Probably the greatest missing element in the public discourse about climate change (to characterize the current rancorous political debate with a more benign phrase) is error bars. Too much of the public expects scientific projections of sea level to be well-defined, but climate scientists know that best estimates reported in the most recent report of the Intergovernmental Panel on Climate Change (IPCC) are likely to be much too low because they neglect “dynamic melting” of the Greenland and Antarctic ice sheets.
The IPCC’s projections are based on melting of the ice sheets from the surface, but there is increasing evidence of melt water flowing through deep holes in the ice sheets known as moulins and showing up below the ice sheets, lubricating their slippage into the ocean. The IPCC’s reported error bars are based on how reliably the surface melt-rate could be calculated and uncertainties in snow accumulation. That gives a relatively small range of error, about equal on the plus and minus side.
But when dynamic melting is factored into the calculations, the error bar on the high side of sea-level rise becomes much greater. Instead of the rise in sea level by 2100 being a dangerous but manageable three feet, the current IPCC upper bound, an increasing number of mainstream scientists are concerned that dynamic melting might turn worst-case scenarios of twice that height or more into reality.
According to an article called “As Glaciers Melt, Science Seeks Data on Rising Seas” by Justin Gillis in the November 13 issue of the New York Times,
Climate scientists readily admit that the three-foot estimate could be wrong. Their understanding of the changes going on in the world’s land ice is still primitive. But, they say, it could just as easily be an underestimate as an overestimate. One of the deans of American coastal studies, Orrin H. Pilkey of Duke University, is advising coastal communities to plan for a rise of at least five feet by 2100.
The article adds this blunt warning from Australian scientist John A. Church, a leading expert on sea level:
I think we need immediately to begin thinking about our coastal cities — how are we going to protect them? We can’t afford to protect everything. We will have to abandon some areas.
It’s those darned asymmetrical error bars he’s talking about. I’m on record as saying that some people who know the science well can go overboard with their scenarios. But I criticize them not because the scenarios are totally out of bounds, but rather that they move critics to shout “alarmist” even when more reasonable projections like Pilkey’s appear in the press.
Even the best case scenarios are worrisome, and the ones slightly on the high range of possibility call for prompt and dramatic action. The time for wishful thinking about sea level rise is past.