On July 10 in Paris a gathering of nearly 2,000 scientists and academics reaffirmed what most climate scientists have been saying for decades: The cost of making cuts in greenhouse gas pollution rises with every day of delay and zero emissions must be the goal for this century. Such was the outcome of the Our Common Future under Climate Change conference held in Paris from July 7–10 by the United Nations Educational, Scientific and Cultural Organization, better known by its acronym, UNESCO, and meant to advise the upcoming international negotiations to curb global warming in Paris this December.

The dangers include sea level rise of more than six meters at an unknown rate, more downpours, heat waves, wildfires and droughts as well as the loss of ice everywhere, among other challenges. To avoid global warming of as much as 4 degrees Celsius by the end of the century, the scientists suggested civilization has a total budget of 900 billion metric tons of carbon dioxide in the atmosphere, and the world has already added roughly 592 billion metric tons since 1780.

Nobel laureate economist Joseph Stiglitz of Columbia University suggested at the conference that the world has already missed an opportunity during the Great Recession to start the necessary shift to a low-carbon energy system by neglecting to invest in more climate-friendly energy infrastructure, which would have both stimulated the global economy and reduced income inequality. He called for an enforceable global price on carbon—not the spotty global cap-and-trade program presently in effect—to drive this shift, along with cross-border carbon tariffs for laggard nations. This rise in carbon taxes could then be used to reduce other taxes, which could mean low or even negative costs to countries. "This reflects the basic economic principle: that it's better to tax bad things than good things," he said.

The scientists also laid out in their final communiqué that climate change is already here, affecting the poor most directly in the form of extreme weather. By 2050, current greenhouse gas emissions of roughly 40 billion metric tons per year will need to drop by at least 40 percent. And by the end of the 21st century, "greenhouse gas emissions must be zero, or even negative."

In fact, all current scenarios to meet such goals require CO2 to be pulled from the air. The leading candidates for that job are plants and rocks, both of which have been absorbing CO2 via photosynthesis and chemical reactions with rain, respectively, for geologic eons, albeit slowly. Burning plants—or biomass, as scientists like to call them—in a power plant outfitted with devices to capture CO2 and bury it could potentially remove more carbon from the atmosphere than they emit, as could artificial trees and other technologies designed for that purpose. There are also dreams of vast algae farms in the ocean, fed concentrated CO2 and producing a carbon-negative fuel.

But a coal-fired power plant with CO2 capture-and-storage (CCS) technology is like nuclear: big, cumbersome and expensive. In addition, unlike a nuclear power plant, the electricity such "clean coal" produces is not cheap and it has to produce more of it to run all the CCS equipment. Adding biomass to the mix puts climate goals in competition with food production for hungry stomachs as well as space for noncrop plants and wild animals, in addition to requiring yet more use of fossil fuels to produce the nitrogen fertilizer needed to grow all these plants.

The argument against CCS is strong. Critics, however, often overlook the fact that making steel and making cement—the fundamental substrates of all power plants, whether wind turbines and solar rooftops or new nuclear—spew copious CO2, as does making the fertilizer that has made more than seven billion humans on the planet today possible. If the goal is zero- or negative emissions, these emissions will have to be eliminated. The only proposal at present to do so is CCS.

In Paris Hans Joachim Schellnhuber, a climate physicist and chair of the German Advisory Council on Global Change, called for "an induced implosion of the carbon economy" over the next few decades, a shift he compared with efforts to end slavery in the last few decades of the 19th century. Without such a momentous shift in the global energy system, there is "not the slightest chance of avoiding dangerous, maybe disastrous climate change."