With 2014 looking like it will go down as the warmest year globally on record, climate scientists now wonder whether 2015 will be the year nations finally take significant steps to reduce greenhouse gas emissions.
They'll have a chance at a climate summit in Paris next December, but the road there goes through two countries: the U.S. and China. The world's biggest carbon dioxide emitters, they've pledged to cut back, and other nations are watching them closely.
"I think the big issue in 2015 will be solutions," says Chris Field, director of the Carnegie Institution's Department of Global Ecology at Stanford University.
"2015 is the year of the Paris meeting to develop the next international agreement," adds Fields, who helped write one of the influential reports by the U.N. Intergovernmental Panel on Climate Change. "It will be a big year for local, state, national, and international discussions."
The momentum began building late this year, notes Jonathan Overpeck, director of the University of Arizona's Institute of the Environment and also a co-author of an IPCC report.
"There seem to have been some real breakthroughs that signal hope," he says, "particularly the work by China and the United States to step up and recognize their outsized rolls in causing the problem, as well as in solving the problem."
"The U.S.-China accord is a very important step," agrees Dennis Hartmann, a University of Washington atmospheric scientist and IPCC co-author.
Presidents Barack Obama and Xi Jinping last month announced a U.S. pledge to reduce carbon emissions by a quarter come 2025 and China's first-ever pledge to stop CO2 emissions growth by 2030.
That was followed by international talks in Peru earlier this month that Overpeck calls "a warm-up act" to get nations "focused on what it will take to finally make a serious bid to curb global emissions."
The key for progress next year, Fields believes, "is building on the recent U.S.-China agreement."
"There is momentum for more progress, but the steps from momentum to real commitments are unclear."
That's partly because the U.S.-China announcement is non-binding -- a point seized on by Republicans to criticize it as hurting the U.S. economy.
"It requires the Chinese to do nothing at all for 16 years, while these carbon emission regulations are creating havoc in my state and other states across the country," complained incoming Senate Majority Leader Mitch McConnell of Kentucky.
That mistrust does not bode well for momentum, especially with Republicans controlling Congress next year.
Still, there are other ways momentum could build. Large developing countries besides China are getting "more involved in being part of the solution," Field says. "This is critically important."
Moreover, climate developments themselves could add to a sense of urgency.
First off: heat. Combined land and sea temperatures from January-November 2014 were the warmest such period in 135 years of recordkeeping, topping the 20th Century average of 57 degrees Fahrenheit by 1.22 degrees.
Record warm oceans are driving that, and this year is likely to overtake 2010 as the warmest on record as long as December is among the 10 warmest on record — a pretty safe bet, federal climate trackers have told NBCNews.com.
What's alarming is that the warmth is happening without the presence of a mature El Nino, the cyclical ocean pattern that pushed 2010 to its record warmth.
"Over time the planet continues to warm" even without El Ninos, showing that those natural patterns "do not dominate the long-term signal," Deke Arndt, climate monitoring chief at the National Climatic Data Center, recently told NBCNews.com.
An El Nino does appear to be forming now and that "would move the global thermometer upward," notes Hartmann, setting 2015 up for the possibility of another record warm year.
A second potential climate development has to do with the Arctic, where temperatures continue to rise at twice the rate of the rest of the world.
Experts track sea ice, which floats on the ocean, as well as the Greenland ice sheet, which if it completely melted into the ocean would raise sea levels by 20 feet.
Arctic sea ice levels continue to be below normal. A coastal survey last September spotted some 35,000 walruses crowded along a shoreline in northern Alaska and, while such haul outs are not new, experts fear they will become more common in the future if sea ice, a favorite spot for walruses to hang out, continues to decline.
As for Greenland, a study earlier this month found that melt predictions are likely too conservative.
Overpeck expects advances in ice sheet modeling will soon provide a much better sense of how much sea level rise to expect from melting Antarctic and Greenland ice sheets.
"Will we see three-plus feet by the end of this century or less?" he asks, referring to some model predictions. "Will stabilizing the Earth's climate at two degrees Celsius above pre-industrial (times) commit us to 20-plus feet of future sea level rise or less?"
The latter, longer-term projection refers to efforts to limit temperature increases to two degrees Celsius, or 3.6 degrees F, but some scientists are now saying that global goal is not doable and that a better approach is to focus on how key countries can reduce emisisons.
Those key countries include the United States, and Overpeck, for one, feels that the dry spell across the U.S. Southwest, especially California, since 1999 could provide impetus for action.
"There is a grim silver lining to worsening drought conditions," he says. "They might serve as a wakeup call to our nation at a critical time when we have a chance to work with all the nations of the world to solve a problem that is especially damaging to our own country."