Accomplishing it is arguably the most difficult problem facing the world, but at least the target is clear. Negotiators gathered in Cancύn, Mexico, are shooting to limit global warming to less than 2 degrees Celsius (3.6 degrees Fahrenheit), a goal set a year ago in Copenhagen.
As international climate talks head toward the two-decade mark, achieving this goal is anything but a certainty. But if you take the leap of faith and envision Earth in 2100 with its overall surface temperature 2 degrees warmer than now, what is the planet like?
Although researchers don't have the details, a broad-strokes portrait is emerging, in which day-to-day life is punctuated by increasingly intense storms, wider-ranging wildfires and increasing drought, among other changes.
"These guidelines, in terms of global mean temperature, they are more like speed limits," Raymond Pierrehumbert, who directs the Climate Systems Center at the University of Chicago, told LiveScience. "The warmer you get, the more bad stuff can happen and the more outside the natural range of the Earth's climate we get." [10 Surprising Results of Global Warming]
A warmer world
The implications depend on where you are. Warming over land is twice as intense as over the ocean, and it is exacerbated over the Arctic, where retreating sea ice reflects less light and so produces less cooling, according to Kevin Trenberth, senior scientist and head of the climate analysis section of the independent National Center for Atmospheric Research (NCAR) in Boulder, Colo.
The more severe precipitation patterns associated with a warmer world are already showing up, Trenberth said.
Last year, the sea surface temperatures in the subtropical Atlantic were as much as 1.1 degrees Celsius (2 degrees Fahrenheit) above normal, and about half of this increase can be attributed to global warming, according to Trenberth. In February, this warm spell contributed to a perfect storm, and "Snowmageddon 2010" shut down a swath of the United States' East Coast, including Washington, D.C.
"The fact that this was a big one has a large element of chance, but the underlying environmental conditions associated with global warming mean that when the conditions are right, the result is bigger and better than anything else seen before," he told LiveScience in an e-mail.
In other words, day-to-day weather in a warmer world may remain about the same, but extreme events become more extreme.
While warming oceans may not produce more tropical storms and hurricanes – they may even produce fewer – those storms will be more intense, and with longer dry spells between them. More sporadic precipitation, combined with earlier snowmelt, particularly in mountains like the Rockies, will increase the risk of wildfires, according to Trenberth.
A study published in 2007 in the journal Climate Dynamics predicted wetter winters for the northeastern United States – with 10 to 15 percent more precipitation – and hotter summers, with increasing drought over the next century as things heat up.
"We are getting a better and better idea of the whole picture, but it is still very difficult to translate general thinking about what is happening into specific results," said Mark Schwartz, a distinguished professor of geography and climate at the University of Wisconsin-Milwaukee who worked on the 2007 study.
A 2010 report by the National Research Council, to which Pierrehumbert contributed, breaks down a series of incremental changes. Each one degree increase could mean up to 10 percent less rainfall during the Mediterranean, southwest North American and southern African dry seasons, and a corresponding increase in Alaska and other high latitudes of the Northern Hemisphere. It also could mean up to 10 percent less stream flow in some river basins, including the Arkansas and the Rio Grande, and an up to 15 percent reduction in the corn crop in the U.S., Africa and wheat in India. Each degree could also bring up to a 400 percent increase in area burned by wildfire in parts of the western U.S. And the dizzying array of impacts the authors project widens as the increases rise above two degrees.
Nature and politics collide
The Earth's average surface temperature has risen 0.7 C (1.3 F) since humans accelerated emissions of carbon dioxide and other greenhouse gases during the Industrial Revolution. The Copenhagen Accord leaves negotiators some wiggle room, in that it didn't specify whether the 2-degree Celsius cap includes the increase so far.
In Cancún, there is a movement afoot to set the threshold lower, to 1.5 C (2.7 F). Leading the push is the Alliance of Small Island States, representing island nations, which are particularly vulnerable to rising sea levels, according to Ramzi Elias, an associate for the European Climate Foundation who is attending the talks. (The Marshalls, Kiribati, and Tuvalu islands are already feeling the effects for rising sea levels.)
But setting a threshold doesn't guarantee the emissions reductions that are necessary to back it up.
In a report issued as talks in Cancύn began, the United Nations Environment Program found that the pledges made a year ago in Copenhagen filled 60 percent of the gap between the "business as usual" scenario and the rate of greenhouse gas emission necessary by 2020 to cap global warming at 2 C. (Elias was a project manager for the UNEP report.)
Some fear that 2 degrees Celsius may no longer be attainable. The Intergovernmental Panel for Climate Change (IPCC) uses a range that goesup to 4 C (7.2 F). However, its most extreme scenario extends well above that.
A series of papers, which will appear in the Jan. 13 issue of the journal Philosophical Transactions of the Royal Society A, looked at the likelihood and the implications of warming well above the 2-degree threshold.
"The continued rise in greenhouse gas emissions in the past decade and the delays in a comprehensive global emissions reduction agreement have made achieving this target extremely difficult, arguably impossible," wrote researchers led by Mark New of the University of Oxford.
An increase of 4 degrees Celsius within the century is more likely, and from disappearing coast cities to increased water stress to changing ecosystems, the changes we see will be larger for 4 degrees than for 2. In fact, the additional warming could have dramatic consequences, such as the collapse of farming in sub-Saharan Africa, the researchers wrote.
When following the progress of the talks in Cancύn, which finish up Friday, neither optimism nor pessimism is productive, Pierrehumbert said.
"The point is, our only hope is to get the carbon under control, and we just have to keep trying until we do," he said.
- Questions Answered: The Basics of the Cancun Climate Talks
- Top 10 Craziest Results of Global Warming
- Earth in the Balance: 7 Crucial Tipping Points
You can follow LiveScience writer Wynne Parry on Twitter @Wynne_Parry