evidence: Limit global temperature rise by reducing greenhouse gas emissions. [...] Human-generated emissions cause global warming. Carbon dioxide, nitrous oxide, and methane are gases that collect in the atmosphere and prevent heat from radiating from earth’s surface into space, creating what’s known as the greenhouse effect. According to the Intergovernmental Panel on Climate Change (IPCC), the leading international scientific body studying the subject, the concentration of these heat-trapping gases has increased substantially since preindustrial times to levels not seen in at least 800,000 years. Carbon dioxide (the chief contributor to climate change) is up by 40 percent, nitrous oxide by 20 percent, and methane by a whopping 150 percent since 1750—mainly from the burning of dirty fossil fuels. The IPCC says it’s "extremely likely" that these emissions are mostly to blame for the rise in global temperatures since the 1950s. Meanwhile, deforestation and forest degradation have contributed significantly to global carbon emissions as well.