We are asking the wrong question about rising sea level, usually stated as:
'How high will sea level get by 2100?', or some other year. While it seems reasonable, it is the wrong focus. There is a better way to frame the question. First, let's review why we can't say exactly how high sea level will be at any specific year in the future:
In the present era much of the rise in ocean height comes from the tens of thousands of glaciers that are melting. In addition to the glaciers there is the big ice sheet on Greenland that is melting faster each decade. Antarctica is starting to go into "melt mode" as well. As the ocean warms, the sea water actually expands very slightly, further adding to the higher ocean. All of those things add to the height of the ocean. Lastly, the local sea level effect can vary greatly depending on land uplift or subsidence.
The fundamental force melting the ice is the warming at the higher lattitudes. Even though the models continually improve, because we are warming faster than in millions of years, the models are almost certainly imperfect. Perhaps more important, we can only make estimates for how much we will warm the atmosphere this century for another key reason. The calculations depend on variables like population, per person energy useage and how we make energy -- for the next 86 years. No one can possibly know those inputs, particularly whether we will be using solar, nuclear, coal, or tar sands, even fifty years from now.
As a result there is no certainty for the exact amount of warming, for the amount of ice that will melt, and how high sea level will rise. Estimates for sea level rise this century now range from one foot, to six feet -- and possibly even higher. It depends on the factors cited above.
Rather than waiting to see how accurate one model is compared to another, and waiting to see what will happen with energy sources, we need a way to start planning for inevitable sea level rise now. Amid all the talk about climate change, preparing for the next "Sandy" and being more "sustainable", a key fact has largely gone unnoticed: It is that it is too late to stop sea level from rising. The oceans are one and a half degrees warmer (F) than a century ago. The ice sheets and glaciers must continue to get smaller. That will directly raise sea level for centuries. That has a multiplied effect to move the shoreline inland, about 300 feet inland for each foot of rise, as a global average.
I now have a suggestion for how to plan, given that uncertainty for how high sea level will be by a specific year. We should start looking towards the future when sea level will be three feet higher than now, even though we are not sure exactly when that will occur.
It surprises many to learn that we will eventually get three feet of sea level rise, regardless of carbon emisssions and other greenhouse gases. In fact it is now almost inevitable that sea level will rise much higher. The excess heat in the ocean essentially guarantees that sea level will just keep rising ten feet and beyond. (For a video about the scientific basis, see http://youtu.be/RaD3ax2j3Ks )
It is not too late to slow the rise, depending on how effective we are at stopping the warming and restoring the CO2 level to the historic range. But given the heat already stored in the ocean and the melting already happening to the glaciers and ice sheets, it is time to adapt to rising sea level. That is the basis for my belief that we will plan better if we start focusing on how to adapt to three feet of sea level rise, without waiting for the definitive answer as to when it will happen. If we are successful with our green and sustainable efforts, it might be a century before we pass the three foot level. On the other hand, if we hit some "tipping points" with the collapse of a few West Anarctic glaciers and the hyper warming from methane, we could see three feet of sea level rise in decades. That is the range of uncertainty leading me to say that we should change the question.
Three feet is a big step upwards. It will challenge us to do better engineering, architecture, and planning. The effects will be profound. But if we plan for the future we can get a better return on our investments. Also if we plan for three feet we will be better prepared for extreme tides and storm damage. And while we are getting our minds around three feet of sea level rise, we can begin looking ahead to the continued rise that will surely follow that amount.
Adapting to rising sea level does not mean we should stop our efforts to slow the warming, the so called "mitigation" efforts to reduce greenhouse gases. We need to be doing both simultaneously.