Answer: A The U.N. Intergovernmental Panel on Climate Change, the leading scientific body on the subject, released a report in 2013 stating that the warming of the climate is “unequivocal” and that it is “extremely likely” that human activity is the “dominant” cause. This position has been publicly endorsed by many leading science organizations globally. A 2017 U.S. government report found that, other than human emissions of greenhouse gases, “there is no convincing alternative explanation supported by the extent of the observational evidence.”

Answer: D Carbon dioxide, a greenhouse gas that traps heat in the atmosphere, is the main cause of global warming. Scientists do not think changes in the sun can explain the warming that we’re seeing. The seasonal ozone hole over Antarctica represents a major and dramatic instance of human activity damaging the Earth’s atmosphere, but it is not a major cause of warming.

Answer: B Greenhouse gases, such as carbon dioxide, methane, nitrous oxide and water vapor, trap heat energy emanating from the Earth and prevent it from going into space. This keeps the planet much warmer than it would be otherwise. But human activities, such as burning fossil fuels, are adding even more carbon dioxide to the atmosphere and causing added warming.

Answer: B A Washington Post analysis found numerous hot spots have already exceeded the critical two-degree Celsius (3.6 degrees Fahrenheit) mark, far above the global average. In the United States, Alaska and Rhode Island are the fastest-warming states. In general, higher latitudes, such as the Arctic, are warming faster than the mid-latitude regions.


Answer: D While the Paris agreement did indeed identify two degrees Celsius (3.6 degrees Fahrenheit) as a line not to be crossed, it also suggested that countries should make an effort to keep warming even lower, to 1.5 degrees Celsius above pre-industrial levels. Since then, additional evidence has suggested that this more stringent limit may be needed to avoid many severe consequences of warming.


Answer: C On June 1, 2017, President Trump spurred global criticism by declaring that the United States would back out of the landmark deal. Trump took additional steps in November 2019 toward withdrawal. The move isn’t final yet, though: The earliest the United States’ withdrawal would take effect is November 2020.


Answer: B A “high-emissions scenario” is one that represents a major failure to curb greenhouse gas emissions. The Intergovernmental Panel on Climate Change (IPCC) predicts that in this case the average global temperature could rise by 4.7° to 8.6° F, an increase that could trigger catastrophic consequences, such as massive sea level rise. But with swift emissions reductions, worst-case scenarios like this can still be avoided. The Paris climate agreement, if fully implemented, would steer the world off this severe pathway.

Answer: A Climate change can worsen the effects of certain types of severe weather events, such as hurricanes, wildfires, heat waves, droughts, floods and even snowstorms.


Answer: C Antarctica is losing 127 billion tons of ice per year at present and Greenland is losing 286 billion tons per year, according to NASA. Scientists have found that the rate of ice loss in Greenland has grown by a factor of six since the 1980s, and in Antarctica a similar acceleration is underway.


Answer: D The remains of woolly mammoths, once lodged in permafrost, are starting to appear in Siberia, one of the fastest-warming places in the world.