Flooding in Portsmouth, Va., after Hurricane Matthew in 2016. (Steve Earley/AP)

The klaxon sound of a government alert went off on cellphones all over Northern Virginia at 3:35 a.m. Monday; the National Weather Service had issued a flash-flood warning for the region. Hours later, the state climatologist fielded phone calls about tornado watches and warnings in the central part of the state.

Such warnings may become more frequent in the future, and they may have major implications for human health, according to a new study by the Natural Resources Defense Council, a 48-year-old environmental organization.

The study, “Climate Change and Health in Virginia,” warns that as heat waves increase, the risk of heat-related illnesses and deaths in Virginia will grow. Coastal flooding, which already threatens Norfolk and the Hampton Roads area, is likely to worsen as sea levels rise, potentially impeding emergency medical services. Allergy season is starting earlier and lasting longer, and asthma attacks are increasing in the southeastern United States.

“Climate change is already affecting the health of Virginians, and it’s getting worse,” said Juanita Constible, the lead author of the NRDC report. “Extreme events are where we’re more likely to see circumstances overwhelm officials’ ability to respond.”

The NRDC is not alone in spotting the risk. The National Climate Assessment, a congressionally mandated report by more than 300 experts, also cited sea-level rise, increased temperatures and greater competition for water in its 2014 look at the southeastern United States. (The 2018 report will be released late this year.)

Philip J. Stenger, the state climatologist and director of the climatology office at the University of Virginia, said the NRDC report “brings up a lot of important points. We’re not going to simply warm up everywhere although temperatures are rising. . . . The idea that we could be facing more intense allergy seasons is likely. . . . Overall, these are issues that should be looked at.”

The study points out that Virginia’s summer temperatures have risen one degree Fahrenheit since 1895, raising the risk of death from the heat by about 2.5 percent. Those most affected are often the most vulnerable: the elderly, children and people with chronic illnesses.

One Norfolk tide gauge reports the equivalent of 18.2 inches of relative sea-level rise in the past 100 years, more than twice as much as the global average, the report says. Local land is also sinking, which makes evacuation and emergency routes dicey. And the report warns that as sea levels rise, so does the risk of saltwater intrusion on local aquifers, which could endanger drinking water.

Longer growing seasons for vegetation mean earlier periods when pollen is in the air, and higher pollen counts, the study says, causing allergy and asthma patients to suffer. Mosquito- and tick-borne infections also are increasing as the insects find new places to live. The NRDC’s Constible, who lives in Loudoun County, said she has contracted Lyme disease three times since moving there in 2009.

She and her co-authors called for more efforts to cut carbon pollution, which drives climate change, by reducing traffic congestion, using wind and solar energy rather than coal, oil or natural gas, and tracking temperature-driven increases in reports of allergies, as called for in the state’s 2008 climate action plan.