It was January, and tennis players at the Australian Open were suffering through the scorcher that would be the year 2014.

They threw up and they fainted from the record-setting heat. They put on ice vests. The soles of their sneakers and the bottoms of their water bottles softened as the mercury marked 109 degrees Fahrenheit. They were knocked flat by the longest Melbourne heat wave in a century.

The new year seemed to be welcoming the world to the new normal.

All year long, from the Antipodes to the Aleutians, the changing global climate plagued the planet. If 2014 does not, in the final reckoning, prove to be the hottest year ever recorded, it will come very close.

As a landmark scientific review published by several U.S. government agencies declared in May: "Climate change, once considered an issue for a distant future, has moved firmly into the present."

Of course, it did not happen all at once. The Australian heat wave had lingered over from 2013. Every decade is now warmer than the one before. Each new year is one of the warmest on record. Young adults have never lived through a month when global temperatures were cooler than the old normal.

"There is no standstill in global warming," said Michel Jarraud, secretary-general of the World Meteorological Organization, which as the year drew to a close estimated that 2014 would turn out to be the warmest year scientists have ever recorded.

It is fair to say that 2014 was the year that climate change undeniably arrived.

That means much more than to say the world's temperature continued to rise. So did the emissions of greenhouse gases and the buildup of those gases in the atmosphere. And so did the gathering sense of crisis, the understanding that the time left for effective action was running out.

On the scientific front, above all, 2014 was a landmark year. The science became more unequivocal than ever: climate change has arrived, and we are its cause.

And as vexatious as it remains to convey that understanding to citizens and their leaders, the science pushed policy and the public along more forcefully than before.

The world's governments—not in every capital, but from towns and cities to the United Nations itself—ended the year seemingly determined to find a new course away from fossil fuels and toward a safer future.

Meanwhile, on the streets, a popular movement to address the crisis found its voice in 2014 as never before.

The Science Points the Way

First, the science.

Researchers have understood for a long time that man-made pollution would warm the planet. As early as 1965, a White House science panel spelled out the theory in considerable detail.

With each year the evidence mounts—and it also becomes sharper. Now, the science describes such urgency that Secretary of State John Kerry likes to declare, "It's screaming at us."

The U.S. National Climate Assessment provided much more detailed evidence in May about how the effects of climate change would play out region by region. New England, for example, should expect to see more rainfall and flooding in the most intense storms. Alaska, where recent warming has been especially pronounced, will see far-reaching changes in its ecosystems.

In addition, scientists are making headway in attributing individual extreme weather events, such as heat waves, to man-made causes.

This was the case with the heat wave in Australia that began in 2013 and lingered into this year, to the distress of those tennis players. Several studies of heat waves published in the journal of the American Meteorological Society this year showed "overwhelmingly" how climate change has loaded the dice in favor of just this kind of event. Such episodes are now as much as 10 times as likely to occur.

But the main development of the past year or so was the solidification of the comprehensive scientific consensus on what is happening to the climate and what needs to be done about it.

Starting in late 2013 and in a stream of reports published throughout 2014, the Intergovernmental Panel on Climate Change, representing the mainstream view of the world scientific community, issued its Fifth Assessment Report. In three massive volumes and a final summing up, the organization warned in stark terms of the grave repercussions of inaction.

Among the principal findings were these:

It is now "extremely likely" that human activities including the burning of fossil fuels are the cause of the warming that is already upon us.

To have a reasonable chance of keeping the world within a relatively safe limit of 2 degrees Celsius of warming from pre-industrial levels, we must live within a carbon budget that is already more than half exhausted.

Without a change in course, the carbon budget will run out in just a few decades.

The harm of greater warming if we stay on the current course could be "severe, pervasive and irreversible."

Warming of a bit more than 2 degrees would cause damages that might cost 2 percent of global economic activity.

The cost of taking steps to avoid that damage would be significantly less than the costs of inaction but would rise if we delay action.

The harm of global warming will fall hardest on the world's poor, would undermine global development and could prevent lifting them out of poverty.

For the world as a whole, a warming planet would mean lower production of food in the coming century, just as the world's growing and more prosperous population needs more food.

Putting the brakes on emissions will require rapid increases in energy efficiency and a tripling or quadrupling of carbon-free energy by mid-century.

The ultimate goal must be to reduce greenhouse gas emissions, especially from the production of energy, to near zero.

It's not too late to act, the panel's leaders said.

The Will To Change

"We have the means to limit climate change," said R. K. Pachauri, chairman of the climate-change panel, when he presented its final roundup of the science. "The solutions are many and allow for continued economic and human development. All we need is the will to change, which we trust will be motivated by knowledge and an understanding of the science of climate change."

The IPCC's work did not land in a vacuum—far from it.

In the past year there was significant progress toward the UN's goal of negotiating a comprehensive new climate treaty that for the first time would commit all of the world's nations to significant actions for eliminating carbon emissions from energy and saving the planet from catastrophic overheating.

The treaty is supposed to be finished in Paris next December, a challenging goal for negotiators who, under the UN Framework Convention on Climate Change (and its Kyoto Protocol) have talked for 25 years about reining in global emissions while accomplishing little.

There were three especially notable steps toward that objective in the last few months.

First was a summit of world leaders on climate change convened by UN Secretary-General Ban Ki-moon in New York. It galvanized nations in a way that the ponderous diplomatic process had failed to do before. And it was not just politicians, but financiers who spoke out more powerfully than before for new solutions, including calls to put a price on carbon through taxation or other market-based limits. The swelling corporate calls for action may ultimately drown out the fossil-fuel industry's naysayers, as the world of business increasingly recognizes the profound risks of inaction.

Second was a set of commitments by a handful of nations that together account for fully half of the world's emissions. They were led by the European Union, which pledged to reduce its emissions 40 percent by the year 2030 and to achieve demanding new targets for energy efficiency and the use of renewables. Then the leaders of China and the U.S.—the world's two biggest economic powers and its two biggest emitters of carbon dioxide—shook hands on an understanding that each would adopt its own ambitious target. The U.S. would reduce emissions by as much as 28 percent by the year 2025. China's emissions would peak by 2030 at the latest.

Europe's targets have the force of law. The U.S. has proposed regulations governing the use of fossil fuels for electricity that will move it well along in the promised direction. China, by establishing new cap-and-trade carbon markets and emphasizing renewable energy in its command-and-control economic system, is now beginning a historic shift away from coal. So these declarations carried significant weight with the rest of the world.

The third step toward Paris was a plenary meeting of negotiators in Lima this month. It began with wind in its sails, and after intense horse-trading that ran two days over schedule, resulted in a compromise that, however fragile, leaves open a path toward a meaningful treaty in 12 months.

Will negotiators have the political will to persevere?

That may depend on one final phenomenon that emerged in a spectacular way in September, on the eve of Ban Ki-moon's UN summit—the coming of age of a new popular movement demanding climate action now.

Hundreds of thousands of marchers filled the streets of Manhattan, curb to curb for 50 blocks or more. Their presence attested to a new dynamic in which inside-the-beltway lobbyists and well-heeled think tanks joined forces with grassroots anti-fracking and anti-pipeline protestors, in which labor unions and school kids found common cause.

They are not yet a force potent enough to tip the political balance in the U.S. The November election showed that as voters expanded the power of Republicans in Congress, who are, by and large, allied with the fossil fuel industry.

If nothing else, the rising voices in the street will embolden the Obama administration as it defends its ambitious climate policies. And because the turnout showed a rising fervor and a resurgent organizational skill that had been lying fallow, the marchers could ultimately wield as much influence as all the scientists of the Intergovernmental Panel on Climate Change, or all the movers and shakers of the UN Framework Convention on Climate Change.

The activists' parade, as much as any heat wave or peer-reviewed publication or diplomatic gambit, could prove to be 2014's defining moment for action on the climate crisis.