People hike on the Perito Moreno glacier in Los Glaciares National Park, part of the Southern Patagonian Ice Field in Santa Cruz Province, Argentina. The Southern Patagonian Ice Field is the third largest ice field in the world. The Los Glaciares National Park is one of more than 1,000 places on Unesco's World Heritage List of precious environmental and cultural sites. The planet's climate has constantly been changing over geological time. The current period of warming is occurring more rapidly than many past events. Scientists are concerned that the natural fluctuation, or variability, is being overtaken by a rapid human-induced warming that has serious implications for the stability of the planet's climate. Most glaciers in temperate regions of the world and along the Antarctic Peninsula are in retreat. Since 1979, satellite records show a dramatic decline in Arctic sea-ice extent, at an annual rate of 4% per decade. In 2012, the ice extent reached a record minimum that was 50% lower than the 1979-2000 average.