Human activity has caused subtle changes in the Earth's atmosphere over the past several decades due to increased emissions of gases such as carbon dioxide (CO₂) and methane (CH₄). These anthropogenic changes in greenhouse gases have the potential for increasing the temperature in the lower atmosphere due to their ability to absorb infrared radiation. The far-reaching consequences of these changes are now being seen at extremely high altitudes in the outer atmosphere of the Earth, especially in the thermosphere, the high-temperature, near-vacuum region ~90–500 km, which is where many satellites are in orbit.