Until a few centuries ago, most people thought that Earth's climate had always been more or less the same. In the early 1800s, that idea began to change. It started with the work of Swiss paleontologist Louis Agassiz. While Agassiz examined the geological features of Switzerland, he noticed signs of glaciers where no glaciers currently existed. He concluded that Earth had once endured a great ice age. That glaciers had once advanced and retreated across Earth suggested that Earth's climate could change. Scientists then had no idea how this had happened. While Agassiz was puzzling over glaciers, other scientists were studying gases. In the 1820s, French physicist Joseph Fourier wondered why Earth stayed so warm His calculations told him that, given Earth's distance from the sun, the planet should be a lot colder. Fourier proposed that, like a sheet of glass, gases in Earth's atmosphere trapped some heat near Earth's surface. This heat - trapping effect later became known as the greenhouse effect. In 1859 Irish scientist John Tyndall performed experiments to measure the ability of different gases to absorb and radiate heat. He found that oxygen, nitrogen and hydrogen have a little of this ability, but water vapor and carbon dioxide have a lot. He said that without the latter, Earth would be "held fast in the iron grip of frost." He also noted that changes in water vapor and carbon dioxide could have caused "all the mutations of climate." But how could the atmosphere change? In the 1890s, the work of Swedish scientist Arvid Hogbom pointed the way to an answer. Hogbom recognized that coal - burning factories were pouring carbon dioxide into the air. Hogbom and his colleagues concluded that as humans burned fossil fuels, they would add carbon dioxide to Earth's atmosphere and raise the planet's average temperature. Hogbom wasn't the least bit worried about this possibility. He thought that in thousands of years burning fossil fuels might help prevent a future ice age. He didn't consider global warming a threat, and neither did most of his peers. Tell as much as you can about what you learned in the text you just read. Be sure to include the main idea and key details.