I was lucky to be out of town for a week during “the greatest statewide heat wave ever recorded in California.” When I arrived in Seattle, I was quickly informed that they had just set a record of 55 consecutive days without rain—and that the record would still be increasing had it not been for 0.02 inches late one night a few weeks earlier. Seattle has also set a number of heat records the past four summers. The same people that bragged about this “beautiful weather” scoffed that I believed in climate change. They asserted that no sea level rise would occur during our, our children’s, or our grandchildren’s lifetimes because, 1) Puget Sound was not really part of the ocean; and 2) those NOAA flood maps are “bureaucratic bullshit”.  These same people live on the water in homes that are a few feet above current maximum high tides. Days later we all swept ashes off decks while marveling at the sun, which was reduced to a rosy red disc by smoke from a record 68 large uncontained fires burning across the West.

Astounded by the number of homes, roads, and railroad tracks located just toe-dipping distance above Puget Sound, I set out to learn about sea level rise, talking to experts and reading published studies and reports from the Intergovernmental Panel on Climate Change (IPCC) .  Here are the basics.

• Increases in sea level lag quite a bit behind climate change. We set the record for the warmest year on earth in 2016, breaking the record from 2015, which broke the record from 2014. For something as variable as weather, which has all kinds of ups and downs, this kind of consecutive record-breaking suggests runaway global warming. It is dramatic. But not so with sea levels.
• Sea level rise is a function of several different factors:
  1. Thermal expansion: This happens because, like air or most anything else, water expands when it is warmer, taking up more space. As ocean temperatures increase, they bulge up a bit, and the sea level rises. This can be quantified with pretty good precision and is already occurring. In fact, this explains nearly all of the sea level rise currently underway.
  2. Antarctic Ice Sheet: While melting ice in the Arctic Ocean affects weather and ocean currents, it doesn’t add to the sea level because the ice was already in the ocean to begin with. An ice cube that melts in a glass of water does not change the water level. But glaciers that are on land, like in Antarctica, will flow into the sea when they melt, thus adding to sea levels. They are like an ice cube perched on the edge of the glass, melting into it.
  3. Greenland Ice Cap: While not as big as the Antarctic Ice Sheet, the Greenland Ice Cap will melt faster. In fact, it is already becoming the next big contributor to sea level rise.
  4. Other glaciers and other factors: Smaller glaciers from Alaska to New Zealand are melting, and also adding to sea level.
• Sea level rise is underway, currently at a rate of 3.2 mm/yr (about a foot per 100 years). It has already risen 2 ½ inches since the year 2000, and 6 ½ inches since 1900.

Sea level rise as measured by NASA satellites, 1993-present.

• The rate of sea level rise is increasing as ice melt from Greenland, Antarctica, and other glaciers begin to contribute. The standard practice is to estimate total sea level rise by the year 2100 as compared to 2000. Because the rate is increasing, it will certainly be more than the one foot described above, because ice melt from Greenland, Antarctica, and other places is just beginning. However, estimating that increase is difficult. The 2013 report from the IPCC estimated total sea level rise by 2100 at 1 foot to 3.2 feet, depending upon assumptions about CO2 levels. Because Greenland and other Arctic glaciers are melting faster than anticipated, the IPCC report has come under criticism from scientists, who have now adjusted their estimates up to 1.7 feet to possibly 6 feet by 2100. (Revision from November 2017:  A new study estimates sea level rise at 6 to 11 feet by 2100.)

The projected range of sea level rise, depicted in scale over a waterfront home.

• The rate of sea level rise will continue to increase at an increasing rate for several hundred years. The Antarctic Ice Sheet and Greenland have a lot of ice, and melting takes time. This melting, which is only just beginning, will increase with time, but may still take hundreds of years to really pick up steam. This appears to be “virtually certain”. The Antarctic Ice Sheet is now considered to be “past the point of no return”, with large scale melting “unstoppable.” Still, the exact timing is unknown. It will begin slowly but then suddenly increase rapidly, possibly during this century. This caveat is included in all predictions. The “conservative” estimate is that ocean levels will rise 3 to 10 feet by the year 2300, depending upon future CO2 levels and temperature increases. At that point, however, it will still be rising at a rate more than double the current rate. It thus appears that a total sea level rise over 10 feet, largely if not entirely due to anthropogenic CO2 emissions, is inevitable in the long run.
• In the next few decades, sea level rise will be mostly felt during acute events, such as during high tides or large storms, or a combination of the two. This will, unfortunately, cause my friends in Puget Sound to attribute their flooded living rooms to unusually high tides or large storms, but not to rising sea levels.

To examine flooding in Puget Sound under various levels of sea level rise, you can surf a map and toggle the level of sea level rise at this interactive website.