When it comes to climate change, it sometimes feels as if there are two planets Earth. On one — the one where people watch An Inconvenient Truth and Al Gore wins Nobel Prizes — there is a sense that a true crisis is gathering. On the other, a studied obliviousness prevails, and a 35-miles-per-gallon fuel-economy standard counts as a "bold" step forward.

In December, President Bush signed an energy bill that brought the first fuel-economy increase in the United States since 1975, calls for a nearly eight-fold expansion of renewable fuels by 2022, and mandates that most standard incandescent light bulbs be phased out over the next six years. The Bush administration hailed the bill as a major step forward in the climate change battle.

Half a world — and maybe more — away, delegates from 187 nations gathered in Bali, Indonesia, to begin negotiating a successor to the Kyoto Protocol, which will expire four years from now. The United States, which emits 22 percent of the world's greenhouse gases, has notoriously refused to sign the protocol, effectively blocking any real effort to rein in climate change. The day the Bali conference opened, Australia — the other holdout on Kyoto — finally signed, effectively defecting from the second Earth to the first and leaving the United States home alone on that other planet. The U.S. government’s emissaries to Bali were nearly booed off the dais for their obstructive tactics.

Meanwhile, back home, James Connaughton, the chairman of the White House Council on Environmental Quality, told reporters that when President Bush signed the energy bill into law, "We translated talk in Bali into very concrete, legally accountable action here in the United States."

Despite the cognitive distance between these two Earths, they do have one thing in common: an atmosphere that is rapidly filling with greenhouse gases that make the planet hotter. Late last year, the International Energy Agency, which serves as a sort of oracle of the future of the planet’s energy system — and, increasingly, that system’s effect on the climate — issued a prophetic proclamation: "The primary scarcity facing the planet is not of natural resources nor money, but time."

Despite the angst in Bali and the bombast back in the U.S., a subtle shift in attitudes is under way, and the two Earths may finally be converging. Even on the Earth of Bush, climate change is an acknowledged fact.

But knowing there's a problem doesn't necessarily equate with knowing how to solve it. Even among people who agree on the need to act quickly and decisively to address climate change — which is to say, even in Gore World — there's deep disagreement about what to actually do. One thing is clear, however: Fighting climate change could prove to be a lot harder than anyone's telling you.

Marty Hoffert is an emeritus physics professor at New York University who has been closely involved in the effort to develop a strategy for stopping global warming. "What we actually have to do and what is being proposed," he says, "are very far apart."

Like any good health-improvement regimen, a plan to fight climate change starts with a target. The most ambitious goal — and the one that has come to serve as the basic, recurring motif in climate discussions — would be reached when the concentration of greenhouse gases in the atmosphere was stabilized at 450 parts per million. (Today, they are around 384 ppm and growing.) Hitting that target would likely keep the average temperature of the planet from rising more than 2.4 degrees Celsius (4.3 degrees Fahrenheit) above pre-industrial levels.

And doing that would prevent a generalized unraveling of the planet. According to the Intergovernmental Panel on Climate Change — which shared last year's Nobel Peace Prize with Al Gore — holding the temperature rise to around 2.4 degrees Celsius could lower the risk of both extinction of as many as 40 percent of Earth's species and "extreme weather events" such as Hurricane Katrina; keep the Greenland and West Antarctic ice sheets from shedding chunks of ice into the oceans, raising sea levels by as much as 33 feet; and prevent melting permafrost from accelerating warming by burping up huge amounts of methane, another greenhouse gas.

To hit the 450-ppm target, world carbon dioxide emissions would have to peak by 2015 and — because they are long-lived and continue to have a powerful effect in the atmosphere — then be reduced 50 to 85 percent below 2000 levels by the middle of the century. "The task we have is whether we can continue to grow the global economy at 3 percent a year," Hoffert says, "and first hold CO2 constant, then phase it out."

Doing that will require a massive technological exorcism to "decarbonize" the energy mix that powers the global economy. Eighty-five percent of human-caused carbon dioxide emissions come from the burning of fossil fuels to make energy. World energy use is projected to continue growing at 1.8 percent per year, about half the rate of GDP growth. The aggregate "burn rate" of the world's energy infrastructure is staggering. Today, humankind produces about 15 terawatts — that's 15 trillion watts — of electricity. That, plus the emissions from cars, planes, ships, cement factories and everything else that burns fossil fuel, pumps around 27 billion tons of greenhouse gases into the air each year. Holding carbon dioxide constant would require eliminating emissions from the equivalent of at least 11,000 coal-fired power plants, more than actually exist on the planet today.

Decarbonizing the energy mix means either replacing coal, which supplies a quarter of the world's energy, with alternative energy sources that don't emit carbon dioxide when they're burned or figuring out a way to capture carbon dioxide from the exhaust of coal and other fossil-fuel electric plants and to then sequester it underground. It also means finding "carbon-neutral" replacements for the gasoline and other liquid fuels that power transportation.

And evidence is mounting that the task of stabilizing atmospheric carbon dioxide may be more challenging than even most climate scientists have thought. Most computer models of how climate change will unfold include an assumption that, even without dramatic efforts to combat global warming, the rate of growth in greenhouse gas emissions will slow somewhat. The energy mix has, in fact, already been decarbonizing for two centuries as efficiency has improved and natural gas, which is cleaner than coal, has come into wider use. The assumption that the trend will continue is a critical one because it forms the "business as usual" scenario from which the quantity of greenhouse gases would be reduced to achieve, say, a 450-ppm target.

But as it turns out, real-world emissions are outpacing even the worst-case scenario in the latest round of projections by the Intergovernmental Panel on Climate Change. Last year, a number of climate researchers discovered that emissions of greenhouse gases have sharply accelerated since 2000, and a group of scientists led by Australian researcher Josep Canadell concluded that the world is now witnessing "the most rapid increase since the beginning of the industrial revolution."

Much of the increase is driven by China. What is sometimes called "the Chinese miracle" may also prove to be a stupendous climatological hex. The country has been powering its record-setting economic growth with a slew of new coal-fired power plants: It builds a new one every seven to 10 days. Last year, China overtook the United States as the planet’s largest emitter of carbon dioxide.

"Things turned around so quickly that it caught everybody by surprise," says Mark Levine, who leads the China Energy Group at the U.S. Department of Energy's Lawrence Berkeley National Laboratory. How China (and, to a lesser extent, India) powers its way into the future is now the single biggest variable in the global climate equation. "It depends so critically on what China does for policy," Levine says, "and if they can tame their energy growth or not."

Per capita, carbon dioxide emissions in China are now one-fifth of those in the U.S., but they are rising. The International Energy Administration estimates that China will be responsible for fully one-third of the growth in world energy demand over the next 22 years.

China's scenario-busting energy-and-emissions growth is also one of the shadow effects of globalization: A number of recent studies contend that the country's exports account for somewhere between 23 and 34 percent of its total greenhouse gas emissions. By transferring much of their manufacturing to China, the U.S. and other developed nations have, in fact, amplified the climate footprint of those processes. Because China is so coal-reliant — and because the country's coal-fired power plants emit 22 percent more carbon dioxide per kilowatt-hour than the global average — manufacturing a Mattel toy in China generates far greater emissions than it would in the U.S.

It doesn't help that, in the rush to power the boom, as many as a quarter of the coal-fired power plants in China are essentially illegal, having been built without the approval of the central government. "There are some people, including Al Gore and Jim Hansen" — the NASA climate scientist whom the Bush administration famously tried to muzzle — "who say we need a moratorium on coal-fired power plants," Hoffert says. "But that’s not going to happen. It's certainly not going to happen in China, where the central government is a dictatorship, and even (it) isn't able to exert control on the provinces to stop building coal-fired plants. We're not gonna be able to get to them by moral suasion."

And not just China but the entire world may well get dirtier. Conventional wisdom holds that high oil and gas prices will spur the development of alternative, renewable energy sources such as solar and wind. But high oil prices have, perversely, put a new shine on coal, which is not only cheap but also available in huge quantities.

"All those (climate) scenarios assume that there's going to be a spontaneous decarbonization of the energy economy," says Ken Caldeira, a Stanford University climate scientist. "But there's far more coal available than anything else, and so the long-term trend has to be toward increased carbonization." Indeed, recent studies indicate that the world energy mix has been recarbonizing since 2000. In its most recent projections, the International Energy Agency predicts that without aggressive climate policy changes, the number of coal-fired power plants in the world will increase 46 percent by 2030.

Similar concerns hold true for gasoline. Many people have assumed that as liquid petroleum is tapped out — as the world experiences "peak oil," which, though its timing is subject to considerable uncertainty, will inevitably happen — greenhouse gas emissions will go down. That view is wrong.

"'Peak oil' is not the answer to the climate-change problem," says Alex Farrell, a member of the Energy Resources Group at the University of California, Berkeley. "It makes it worse." The explanation for this counterintuitive reality is simple: Although conventional liquid petroleum may be dwindling, plenty of "unconventional" sources of petroleum, including tar sands and oil shale, lie waiting. All told, the amount of liquid hydrocarbons remaining on the planet — the tarry dregs in the genie's lamp — may actually be 19 times more than has been burned to this point.

And, greenhouse gas-wise, it’s extremely dirty petroleum. Refining unconventional petroleum releases up to twice the amount of greenhouse gases involved with "regular" petroleum — which means that a transition to unconventional fuels would torpedo efforts to rein in climate change. "We're not running out of oil," Farrell says. "We're running out of atmosphere."

Several teams of climate scientists have proposed blueprints for the kind of massive transformation of the global energy system that would need to happen to stabilize carbon dioxide levels. The best known of the bunch comes from Stephen Pacala and Robert Socolow, two researchers at Princeton University who built their plan around existing energy technologies. "I was so infuriated by the Secretary of Energy's (Spencer Abraham's) speeches that we needed a discovery as revolutionary as the (invention of electric motors) by (Michael) Faraday in order to get started," Pacala says. "I thought, 'That's just preposterous.' We had every tool we needed right now to put ourselves on the track we needed to get on."

In the original formulation of their plan, published in Science in 2004, Pacala and Socolow proposed assembling a package of greenhouse gas reduction measures, or "stabilization wedges." Each wedge can ultimately replace 1 billion tons — or one gigaton — of carbon emissions per year. The two researchers proposed 15 possible wedges and suggested that seven would be needed to achieve a roughly 450-ppm target.

If, for example, the fuel economy of 2 billion cars rose from 30 miles per gallon to 60 over the next half century, that would generate one wedge of emissions reductions. Conservation measures, such as improving the energy efficiency of appliances and buildings, could yield a wedge. Another wedge could be generated by sequestering the carbon dioxide from 800 large coal-fired power plants. Increasing ethanol production by 50 percent would yield a wedge, as would adding 2 million one-megawatt wind turbines. Doubling the size of the world’s existing fleet of nuclear fission reactors would create yet one more wedge — but a politically problematic one, given public concern about nuclear safety and waste disposal.