Geography, climate, and environment

The 48 contiguous states and the District of Columbia occupy a combined area of 3,119,885 square miles (8,080,470 km2). Of this area, 2,959,064 square miles (7,663,940 km2) is contiguous land, composing 83.65% of total U.S. land area. Hawaii, occupying an archipelago in the central Pacific, southwest of North America, is 10,931 square miles (28,311 km2) in area. The populated territories of Puerto Rico, American Samoa, Guam, Northern Mariana Islands, and U.S. Virgin Islands together cover 9,185 square miles (23,789 km2). Measured by only land area, the United States is third in size behind Russia and China, just ahead of Canada.

The United States is the world's third- or fourth-largest nation by total area (land and water), ranking behind Russia and Canada and nearly equal to China. The ranking varies depending on how two territories disputed by China and India are counted, and how the total size of the United States is measured.d

The coastal plain of the Atlantic seaboard gives way further inland to deciduous forests and the rolling hills of the Piedmont. The Appalachian Mountains divide the eastern seaboard from the Great Lakes and the grasslands of the Midwest. The Mississippi–Missouri River, the world's fourth longest river system, runs mainly north–south through the heart of the country. The flat, fertile prairie of the Great Plains stretches to the west, interrupted by a highland region in the southeast.

The Rocky Mountains, west of the Great Plains, extend north to south across the country, peaking around 14,000 feet (4,300 m) in Colorado. Farther west are the rocky Great Basin and deserts such as the Chihuahua and Mojave. The Sierra Nevada and Cascade mountain ranges run close to the Pacific coast, both ranges reaching altitudes higher than 14,000 feet (4,300 m). The lowest and highest points in the contiguous United States are in the state of California, and only about 84 miles (135 km) apart. At an elevation of 20,310 feet (6,190.5 m), Alaska's Denali is the highest peak in the country and in North America. Active volcanoes are common throughout Alaska's Alexander and Aleutian Islands, and Hawaii consists of volcanic islands. The supervolcano underlying Yellowstone National Park in the Rockies is the continent's largest volcanic feature.

The United States, with its large size and geographic variety, includes most climate types. To the east of the 100th meridian, the climate ranges from humid continental in the north to humid subtropical in the south. The Great Plains west of the 100th meridian are semi-arid. Much of the Western mountains have an alpine climate. The climate is arid in the Great Basin, desert in the Southwest, Mediterranean in coastal California, and oceanic in coastal Oregon and Washington and southern Alaska. Most of Alaska is subarctic or polar. Hawaii and the southern tip of Florida are tropical, as well as its territories in the Caribbean and the Pacific. States bordering the Gulf of Mexico are prone to hurricanes, and most of the world's tornadoes occur in the country, mainly in Tornado Alley areas in the Midwest and South. Overall, the United States receives more high-impact extreme weather incidents than any other country in the world.

Wildlife and conservation

The U.S. ecology is megadiverse: about 17,000 species of vascular plants occur in the contiguous United States and Alaska, and more than 1,800 species of flowering plants are found in Hawaii, few of which occur on the mainland. The United States is home to 428 mammal species, 784 bird species, 311 reptile species, and 295 amphibian species, as well as about 91,000 insect species.

There are 62 national parks and hundreds of other federally managed parks, forests, and wilderness areas. Altogether, the government owns about 28% of the country's land area, mostly in the western states. Most of this land is protected, though some is leased for oil and gas drilling, mining, logging, or cattle ranching, and about .86% is used for military purposes.

Environmental issues include debates on oil and nuclear energy, dealing with air and water pollution, the economic costs of protecting wildlife, logging and deforestation, and international responses to global warming. The most prominent environmental agency is the Environmental Protection Agency (EPA), created by presidential order in 1970. The idea of wilderness has shaped the management of public lands since 1964, with the Wilderness Act. The Endangered Species Act of 1973 is intended to protect threatened and endangered species and their habitats, which are monitored by the United States Fish and Wildlife Service.

The United States is ranked 24th among nations in the Environmental Performance Index. The country joined the Paris Agreement in 2016 and has many other environmental commitments. It left the Paris Agreement in 2020.