Threat : Changes in Intensity and Frequency of Heavy Precipitation

Precipitation is falling in more intense events across the continental US. This trend is especially strong in the northeastern US, which has seen a 71% increase in heavy precipitation events from 1958-2012. These events are expected to double in the Northeast by 2100, with a corresponding 10-15% increase in intensity.
Heavy Precipitation Events are defined as the heaviest 1% of all precipitation events in a year

Coastal areas have the highest amounts of precipitation within the Northeast region. Some of these heavy precipitation events will be in the form of severe storms (e.g. hurricanes and winter northeasters).

Studies have found that there could be as much as a 50 percent increase in severe storm occurrences over the next century given the current rate of climate change. The intensity of hurricanes in the Atlantic will increase along with warming ocean waters: for each 1.8°F increase in tropical sea surface temperatures the rainfall rates of hurricanes could increase by 6-18% and the wind speeds of the strongest hurricanes could increase by about 1-8%.

Seasonal variations are also expected. More winter precipitation will fall as rain, and less as snow; this is due in part to warmer winter air temperatures. Under one of the climate change scenarios an average of 5%-20% increase in winter precipitation is expected by 2100.

The predictions for seasonal precipitation used by the Commonwealth of MA (MA EOEA 2011) are shown in Table 1. The effects of these possible changes on shellfisheries are not well understood. For example, while heavy precipitation events may decrease salinity levels in near-shore embayments, sea level rise may increase salinity levels. Thus, changes in salinity (and other aspects of water chemistry) may occur in pulses, having to do with the timing of tides and runoff from precipitation events.

Heavy Precipitation Change Impacts Relevant to Shellfishing

1. Reduced salinity in estuaries following high precipitation events, which can decrease biodiversity.
2. Increased freshwater runoff into estuaries, which can lessen the prevalence of MSX infections among oysters and QPX mortality in clams, in contrast to increasing levels of salinity associated with rising sea levels.
3. Increased pollution from run-off, septic systems, and wastewater treatment facilities, which can degrade estuarine water quality.
4. Increased nutrient loading, which can lead to hypoxia and degradation of coastal ecosystems, including coastal salt marshes, mortality of benthic animals, and increased incidences of harmful algal blooms. Larger or more frequent Harmful Algal Blooms can lead to increased closures of shellfishing beds, resulting in economic losses.
5. Increased stratification of estuarine waters with respect to salinity and temperature.
6. Changes in sediment transport and coastal erosion, resulting from storm surge associated with heavy precipitation events.