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.

Greenhouse gas emissions are still rising, while tangible impacts from climate change are already being felt around the world.

But there is some progress. Before the Paris deal, the world was heading for 4.5C (8.1F) of warming by the end of the century compared with pre-industrial times. Recent forecasts have that down to 2.6C (4.7F) thanks to measures taken or firm commitments governments have already made.

That is far above the 1.5C (2.7F) limit countries agreed to seven years ago, however, and the time for keeping that target is fast running out.