The science of climate change is complex and often uncertain. So are the politics around it.
Starting Monday (Nov. 29) negotiators from around the globe gathered in Cancun, Mexico, for two weeks to haggle out measures to mitigate and adapt to the changes associated with global warming. These could be dramatic. According to the Intergovernmental Panel on Climate Change droughts, accelerated extinctions, rising sea levels and many other prospects could be on the horizon if something isn't done to curb rising greenhouse gas emissions.
There is no easy solution; this is a problem with a history that stretches back to the Industrial Revolution. So, as greenhouse gases accumulate in the atmosphere, negotiations continue. This is the 16th session of climate talks under the United Nations framework.
Here's a brief guide to talks so far, what's going on and the implications.
When did climate talks begin?
In 1992, the international community met in Rio de Janeiro where they adopted the United Nations Framework Convention on Climate Change, which acknowledged the risks greenhouse gas emissions posed to global climate and established a framework for efforts to tackle it. Nations agreed to share information, launch national strategies to address greenhouse gas emissions and cooperate in preparing to adapt to the impacts of climate change, according to the UNFCCC website. It did not, however, set targets to reduce greenhouse gas emissions. . [The History of Climate Change Science]
What's the Kyoto Protocol?
In 1997, 37 industrialized countries and the European states pledged to reduce their greenhouse gas emissions by an average of 5 percent by 2012, as compared with 1990 levels. The protocol also established guidelines for a carbon emissions market – where users of cleaner technology can sell carbon credits to polluters, mandated monitoring of countries' actual emissions and established a fund to finance projects in developing countries to help residents cope with a warming planet.
Though a legally binding treaty, a number of nations aren't on track to meet their commitments, according to the UNFCCC.
Early on in the first week of talks in Cancun, the Japanese announced they would not support an extension of the Kyoto Protocol after it expires next year. Japan's greenhouse gas emissions have grown — not declined — between 1990 and 2010, according to the UNFCCC.
"There have been some successes with the Kyoto Protocol, but, overall, the Kyoto Protocol hasn't seemed to make a difference in the overall emission trajectory of the world," said Nathan Hultman, a professor of public policy at the University of Maryland, who is attending the Cancun talks.
What happened in Copenhagen?
A more recent round of talks, held in Copenhagen, offered the international community a sort of do-over, where nations could make nonbinding and more flexible pledges to reduce emissions, Hultman told LiveScience.
The U.S. had pledged to reduce its emissions by 17 percent by 2020. However, this pledge isn't off to a good start; after a regulatory cap and trade system to reduce emissions failed to pass in the U.S. Senate earlier this year, Congress has yet to pass climate legislation.
Copenhagen was a watershed, because for the first time, nearly all of the world's largest economies, including China, India, Brazil and the United States made pledges, said Ramzi Elias, an associate with the European Climate Foundation and the project manager on a recent U.N. Environment Program report on emissions projections.
"People forget about that," Elias, who is at the talks in Cancun, told LiveScience.
In spite of this, the talks in Copenhagen were widely regarded as a disappointment, since no treaty to succeed the Kyoto Protocol was established. In framing the latest round in Cancun, the UNFCCC acknowledges these "negative headlines," but notes one achievement made in 2009: The resulting accord states that the global temperature increase should be below 3.6 degrees Fahrenheit (2 degrees Celsius).
What's going on in Cancun?
After Copenhagen, expectations for the Cancun talks are not high.
"Most people have [a] pretty good idea of what is going to come out of Cancun, which is not much," Hultman said. "There will not be any big treaty here."
The biggest challenge to climate negotiators is how to distribute the burden associated with reducing emissions, or in other words, determining a fair way for each country to pull its own weight, or at least the richer nations, in response to the problem.
"That's the big problem, the big problem stuff is not being discussed in detail at the Cancun meeting," he said. Rather negotiators are focusing on smaller, easier-to-tackle issues.
Talks are focusing on sharing clean energy technology, helping poor countries adapt to the effects of climate change, slowing deforestation, funding and solving methodological issues such as how to monitor any progress made, he said.
Although less than glamorous, the accounting rules that allow countries to take advantage of carbon credits, surplus emission units and carbon offsets could have a big impact on how much greenhouse gas emissions are actually reduced, according to Elias.
Currently, the rules around credits for carbon removal by existing forests are a hot topic, he said. Since forests suck up carbon, the question is how to account for the removal of greenhouse gases by forests and emissions caused by deforestation.
What are the prospects for cutting emissions to a point where a global temperature increase of 3.6 degrees F (2 degrees C) by 2100 is realistic?
The average temperature of the Earth's surface has risen by 1.3 degrees F (0.74 degrees C) since the late 1800s, and it is expected to increase by as much as 7.2 degrees F (4 degrees C) by 2010, according to the UNFCCC. [2010 Tied for Warmest on Record]
The 2-degree-Celsius target may bring changes across the globe – increasing drought, increasing damage from floods and storms along coastlines, increased extinctions and range shifts among species, more risk of wildfires, changes in distribution of disease-spreading insects, and so on, according to the Intergovernmental Panel on Climate Change, 2007 Synthesis Report.
The report shows the severity of these impacts grows along with the projected temperature increase.
In order to keep global warming below the 2-degree mark, the rate of global greenhouse gas emissions must decline to 44 gigatons of carbon dioxide equivalent by 2020, according to a report issued by the United Nations Environment Program. By comparison, the global rate of emission in 2009 was 48 gigatons. (Greenhouse gas emissions are expressed as the amount of carbon dioxide that would generate the same amount of warming.)
However, the Copenhagen pledges bring the world up at least 5 gigatons short of reaching this goal, creating an "emissions gap," according to the report. The solution it offers is a combination of stronger pledges and tighter accounting rules.
There is a push underway in Cancun for the pledges to be anchored in a formal document, acknowledged by all of the parties, according to Elias, a project manager on the report. In the future, countries could review their pledges and ramp them up, he said.
"What our study shows is negotiations really do matter," he said.
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You can follow LiveScience writer Wynne Parry on Twitter @Wynne_Parry.