If you're planning to follow the United Nations Framework Convention on Climate Change's Conference of the Parties -- which starts today and runs through Dec. 11 in Paris -- you'll likely hear a lot about the threshold of 2 degrees Celsius (3.6 degrees Fahrenheit).
That number separates us from climate change-induced floods, droughts, storms, heat waves and rising sea levels more severe than those we've already seen, according to some scientists. Keeping our world from warming more than 2 degrees Celsius by 2100 -- so the average global temperature doesn't rise above roughly 61.6 degrees Fahrenheit -- may help us maintain the somewhat stable climate conditions that humans have adapted to since we started burning fossil fuels in the late 1800s, they say.
But not everyone agrees.
Other scientists and some world leaders want to push for a stricter limit. They say the threshold should be 1.5 degrees to help protect countries, such as island nations, that are most vulnerable to these effects.
Tensions between these two camps are running high as the global average temperature reached about 58.86 degrees Fahrenheit in October, making last month the warmest October ever recorded and putting 2015 on track to be the hottest year.
The 2-degree threshold emerged in the 1970s, when Yale University economist William Nordhaus published research suggesting damage to economic growth and environmental quality can intensify once the global average temperature rises by more than 2 degrees above pre-industrial levels.
"He wrote that ... 'this would take the climate outside of the range of observations which have been made over the last several hundred thousand years,'" Kelly Levin, senior associate in the climate program at the World Resources Institute in Washington, D.C., told The Huffington Post. "So it wasn’t about a proposed target or that there was a 'safe' level below this -- just that it did not have a recent precedent."
Subsequent papers and studies have supported the 2-degree idea, but many scientists argue that it is essentially an arbitrary threshold. Now, world leaders are prepared to deliberate over the threshold at the climate conference in Paris.
"Paris is all about putting in the pathways, policies and actions that will eventually lead to the world making the kinds of deep emission reductions able to keep under that 2 degrees Celsius temperature rise," said Nick Nuttall, head of communications and outreach at the UNFCCC.
"Some vulnerable nations, including small island developing states, consider the 2 degrees Celsius goal to be too high," he said. "So in short, 2 degrees Celsius is not ideal, impacts are occurring and will occur at even lower warming: the 2 degrees Celsius goal has been described by some as a 'speed limit' for global warming, we would call it a defense line, which we approach and cross at our peril."
Furthering this analogy, Dr. Gavin Schmidt, a climatologist and the director of NASA's Goddard Institute for Space Studies in New York, told HuffPost that a 2-degree threshold is comparable to setting a speed limit of 70 mph.
"Nothing special happens at 70 mph, and bad accidents can happen at slower speeds," he said, "but we know that accidents increase steeply if people are driving faster."
So in short, 2 degrees Celsius is not ideal..." Nick Nuttall, head of communications and outreach at the UNFCCC
For some nations, the 2-degree threshold seems like too much of a risk.
The 1.5-degree threshold seems to fall in line with a 2012 study published in the journal Nature Climate Change, which found that the Greenland ice sheet may melt completely if we don't limit warming to around 1.6 degrees. The more we go over that threshold, the faster the ice melts, causing rising sea levels that pose a threat to islands.
"The science clearly says that we have to go to a long-term goal of 1.5 degrees," Amjad Abdulla, lead climate negotiator for the Maldives, an island chain in the Indian Ocean that is severely threatened by rising sea levels, told The Wall Street Journal. "Where do the islands end up? We’re part of the world community."
More than 190 countries are meeting in Paris to discuss what they each are willing to do to help curb climate change, which they have outlined in plans called Intended Nationally Determined Contributions.
The INDCs so far have the potential to possibly reach the 2-degree threshold, but only if they are followed by more ambitious plans in the future, according to a study that was published in the journal Science on Thursday.
"Long-term temperature outcomes critically hinge on emissions reduction efforts beyond 2030," Gokul Iyer, the study's lead scientist at the Joint Global Change Research Institute, a collaboration between the U.S. Department of Energy's Pacific Northwest National Laboratory and the University of Maryland, said in a statement.
"If countries implement their INDCs through 2030 and ramp up efforts beyond 2030," he said, "we'll have a much better chance of avoiding extreme warming and keeping temperature change below 2 degrees Celsius. It's important to know that the INDCs are a stepping stone to what we can do in the future."
If there are no further efforts to reduce emissions in years to come, there is "virtually no chance" that global temperatures will stay below the 2-degree threshold, The Washington Post reported. Not to mention, the proposed 1.5-degree threshold.
"Many are hoping that the Paris agreement will have a strong and clear complementary goal to the 2 degrees Celsius goal, which communicates to governments and the private sector the scale of transformation that is required," Levin told HuffPost. "Regarding what is unavoidable, given thermal inertia and lag in the climate system, we already have some 'climate change commitment' and warming that we are locked into."
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