Over the past 200 years greenhouse gas levels in our atmosphere have been steadily increasing due to our uninhibited consumption of fossil fuels like coal and oil, as well as increasing deforestation all over the world. These gases prevent heat from escaping to space, somewhat like the glass panels of a greenhouse.
Greenhouse gases are necessary to life as we know it, because they keep the planet's surface warmer than it otherwise would be. But, as the concentrations of these gases continue to increase in the atmosphere, the Earth's temperature is climbing above past levels, causing sea levels to rise, deserts to expand, and changes to the global ecology that we still have yet to discover.
Image courtesy of National Park Service