From: Ars Technica
A backward-looking model that incorporates 22,000 years' worth of data could be a more accurate predictor of future sea level rise than many forward-looking ones.
By Jeremy Jacquot | Last updated July 30, 2009 11:45 AM CT
How high sea levels may rise has remained a subject of considerable debate among the climate cognoscenti. Even as their models have grown ever more sophisticated, researchers have struggled to make sense of some of the underlying physical mechanisms driving sea level fluctuations—particularly the response of the Greenland and Antarctic ice sheets to global warming. This, on top of the intricacies inherent to a complex system, has left sea level predictions subject to significant uncertainties.
The most recent report of the Intergovernmental Panel on Climate Change (IPCC), published in 2007, estimated that sea levels would rise by 18 to 59 cm by 2100. It couched its forecast in caveats, however, acknowledging that even the most advanced models did not consider the full range of changes in ice sheet flow and uncertainties in climate-carbon cycle feedbacks. Devising a single model that accurately incorporates the full array of parameters and processes that shape the global climate system has been a continual challenge for researchers, leaving most models focused on a particular variable, such as temperature, around which they center their prognostications.
To tackle changes in sea levels, a team of researchers led by Mark Siddall of Columbia University opted for a different approach, looking to the past by drawing on reconstructions of sea-level rise, derived from fossil corals, which span the past 22,000 years—since the Last Glacial Maximum (LGM). They used this to create a simpler, integrated model of sea-level response, which they detailed in the latest issue of Nature Geoscience.
The advantages of this approach are two-fold. First, because the model covers such a lengthy period, the authors were confident that it at least implicitly includes the contributions from thermal expansion, which occurs when water expands as it heats up, and the changes in glaciers and ice sheet flows. Second, and, perhaps, more importantly, it enabled them to carefully scrutinize the non-linear response of sea levels to large temperature fluctuations on centennial timescales— most current models are only accurate on shorter time scales.
Because temperatures in the Northern and Southern hemispheres do not change in synchrony, Siddall and his colleagues considered two temperature proxy records to track the impacts of temperature variations on sea level: the oxygen isotope record of the North Greenland Ice Core Project (NGRIP), which is representative of the Northern Hemisphere, and the deuterium record of the European Project for Ice Coring in Antarctica (EPICA) Dome C, which is representative of the Southern Hemisphere.
After inputting the data and letting the models run, they found that the NGRIP data tracked the structure of the sea-level record very closely during the different phases of the deglaciation, while the EPICA Dome C data do not, failing even to track the transition to the Holocene, the current interglacial period, which began around 10,000 years ago. The NGRIP scenario estimated 4 to 24 cm of sea-level rise during the twentieth century, in good agreement with the IPCC’s report, and 2 to 6 cm per century before the industrial period. However, it was unable to resolve sea level variations on a decadal timescale.
Projecting their model ahead to 2100, they concluded that sea levels could rise by as much as 82 cm and as little as 7 cm, depending on the warming estimates drawn up by the IPCC (6.4°C and 1.1°C, respectively). When one includes the additional 9 to 17 cm rise that the IPCC estimates will come from accelerated ice-sheet flow over the next century—this raises the upper bound of sea-level rise from 59 cm to 76 cm—their numbers compare favorably to the IPCC’s.
Looking even further ahead, their model predicts that the impact of twentieth century warming will continue to drive sea level rises over the coming centuries. In that sense, the IPCC’s focus on establishing a value for the end of the current century doesn’t provide a complete picture of the challenges.
Nature Geoscience, 2009. DOI: 10.1038/ngeo587