While the sun plays a role in Earth's climate by warming the planet and influencing ice ages through changes in the planet's orbit, the current warming trend is too rapid and intense to be explained by solar activity alone. According to the IPCC, human-caused greenhouse gas emissions account for most modern warming, as solar variations only have a minor impact. Researchers have also ruled out a primary solar influence because if the sun's energy output increased, all layers of the atmosphere would be expected to warm uniformly, which is not observed.