he 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 southern tip of Florida is tropical, as is Hawaii. The Great Plains west of the 100th meridian are semi-arid. Much of the Western mountains are alpine. The climate is arid in the Great Basin, desert in the Southwest, Mediterranean in coastal California, andoceanic in coastal Oregon and Washington and southern Alaska. Most of Alaska is subarctic or polar. Extreme weather is not uncommon—the states bordering the Gulf of Mexico are prone to hurricanes, and most of the world's tornadoes occur within the country, mainly in the Midwest's Tornado Alley.

The U.S. ecology is considered "megadiverse": about 17,000 species of vascular plants occur in the contiguous United States and Alaska, and over 1,800 species of flowering plants are found in Hawaii, few of which occur on the mainland  The United States is home to more than 400 mammal, 750 bird, and 500 reptile and amphibian species.About 91,000 insect species have been described.

There are 58 national parks and hundreds of other federally managed parks, forests, and wilderness are Altogether, the government owns 28.8% of the country's land area] Most of this is protected, though some is leased for oil and gas drilling, mining, logging, or cattle ranching; 2.4% is used for military purposes

Environmental issues have been on the national agenda since 1970. Environmental controversies 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.] Many federal and state agencies are involved. The most prominent 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.