Description: The issue of global warming is discussed. Recent climate change over the last half of the 20th century is thought to be driven largely by greenhouse gas emissions, with carbon dioxide playing a large role. The carbon cycle describes the reservoirs of carbon (atmosphere, terrestrial biomass and ocean) and the exchanges that occur between these reservoirs. Inputs of carbon to the atmosphere include burning of fossil fuels and respiration from biomass. Vegetation also removes carbon from the atmosphere through photosynthesis, and a similar uptake of carbon from the atmosphere occurs in the ocean through biological processes. Residence time for carbon in the atmosphere can be computed and is estimated to be a few hundred years. Atmospheric carbon dioxide has been measured directly since the 1950s, and longer records exist over geologic time from ice core data.