local climate projections
Today’s news is full of stories of extreme weather events. California wildfires. Hurricanes. Heat waves.
Mystic River communities are also beginning to experience the effects of climate change: too much water (flooding), too little water (drought), summer heat waves and unpredictable winter weather. What do these trends mean for our watershed?
In May 2016, the City of Boston released Climate Ready Boston Climate Projections Consensus, a report written by the the region’s top climate scientists. Using the best available science at the time, the Boston Research Advisory Group (BRAG) developed mid- and late-century climate projections for extreme temperatures, sea level rise, heavy precipitation and coastal storms.
Recognizing substantial uncertainty regarding the ability of humanity to collectively curb global greenhouse gas emissions, they developed low, medium and high scenarios that reflected future human behavioral paths. Below are the summary projections for Boston; this memo assumes that these projections are also valid for the full Mystic River Watershed. Unless otherwise noted, all projections are taken from the Climate Ready Boston report.
Downstream communities in the Mystic River watershed face serious threats of coast flooding. These threats will be exacerbated as sea levels rise and storms intensify. This map series show the probabilities and depths of flooding in 2013, 2030, and 2070. The datasets are derived from the Boston Harbor Flood Risk Model (BH-FRM) for sea level rise and coastal storm simulations. Download PDF maps here, scroll through map series below. Data provided by Woods Hole Group, more information here.
% Probability: percent annual chance of occurrence of flood event
1% Storm: flood depths (in feet) for a 100-year storm
.2% Storm: flood depths (in feet) for a 500-year storm
BRAG scientists projected that Boston’s average (i.e., both day and night) summer temperature could rise from approximately 69 degrees Fahrenheit in the year 2000 to as high as 76 degrees by 2050 and 84 degrees by 2100 (note: at the low end of carbon emissions, temperatures are not projected to significantly increase).
They also found that the number of very hot days (above 90 degrees Fahrenheit) could increase from an average of 11 days per year in 1990 to nearly all summer by 2070 (see graphic). With hotter summers, stress on aquatic ecosystems and demand for potable water and opportunities to cool off will all increase.
Sea Level Rise
Depending on human behavior and modeling uncertainty, sea level rise predictions for Greater Boston range from less than one foot to over ten feet by 2100. BRAG scientists put the most-likely scenario as up to 1.5 feet by 2050 and between 2.4 and 7.4 feet by 2100 (see graphic).
The top of the Amelia Earhart Dam is located about 6.5 feet above current high tide. January 2018’s “Bomb Cyclone” Grayson came within 18.5 inches of the top of the dam. With typical winter storm tides exceeding 2 to 3 feet, the dam could begin regularly overtopping (or flanking) by the 2080s. Related concerns include riverbank scouring, toxic releases from flooded brownfields and storage tanks, and chronic saltwater inundation of and damage to built and natural resources.
The rainfall projections described in the Climate Ready Boston report are not as well-understood and thereful not as useful as those for projections for heat or sea level rise. The report provides a graphic showing a gradual increase in the 10-year, 24-hour design storm over time. Not depicted but more damaging is the increasing frequency of “cloud bursts” or “cloud bombs” that overwhelm existing stormwater systems and cause substantial damage to built and natural environments.
For communities, this can mean contaminated stormwater/sewage inundation of basements and streets and flood damage to private and public property. For the Mystic River, this can cause damage both through riverbank erosion and acute toxin and nutrient pollution from surface flooding and combined sewer overflows. In addition, as precipitation becomes more intense, so will short-term droughts, such as occurred in summer 2016, when 98% of Massachusetts was under moderate to extreme drought conditions. These flash-floods/flash droughts stress natural systems and can lead to increased urban forest mortality and even local extinction of native plant and animal species.
In addition, Greater Boston is experiencing warmer winters, with more rainfall during times when the ground is frozen. A recent paper in the journal Nature correlated a warming Arctic with more frequent late-winter cold and snow in the Northeastern US. These changes in winter weather have implications for e.g., the ability of green infrastructure to control pollutant and nutrient runoff into the Mystic.
In Greater Boston, winter Nor’easters tend to cause more damage than hurricanes, (though see Hurricanes Sandy and Irene). Nor’easters are most prevalent during the winter, formed when colder air over land hits warmer ocean air and forms a low-pressure area.
After the record 2018 coastal floods, MyRWA scientist Andy Hrycyna examined historical tide gauge data at the Amelia Earhart Dam and in Boston Harbor. Some observations: The monthly maximum tide is trending upward (note: this NOAA chart does not include 2018 record storm tides). Of 54 historic high tides in Boston ranked by NOAA, most occurred between December and March, likely due to Nor'easters.
As a result, climate change—especially Arctic warming--may mean that precisely the kinds of storms that historically have posed the greatest coastal flooding threat will become more frequent and more intense. That is, against a backdrop of gradually rising average sea levels, extreme events that cause the most damage may become disproportionately more likely.
Want a quick review of the latest climate projections? Below are links to NASA, National Climate Assessment, and US EPA explanations of the latest global scientific consensus.
For local "downscaled" climate projections (as of 2016) for the Boston area, read the Climate Ready Boston executive summary
 Executive summary is available at https://www.boston.gov/sites/default/files/document-file-12-2016/brag_report_summary.pdf.