14.11: What Have We Learned About Global Sea Level Rise?
- Page ID
- 31703
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Measurements of sea level over more than a century have revealed two trends: (1) global mean sea level is rising; and (2) the rate at which it is rising is accelerating (Oppenheimer et al. 2019). Over the period from 1901 to 1990, global mean sea level increased at about 0.055 inches (1.4 mm) per year, as measured from tide gauges. Over the period from 1993 to 2015—using data from tide gauges and satellites—scientists measured a rate of global mean sea level rise of about 0.126 inches (3.2 mm) per year, more than double the historical rate. Using data from tide gauges and satellite altimetry with independent estimates based on GRACE and Argo data from 2006 to 2015, scientists determined a rate of rise of global mean sea level of 0.14 inches (3.6 mm) per year (Oppenheimer et al. 2019). If we use these data to calculate how much global mean sea level has risen from 1901 to 2020 (using 0.055 in y-1 for 1901–1992, 0.126 in y-1 for 1993–2005, and 0.14 in y-1 for 2006–2020), we come up with a global mean rise of sea level of about 8.6 inches (218 mm). This roughly matches the eight inches of sea level rise from 1880 to 2009 reported by Church and White (2011). If we assume the fastest rate (no further acceleration) of sea level rise until 2050, we add another four inches (108 mm). And if we project that out to 2100, we get an additional seven inches (180 mm) for a total projected sea level rise of about 19 inches (about 0.5 m).
However, scientists don’t expect a constant rate of sea level rise; they expect it to continue to accelerate as the data above show (e.g., Nerem et al. 2018). Assuming a slow-but-steady acceleration of sea level rise, scientists project a range of two to four feet (0.61–1.1 m) of global mean sea level rise by 2100 (Oppenheimer et al. 2019; Palmer et al. 2020). Of course, projected global sea level rise depends on whether we begin to reduce atmospheric greenhouse emissions or not. If we start on a path toward reductions, sea level rise may be more modest. If not, then it could be even worse. Under the worst-case scenario, sea level could rise by as much as 6.5 to 10 feet (2–3 m; e.g., Jevrejeva et al. 2014; Le Bars et al. 2017).
To give these numbers a little perspective, imagine that you’re on the freeway going 40 mph to a special someone’s house for Thanksgiving. If your destination is 120 miles away, it will take you three hours to get there at 40 mph. That’s a steady speed with no acceleration, like what would happen if sea level simply rose eight inches every 100 years or so. But imagine that your rate of speed doubles every hour. In your first hour you’ll travel 40 miles, but in your second hour—traveling at 80 mph (double your previous speed)—you’ll make 80 miles and be done with your trip (40 mi + 80 mi = 120 mi). What would have taken three hours only takes two hours when your speed is accelerating. Imagine the surprise of that special someone when you arrive an hour early. This is what happens when a process accelerates with time. Acceleration makes things happen faster than we might otherwise have expected. Faster-than-expected sea level rise falls into this category.