Natural events cause changes in climate. For example, large volcanic eruptions put tiny
particles in the atmosphere that block sunlight, resulting in a surface cooling of a few years'
duration. Variations in ocean currents change the distribution of heat and precipitation. El Niño
events (periodic warming of the central and eastern tropical Pacific Ocean) typically last one to
two years and change weather patterns around the world, causing heavy rains in some places
and droughts in others. Over longer time spans, tens or hundreds of thousands of years, natural
changes in the geographical distribution of energy received from the sun and the amounts of
greenhouse gases and dust in the atmosphere have caused the climate to shift from ice ages to
relatively warmer periods, such as the one we are currently experiencing.