Since 1992, when world leaders first came together to address global warming, humanity has spewed more than one trillion tonnes of heat-trapping carbon dioxide from fossil fuels into the air. The world got 0.6 degrees Celsius (1.1 degrees Fahrenheit) hotter.
Each year there are high hopes for the two-week United Nations climate gathering and, almost inevitably, disappointment when it does not deliver another landmark pact such as the one agreed on in 2015 in Paris.
Scientists are more concerned than three decades ago when governments first came together to discuss the problem. The pace of warming in the past decade is 33 percent faster than in the 1990s.