Here's a comprehensive summary of the Wikipedia article on Earth's atmosphere:
Earth's atmosphere is a vital layer of mixed gases, primarily 78% nitrogen and 21% oxygen, held by gravity around the planet. It acts as a crucial protective buffer, shielding us from meteoroids and harmful ultraviolet radiation, while also moderating extreme temperatures and maintaining a warm climate through the greenhouse effect, all essential for supporting life. This dynamic system, weighing approximately 5.15×10^18 kg, gradually thins with altitude, conventionally considered to transition into outer space at the Kármán line, 100 km above the surface.
Over millennia, the atmosphere's composition has dramatically evolved, influenced by natural events like volcanism and the emergence of life; however, modern human activities, particularly fossil fuel use, are now significantly altering it, contributing to climate change and ozone depletion. The atmosphere is structured into distinct layers, including the troposphere and stratosphere, each with unique characteristics that are studied by atmospheric science to understand our planet's complex weather and climate systems.