and scattering of radiation from the Sun and from the Earth). Five years ago, a comparison was made of how a variety of models treated atmospheric radiative processes, and the results


are beginning to do runs on the new petascale machines, such as Blue Waters at the University of Illinois, and even without some of the improvements fully represented in the model yet, such as the treatment of ice, I do believe there will be an overall much better representation of the Earth. Higher resolutions that are as fine as 25km will provide us with far more detail on regional and local scales than is currently available with global climate models, but they do come with their own sets of problems. Initial runs at NCAR found that while these higher resolutions did improve a lot of aspects, they also found that


THE BASIC MESSAGE OF THE CONCERNS ABOUT


CLIMATE CHANGE REMAINS THE SAME – THIS IS ONE OF THE BIGGEST ISSUES CONFRONTING HUMANITY THIS CENTURY


were very surprising in that many of the models were not very accurate. We also know that when it comes to aspects such as sea ice and land ice, current models aren’t capturing the processes correctly. Within the next five years, we hope that the relationships between ice on land and in oceans, and the effects of atmospheric particles, will all be greatly improved. One thing we are already beginning to see is that the treatment of surface hydrology and how models represent what’s happening with rivers and lakes are getting better. Beyond that, we want to see models reflect the impact of agriculture on the land and, also importantly, how urban environments affect the climate system. With our research partners at NCAR (Te National Center for Atmospheric Research), we


subgrid parameterisations needed to be further improved. Te core of the models are changing and


moving away from the linear grids of the past and going to grids that are better able to handle the message passing and relationship between one node and another. In terms of the core structure of the models, this will be a substantial transition. Te question is what type of runs we will be doing ten years from now and how we should be designing our codes in preparation. Tere is a lot of uncertainty when it comes to exascale and over the next year we want to do a run with the NCAR Earth system model to look at using the proposed coding aimed at changes needed for running on an exascale machine. Interviews by Beth Harlen


www.scientific-computing.com


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