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Carbon Brief Staff

22.11.2013 | 6:30pm
ScienceIPCC sea level rise projection probably too low, says expert survey
SCIENCE | November 22. 2013. 18:30
IPCC sea level rise projection probably too low, says expert survey

A new survey of expert opinion suggests 21st century sea level rise might be higher than the latest UN climate report projects. More than two thirds of the researchers interviewed for the study said higher and faster rises are possible – implying the report’s estimates could be too conservative.

Upper limit underestimated

The most comprehensive projections on sea level rise are those contained in the report from Intergovernmental Panel on Climate Change (IPCC). The IPCC evaluates evidence from all the published literature and combines the estimates into a single set of projections.

In its most recent report, the IPCC predicted sea levels are likely to rise by between 0.28m and 0.98m by 2100 – a range encompassing both its highest and lowest emissions scenarios.

But according to a new survey of sea level experts, that range might be an underestimate. Scientists from the Potsdam Institute of Climate Impact Research (PIK) asked 90 researchers from 18 different countries for their expert opinion on future sea level rise.

Two thirds of those questioned said they thought sea levels could rise higher than the IPCC’s upper estimate for the end of the century.

When asked what they thought the likely range of sea level rise would be under a low emissions scenario, the experts came up with similar estimates to the IPCC.

When it came to predicting sea level rise under a high emissions scenario, the experts’ assessment was considerably higher. They projected a maximum sea level rise of 1.2m by 2100:

Low High Em Table

Modelling sea level rise

The reason the expert assessment and the IPCC figures don’t match up is because there are different ways of predicting sea level rise.

The IPCC relies on ‘process models’ to work out how a number of factors affecting sea level rise might change in the future. That includes, for example, predictions about different mechanisms leading to ice loss from the ice sheets in Antarctica and Greenland, and how much the oceans will expand as temperatures rise. The process models sum up each of these estimates to come up with a total estimate.

But that’s not the only way of anticipating what could happen. A number of researchers use what are known as ‘semi-empirical models’, which make predictions about future sea level rise based on the way it fluctuated in relation to past temperatures. And these models suggest higher levels of change.

The IPCC doesn’t use include these semi-empirical models in its estimates, however, as scientists can’t be certain how long the relationship between temperatures and sea levels, which these models are based on, will hold true. In its recent report, the IPCC concluded:

“There is low agreement in their projections and no consensus in the scientific community about the reliability of semi-empirical models’ projections”

The decision not to include sea level rise projections from semi-empirical models led some researchers to conclude the IPCC was being too conservative in its estimates. The findings of this  new study may confirm that sentiment.

While the IPCC doesn’t attach much weight to the high estimates of sea level rise calculated from semi-empirical models, it doesn’t rule out higher projections of sea level rise altogether.

The IPCC report says a collapse of a large part of the antarctic ice sheet could cause global mean sea level to rise substantially above the likely range during the 21st century. But scientists aren’t confident yet how likely that it to happen – and so don’t include it in projections.

Gap remains

Projecting sea level rise is a work in progress. There have been enormous improvements in scientists’ understanding of the physical processes linking temperature rise, ice melt and sea level rise since the previous IPCC report back in 2007, and scientists have more confidence now that their estimates are realistic.

But this new survey confirms there’s still a gap between what some scientists think the worst case scenario could be, and what the models used by the IPCC project.

 

Horton et al. (2013) Expert assessment of sea-level rise by AD 2100 and AD 2300. Quaternary Science Reviews. DOI: 10.1016/j.quascirev.2013.11.002

 

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