Sometimes, before there were people, the Earth's climate was much hotter than it is today. For example about 60 million years ago there were a lot of volcanoes, which burnt a lot of underground "organic matter" (squashed and fossilized dead plants and animals like coal, gas and oil) so a lot of carbon dioxide and methane went up in the air like nowadays. This made the Earth hot enough for giant tortoises and alligators to live in the Arctic.