Before the 1980s, when it was unclear whether the warming effect of increased greenhouse gases were stronger than the cooling effect of airborne particulates in air pollution, scientists used the term inadvertent climate modification to refer to human impacts on the climate. In the 1980s, the terms global warming and climate change became more common. Though the two terms are sometimes used interchangeably, scientifically, global warming refers only to increased surface warming, while climate change describes the totality of changes to Earth's climate system. Global warming—used as early as 1975—became the more popular term after NASA climate scientist James Hansen used it in his 1988 testimony in the U.S. Senate. Since the 2000s, climate change has increased in usage. Climate change can also refer more broadly to both human-caused changes or natural changes throughout Earth's history. Various scientists, politicians and media now use the terms climate crisis or climate emergency to talk about climate change, and global heating instead of global warming.