Why today’s National Climate Assessment understates sea level risk
Today the US Government issued the National Climate Assessment. It has been covered extensively in the media today from the Wall Street Journal to the environmental press. It is a well done, sobering report, that describes how things are getting worse and what is at risk in very understandable stark terms.
Yet ironically, the one topic where they largely miss the concept of risk is rising sea level. It is worth understanding why. Last week Mark Trexler and I published an article in GREENBIZ titled, “Are Rising Seas a Business Risk? Don’t Ask the IPCC” referring to the international scientific body, the International Panel on Climate Change, organized by the U.N. Largely the same limitations apply to the National Climate Assessment (“NCA”).
Briefly here is the problem: The scientific community is cautious. They are extremely reluctant to state anything they can’t quantify and prove with a high degree of accuracy and certainty — perfectly understandable. But they are not being asked to assess “risk.”
It is worth understanding the difference between known, quantifiable fact, and the concept of risk.
There is no doubt that the oceans have warmed over the last century. The warmer ocean means the ice sheets and glaciers will keep shrinking to levels that have not happened in 120,000 years, raising sea level, moving shorelines inland. There is wide consensus on those points.
The problem is the “tipping points”, the positive feedback loops. Those are very hard to pinpoint in terms of what year they will occur, and even harder to quantity.
By contrast there are some factors of sea level that we can quantify, such as the thermal expansion as the oceans warm; that is a simple equation – but only amounts to a modest amount this century, several inches, not several feet. The disappearing glaciers — more than 170,000 in the world — are also relatively easy to quantify, yet all of them combined would only raise global average sea level by two or three feet.
It is the ice sheets of Greenland and Antarctica that hold enough ice to raise global sea level by more than two hundred feet. While it will take centuries, or possibly millennia to melt all the ice, it is very difficult to quantify how much of those ice sheets will melt this century. Even partial collapse of one of the major ice sheets will be catastrophic, causing sea levels not seen in human history.
Estimates for rising sea level presently range from as little as one foot to sixteen feet by the year 2100. Even a few feet of higher sea level will be catastrophic. Since the Earth is warming to a level not seen in millions of years, it is very difficult to say with precision, how high sea level will rise in a particular decade or even by mid century or the year 2100.
Thus without intending to “low ball” the figure, scientific methodology often puts a low figure on rising sea level rise. Essentially they omit the “wild cards”, those variables that have too much range to quantify with certainty. So, they just leave them out — they are covered in the text, but not in the summary figures and graphs, including the graph of 1-4 feet in today’s NCA.
As Dr. Trexler and I pointed out in the GREENBIZ article mentioned, the IPCC report — and the NCA released today — do not focus on risk. They look at factors that can be quantified with a high degree of certainty. Here are a couple of other situations to see the difference. There is no certainty that an earthquake or tsunami will occur in any location this decade, or even this century. But in many areas we plan for it and design buildings for it, because the results WHEN it happens will be disastrous. Another risk that we are all familiar with is fire. The risk of a building in the US having a fire this year, requiring the fire department is about one in three hundred — a very low risk. Yet we have fire insurance because if, or when it happens, the damage is so devastating.
The risk of high sea level rise, perhaps defined as ten feet this century– can not be quantified, largely because of the uncertain instability in the Antarctic ice sheets. Yet many experts believe that the chance of it happening is very real, even if they can not definitively say whether it is a two percent or ten percent risk. We simply do not know. The fact that we can not quantify the risk is no reason to pretend the risk does not exist.
Rising sea level is now inevitable because we have warmed the ocean — about one and half degrees F over the last century. The ice sheets will melt as they did hundreds of thousands of years ago. We have decades to adapt to higher sea level, but the enormity of the task is so great that there is no time to waste. The time to begin planning is now.
The first step is for each of us to understand and explain to others the difference between scientific projections, like those in today’s National Climate Assessment, and risk. Once we understand that distinction, we need to explain it to wider audiences. It is even worth explaining to the scientific community, who largely miss the chasm of communication that presently exists. Today’s report is a good example.