Like many low-lying coastal cities around the world, Miami is threatened by rising seas. Whether the majority of the cause is anthropogenic or natural, the end result is indisputable: sea level is rising. It is not a political issue, nor does it matter if someone believes in it or not.
The mean sea level has risen noticeably in the Miami and Miami Beach areas just in the past decade. Flooding events are getting more frequent, and some areas flood during particularly high tides now: no rain or storm surge necessary. Perhaps most alarming is that the rate of sea level rise is accelerating.
Diving Into Data
Certified measurements of sea level have been taken at the University of Miami’s Rosenstiel School on Virginia Key since 1996 (Virginia Key is a small island just south of Miami Beach and east of downtown Miami). Simple linear trends drawn through annual averages of all high tides, low tides, and the mean sea level are shown below, and all three lines are about 4.2 inches (11cm) higher in 2015 than they were in 1996.Zooming in to daily data, let’s look at two representative months (nothing unique about them): May 1996 and May 2014. Tidal predictions are calculated to high precision using dozens of known astronomical factors, but do not account for non-astronomical factors such as weather or sea level rise. In 1996, the observed water levels were typically close to the predicted values… sometimes slightly higher, sometimes slightly lower due to meteorological influences. In May 2014, however, there was still variability, but the tides were always higher than predicted — the baseline, or mean sea level, has increased.
For the following chart, only the daily high water mark (highest of the two high tides) for 20 years is plotted. The water levels at high tides are the most relevant because that is when flooding events are more prone to occur. For reference, the average seasonal cycle is shown by the thick black line and is calculated using a 31-day running mean of all 20 years of daily data. The daily high tide values are plotted with a thin blue line, and the thin red line is a smoothed version of the blue line (91-day running mean). The highest water marks are annotated… they have historically been associated with the passage of hurricanes, until September 2015 when a very high water level was reached without a storm nearby.As eluded to in the introduction, sea level is not just rising here, the rate of the rise is accelerating. If the seasonal cycle (black line in the figure above) is subtracted from the data, as well as the mean of all of the data, a series of trendlines can be generated (see figure below). Removing the dominant annual and semi-annual cycles from the time series leaves only daily variability, miscellaneous cycles, and trends. The data are color-coded by arbitrary 5-year periods (red is 2011-2015, green is 2006-2010, blue is 2001-2005, and purple is 1996-2000). The trendlines are drawn through the past 5 years (red), 10 years (green), 15 years (blue), and 20 years (purple). There is plenty of daily and intra-annual variability of course, but what stands out is the increasing slopes of the linear trends in more recent periods. Over the past 20 years, the average high tide has increased by 0.22 inches/year, which agrees very closely with the trend shown in the first chart using annual averages (0.21 inches/year). However, notice that the trends over shorter and shorter periods become increasingly rapid.
Be advised that simple linear trends of noisy time series are not reliable for extrapolating very far into the future, nor are the trend values reliable for shorter time periods. Longer data records allow for greater confidence in a linear trend, but cannot account for accelerating rates.Clearly, the 0.92 inches/year rate for the past five years is too high, but what would a representative recent trend be? If we use data from the first chart (annual averages) and rather than relying on a trend through just the past five years, we use the past five five-year periods from each of the three time series (2011-2015, 2010-2014, 2009-2013, 2008-2012, 2007-2011), then we average out some of the interannual variability and eliminate the dependence on specific endpoints. The average trend of the three time series (high tide, low tide, and mean sea level) for the past five five-year periods comes out to be 0.36 inches/year, or nearly double that of the full twenty-year record. Exposure
The Miami metropolitan region has the greatest amount of exposed financial assets and 4th-largest population vulnerable to sea level rise in the world. The only other cities with a higher combined (financial assets and population) risk are Hong Kong and Calcutta .
Using a sea level rise projection of 3 feet by 2100 from the 5th IPCC Report  and elevation/inundation data, a map showing the resulting inundation is shown below. The areas shaded in blue would be flooded during routine high tides, and very easily flooded by rain during lower tides. Perhaps the forecast is too aggressive, but maybe not… we simply do not know with high confidence what sea level will do in the coming century. But we do know that it is rising and showing no sign of slowing down.
An Attack from Below
In addition to surface flooding, there is trouble brewing below the surface too. That trouble is called saltwater intrusion, and it is already taking place along coastal communities in south Florida. Saltwater intrusion occurs when saltwater from the ocean or bay advances further into the porous limestone aquifer. That aquifer also happens to supply about 90% of south Florida’s drinking water. Municipal wells pump fresh water up from the aquifer for residential and agricultural use, but some cities have already had to shut down some wells because the water being pumped up was brackish (for example, Hallandale Beach has already closed 6 of its 8 wells due to saltwater contamination).
The wedge of salt water advances and retreats naturally during the dry and rainy seasons, but the combination of fresh water extraction and sea level rise is drawing that wedge closer to land laterally and vertically.
In other words, the water table rises as sea level rises, so with higher sea level, the saltwater exerts more pressure on the fresh water in the aquifer, shoving the fresh water further away from the coast and upward toward the surface.
An Ever-Changing Climate
To gain perspective on the distant future, we should examine the distant past. Sea level has been rising for about 20,000 years, since the last glacial maximum. There were periods of gradual rise, and periods of rapid rise (likely due to catastrophic collapse of ice sheets and massive interior lakes emptying into the ocean). During a brief period about 14,000 years ago, “Meltwater Pulse 1A”, sea level rose over 20 times faster than the present rate. Globally, sea level has already risen about 400 feet, and is still rising.
With that sea level rise came drastically-changing coastlines. Coastlines advance and retreat by dozens and even hundreds of miles as ice ages come and go (think of it like really slow, extreme tides). If geologic history is a guide, we could still have up to 100 feet of sea level rise to go… eventually. During interglacial eras, the ocean has covered areas that are quite far from the coastline today.
As environmental author Rachel Carson stated, “to understand the living present, and promise of the future, it is necessary to remember the past”.
What Comes Next?
In the next 20 years, what should we reasonably expect in southeast Florida? The median value of sea level from various observed trends in 2034 is around 5 inches, with a realistic range of 3-7 inches.
Year by year, flooding due to heavy rain, storm surge, and high tides will become more frequent and more severe. Water tables will continue to rise, and saltwater intrusion will continue to contaminate fresh water supplies.
This is not an issue that will simply go away. Even without any additional anthropogenic contributions, sea level will continue to rise, perhaps for thousands of years. But anthropogenic contributions are speeding up the process, giving us less time to react and plan.
Coastal cities were built relatively recently, without any knowledge of or regard for rising seas and evolving coastlines. As sea level rises, coastlines will retreat inward. Sea level rise is a very serious issue for civilization, but getting everyone to take it seriously is a challenge. As Dutch urban planner Steven Slabbers said, “Sea level rise is a … storm surge in slow motion that never creates a sense of crisis”. It will take some creative, expensive, and aggressive planning to be able to adapt in the coming decades and centuries.
Special thanks to Dr. Keren Bolter at Florida Atlantic University and Dr. Shimon Wdowinski at University of Miami for their inspiration and assistance.