Climate change: Can we change in time?

In his inaugural address last week, President Barack Obama made climate change a priority of his second term. It might 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 Barnosky, a biology professor at the University of California at Berkeley 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 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 summer 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 a 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 such as 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.”

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. The northern permafrost is thawing, with the potential to release methane, a potent greenhouse gas, and CO2 stored in soils.

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

clochhead@sfchronicle.com