By Gary Griggs, Distinguished Professor of Earth and Planetary Sciences, UCSC
While we know the history of sea-level rise for the past 150 years or so quite well, the future of sea level is much more uncertain, and the farther we go out into the future, the greater the uncertainties. Because sea level is intimately connected to climate, whatever happens to climate in the future is going to drive the level of the oceans.
The warming of the Earth, the oceans, and atmosphere are well documented. As long as we depend upon fossil fuels for most of our energy, we will continue to increase the carbon dioxide content of the atmosphere, and the Earth will continue to heat up. A warmer planet means more ice melt and more seawater expansion, which both raise sea level.
So, how much can we expect? When do we need to start worrying?
Well, there are lots of people who either are or should be worried now. Anybody living anywhere in the world that has had their home, business, community or city damaged or destroyed by high tides, storm waves, El Niño events, hurricanes, typhoons, cyclones or tsunamis should be concerned. There are about 150 million people living now within three feet of high tide, and extreme events like those listed above can push water levels well above high tide. Recent examples of these locations in the USA include New Orleans during Katrina, and the shorelines of New York and New Jersey during Super Storm Sandy.
In the intermediate term, to perhaps 2050, the hazards of these extreme events are going to present greater risks to most of us than the slow, but gradual rise of sea level. We can think of sea-level rise as a ramp, with storms, extreme high tides, El Niño events or hurricanes riding on top of the ramp. The further we go out into the future, the higher sea level and the ramp will be, and the further inland and the greater the elevation these severe events will extend and reach.
How high sea level is likely to be at various times in the future should be of major concern to many coastal communities, states or nations. The future sea-level rise elevations being relied on in California come from a study completed by the National Research Council in 2012 (Sea-Level Rise for the Coasts of California, Oregon and Washington: www.nap.edu/catalog/13389/sea-level-rise-for-the-coasts-of-california-oregon-and-washington).
You can download or read the report on line free, and while its fairly long at 274 pages, the summary is just eight pages.
The report projected sea levels for 2030, 2050 and 2100, along the shoreline of the west coast states, and included all of the existing research, data, and studies to arrive at the most scientifically reasonable future levels.
The major questions that the scientists who put together this report had to consider were:
- How much will each of the major contributors to sea-level rise (ocean warming and expansion of seawater, retreat of mountain glaciers and melting of ice sheets in Greenland and Antarctica) add to sea-level rise by 2030, 2050 and 2100, and what are the uncertainties involved with each?
- How important are short-term contributions to sea-level rise (such as El Nino events, storm surges, and hurricanes)?
There are some who might ask, what difference does a few inches make anyway? Well, the difference between whether sea level rises 6 inches by 2050 or 18 inches isn’t anything to lose sleep over if you are living comfortably most places in Santa Cruz, at least most places at least twenty feet above sea level.
But if you happen to live in the Venetian Courts on the beach in Capitola, at Potbelly Beach, or along Las Olas or Beach Drive in Rio Del Mar, and have been through any of the winters of the past 40 years, you probably think a bit more about the possibility of high tides, storm waves and El Niño events knocking loudly on your ocean view windows and sliding glass doors.
It has been said that prediction is really difficult, especially about the future. And sea-level rise predictions are no different.
There are several different ways that we can make projections for future sea level: 1) extrapolate historic trends; 2) use climate models that incorporate a range of greenhouse gas emissions (carbon dioxide, nitrous oxide and methane); or 3) Develop historical relationships between global temperature and past sea level, and then use estimates of future temperatures to predict future sea levels. The West Coast Sea-Level Rise Committee used a combination of all of these approaches.
The Committee concluded, based on all existing data and information, that we can expect global sea level to rise somewhere between 3 and 9 inches by 2030, with a mid-point or best estimate of 6 inches, compared to levels for the year 2000; a rise of 7-19 inches by 2050, with a midpoint of 12 inches; and a rise of 20 to 55 inches by 2100, with a best estimate of 36 inches.
The farther we go out into the future, the greater the uncertainties become, simply because of the unknowns in things like global production of greenhouse gases. How much more coal, oil and gas will the U.S., China and India use in the decades ahead? And no one knows the answer to that question yet, but this will have a huge impact on how fast sea level rises, what gets inundated and when.
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