Things have changed dramatically over the past 18 years.
This time, it's a hotter, waterier, wilder Earth that world leaders are trying to save.
The last time that the nations of the world struck a binding agreement to fight global warming was 1997, in Kyoto, Japan.
As leaders gather for a conference in Paris on Monday to try to do more, it's clear things have changed dramatically over the past 18 years.
Some differences can be measured: degrees on a thermometer, trillions of tons of melting ice, a rise in sea level of a couple of inches. Epic weather disasters, including punishing droughts, killer heat waves and monster storms, have plagued Earth.
As a result, climate change is seen as a more urgent and concrete problem than it was last time.
"At the time of Kyoto, if someone talked about climate change, they were talking about something that was abstract in the future," said Marcia McNutt, the former US Geological Survey director who was picked to run the National Academies of Sciences.
"Now, we're talking about changing climate, something that's happening now. You can point to event after event that is happening in the here and now that is a direct result of changing climate."
Other, nonphysical changes since 1997 make many experts more optimistic than in previous climate negotiations.
For one, improved technology is pointing to the possibility of a world weaned from fossil fuels, which emit heat-trapping gases. Businesses and countries are more serious about doing something, in the face of evidence that some of science's worst-case scenarios are coming to pass.
"I am quite stunned by how much the Earth has changed since 1997," Princeton University's Bill Anderegg said in an email. "In many cases (e.g. Arctic sea ice loss, forest die-off due to drought), the speed of climate change is proceeding even faster than we thought it would two decades ago."
Some of the cold numbers on global warming since 1997: The West Antarctic and Greenland ice sheets have lost 5.5 trillion tons of ice, or 5 trillion metric tons, according to Andrew Shepherd at the University of Leeds, who used NASA and European satellite data.
The five-year average surface global temperature for January to October has risen by nearly two-thirds of 0.36 degrees Celsius, between 1993-97 and 2011-15, according to the US National Oceanic and Atmospheric Administration.
In 1997, Earth set a record for the hottest year, but it didn't last. Records were set in 1998, 2005, 2010 and 2014, and it is sure to happen again in 2015 when the results are in from the year, according to NOAA.
The average glacier has lost about 39 feet, or 12 meters, of ice thickness since 1997, according to Samuel Nussbaumer at the World Glacier Monitoring Service in Switzerland.
With 1.2 billion more people in the world, carbon dioxide emissions from the burning of fossil fuels climbed nearly 50% between 1997 and 2013, according to the US Department of Energy. The world is spewing more than 100 million tons of carbon dioxide a day now.
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