The San Fernando Valley Generating Station is seen December 11, 2008 in Sun Valley, California. (David McNew/Getty Images)

The debate is over.



On Friday, an international panel of hundreds of scientists will issue its fifth (and perhaps final) comprehensive scientific assessment of what we now know about climate change. Its central conclusion will be certain and unequivocal – human beings are altering the climate, with impacts starting to occur now.



Yes, there are still a handful of scientists who like to take slightly contrarian positions, which allows them to be quoted in media stories. And, yes, scientists are still trying to determine how much things like La Nina, excess water vapor and ocean sinks (the ability of the oceans to store carbon dioxide and warming) mask the extent of how rapidly the planet is warming from greenhouse gases.



But the central portion of the artificial science debate – the one that has vexed policy makers for decades – is now over. Climate change is real, human beings are responsible for a good portion of it, and we need to take the issue seriously, sooner rather than later, and start to do something about it.



One of the reasons that global warming has traditionally polled so low with the American public is that it's perceived as a distant threat – not a present one. There are other things, like lack of health insurance or a struggling economy, that matter more to them right now. But that, too, is changing.



As science settles on the ways in which climate change drives extreme weather events like Superstorm Sandy, massive wildfires in the west, extended droughts that are causing water shortages or one in 1000 years flood events that no one could have imagined until recently, the public gets it.



When the plenary session of the Intergovernmental Panel on Climate Change (IPCC) finishes its work late Thursday night and issues its report on the science basis of climate change to nearly 200 governments, it will essentially end the climate science portion of the debate for policy makers and government officials.



The IPCC is the most comprehensive and robust assessment of existing climate change research we have available. Its last report six years ago – which won the Nobel Peace Prize in 2007 - had 498 authors from 28 countries, who reviewed more than 6,000 peer-reviewed studies to reach conclusions about the science basis for climate change. This new report, the one that will be issued Friday, is broader and deeper - more than 600 authors from 32 countries contributed to the report. They assessed 9,200 peer-reviewed studies, undergirded by a staggering two million gigabytes of numerical data.



And what it will say is this: it is "extremely likely" that human behaviors (burning fossil fuels) are driving climate change. That's as close to a consensus as you will ever get in the scientific community, which uses an argumentative, peer-review process as its gold standard for information- and knowledge-sharing.



It will also confirm the accelerated rate of change for impacts such as sea level rise, the steady retreat of Arctic sea ice and quickened melting of ice sheets and glaciers, as well as offer more detail on scenarios that will shape international negotiations over both short-term and long-term greenhouse gas emissions, including how long "business as usual" can be sustained without dangerous risk.



World leaders who struggled to get out from under a global economic slowdown are now paying attention again. Climate change is back on the agenda for Davos in January. The secretary general of the United Nations has now said he will invite world leaders to New York in the fall of 2014 to deal with climate change.



Led by leaders in the United States and China, developed nations have now agreed in principle to deal with short-term greenhouse gas pollutants like HFCs that can forestall nearly a degree Fahrenheit of warming during the coming decades.

A clear path forward is now, finally, starting to emerge on climate change. The Clean Air Act allows White House and State Department officials to negotiate bilaterally with China on significant greenhouse gas cuts. And if China and the United States – which, together, represent 40 percent of the world's emissions – can agree, then it is unlikely that others will stand in the way at this juncture in history. We won't need an international treaty.



Difficult, complicated science questions like the interplay between natural climate variability (e.g., volcanic eruptions, solar minimums, the el Nino-La Nina cycle) and man-made, fossil fuel-based energy consumption that accelerates greenhouse gas emissions and drives climate change still need to be further clarified.



But the first decade of this century was the hottest ever in recorded history — despite natural factors like a solar minimum and an unusual, double La Nina cycle exerting a cooling influence. History tells us that once the current natural cycles revert, surface warming will jump upward, as it did at the peak of the solar cycle and el Nino in 1998.



Even the Republican leadership in . Congress – which is still fighting a losing "war on coal" political narrative that fared poorly in the 2012 elections - has decided that it's no longer productive to argue the science of man-made climate change.



"Climate policy will play a major role in the campaign in specific (local) areas," Jordan Davis, policy director for the National Republican Congressional Committee, told National Journal Sunday when explaining how it will argue against President Barack Obama's national climate plan in a handful of coal districts. "It's not so much about the climate science," Davis said. "We have a lot of members in our caucus who are not crazy climate deniers. It's about the policy."



Just as it finally became apparent years ago that it was no longer logical to doubt the science explaining nicotine addiction and cancer risk from cigarettes, we have now reached the same point on climate change. There is a scientific consensus, however inconvenient that might be for governments and businesses forced to deal with it.



What we do about pricing and dealing with climate risk – and how politicians argue about it in federal election cycles – may be very much in play. But the science itself is no longer in doubt. That debate is over.