There are many causes of environmental changes on Earth. Natural events cause changes in climate. For example, large volcanic eruptions release tiny particles into the atmosphere that block sunlight, resulting in surface cooling that lasts for a few years. Variations in ocean currents such as El Niño can also change the distribution of heat and precipitation. Over longer time spans, tens to hundreds of thousands of years, natural changes in the geographical distribution of energy from the Sun and in the amounts of greenhouse gases and dust in the atmosphere have caused the climate to shift from ice ages to relatively warmer periods. On a longer timescale the presence of life on Earth has changed the environment of the planet radically, transforming a predominantly reducing atmosphere made up of methane and ammonia to today's oxygen-rich gaseous envelope.

Human activities can also change the environment...