Key facts: Global sea levels are forecast to rise this century. By the year 2100, there is a 96% chance that Boston sea level will have risen by at least 1.8 ft. (0.56 m), a 1.3% chance of the rise exceeding 6.3 ft. (1.92 m) and a 0.1% chance of exceeding 11 ft. (3.4 m).[*]

Background

• Human emissions of greenhouse gases are altering the global climate. For coastal cities like Boston, rising sea levels will be one of the most important consequences of climate change.
• Global mean sea level (GMSL) has risen by 8-9 inches since 1880 [1] and could rise by as much as 8.2 ft. (2.5 m) by 2100 [1]. Sea level rise is primarily caused by thermal expansion of the warming ocean, with a (so far) small contribution from melting of land-based ice sheets and glaciers.
• GMSL is expected to continue to rise throughout the 21^st century and beyond due to legacy emissions already in the atmosphere, even without any future emissions.
• Rising sea levels threaten lives and livelihoods in coastal areas, where they can cause increased flooding and permanent inundation of low-lying areas, which would damage homes, businesses and infrastructure through wave action, erosion and encroachment of corrosive salt water [2].
• In the near term, the primary effect of increased sea levels will be experienced through extreme events: a local rise of just 1.2 ft. can cause a 25-fold increase in the frequency of flooding (not accounting for increases in the frequency and severity of storms caused by climate change) [1].
• Climate scientists use standardized projections of future human greenhouse gas emissions called Representative Concentration Pathways (RCP) [3]:
  • RCP2.6: Dramatic reduction in emissions (zero or negative emissions by 2100).
  • RCP4.5: Moderate reduction in emissions (21-54% lower than 2010 levels by 2100).
  • RCP8.5: Business-as usual (continued high emissions).
• The highest astronomical tide in Boston was 7.2 ft. above mean sea level [4].[†]
• The top of the Charles River Dam, which prevents seawater from flowing into the Charles River Basin, is 12.5 ft. above mean sea level [5].

Sea level rise in Boston

• Sea level along the East Coast of the U.S. is expected to rise faster than GMSL, due to a variety of geophysical factors.
• In all emissions scenarios, sea level in Boston is almost guaranteed to rise by at least 1.4 ft. (0.42 m) by 2100 (>94% likelihood).
• Future emissions have a strong effect on the extent of the rise and the likelihood of extreme outcomes.
• These predictions are based on conservative estimates of the rate of loss of land-based ice, potentially underestimating the risk of extreme sea level rise by the end of this century. This risk is difficult to quantify due to complex nonlinear effects in the behavior of land-based ice.
• According to the interactive NOAA Sea Level Rise Viewer, a 5-ft. increase in sea level appears to be the threshold to significant inundation at high tide around the Charles River Basin (including Back Bay) as well as upstream. See visualization for 4 ft. and 5 ft. [7].

Sea Level Rise (ft.)		Emissions Scenario [1]
Global	Boston [6]	RCP2.6 (low)	RCP4.5 (moderate)	RCP8.5 (high)
1.0	1.4	94%	98%	100%
1.6	1.8	49%	73%	96%
3.3	4.1	2%	3%	17%
4.9	6.3	0.4%	0.5%	1.3%
6.6	9.0	0.1%	0.1%	0.3%
8.2	11.1	0.05%	0.05%	0.1%
Table 1: Six possible values of GMSL rise, the predicted corresponding rise in Boston using year 2000 as a baseline [6], and the probability of exceeding that rise under three standardized emissions scenarios (RCP) [1].

Adaptation

• Although it is important to mitigate climate change by reducing greenhouse gas emissions, it is too late to prevent significant consequences. Therefore, is it critical that policymakers at all levels plan for climate change adaptation. Planning for rising seas must be incorporated into almost all types of decisions from land-use to infrastructure at the local, regional and national level.
• Even the most moderate plausible sea level rise scenarios will require significant adaptation, but global greenhouse gas emissions are largely out of the hands of local planners. Therefore, the best strategy may be to plan for the highest emissions scenario (RCP8.5).
• The uncertainty about melting of land ice provides additional motivation to consider the worst-case scenarios under RCP8.5.
• In managing risk, it is essential to consider the full range of possible outcomes rather than just the most likely outcome, since a large share of the risk will come from unlikely severe outcomes.
• Decision-makers must choose an amount of sea level rise to use for adaptation planning. The best choice will depend on the type of project, the timeframe and the level of acceptable risk. Critical infrastructure with long service lives may warrant considering even extremely unlikely scenarios.
• For example, when building a seawall, where the cost of failure may be catastrophic, planners may want to consider the most extreme physically plausible scenario. Although planning for extreme scenarios imposes additional cost, there will often be other benefits. In the seawall example, a higher wall provides increased resilience to storm surge and extended service life (especially since sea levels are expected to continue rising beyond the end of the 21^st century).

Guide to additional resources:

• Most of the data reported here can be found in a 2017 NOAA report [1], which incorporates the latest data on expected GMSL rise with local geophysical data to produce predictions for local sea level changes along U.S. coastlines. This report, especially Sec. 6, is written for policymakers concerned with managing the risks of rising seas.
• The 2014 National Climate Assessment [2] features an excellent summary of the latest climate science and the impacts of climate change in the United States along with advice for policymakers. The following sections are relevant: Key Message 10: Sea Level Rise, Key Message 11: Melting Ice and 16 Northeast.

Supplemental Material

This supplement provides additional details on the uncertainty in sea level rise predictions arising from loss of land-based ice as well as a brief summary of the mechanisms of storm surge.

Land Ice

• Climate change increases global mean sea level (GMSL) through thermal expansion of existing seawater and by loss of land-based ice such as glaciers and ice sheets.[‡]
• Ice sheets are the size of continents and they respond to changes in climate over thousands to hundreds of thousands of years.
• Earth’s two major ice sheets (in Greenland and Antarctica) contain enough water to raise GMSL by 230 ft. (70 m.) [8]. Therefore, the loss of even a small fraction of their mass could lead to massive increases in sea level.
• Ice sheets gain mass from precipitation (snowfall) and lose mass through ablation,[§] surface melt, melting from contact with the ocean, and iceberg calving.[**] The difference between the gain and loss is known as mass balance [9].
• It is difficult to predict how ice sheets will respond to climate change. It’s even possible that they could gain mass (through increased precipitation) faster than they would lose it, but the consensus among experts is that ice sheets are shrinking and will continue to shrink.
• There is considerable uncertainty over the rate of future ice loss. Predicting how ice sheets will respond to changes in climate requires a detailed understanding of the physical structure of the ice, ocean currents, air temperature, and precipitation.
• There is good reason to believe that these ice sheets are highly sensitive to small changes in climate. During the most recent interglacial period (130,000 to 115,000 years ago) global temperatures were only 0-4° F (0-2° C) warmer than today, but GMSL was up to 30 ft. (9.3 m) higher than today, almost all from land-based ice [10].
• Sensitivity to temperature changes is due to nonlinear effects in the behavior of the ice. For example, as surface meltwater percolates through a glacier, it may erode the structural stability of the glacier, increasing the rate of calving.
• A recent paper by DeConto et al. attempted to include detailed physical models of the ice shelves on a large scale [10]. The authors emphasize that their models should not been interpreted as predictions, but “as possible envelopes of behavior.” They found:
  • Antarctica alone could potentially contribute more than 3 ft. (1 m) of GMSL rise by the end of the century and more than 50 ft. (15 m) by 2500.
  • Major ice-sheet retreat could begin as early as 2050 for RCP8.5.
  • Only the dramatically-curtailed emissions scenario (RCP2.6) avoids ice-sheet collapse.

Land Ice: Considerations for policymakers

• Glaciers and ice sheets change very slowly. Total melting of an ice sheet would take hundreds to thousands of years.
• The sea level rise projections cited in the main report assume a constantly increasing rate of mass loss for land-based ice sheets [1]. Linear response predictions like this will likely be accurate on the scale of a few decades. However, the authors of Ref. 1 acknowledge that ice loss may occur more rapidly, in which case sea level rise could transition from the intermediate scenario mid-century to the high or extreme scenario by the end of the century.
• The risk of a rapid acceleration of ice loss is difficult to quantify, but it can still be incorporated into planning decisions.
• As scientists continue to improve their understanding of land-based ice, they will revise their predictions for future sea level rise.
• By the time it is clear that ice sheet collapse will occur, it will likely be too late to prevent it. This is a strong argument for quick action to curb climate change, since we may already be causing irreversible changes in the climate and especially GMSL.

Storm Surge

• Storm surge is high water in excess of astronomical tides caused by storms, especially hurricanes and nor’easters. In such storms, the surge often responsible for the largest losses of life and property.
• The total water level is known as storm tide and includes contributions from storm surge, astronomical tides, and rainfall. Timing with tides has a huge effect on the impact of storm surge.
• Storm surge is primarily caused by wind-induced currents which blow water towards the shore. Surges therefore tend to be largest where the winds are blowing directly onshore.
• The amount of storm surge depends on a number of factors and can vary significantly over relatively small distances. These factors include:
  • Central pressure: The low-pressure region in the center of a storm increases the water level. This effect is small compared to the effect of the wind.
  • Storm intensity and storm size: Larger storms and storms with more powerful winds tend to produce greater storm surge.
  • Storm forward speed and angle of approach to coastline.
  • Shape of the coastline: Concave coastlines can amplify the surge.
  • Slope of ocean bottom: A gently sloping ocean floor produces the largest surge.
  • Other local features such as islands, rivers, inlets, etc.
• Scientists expect that the storms that cause storm surge are will become more frequent and more powerful as the planet warms. Rising seas will compound this problem.

Storm Surge Resources

• The summary presented here is based on Introduction to Storm Surge [11] by NOAA.
• The National Hurricane Center website has general information on storm surge [12].
• For information on the risk of storm surge in Massachusetts, see National Storm Surge Hazard Maps [13]. These maps depict worst-case scenario inundation for hurricanes by strength. These data are calculated using SLOSH (Sea, Lake, and Overland Surges from Hurricanes), a sophisticated numerical model of storm surge. To make each point they consider up to 100,000 simulations of hypothetical storms and choose the highest inundation.

References:

1. NOAA Technical Report NOS CO-OPS 083: Global and Regional Sea Level Rise Scenarios for the United States. January 2017. https://tidesandcurrents.noaa.gov/publications/techrpt83_Global_and_Regional_SLR_Scenarios_for_the_US_final.pdf
2. Climate Change Impacts in the United States: The Third National Climate Assessment (2014), U.S. Global Change Research Program. http://nca2014.globalchange.gov/report
3. Table 3.1 of IPCC, 2014: Climate Change 2014: Synthesis Report. https://www.ipcc.ch/report/ar5/syr/
4. NOAA Tides and Currents: Datums for 8443970, Boston, MA. https://tidesandcurrents.noaa.gov/datums.html?id=8443970
5. US Army Corps of Engineers: Charles River Dam Local Protection Project. http://www.nae.usace.army.mil/Missions/Civil-Works/Flood-Risk-Management/Massachusetts/Charles-River-Dam/
6. USACE Sea Level Change Curve Calculator (2017.42) http://www.corpsclimate.us/ccaceslcurves.cfm
7. NOAA Office for Coastal Management, Sea Level Rise Viewer. https://coast.noaa.gov/slr/
   Direct link to 4-ft. rise: http://bit.ly/2s5FMYu Direct link to 5-ft. rise: http://bit.ly/2s632pk
8. IPCC 2001, WG1: The Scientific Basis, Ch. 11, pp 650. https://www.ipcc.ch/ipccreports/tar/wg1/
9. “State of Cryosphere: Ice Sheets,” National Snow and Ice Data Center, 9 Aug. 2017, https://nsidc.org/cryosphere/sotc/ice_sheets.html
10. Robert M. DeConto and David Pollard. “Contribution of Antarctica to past and future sea-level rise”, Nature 531, 591-597 (2016). https://www.nature.com/nature/journal/v531/n7596/full/nature17145.html
11. “Introduction to Storm Surge,” National Hurricane Center, NOAA (7 Aug. 2017). http://www.nhc.noaa.gov/surge/surge_intro.pdf
12. “Storm Surge Overview,” National Hurricane Center, NOAA (9 Aug. 2017). http://www.nhc.noaa.gov/surge/
13. “National Storm Surge Hazard Maps–Version 2,” National Hurricane Center, NOAA (7 Aug. 2017). http://www.nhc.noaa.gov/nationalsurge/

 

[*] Compared to year 2000 sea levels, assuming business-as-usual greenhouse gas emissions (RCP8.5).

[†] In the most recent period for which data was available, 1983-2001.

[‡] Melting of already-floating sea ice does not increase GMSL.

[§] Evaporation of the ice.

[**] When chunks of ice break off and fall into the sea, forming icebergs.