Spin the wheel and whatever number the ball lands on will be the new tipping point we must get below; if not, catastrophic global warming to cause 2012-style disasters on our planet. A few years ago the upper limit on carbon dioxide was 450 parts per million (ppm), which meant an 80 percent cut in greenhouse gas emissions by 2050. Now it’s 350 ppm:

In the past four years, climate scientists, led by NASA’s James Hansen, have dramatically altered the goal. To avoid the collapse of the continental ice-sheets and a dangerous rise in sea levels, many scientists are now saying we have to get down to 350 ppm, and quickly.

This means what was already a heroic (and to many, impossible) target has become mind-boggling. Reaching 350 ppm would require a 97 percent reduction in emissions, entailing a complete conversion to renewable energy systems by mid-century, with the world economy virtually free of carbon emissions. Such a goal is far more demanding than any of the leading policy proposals under discussion.”

Recently a group of scientists wrote an open letter to Congress stressing that cap and trade is a good first step, but Waxman-Markey should just be the beginning: “The Waxman-Markey bill now being considered by the Congress offers a powerful advance and must be enacted this year. But at its best it will be only a first step in the direction that scientists now recognize as necessary to protect local and regional climates.”

What would be necessary to obtain these goals is an energy transition of unthinkable magnitude. Although authors of a report Economics for Equity and the Environment Network say stopping global warming is something we can afford and “remains fundamentally a problem of political will”, others suggest that’s not the case.

Energy chemist Nate Lewis of the California Institute of Technology says just the opposite: “It’s not true that all the technologies are available and we just need the political will to deploy them.” Lewis shows just how much political will we’d need to even have a shot at this working:

The world used 14 trillion watts (14 terawatts) of power in 2006. Assuming minimal population growth (to 9 billion people), slow economic growth (1.6 percent a year, practically recession level) and—this is key—unprecedented energy efficiency (improvements of 500 percent relative to current U.S. levels, worldwide), it will use 28 terawatts in 2050. (In a business-as-usual scenario, we would need 45 terawatts.) Simple physics shows that in order to keep CO2 to 450 ppm, 26.5 of those terawatts must be zero-carbon.

That’s a lot of solar, wind, hydro, biofuels and nuclear, especially since renewables kicked in a measly 0.2 terawatts in 2006 and nuclear provided 0.9 terawatts. Are you a fan of nuclear? To get 10 terawatts, less than half of what we’ll need in 2050, Lewis calculates, we’d have to build 10,000 reactors, or one every other day starting now. Do you like wind? If you use every single breeze that blows on land, you’ll get 10 or 15 terawatts. Since it’s impossible to capture all the wind, a more realistic number is 3 terawatts, or 1 million state-of-the art turbines, and even that requires storing the energy—something we don’t know how to do—for when the wind doesn’t blow. Solar? To get 10 terawatts by 2050, Lewis calculates, we’d need to cover 1 million roofs with panels every day from now until then. “It would take an army,” he says.”

And Marlo Lewis at the Competitive Enterprise Institute points out that “sacrifices required of developing countries would be immense, because 90% of the growth in global CO2 emissions is expected to occur in developing countries.”

This is for 450ppm. They want 350ppm. To put this in some perspective, Sharon Begley notes in her Newsweek column that we are currently at 386 ppm; we were at 280ppm before the Industrial Revolution.

We shouldn’t forget there was once a time when it was 550ppm. The much trumpeted Stern Review offers 550 parts per million of atmospheric CO2 as a magical threshold. Beyond 550 ppm, there is climate catastrophe.

The shift from 550ppm to 450ppm to 350ppm demonstrates how arbitrary and inconclusive the science on global warming is. MIT’s Richard Lindzen Yong-Sang Choi recently published a study that says the impact of carbon dioxide emissions from fossil fuels may affect the global temperature far less than originally thought. It also goes to show how capping carbon dioxide emissions could strangle the global economy and adaptation could be a much less costly but much more effective approach to dealing with climate change.