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).[*]
- 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  and could rise by as much as 8.2 ft. (2.5 m) by 2100 . 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 21st 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 .
- 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) .
- Climate scientists use standardized projections of future human greenhouse gas emissions called Representative Concentration Pathways (RCP) :
- 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 .[†]
- 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 .
Sea level rise in Boston
|Sea Level Rise (ft.)||Emissions Scenario |
|Table 1: Six possible values of GMSL rise, the predicted corresponding rise in Boston using year 2000 as a baseline , and the probability of exceeding that rise under three standardized emissions scenarios (RCP) .|
- 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 21st century).
Guide to additional resources:
- Most of the data reported here can be found in a 2017 NOAA report , 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  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.
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.
- 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.) . 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 .
- 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 .
- 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 . 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 . 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 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  by NOAA.
- The National Hurricane Center website has general information on storm surge .
- For information on the risk of storm surge in Massachusetts, see National Storm Surge Hazard Maps . 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.
- 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
- Climate Change Impacts in the United States: The Third National Climate Assessment (2014), U.S. Global Change Research Program. http://nca2014.globalchange.gov/report
- Table 3.1 of IPCC, 2014: Climate Change 2014: Synthesis Report. https://www.ipcc.ch/report/ar5/syr/
- NOAA Tides and Currents: Datums for 8443970, Boston, MA. https://tidesandcurrents.noaa.gov/datums.html?id=8443970
- 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/
- USACE Sea Level Change Curve Calculator (2017.42) http://www.corpsclimate.us/ccaceslcurves.cfm
- 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
- IPCC 2001, WG1: The Scientific Basis, Ch. 11, pp 650. https://www.ipcc.ch/ipccreports/tar/wg1/
- “State of Cryosphere: Ice Sheets,” National Snow and Ice Data Center, 9 Aug. 2017, https://nsidc.org/cryosphere/sotc/ice_sheets.html
- 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
- “Introduction to Storm Surge,” National Hurricane Center, NOAA (7 Aug. 2017). http://www.nhc.noaa.gov/surge/surge_intro.pdf
- “Storm Surge Overview,” National Hurricane Center, NOAA (9 Aug. 2017). http://www.nhc.noaa.gov/surge/
- “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.
Wow! Thanks for this thoughtful (as always) report. I know there is quite a bit to consider and meanwhile, building in low-lying areas continues apace.
Thanks for this very useful data, Will.
I am however increasingly concerned by the frequent building on former wetlands e.g. around the Fresh Pond area. As we have seen in Houston, former wetlands would have greatly mitigated the effects of Harvey had they not been developed into housing. Let’s get stronger legislation going to prevent this here, albeit rather late for some areas.
Thank you for the bringing all of the parts of the sea rise question into focus. I am gratified that Boston already has been looking into the issue of how to adapt to rising sea level.
I teach the physics of global warming on the global scale. On this scale, the warming of our atmosphere becomes an obvious consequence of pumping more CO2 and other “greenhouse gases” into our air. The physics can be understood by high school students who have had a substantial course in physics.
Could the members of our congress be taught the physics and hence clearly see the implications??
Excellent information for the activist, policy maker, and average citizen.
Thank you indeed for this account that should stand as the baseline for planning from this minute forwards.
But one question: although legacy emissions will hang in in the atmosphere for some long time, the rate of its release is far less than the current rate of accumulation. Are these projections based upon the current atmosphere or one that continues to increase in its heat trapping? – although we HOPE engineering and its applications will change that.
Fantastic information. Thank you for this extremely informative and detailed summary.
People tend to understand things better when they are provided with a local context they can relate to. When people ask me about global warming in the future (and they will), I will point them this post.
It seems to me that the best long term solution to sea level rise in Massachusetts would be to require that grade above sea level everything is built is increase 10 or 15 feet above sea level above todays grade above sea level within 1/2 a mile of the coast. The time line for achieving this should be 50-100 years. But if started today within a decade or two our facilities public and private would begin to be much more sea level rise and storm surge proof. Public Infrastructure should be required to be built going forward with this standard in mind. Private development should be required to be designed accordingly. By doing this we will end up over time with a barrier that is designed counter sea level rise and storm surges. By adopting a long time horizon, we not break the bank dealing with the issue. While it may be complex to implement, the barrier simple in concept and fairly inexpensive to maintain. We would have to deal with the occasional flood until the project is finished, but we would have a plan in place that solves the problem.
This idea should be in addition to doing what we can to slow climate change by limiting carbon dioxide emissions.
Thanks, Adam. Lately–post-Harvey–I’ve been brooding about the combined effect of storm surge and a major rain event (something not directly addressed here although touched on in the resource links). I’d be curious to know if anyone has calculated and mapped the effect of, say, a 3′-4′ rainfall on any of the towns in the Mystic or Charles Basins showing the depth of accumulated surface runoff after (complete) storm sewer failure. Could make interesting viewing.
Thank you for this Will,
A very concise and timely report and a subject we all need to pay more attention to. Sea level rise and an increased frequency of extreme weather with record breaking rainfall could have devastating effects far inland. Low lying areas along the rivers will probably see increased episodic and permanent flooding.
These changes are happening rapidly in terms of geologic time and our coastline will look very different in 100 years.
Thank you for your interest and research on this important subject. Sadly, the rise in sea level will be effecting many of us negatively.
Is there an “Economic analysis” for determining how much resources to put at mitigation of sea rise and when to just let the sea claim areas and focus scarce resources elsewhere
Whew, a ton of useful information. The ‘Adaptation” section with its sentence “Although it is important to mitigate climate change by reducing greenhouse gas emissions, it is too late to prevent significant consequences.” should be echoed in as many forums as possible.
The explanation of “Storm Surge” is particularly useful since it has been used hundreds of times in the past several weeks.
In the first read, I was overwhelmed by the data. I took a breath and decided to support the kinds of Adaptation initiatives that are well founded by data and will fly politically.
Thanks to Adam Iaizzi for his well documented and clearly explained report.
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