Humanity has yet to reach the point of no return when it comes to catastrophic climate change, according to new calculations. If we content ourselves with the existing fossil-fuel infrastructure we can hold greenhouse gas concentrations below 450 parts per million in the atmosphere and limit warming to below 2 degrees Celsius above preindustrial levels—both common benchmarks for international efforts to avoid the worst impacts of ongoing climate change—according to a new analysis in the September 10 issue of Science. The bad news is we are adding more fossil-fuel infrastructure—oil-burning cars, coal-fired power plants, industrial factories consuming natural gas—every day.

A team of scientists analyzed the existing fossil-fuel infrastructure to determine how much greenhouse gas emissions we have committed to if all of that kit is utilized for its entire expected lifetime. The answer: an average of 496 billion metric tons more of carbon dioxide added to the atmosphere between now and 2060 in "committed emissions".

That assumes life spans of roughly 40 years for a coal-fired power plant and 17 years for a typical car—potentially major under- and overestimates, respectively, given that some coal-fired power plants still in use in the U.S. first fired up in the 1950s. Plugging that roughly 500 gigatonne number into a computer-generated climate model predicted CO2 levels would then peak at less than 430 ppm with an attendant warming of 1.3 degrees C above preindustrial average temperature. That's just 50 ppm higher than present levels and 150 ppm higher than preindustrial atmospheric concentrations.

Still, we are rapidly approaching a point of no return, cautions climate modeler Ken Caldeira of the Carnegie Institution for Science's Department of Global Ecology at Stanford University, who participated in the study. "There is little doubt that more CO2-emitting devices will be built," the researchers wrote. After all, the study does not take into account all the enabling infrastructure—such as highways, gas stations and refineries—that contribute inertia that holds back significant changes to lower-emitting alternatives, such as electric cars.

And since 2000 the world has added 416 gigawatts of coal-fired power plants, 449 gigawatts of natural gas–fired power plants and even 47.5 gigawatts of oil-fired power plants, according to the study's figures. China alone is already responsible for more than a third of the global "committed emissions," including adding 2,000 cars a week to the streets of Beijing as well as 322 gigawatts of coal-fired power plants built since 2000.

The U.S.—the world's largest emitter of greenhouse gases per person, among major countries—has continued a transition to less CO2-intensive energy use that started in the early 20th century. Natural gas—which emits 40 percent less CO2 than coal when burned—now dominates new power plants (nearly 188 gigawatts added since 2000) along with wind (roughly 28 gigawatts added), a trend broadly similar to other developed nations such as Japan or Germany.

But the U.S. still generates half of its electricity via coal burning—and what replaces those power plants over the next several decades will play a huge role in determining the ultimate degree of global climate change. Coal-burning poses other threats as well, including the toxic coal ash that can spill from the impoundments where it is kept; other polluting emissions that cause acid rain and smog; and the soot that causes and estimated 13,200 extra deaths and nearly 218,000 asthma attacks per year, according to a report from the Clean Air Task Force, an environmental group. "Unfortunately, persistently elevated levels of fine particle pollution are common across wide swaths of the country," reveals the 2010 report, released September 9. "Most of these pollutants originate from combustion sources such as power plants, diesel trucks, buses and cars."

Of course, those are the same culprits contributing the bulk of greenhouse gas emissions. Yet "programs to scale up 'carbon neutral' energy are moving slowly at best," notes physicist Martin Hoffert of New York University in a perspective on the research also published in Science on September 10. "The difficulties posed by generating even [one terawatt] of carbon-neutral power led the late Nobel laureate Richard Smalley and colleagues to call it the 'terawatt challenge'."

That is because all carbon-free sources of energy combined provide a little more than two of the 15 terawatts that power modern society—the bulk of that from nuclear and hydroelectric power plants. At least 10 terawatts each from nuclear; coal with carbon capture and storage; and renewables, such as solar and wind, would be required by mid-century to eliminate CO2 emissions from energy use. As Caldeira and his colleagues wrote: "Satisfying growing demand for energy without producing CO2 emissions will require truly extraordinary development and deployment of carbon-free sources of energy, perhaps 30 [terawatts] by 2050."