Got your phone handy? Over the last month, coastal residents from Hawaii to Rhode Island wielded their smartphones and snapped dozens of shocking photos at high tide showing neighborhoods, parking lots, and public parks underwater. Meanwhile, scientists have published a spate of sobering sea level rise studies. We spend hours cradling our phones in our hands…let’s put them to use for a moment (screens off!) to put the latest sea level rise science into perspective.
How fast is sea level rising?
Hold your phone flat at eye level. If you’ve got an iPhone 6 or 7, it’s about 6 mm thick (Androids are a little thicker, at 7-9 mm). The latest research published in PNAS by Sönke Dangendorf and others, based on both tide gauge measurements and satellite altimeter data, shows that, averaged over the globe, the sea level is rising by just over 6 mm every two years. That amounts to 3.1 mm/yr.
There are a lot of wiggly lines here, but focus in on the solid black line labeled GMSL [this study, all corrections] in panel A. Over the course of the 20th century, the slope of that line–which is essentially what’s shown in panel B–increases. That’s the recent acceleration in the pace of sea level rise. Source: Dangendorf et al. 2017
Is sea level rising faster than it used to?
Yes. Take a look at the home button on your iPhone. Dangendorf’s study shows that over the course of the 20th century, sea level rose by an average of 1.1 mm/year. So every decade during the 20th century, sea level rose by the width of the home button on your phone (1.1 cm wide). And over the course of the 20th century, it would have taken about 6 years for sea level to rise by the thickness of your phone compared to just two years currently.
So the 20th century average sea level rise rate was 1.1 mm/yr, but now sea level is rising at 3.1 mm/yr. This means that, in the last 25 years or so, sea level rise has accelerated dramatically. Our appreciation of just how dramatic this acceleration is has been growing over the last few years.
As of the writing of the last IPCC report in 2013, the widely quoted 20th century sea level rise rate was 1.7 mm/yr. Compared to the present day rate of 3.1 mm/year, that implied some recent acceleration. But the latest estimates of 20th century sea level rise are significantly lower, which makes the difference between then (1.1 mm/yr) and now (3.1 mm/yr) starker.
Estimating 20th century sea level rise rates has long been a challenge for earth scientists. For one thing, sea level does not change uniformly around the globe. And tide gauges, which were the primary basis for sea level rise measurements until the early 1990s, aren’t evenly distributed on all coastlines. The farther back in time you go, the more these problems compound because there are fewer tide gauge records to rely on.
Locations of tide gauges around the world. Source: The Global Sea Level Observing System
So over the years, scientists have used a variety of methods to try to account for the spotty nature of tide gauge observations to come up with a single global average. Dangendorf’s new estimates are well-aligned with those published by Carling Hay and others in 2015 despite using very different methodologies, which suggests that we’re homing in on the right number for the 20th century, and experiencing a much faster rise in sea levels than our parents and grandparents.
Even faster sea level rise in store
The rapid sea level rise we’ve been experiencing for the last quarter century was the impetus for NOAA to revise its baseline estimate for future sea level rise through the year 2100. Back in January, NOAA released a new set of sea level rise projections that are being used for the Fourth National Climate Assessment.
This suite of projections was developed by NOAA scientists for use in the forthcoming Fourth National Climate Assessment. Source: Sweet et al. 2017
So pull out that phone again! The new lowest sea level rise scenario from NOAA projects 0.3 m (1 ft) of rise above 2000 levels by 2100. That’s just over the height of 2 iPhones stacked up end to end. And it represents a 10 cm increase compared to the projections used for the Third NCA report in 2014.
Even if global greenhouse gas emissions were to peak before 2020 and decline thereafter, NOAA scientists report that there’s a 94% chance of sea level exceeding that 0.3 m rise.
Because the most recent research coming out of Antarctica points to a potentially large contribution of Antarctic ice to sea level rise this century, NOAA has also added an extreme sea level rise scenario that projects 2.5 m (about 8 ft) of rise. That’s 18 iPhones stacked up end-to-end, which would reach from the floor to the ceiling in an average room.
What that all spells is more coastal flooding
Several studies have shown that sea level rise in the coming decades will increase the frequency of “sunny day” flooding of the sort that much of the US experienced last weekend during king tides. But recent research by Sean Vitousek and others highlights just how little sea level rise it takes to cause drastic changes in coastal flooding in other parts of the world.
With just 10 cm of sea level rise—less than the length of your iPhone—coastal flood frequency in the tropics would double. If we were lucky enough to continue on the sea level rise trajectory we’ve been on, that 10 cm rise would take place in about 30 years’ time. But most projections suggest a continued acceleration of sea level rise such that we could reach that 10 cm mark much sooner, and cities like Annapolis, Maryland, and Charleston, South Carolina are already starting to prepare.
The yellow and red areas of the map show places where a 10 cm or less increase in sea level–less than one iPhone length–would double the frequency of coastal flooding. Source: Vitousek et al. 2017
While relatively well-heeled places like Miami Beach have been investing heavily to reduce recurrent flooding issues that have long plagued the city, many low-lying tropical island communities have fewer resources to invest in flood mitigation measures. With the US exiting the Paris Agreement and reneging on its pledge of $3 billion to the Green Climate Fund, developing countries will have even fewer resources available to protect themselves from the floods to come. With the budgets for FEMA, the EPA, and the Department of the Interior in the crosshairs, communities here in the U.S. could be operating with limited resources as well as they watch the water rise.