Average annual temperatures have increased by more than 2°F in Delaware over the past century. Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century. Heat waves are projected to be more intense and cold waves less intense.
Precipitation is projected to increase, as are the number and intensity of extreme precipitation events.
Delaware sea level has risen at the rate of more than 1 foot per century, much greater than the global rate. The number of tidal floods has been increasing. Global sea level is projected to rise another 1 to 4 feet by 2100; following historical trends, even greater rises are possible on Delaware’s coasts. The low elevation areas of Delaware are highly vulnerable to sea level rise.
Delaware’s mid-latitude location and proximity to the Atlantic Ocean has great influence on its overall climate, which is characterized by cold winters and warm summers. The moderating influences of the Atlantic Ocean and Delaware Bay tend to lessen temperature extremes in the state compared to temperatures in the interior of adjoining states. There are only small variations in average temperatures across this small, relatively flat state, ranging from 54°F in the northern portion to 58°F along coastal Delaware in the south. The statewide average annual precipitation is about 45 inches with large inter-annual variability. Annual statewide precipitation has varied from 27.39 inches in 1930 to 60.01 inches in 1948. Average annual snowfall ranges from 19 inches in the north to 14 inches in the south.
Geography also plays an important role, specifically its position on the eastern coast of the North American continent. Delaware’s mid-latitude location places the state in frequent close proximity to the jet stream, particularly in winter and spring. The storm systems tracking with the jet stream bring frequent precipitation and fluctuating temperatures. Strong winter storms that derive their energy from the contrast between cold air in the continental interior and warmer air over the western Atlantic Ocean, otherwise known as nor’easters often affect the state. Delaware has the lowest elevation of all states and also experiences land subsidence. All of Delaware is also classified as a coastal zone due to the proximity of inland areas to tidal waters. The shoreline of Delaware spans more than 250 miles with no geographic location within the state more than 8 miles from tidal waters.
There are no long-term trends in statewide annual precipitation since 1895, although relatively consistent above average precipitation has occurred since the mid-1990s (Figure 2c). Heavy precipitation events (days with more than 2 inches) in Dover have generally been somewhat above the long-term average since the early 1990s (Figure 2d).
The state’s coastline is highly vulnerable to damage from coastal and tropical storm events. Nor’easters are the most common coastal storm, bringing strong winds, heavy precipitation and coastal flooding. They are most active from mid-winter through spring, with peak activity occurring each year in March. The worst Nor’easter in Delaware history is the Ash Wednesday storm of 1962 (March 6–8) and illustrates the potential danger of such storms. The strong northeast winds caused record flooding and beach erosion in Delaware and up and down the eastern seaboard extending from New England to Florida. The strong northeast winds, broad fetch, and high angle of wave approach caused record flooding and beach erosion down the eastern seaboard extending from New England to Florida. Most houses near the beach not protected by a wide beach and dunes were destroyed. Tropical storms and hurricanes occasionally affect Delaware in the late summer and fall. Densely populated areas in New Castle County located along major streams are at significant risk of flooding related to heavy precipitation and from possible surges up Delaware Bay. The state was severely affected by Hurricane Sandy (commonly known as Superstorm Sandy) and Hurricane Irene, causing significant economic and infrastructure damage. Hurricane Sandy made landfall in New Jersey as a post-tropical storm and caused record flooding along the Atlantic and Delaware Bay coasts. Tornadoes and heavy rains trailing Hurricane Irene resulted in power outages for at least 119,000 residents and economic damage estimates totaling $43.2 million for the state.
Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century (Figure 1). Even under a pathway of lower greenhouse gas emissions, average annual temperatures are projected to most likely exceed historical record levels by the middle of the 21st century. However, there is a large range of temperature increases under both pathways, and under the lower pathway, a few projections are only slightly warmer than historical records (Figure 1). According to a recent state-level analysis, annual maximum (daytime) temperature is projected to increase by an average of 2°F to 2.5°F and annual minimum (nighttime) temperature by an average of 1.5°F to 2.5°F by 2039. In the near-term (2020–2039) projections are for heat waves to occur 3 out of every 5 years. Projections for mid-century show an average of 1 heat wave per year and up to 10 extreme heat waves occurring each year by the end of the 21st century. Higher temperatures and extreme heat events in the future may result in decreased air quality and affect the health of Delaware residents. However, future cold waves are projected to be not as cold.
Annual average precipitation is projected to increase for Delaware, with the increases occurring in winter and spring (Figure 4). This change is characteristic of a large area of the Northern Hemisphere in the higher middle latitudes projected to see increases in total precipitation, as well as increases in heavy precipitation events. On average, the state experiences 2 days each year with 2 or more inches of rain. State-level projections show an increase of 0.5 to 1 day each year with 2 inches of rainfall by the end of the century. Projections of above average precipitation amounts and more frequent extreme precipitation events may also result in increased flooding risks throughout the state.
Since 1880, global sea level has risen by about 8 inches. The rise on Delaware’s coasts has been greater than 1 foot per century due to a variety of factors including subsidence and ocean and atmospheric dynamics. Sea level rise has caused an increase in tidal floods associated with nuisance-level impacts. Nuisance floods are events in which water levels exceed the local threshold (set by NOAA’s National Weather Service) for minor impacts. These events can damage infrastructure, cause road closures, and overwhelm storm drains. As sea level has risen along the Delaware coastline, the number of tidal flood days (all days exceeding the nuisance level threshold) has also increased, with the greatest number occurring in 2009 and 2011 (Figure 5). Global sea level is projected to rise another 1 to 4 feet by 2100 as a result of both past and future greenhouse gas emissions from human activities (Figure 6). Findings from the 2012 Sea Level Rise Vulnerability Assessment for the state show projections for increases in sea level rise based on three levels of global warming: (1) 1.6 feet rise for low levels of global warming; (2) 3.3 feet for moderate levels; and (3) 4.9 feet for high levels of global warming (see Delaware Climate Change Impact Assessment for more Sea Level Rise Resources). Sea level rise has important future cross-sector implications for public health, water resources, coastal ecosystems and wildlife, agriculture, and transportation infrastructure. Demographic trends may increase the risks of coastal flooding. Coastal communities are experiencing an increase in the vulnerable elderly population, due to relocation of retirees.