Human Health and Climate Change

Close up image of a deer tick on a blade of grass
Warming winters increase the risk of exposure to Lyme disease.

NPS Photo

These maps show the distribution of reported cases of Lyme disease in 1996 and 2014. Each dot represents an individual case placed according to the patient’s county of residence, which may be different than the county of exposure.
These maps show the distribution of reported cases of Lyme disease in 1996 and 2014. Each dot represents an individual case placed according to the patient’s county of residence, which may be different than the county of exposure.

Centers for Disease Control

Disease and Climate

Climate determines several factors in the maintenance of pathogens in nature, including incubation times, geographic range, and presence of host species. It is expected that current warming trends will alter the distribution of many diseases transferred by fleas, ticks, and mosquitos. Some of these changes are already well underway.

Tick-borne Diseases

Lyme disease is the most common vector-borne disease (that is, a disease transmitted by mosquitoes, ticks, or fleas) in the United States. It is a bacterial illness spread by ticks that can cause fever, fatigue, joint pain, and skin rash, as well as more serious joint and nervous system complications.

Warming winters are allowing more deer ticks to survive through to the spring. Because tick activity depends on temperatures being above a certain minimum, shorter winters could also extend the period when ticks are active each year, increasing the time that humans could be exposed to the disease. Take precautions to protect yourself from disease when hiking in tick habitat.


Japanese Barberry and Ticks

Invasive Japanese barberry was introduced as an ornamental plant to the United States in 1875 and can be found throughout Catoctin Mountain Park. A hardy shrub, barberry is shade tolerant, drought resistant, and highly adaptable. Since introduction, it has spread across our backyards, forests, and fields, where it out-competes and takes space from native species. It’s harsh taste and spiny branches have left it untouched by browsing deer, further aiding its spread. It is perfect habitat for ticks. In areas where barberry is not contained, there can be 120 infected ticks per acre, vs only 10 infected ticks in an area where there is not barberry at all.

Several factors contribute to high tick density in areas with barberry. Dense foliage, and an umbrella shape, allows the plant to retain high humidity levels that deer ticks need to remain active during the day. In the open, ticks can only be active for 15-16 hours of the day before drying out. When they’re protected by barberry, that number can increase to 23 or 24 hours of tick activity. This means more time can be spent waiting for a host (like a raccoon, or a hiker) to pass by. Rodents also like to make their homes in barberry bushes; they are the primary source for larval ticks’ first blood meal, and reservoirs for Lyme disease causing bacteria.

Removing barberry and other invasive plants from the park is a key part of many volunteer projects in the park. Reducing the impact of invasive plants creates healthier forests and increases the resilience of native plants to pollution, climate change, and pests like the emerald ash borer. Get involved in your forests!

Infographic illustrates how lyme disease is connected to rodents and the barberry bush that shelters them and disease carrying ticks.
Invasive Japanese barberry plants have been linked to ticks that can carry Lyme disease.

New York Department of Environmental Conservation


Mosquito-borne Diseases

As average temperatures and humidity increase, mosquitos are faring better in the region, increasing your risk of exposure to mosquito-borne illnesses. The mosquito season in Baltimore has grown faster than any other US city in recent decades. There, the invasive Asian tiger mosquito is now active for 152 days of the year, an increase of 37 days since 1980. First accidentally introduced into the US in 1985 on cargo from Asia, the mosquito is now found in 37 states and is known to spread Zika virus, west Nile virus, dengue, chikungunya, and yellow fever.

A park ranger firefighter starts a prescribed burn in a dry field with a drip torch
NPS firefighters use prescribed burns to help reduce dry fuel loads and prevent uncontrolled wildfires.

NPS Photo


Due to a longer growing season and increased risk of drought, the fire season has become longer and your risk of exposure is greater. Additionally, the spread of invasives increases fuel loads through the increase of biomass and mortality of native plants, while winter storms and wind events cause more downed woody debris.

A black bear rummages through food left by campers on a picnic tables
Properly storing food and disposing of garbage can be life or death to a bear.

NPS Photo

Bear Encounters

As winters warm, and become compressed, hibernation cycles are disrupted. Bears are going into hibernation later in the winter, and waking up earlier in the spring. This increases the risk for human-bear encounters. As the animals are more active throughout the year and changes in food availability force them to forage in closer proximity to human populations. Bears can grow larger due to the abundance of available food year round.

Black bears prefer to avoid people, and rarely cause human injury. You may have even happened across one on the trail without even knowing it! A black bear will get away at the first sign of an approaching hiker, well before you notice it.

Storing your food and disposing of garbage properly can mean life or death to a bear. Bears learn quickly and will return to areas where they find food. Not only can this be dangerous for people, but it is also harmful to bears that may need to be euthanized if they become too habituated to people and forage in campgrounds. Follow bear safe practices when traveling in bear country and camping in the park.

An electric car is plugged in and charging.
Sustainability in the Park

From alternative fuels to energy saving appliances, the park is working towards a low carbon future.

Fall foliage on display at the Hog Rock Overlook
Eastern Forests and Climate

Changes in seasonal weather could have an impact on eastern forests, and the animals that call them home.

A group of volunteers holds shovels and loppers.
How You Can Help

You can be a part of the solution. At home, on the road, and in the park – get involved!

Close up of a solar flare
Climate Change

The greenhouse effect is becoming stronger due to human activity.



Brownstein, John S., Theodore R. Holford, and Durland Fish. “Effect of Climate Change on Lyme Disease Risk in North America.” EcoHealth 2.1 (2005): 38–46. PMC.

Climate Change Indicators: Lyme Disease. Environmental Protection Agency, 22 Feb. 2017.

Foran, Shiela. “Controlling Japanese Barberry Helps Stop Spread of Tick-Borne Diseases.” UConn Today, University of Connecticut, 22 Feb. 2012.

Jones, Beth. “Barberry, Bambi and Bugs: The Link between Japanese Barberry and Lyme Disease.” Scientific American Blog Network, 30 Mar. 2011.

Fritz, Angela. “Climate Change Is Extending Mosquito Season in the D.C. Region - Bad News for Zika.” The Washington Post, WP Company, 28 July 2016.

McKenna, Maryn. “Why the Menace of Mosquitoes Will Only Get Worse.” New York Times, 20 Apr. 2017.

U.S. Forest Service. Central Appalachians Forest Ecosystem Vulnerability Assessment and Synthesis: A Report from the Central Appalachians Climate Change Response Framework Project. Northern Research Station, 2015 (General Technical Report NRS-146).

Inkley, Doug, et al. Nowhere to Run: Big Game Wildlife in a Warming World. National Wildlife Federation, 2013.

Last updated: November 20, 2018

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