The target to keep global warming to 2 degrees Celsius (3.6 degrees Fahrenheit) above preindustrial levels – as agreed on during climate talks in 2010 – may be unachievable. Even if it is possible, scientists argue that it may not be enough to save many of the planet's species.

A United Nations scientific panel is assessing the chances of significantly limiting greenhouse gas emissions to meet the 2-degree goal, ahead of December climate talks in Paris. The panel is gauging if it will be effective — and whether an even more ambitious 1.5-degree limit should be considered.

When the 2-degree limit was agreed to at a 2010 conference in Cancún, Mexico, it faced intense opposition from the low-lying island nations and poor countries at greatest risk from rising sea levels and extremes of weather. They pushed instead for the stricter 1.5-degree limit.

The majority of support for the 2-degree plan came from industrialized nations better equipped to adapt to climate change.

Geographer Petra Tschakert of Penn State University, a scientist involved in the new assessment, said she supports the lower limit.

"Without a doubt, it is in the utmost interest of a large number of countries to pursue the 1.5-degree C target, as ambitious or idealistic it may appear to date, and to see it anchored as a binding goal..." she wrote in an article for Climate Change Responses.

Other scientists have also urged consideration for the lower figure.

At a climate conference last year in Lima, Peru, Hans-Otto Pörtner from the Alfred Wegener Institute in Bremen argued, "some species would struggle to cope with the speed of 2-degree C warming, but that most organisms should be able to move to a different place under 1.5-degree C."

There is a strong political element to the desire of many nations for the consideration of a lower limit, Tschakert said.

Such a benchmark would allow them to make a push at the Paris talks for "loss and damages" — compensation for poorer countries impacted by global warming from the larger greenhouse-gas emitting nations.

"The real question is whether or not the high-income countries, the big polluting countries, are willing to pay loss and damages to countries that bear the brunt of the impacts," Tschakert said. "Vulnerable countries have no other leverage within this political process."

Even if a 1.5-degree cap is unattainable – many scientists believe that's the case, with the world already having warmed 0.85 degrees C – it could provide the necessary leverage. Tschakert said that would allow them to make a strong case for a payout in the event of climate disaster.

"The stakes," she added, "are enormous."

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