PAST: increasing, and increasingly erratic, precipitation in Allegheny County

11/13/2017

Mean annual temperatures in Pennsylvania since 1895. Source: NOAA State Annual and Seasonal Time Series.

Climate changes and the local environment:

Average annual temperatures in Pennsylvania have been generally rising since the beginning of the twentieth century. Since 1965, Pennsylvania’s average annual temperature has risen by almost one full degree celsius. While this sharp increase in temperature and rate of change have had relatively few direct impacts on the environment of Allegheny County, the secondary effects have been substantial.

Allegheny County has typically had high rates of precipitation, and the county seat, Pittsburgh, lies at the intersection of three major rivers. As a result, it has long been vulnerable to flooding. Asphalt cover in the city has increased the rate of runoff, allowing precipitation to quickly raise water levels in the river while carrying pollution from the streets into the water supply. Flooding in and around Pittsburgh therefore endangers both freshwater ecosystems and human health.

Flooding is usually especially severe in the spring, when sudden thawing after heavy snowfall can send meltwater draining into the rivers. Spring temperatures across Pennsylvania have not warmed as quickly as temperatures in other seasons, but they have increased erratically since the 1950s. Meanwhile, annual precipitation has fluctuated from year to year but nevertheless generally increased since 1948. Springtime precipitation has actually decreased slightly, but storms that drop a great deal of rain or snow have become more common.

The picture is therefore very complex, but overall humans have made floods more likely in Allegheny County, both by changing Earth's climate and by building infrastructure that cannot absorb precipitation like natural environments can.

Average annual rainfall in Pittsburgh, relative to a twentieth-century base period, showing an upward trend. Source: NOAA Climate at a Glance.

How we know:

The reconstructions and historical data compiled to track temperature and precipitation come from mass data-collection efforts by the National Oceanic and Atmospheric Administration (NOAA) State Annual and Seasonal Time Series, the NOAA Temperature, Precipitation, and Drought tools, and the NOAA Climate Resilience Toolkit, as well as several local historical sources. Projections of flooding in Pennsylvania's future draw from the Climate Central/Inner City Fund (ICF) States at Risk tools.

These freely available digital resources, combined with local historical sources detailing past floods and incidences of health risk from infrastructural failures, help create a full picture of climate change in Allegheny County. The historical sources rely on contemporary reports of climate-influenced events from newspapers and firsthand accounts, as well as official records pertaining to government regulatory or responsive action.

Modeled (grey shadow) and observed (grey bars) precipitation in Allegheny County. Observed values are relative to a 1960-89 baseline. Source: NOAA U.S. Climate Resilience Toolkit (Climate Explorer).

Human consequences:

The increasingly frequency of precipitation extremes across Pennsylvania are a risk to Allegheny County infrastructure. The county's water treatment center, ALCOSAN, admits that any more than one-tenth of an inch of rain can overflow the sewer system and send raw sewage into local waterways. The county currently copes with these disasters by posting Combined Sewer Overflow (CSO) warnings at local beaches or boat-accessible waterways to keep individuals from coming into contact with contaminated water.

In the past three years, time lost to CSO days has increased significantly. In 2014, there were 50 CSO alerts, 54 in 2015, and 65 in 2016, and CSOs have lasted an average of 4-5 days since 1995. That adds up to over half the days of the recreational boating season. Every CSO period limits recreational activities and revenue from those recreational activities. Rehabilitating the sewage system to prevent CSO events will cost around $3 million.

CSOs are a frequent occurrence even with moderate rainfall, but Pittsburgh is also at risk for more severe flooding. Significant floods in 1889, 1907, 1936, 1964, and 1996, as recorded by the Heinz History Center, have deeply influenced the community’s view of its environment and caused major infrastructural and financial losses.

Flooding in and around Allegheny County is likely to have even more dramatic impacts in the future. According to FEMA’s 100-year floodplain projections, over 430,000 Pennsylvanians live in areas likely to suffer flood damage in the next century, and the risk of flooding is projected to increase with each year. On the edge of three rivers, with poor flood-protection infrastructure and a weak water treatment system, Pittsburgh has been and continues to be vulnerable to the impacts of climate change.