Is climate change really happening? Scientists have been debating this question for about 20 years now. The BBC has produced a handy, one-page summary called Climate Change Explained in Six Graphics, which contains charts showing climate trends over the last few years. Browse those charts and you will see that Earth's temperature has increased systematically over the last century, sea levels have risen significantly, and carbon dioxide emissions from burning fossil fuels have increased almost exponentially. Most climate scientists believe these things are connected: they consider that the burning fuels cause the carbon dioxide emissions, which make the temperature increase, which causes the sea levels to rise. Since records of the weather date back only a hundred years or so, how can scientists confidently make claims that the climate has been changing over a much longer period? In turns out that Earth keeps a natural record of its own climate in many surprising ways. For example, as ice has formed year upon year at the poles, old ice has been buried underneath with bubbles of air trapped inside it. The bubbles act as a record of what the air was like on Earth when the ice formed—and thus what the climate was like in years gone by. Using drills, scientists can extract ice cores (long thin pipes full of ice), study the air bubbles at different depths, and calculate how much carbon dioxide they contain. If they figure out how old the ice is, they can use an ice core as a kind of graph of how carbon dioxide has changed over time. Scientists can also study changes in the climate using ocean sediments, samples of buried pollen, and other, once-living matter. Research like this can tell us what the climate was like hundreds of thousands of years ago. Although most scientists believe in global warming, it's important to note that a very small minority do not. Most agree that Earth is warming but not that fossil-fuel burning, and human carbon dioxide emissions are responsible. The "climate-change sceptics" argue that increases in Earth's temperature are either not happening at all or may be caused by other things, including natural variations in the climate that have been happening for millennia. In recent years, however, fewer and fewer scientists have dissented from the widely held position that global warming and climate change are really happening. People could still be wrong about global warming—but that's becoming increasingly unlikely.