In his inaugural address Monday, President Obama made climate change a priority of his second term. It may be too late.

Within the lifetimes of today's children, scientists say, the climate could reach a state unknown in civilization.

In that time, global carbon dioxide emissions from burning fossil fuels are on track to exceed the limits that scientists believe could prevent catastrophic warming. CO2 levels are higher than they have been in 15 million years.

The Arctic, melting rapidly and probably irreversibly, has reached a state that the Vikings would not recognize.

"We are poised right at the edge of some very major changes on earth," said Anthony Bar nosky, a UC Berkeley professor of biology who studies the interaction of climate change with population growth and land use. "We really are a geological force that's changing the planet."

The Arctic melt is occurring as the planet is just 0.8 degree Celsius (1.4 degrees Fahrenheit) warmer than it was in pre-industrial times.

At current trends, the earth could warm by four degrees Celsius in 50 years, according to a November World Bank report.

The coolest winter months would be much warmer than today's hottest summer months, the report said. "The last time earth was four degrees warmer than it is now was about 14 million years ago," Barnosky said.

Experts said it is technically feasible to halt such changes by nearly ending the use of fossil fuels. It would require a wholesale shift to renewable fuels that the United States, let alone China and other developing countries, appears unlikely to make.

Indeed, many Americans do not believe humans are changing the climate.

"Science is not opinion, it's not what we want it to be," said Katherine Hayhoe, an evangelical Christian, climatologist at Texas Tech University and lead author on the draft climate assessment report issued this month by the National Climate Assessment and Development Advisory Committee.

"You can't make a thermometer tell you it's hotter than it is," said Hayhoe, who with her husband, a linguist and West Texas pastor, has written a book on climate change addressed to evangelicals.

"And it's not just about thermometers or satellite instruments," she said. "It's about looking in our own backyards, when the trees are flowering now compared to 30 years ago, what types of birds and butterflies and bugs we see that ... used to be further south."

Robins are arriving two weeks early in Colorado. Frogs are calling sooner in Ithaca, N.Y. The Sierra Nevada snowpack is melting earlier. Cold snaps like the one gripping the East still happen, but less often. The frost-free season has lengthened 21 days in California, nine days in Texas and 10 in Connecticut, according to the draft climate assessment.

Scientists are loath to pin a specific event such as Hurricane Sandy or floods in England to global warming.

But "the risk of certain extreme events, such as the 2003 European heat wave, the 2010 Russian heat wave and fires, and the 2011 Texas heat wave and drought has ... doubled or more," said Michael Wehner, a staff scientist at Lawrence Berkeley National Laboratory and co-author of the climate assessment report. "Some of the changes that have occurred are permanent on human time scales."

The continental United States last year was the hottest in the 188-year record. Globally the first 12 years of the 21st century were among the 14 warmest ever.

Connecticut was four degrees warmer than the 20th century average. At current rates of CO2 emissions, scientists expect New England to have summers resembling the Deep South within decades.

The pine bark beetle, long held in check by winter freezes, is epidemic over millions of acres of forests from California to South Dakota.

Oceans, which absorb CO2, have increased in acidity, damaging coral reefs, shellfish, and organisms at the bottom of the food chain. Washington state shellfish growers have seen major failures in oyster hatcheries because the larvae don't form shells.

A National Research Council report this month said such changes in ocean chemistry in the geologic past were accompanied by "mass extinctions of ocean or terrestrial life or both."

A key question is when greenhouse gas emissions might reach a tipping point, where changes become self-reinforcing and out of human control.

Arctic sea ice reflects the sun. As it melts, the dark ocean absorbs more solar heat, raising temperatures. Similarly, the Greenland ice sheet is melting rapidly, reducing reflectivity, and possibly speeding up the melting of the West Antarctic ice sheet. The northern permafrost is thawing, with the potential to release methane, a potent greenhouse gas, and CO2 stored in soils. These can produce sudden, so-called non-linear changes that are hard to predict.

"We could be at a tipping point where the climate just abruptly warms," said Mark Z. Jacobsen, director of Stanford University's atmosphere/energy program. An Arctic melting "would make it more difficult for the Northern hemisphere to cool down, so all then Greenland would be next. Greenland stores about five to seven meters of sea level. The Antarctic in the Southern Hemisphere stores another 65 meters of sea level. That would take longer, but even that is starting to crack."

"We could be at a tipping point where the climate just abruptly warms," said Mark Z. Jacobsen, director of Stanford University's atmosphere/energy program.

A new ice core study in Nature, however, found that the Greenland ice sheet might be able to withstand temperatur e rises as high as eight degrees Celsius that existed in the Eemian period, when temperatures were warmer than now. But it also suggests that melting in Antarctica might have been higher.

UC Berkeley's Barnosky said tipping points could come earlier than anticipated when factoring in population growth and land use.

More than 40 percent of the earth's land surface has been covered by farms and cities. Much of the rest is cut by roads. By 2025, the percentage could reach half, a level that on smaller scales has led to ecological crashes.

"It's just sort of simple math: the more people, the more footprint," Barnosky said. "If we're still on a fossil fuel economy in 50 years, there is no hope for doing anything about climate change. It will be here in such a dramatic way that we won't recognize the planet we're on."

Not all climate scientists are so gloomy. Ashley Ballantyne, a bioclimatologist at the University of Montana who studies paleoclimate records, said t he climate has always changed, with ice ages, warmings and mass extinctions. He said at current CO2 concentrations, the Arctic and Greenland are likely to become ice free, as they were four million years ago.

Polar bears are poorly adapted to such conditions, he said, "but it wasn't bad for boreal trees. They were quite happy."

An international political consensus set as a danger zone two degrees of warming, expected in 25 years on current trends when atmospheric concentration of CO2 reaches 450 parts per million. It is now 400 parts per million.

Two degrees is "an arbitrary number," said Alan Robock, director of the Center for Environmental Prediction at Rutgers University. "On our current path we will go zooming way past that."

Climatologist James Hansen, head of NASA's Goddard Institute for Space Studies and activist Bill McKibbon, founder of, believe the only way to preserve the climate humans are used to is to cut CO2 concentrations to 350 parts per million, last seen around 1988.

Ballantyne dismissed the 350 goal: "That's like a 70-year-old old alcoholic saying, 'I'm going quit drinking when I'm 60 years old.'"

McKibbon and Hansen propose a tax on fossil fuels at their source, to be reimbursed to all U.S. residents. Sen. Bernie Sanders, I-Vt., plans to propose that in a "fee and dividend" scheme modeled on Alaska's oil royalty rebates to state residents.

Asked about the Sanders bill Wednesday, White House press secretary Jay Carney said, "We have not proposed and have no intention of proposing a carbon tax."

It would have to be a big tax, McKibbon said, "that drives up the price quickly. Maybe you go to the pump someday and you're paying what people in Europe pay for gasoline, which is good, because then it reminds you every time you go to the pump that you don't really need a semi- military vehicle to go to the grocery store."

Jacobson maintains that wind and solar could power the world many times over. He calculated that the world would need to install 1.7 billion solar rooftops and four million wind turbines.

Jane Long, chair of the California Council on Science and Technology, said any such conversion would be costly and difficult at best. Still, she said, "one way to get out of the hole is to stop digging."

Carolyn Lochhead is the San Francisco Chronicle's Washington correspondent.