In his inaugural address Monday, President Barack 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. Carbon dioxide 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 Barnosky, a University of California, 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, Earth could warm by 4 degrees Celsius in 50 years, according to a November World Bank report.

The coolest summer months would be much warmer than today's hottest summer months, the report said. "The last time Earth was 4 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 back yards, 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. The Sierra Nevada snowpack is melting earlier.

Cold snaps like the one gripping the East still happen, but less often.

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."

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 carbon dioxide stored in soils. These can produce sudden, so-called nonlinear 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 Greenland would be next. Greenland stores about five to seven meters of sea level."

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 Earth's land surface has been covered by farms and cities.

Much of the rest is cut by roads. By 2025, the percentage of development 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 the climate has always changed, with ice ages, warmings and mass extinctions. He said at current carbon dioxide concentrations, the Arctic and Greenland are likely to become ice-free, as they were 4 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."

Clochhead@sfchronicle.com