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Weather refers to atmospheric conditions that occur locally over short periods of time- from minutes to hours or days. Familiar examples include rain, snow, clouds, winds, floods, or thunderstorms. Climate, on the other hand, refers to the long-term regional or even global average of temperature, humidity and rainfall patterns over seasons, years, or decades. Climate change is a long-term change in the average weather patterns that have come to define Earth's local, regional, and global climates. These changes have a broad range of observed effects that are synonymous with the term.

Changes observed in Earth's climate since the early 20th century are primarily driven by human activities, particularly, fossil fuel burning, which increases heat-trapping greenhouse gas levels in Earth's atmosphere, raising Earth's average surface temperature. These human produced temperature increases are commonly referred to as global warming. Natural processes can also contribute to climate change, including internal variability (e.g., cyclical ocean patterns like El Niño, La Niña and the Pacific Decadal Oscillation) and external forces (e.g., volcanic activity, changes in the Sun's energy output, and variations in Earth's orbit). Scientists use observations from the ground, air, and space, along with theoretical models, to monitor and study past, present and future climate change. Climate data records provide evidence of climate change key indicators; such as increase in global land and ocean temperature, rising sea levels, ice loss at Earth's poles and in mountain glaciers, frequent and severe changes in extreme weather; such as hurricanes, heat waves, wildfires, droughts, floods and precipitation, and cloud and vegetation cover changes, to name but a few.