John’s Blog/Newsletter: SEA LEVEL RISE NOW

Ten to thirty inches of sea level rise this century. That is what the latest report of the Intergovernmental Panel on Climate Change (IPCC) says. We should be so lucky. Those summary ‘headline’ numbers are misleading to the point of being irresponsible.

I don’t mean to be disrespectful of the IPCC’s generally excellent review of climate science, but the way they portray sea level rise is deceptively low, just like their last report issued in 2007. This really is worth understanding and sharing so that we can realistically assess what is possible for sea level rise in the decades ahead.

Let me explain  what the IPCC includes in those figures, what they don’t, and why many scientists think they are creating a very wrong impression.  Even most journalists and policy makers are left with the impression that three feet of sea level rise (“SLR”) is the worst that could happen this century. That could very well be the best case on our present course. (BTW, even three feet of SLR would be a huge problem, considering that the shoreline moves about 300 feet inland for each foot of SLR, as a global average.)

The 10-32 inches (26-82 cm) cited in their table SPM2 is a just a summary of the chapter about sea level, but it is as far as most readers get. You can see the full report online, or I will reproduce the relevant part of that table here.

The range of  projections covers four different scenarios as to what might happen this century. Most do not read the accompanying pages of text — the fine print.

These numbers only include the effects that are so certain that it can be said they will almost definitely occur by the year 2100, with a 95% confidence, a very high “bar” to reach. The variables that meet that certainty are:

• Thermal expansion of seawater as the oceans continue to warm from higher levels of greenhouse gases
• Melting of most of the world’s glaciers, more than 170,000 of them on six continents (none on Australia)
• The quickening pace of melting of the Greenland Ice Sheet (the flat two mile thick ice sheet covering about 90% of the island, distinct from the hundred or so glaciers on the perimeter)
• Continuation of the modest melting of Antarctica that started a few years ago.

Here is what those numbers do not include:

1. Accelerated, nonlinear, melting of Greenland. Looking back over thousands of years of record, sea level rise often takes big jumps that our recent times do not allow us to accurately predict. For example, about 14,000 years ago, sea level rose approximately 65 feet (20m) in just four centuries. While it is true that there was a lot more ice on the planet to melt back then, it proves that even by natural cycles, we can not yet put all the factors into the models. There are many reasons to expect some nasty surprises in the melt rates of Greenland this century.

2. West Antarctica has some glaciers that go far underwater, rather than the typical glacier that rests solely on land. These “marine glaciers” are very unstable and prone to much more rapid melting due to the higher heat content of seawater compared to air. As I describe in my book, “High Tide On Main Street” (page 58 of the 2nd Edition) two glaciers in West Antarctica have the potential to raise global ocean levels about ten feet and could happen over a period of a decade or so. We simply do not know if that could happen this century yet with a high confidence, so they omit the possibility from the numerical table that most look at. In the text — the “fine print” — they do note the possibility but relegate it as something of only “medium confidence”

3. Methane has a far greater warming effect than carbon dioxide. In fact, averaged over twenty years, each molecule of methane is 86 times more powerful as a warming agent than CO2. Even most scientists have not recognized this incredible potential for warming. That updated figure of eight six comes straight out of the latest IPCC report. But again, because they can not quantify how much methane will get into the atmosphere by the year 2100, they simply omit it when calculating how fast things can warm, and how high sea level can rise.

If those factors kick in this century, we could get sea level rise of three feet by the middle of this century and could very well get sea level rise of six feet, or more by the year 2100. Experts like Dr. Stefan Rahmstorf and Dr. James Hansen argue that the IPCC needs to follow “semi-empirical” models to look at what is possible this century, rather than strictly what we know will happen for sure based on recent measurements, with the 95% confidence that the IPCC is using.

I appreciate that the IPCC has a dilemma. They do not want to scare with things that are just possibilities. They want to cite things that will occur using the highest levels of certainty. But as a result they are implying that the outlook is much better than it truly is. The best proof of that is their own track record. If we look back at their projections for sea level rise from 1990 and 2000, we can see that there are consistently low (Graphic adapted from Rahmstorf et al 2012). As you can see on this chart, actual sea level (the gold line) is near or above the highest projections done by the IPCC for the same time period, the last two decades. This is the same methodology that now has them predicting a range of 10 – 32 inches for the rest of this century.  It is fair to assume that the future projections will also be on the low side.

For more information on this, see the revised and updated 2nd Edition of my book “High Tide On Main Street: Rising Sea Level and the Coming Coastal Crisis” available in print and e-book on Amazon.

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