A brief walk with Jeff Mitton and Scott Ferrenberg through the pine forests of Niwot Ridge, nearly a mile above the town of Boulder, Colorado, makes it easy to understand why people are worried about climate change. The world has been getting hotter, and so has Niwot Ridge.

Mitton, the model of a naturalist with his white beard and big smile, studied evolutionary biology after getting his Ph.D. in 1973. When I visit, he is toting a tripod-mounted camera. Ferrenberg, carrying a small ax, is tall and lanky with a salt-and-pepper goatee; he spent time in California, Pennsylvania and Arizona, working jobs from insect research to technical support for forest-fire control, before moving to Boulder and eventually becoming a graduate student in Mitton’s lab. They were then part of a team studying the effect of higher temperatures on alpine trees by planting big heating lamps in the hills above Boulder and watching to see what happened.

One late spring day in the forest, the two men spotted something odd: Mountain pine beetles were moving about. These bugs had recently been in the news. By 2010, pine beetles had damaged 4 million acres of Rocky Mountain forest in northern Colorado and southern Wyoming, and beetle watchers were warning of worse to come. It wasn’t unusual to see mountain pine beetles in the Colorado hills: They had been there for as long as anyone could remember. But everyone knew they couldn’t live at the high altitudes of Niwot Ridge and that the beetles usually didn’t come out until July.

Two Generations

Ferrenberg and Mitton realized what was happening. The life cycles of the cold-blooded beetles are controlled by heat; add more high-temperature days, and their lives speed up. With rising temperatures, altitudes that had always been too cold to support pine beetles were suddenly warm enough. More troubling, though, was how quickly the beetles were multiplying. It used to be that they would breed one generation, then die off in the cold. Now, they were breeding two generations a year, one in the late spring and one in summer. If one generation of beetles amounts to 60 insects, two generations means 3,600. A small change in climate meant big and devastating consequences on the ground.

And it was not just the lowly mountain pine beetles that were feeling the heat. Scientists have been gingerly attributing extreme weather events, such as heat waves and droughts, to a changing climate, and confidently predicting worse effects to come. Whatever their individual causes, wildfires, droughts and storms with the power of Hurricane Sandy have made vivid the dangers of a warming world. No corner of the Earth was being spared by these and other looming dangers, and yet people continue to use energy largely as if nothing is amiss.

For decades, dedicated lawmakers and activists have been trying to change things, through new laws that would shift the way Americans use energy. By 2011, though, many of those efforts had stalled, and some people decided to change tactics: They would go after the Keystone XL pipeline and campaign to keep North American oil in the ground.

It’s true that Americans are rapidly tapping into ever-bigger pools of petroleum, and that hardly seems instinctively wise if we want to limit greenhouse-gas emissions. But how worried about it should we be?

Carbon Cost

There are two reasonable ways to gauge the climate danger of U.S. oil development. The first uses something called the social cost of carbon, which puts a dollar figure on the damage that a ton of carbon-dioxide emissions could be expected to cause. The methods that scientists and economists use to estimate this are complicated and controversial. In a 2009 review of the numbers, the U.S. government concluded that every ton of emissions causes a little more than $20 worth of harm -- though it acknowledged that the real figure might be three times larger, or possibly several times smaller. Others have argued that the actual damage is considerably higher.

These figures can help us compare the climate costs of rising U.S. oil production with the economic benefits. Keep in mind that when U.S. oil production increases by one barrel, some OPEC members are likely to cut their own output to stabilize prices.

Imagine, for instance, that every barrel the U.S. adds to the market is offset by four-fifths of a barrel that others remove, so that the net increase is only one-fifth of a barrel. Since producing and burning a barrel of oil leads to about half a ton of carbon dioxide emissions, the net climate damage from every added barrel of U.S. oil works out to about $2. Even if the U.S. government analysts are way off, and the social cost of carbon is $100 a ton, the net climate damage from every extra U.S. barrel is probably less than $10. Unless oil production depends on subsidies that are much larger than the ones currently in place, the economic benefit gained from a barrel of U.S. oil production will almost always exceed these costs by more than that.

It’s useful to also measure big fossil-fuel developments against big goals. In recent years, international negotiations have focused on limiting atmospheric concentrations of carbon dioxide to 450 parts per million. The current concentration is almost 400 ppm. Measured against this benchmark, burning all the oil in North America, or even in the U.S., would be catastrophic for climate change. The Canadian oil sands alone contain enough carbon to raise concentrations by about 60 parts per million, blasting right through the 450 ppm limit. Shale oil, offshore oil and Alaskan oil would each push the level by a smaller amount, but their effects add up.

Burn Rate

The logic that equates tapping these resources with destroying the climate is weak, however, because what matters isn’t how much oil is in the ground but how much will be burned in the coming years. It would take 3,000 years to extract all the fuel from the Canadian oil sands at today’s pace, and it would take even longer to tap out U.S. oil. The future course of climate change will be determined long before then.

So the question is, what are the climate consequences of the amount of oil that could plausibly be produced in the near future? We can put some numbers on this. As of 2011, the U.S. produced 5.7 million barrels of oil a day. Imagine that this amount suddenly doubled. How much extra carbon dioxide emissions would there be? An added 5.7 million barrels of oil a day translates into about 1 billion tons of carbon dioxide a year (and this ignores any compensating cutbacks from elsewhere in the world). This compares with about 6 billion tons a year of total U.S. emissions and almost 40 billion tons a year of global emissions (so far). It’s a decent addition, particularly to the U.S. contribution, but measured against the global total it’s still relatively small.

Figured another way, 1 billion tons of carbon-dioxide emissions adds about 0.07 ppm to the atmospheric concentrations of carbon dioxide. Over 60 years, that would be about four parts per million. Again, this is far from trivial, but in a world that’s on course to see carbon-dioxide concentrations rise by many hundreds of parts per million, it’s not game-changing, either.

To be sure, such arguments can be slippery. No single action, even a big one, will make the difference when it comes to dangerous climate change. And the right U.S. actions can help build international support for cooperative efforts. Yet U.S. restraint is unlikely to influence other big oil-producing countries. Most of the biggest ones -- including Iran, Iraq, Kuwait, Russia, Saudi Arabia and the United Arab Emirates -- depend on oil production for their survival. They can’t slash their production without risk to their economies.

Ultimately, if you really want to cut emissions, you need to reduce oil demand directly. That would reduce production too: slashing demand would lower prices, cutting producers’ incentives to pump more oil. Attacking that demand is the only real path to confronting the climate consequences of abundant oil.

(Michael Levi is the David M. Rubenstein senior fellow for energy and the environment at the Council on Foreign Relations. This is the second of three excerpts adapted from his new book, “The Power Surge: Energy, Opportunity, and the Battle for America’s Future,” which will be published May 2 by Oxford University Press. The opinions expressed are his own. Read Part 1 and Part 3.)

To contact the writer of this article: Michael Levi at mlevi@cfr.org

To contact the editor responsible for this article: Mary Duenwald at mduenwald@bloomberg.net