Abrupt climate change is not only imminent, it's already here. The rapid dwindling of summer Arctic sea ice has outpaced all scientific projections, which will have impacts on everything from atmospheric circulation to global shipping. And plants, animals and other species are already struggling to keep up with rapid climate shifts, increasing the risk of mass extinction that would rival the end of the dinosaurs. So warns a new report from the U.S. National Research Council.

That's exactly why longtime climate scientist James Hansen and a panoply of scientists and economists are urging in another new paper that current efforts to restrain global warming are woefully inadequate. In particular, global negotiations to limit global warming to no more than 2 degrees Celsius risk "wrecking the planet," in the words of lead author Hansen, recently retired head of the Goddard Institute for Space Studies and a researcher at Columbia University's Earth Institute.

"We started this paper to provide a basis for legal actions against governments for not doing their jobs and protecting the rights of young people and future generations," Hansen said of the paper, entitled "Assessing 'Dangerous Climate Change.'" "We can't burn all these fossil fuels. There is no recognition of this in government policies."

The paper, published in PLOS ONE, lays out the case for why fossil fuel emissions to date are dangerous enough to permanently alter the planet's climate—raising CO2 concentrations in the atmosphere above 400 parts-per-million, or levels not seen in at least 3 million years. Global emissions of CO2 from burning fossil fuels—which set another new high in 2012, according to the Global Carbon Project—must decline to zero new pollution within the next few decades, according to the analysis. "Affordable, clean energy is probably the biggest requirement that the planet has," Hansen noted at a gathering of journalists at Columbia University to discuss the new analysis.

Given the size of the problem—the fossil fuels of coal, oil and natural gas still provide more than 80 percent of the world's energy—an "all of the above" clean energy effort will be required, according to Hansen and his co-authors. That includes geothermal, hydropower, nuclear, solar, wind and further development of technologies to capture CO2 from fossil fuel burning and permanently store it in some way. Increasing efficiency in the use of such energy as well as switching cars from running on gasoline to electricity will also be vital. "What's called normal is completely reckless," said co-author and economist Jeffrey Sachs of the Earth Institute, addressing the growth rates in fossil fuel pollution of roughly 3 percent per year, putting the world on a pathway to roughly 4 degrees C of warming by century's end. "To decarbonize the energy system very deeply would require a scale of effort unlike anything seen almost anywhere in the world."

In fact, by the paper's math, emissions must start to fall by 6 percent per year globally starting now, falling below 350 ppm as soon as possible. Delay increases the need as well as making the return to 350 ppm even harder. To date, the fastest decarbonizations—such as France's switch to nuclear for electricity generation or Denmark's drive for wind power—have managed a top speed of roughly 2 percent a year, and in relatively small countries. That rate would have been fine if it had begun worldwide in 1995 but something much faster is now required, according to the analysis.

The key to accelerating that change may be a tax or other fee on carbon that would force fossil fuels to pay the full costs of their environmental impacts. Even at a price as high as $40 per metric ton that would impose a cost of just 1 percent of global economic output, Sachs notes. Already, the U.S. government has instituted a societal cost of continuing CO2 pollution—assumed now to be $35 per ton—in its economic modeling for cost-benefit analyses of various policies.

But the most necessary policy may be a plan for how to achieve such reductions, including measures to pay for it, whether a carbon tax or some kind of clean energy bond to be paid off in future. "At low cost it is possible to avoid devastating risks," Sachs argued. "This is a winnable proposition. It's got the makings of a success story but it's hard."

Starting that now is more important than ever. Already, the world has warmed by 0.8 degree C over the course of the 20th century. "It's true that we're going to pass 500 gigatonnes of carbon and one degree" of warming, Hansen said. "But that doesn't mean we have to pass the two degree threshold."

There is good news. Scientific study suggests that threats like a shift in ocean currents or a rapid meltdown of Arctic permafrost and frozen methane in the oceans is unlikely to happen abruptly. "There is enough carbon in the permafrost and oceans to equal all the coal, oil and gas and there's no reason to think it won't come out as the climate warms up," explained climatologist Jim White of the University of Colorado–Boulder, chairman of the NRC study examining the relative risks of rapid climate change, during a press conference presenting the study. But "it will not come out abruptly,” he said, adding that in the meantime, “we still have to adapt to all that carbon coming out if we don't keep the temperature down."

Surprises are inevitable—perhaps best summarized by old mapmakers in the phrase "here be dragons" for areas of the unknown—especially given the paucity of monitoring some of the biggest threats. Relatively few measurements of methane in the Arctic are taken, nor is there any oversight of the ongoing meltdown of the West Antarctic Ice Sheet as warmer waters lap at its base. "We don't do that," White noted. "There are areas of observations where we are largely blind but we know we should be watching."

The report calls for an Abrupt Change Early Warning System, potentially gathering together the monitoring and modeling efforts of an array of federal agencies—National Oceanic and Atmospheric Administration, National Aeronautics and Space Administration, the National Science Foundation, and U.S. Geological Survey, for example—and then feeding that physical scientific information into social science efforts to enhance warnings, much like the "cone of uncertainty" now used in hurricane warnings. After all, just 12,000 years ago the Earth's climate seems to have warmed by several degrees C—bringing to an end an episode of colder average temperatures known as the Younger Dryas—in a matter of decades.

The reality is, however, that the U.S. is further cutting back on such efforts, including cuts to NOAA's greenhouse gas monitoring network, delayed satellite launches for both civilian and military weather observations and other monitoring systems. "As a scientist, I like to think we study the planet well enough that we're not going to be blindsided," White said. "As a realist, I'm pretty sure we're going to be blindsided."

Plus, as recent United Nations negotiations in Warsaw proved yet again, the international argument on climate change is about who is responsible rather than what needs to be done. Surprises like the rapid meltdown of Arctic sea ice are therefore inevitable, but extreme global warming still isn't, necessarily. "It's not tipping points I worry about but points of no return," Hansen said. "It's not whether things happen quickly but that they're guaranteed."