The Earth’s climate has been changing over the past century, but it has accelerated over the last 30 years. Climate change indicators include rising temperatures, shifting rain and snow patterns, and other severe climate events. These changes are associated with elevated atmospheric concentrations of greenhouse gases like carbon dioxide, nitrous oxide, methane, and a number of fluorinated gases. These gas emissions are triggered by human activities.
Factors that affect the amount of greenhouse gases emitted into the atmosphere include pollution from industrial/economic activity, travel activity, population, energy prices, technology, consumption patterns, and land use.
Average temperatures have increased worldwide, especially during the last 30 years. In the United States, eight of the top 10 warmest years have been recorded following 1998. The top 10 warmest years that occurred worldwide have been recorded following 1998.
Hotter than normal summer days and nights have become more common in the United States over the last 30 years. Extremely cold winter days and nights in the United States have become correspondingly less common. Twice as many record highs have been catalogued as record lows in the years from 2000 to 2009.
Yearly precipitation in the United States and around the world has increased. It has increased by 0.08 inches each decade over landmasses since 1901.
Tropical cyclone events in the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico have become more frequent over the last 20 years. Changes in storm observation techniques over the years render it difficult to ascertain if a long-term increase in storm happenings has set in, though. Records from the late 1800s indicate that the annual number of hurricanes has not risen.
The severity and frequency of river floods varies regionally. River flooding has become more severe in the Northeast and Midwest, while it has lessened in southern Appalachia, northern Michigan, and the West. It has become more commonplace in the Northeast, Pacific Northwest, and areas of the northern Great Plains. It is less frequent in the Rockies and Southwest.
Drought events and severity vary across the United States. The severest droughts occurred during the 1930s and 1950s even though the last 50 years have been wetter than normal. A new index indicates that approximately 20-70% of land in the United States has undergone drought conditions from 2000-2015. This index has not been used long enough to compare data from older draught records, though.