The public has cooled in its concern over climate change, recent surveys and polls show. But a strong interest in alternative energy continues, and Americans are keen on improving energy efficiency and saving on gasoline.

As with other issues, the public’s understanding of details is thin. Half of those Americans surveyed could not identify a renewable resource such as wind or solar power, and 39 percent could not name a fossil fuel – oil or coal, for example.

The Public Agenda survey, conducted last year, (see this article for more details) makes studies such as those of Caltech researcher Nathaniel Lewis that much more important. The simple reason: he offers a stark reality check on the nation’s high rhetoric and crawling progress toward alternative energy.

At the February annual meeting of the American Association for the Advancement of Science in San Diego, Lewis summarized challenges the world faces in curbing its addiction to fossil fuels and transforming world economies toward a carbon-neutral future. And doing so over the next 40 years.

Lewis’ premise is pretty straightforward. If the world’s nations aim to hold carbon dioxide emissions to between 350 to 500 parts per million at mid-century, the trajectory for CO2 emissions needs to come down – fast.

Coming to terms with the scale of the challenge is critical, Lewis said. In 1990, the worldwide annual consumption rate was 12 trillion watts, or 12 terawatts (TW). The figure is calculated by taking the number of joules consumed per year by humans, and dividing it by the number of seconds in a year. The result is an average burn rate of 12 TW for 1990.

More recent studies estimate the average consumption rate at 15 TW.

Most studies project the 1990 figure will double in our lifetimes, Lewis said, primarily driven by population growth and a commensurate rise of Gross Domestic Product around the world.

“Whether it’s level or double or triple is actually irrelevant,” Lewis said. “It’s a huge number no matter what.”

(This “terawatt challenge” is a subject often tackled by Richard Smalley of Rice University.)

Turning next to the trajectory of rising CO2 emissions, Lewis told an AAAS session that many people are confused by climate science graphs, so-called WRE curves, that chart the rate of CO2 emissions over time. He pointed to the accumulated build-up of carbon dioxide emissions in the global atmosphere, the area under the curves, as being most important. To stabilize CO2 concentrations, those WRE curves need to come down in a steady glide slope beginning now, or in a precipitous decline as we move closer to mid-century.

That may appear to be “climate science 101” for people familiar with the issue, but it’s lost on most of the public. (WRE what?)

Here’s how Lewis sums up the challenge:

“Just the physics and chemistry of our planet say that if you’re going to follow the 350 ppm trajectory, that means you’re allowed zero carbon emissions forever by the year 2040,” he said.

“We essentially have to think about an energy system that goes to zero carbon emissions, sometime not too far away.”

He followed with a point-by-point review of the grave limitations of individual sources of alternative energy – at least as currently pursued by the United States and much of the rest of the world.

Before launching into that, Lewis reminded his AAAS audience of the climate feedbacks over which humans have no control going forward: melting permafrost and the potential destabilization of methane clathrates, for example, that could accelerate the rise of greenhouse gas concentrations in the global atmosphere.

“Now I’m going to get every constituency upset at one point or another during the talk,” Lewis continued.

He began with hydroelectricity. The total potential energy generated from all the world’s lakes, rivers, and streams is 4.6 TW. Of that total, about 0.9 TW is a plausible goal – from places where hydroelectricity would even be practically feasible. We’re already producing 0.6 TW.

“It makes us feel good [to think] about hydroelectricity … (but) don’t think about it anymore,” Lewis said.

Next came geothermal power. The amount of heat that can be used is equal to the total geothermal heat flux at the Earth’s surface, which is equal to 57 megawatts per square meter of land. Multiply that figure by the area of all the land on Earth and you get 11.6 TW.

That energy output figure, however, assumes that we can develop 100-percent efficient heat engines. For all practical purposes, we’d be able to extract only a few terawatts sustainably.

Ocean and tidal energy? All of the energy from all the currents, tides and waves on the planet combined falls far short of the 10 to 20 TW needed.

Natural gas, Lewis said, is a “dead end.” Using natural gas to fuel global transportation could cut emissions by 10 to 20 percent, but “let’s keep our eye on the ball,” Lewis said.

And forget about using natural gas for home heating, he added. “We have to tell our natural gas utilities not to ship anymore natural gas to homes for heating, ever again,” Lewis said. “Those are dead ends. We have to forget about them. They are not allowed in any portrait that gets us to zero emissions by 2050.”

The only place where natural gas could be used is where it’s completely captured and stored, he said.

Biomass could play a significant role, Lewis said. If half the farmable land on the planet were devoted to growing biomass, the energy generated from that could be in the range of 7 TW. But most experts agree that the practical amount is more like 1 TW, he said.

“Now that’s an important terawatt, but we shouldn’t be foolish and put them in cars with yellow caps on their gas tanks,” Lewis said. Forty percent of global transportation relies on trucks, ships and airplanes.

“We should not be wasting our precious allocation of biofuels by even thinking about putting them in light-duty vehicles if we’re going to need them all to fly our airplanes, move our ships around, and move our heavy duty-trucks around,” Lewis said.

While he didn’t talk about wind energy in his talk, Lewis has written that wind power could play a role. But it has enormous constraints. There is of course the problem of transmitting electricity from every deployed wind turbine. But Lewis has said there are even more basic challenges.

On a global scale, deploying wind turbines in all the marginal-to-high wind areas available for construction would generate 2 to 4 TW, according to Lewis.

“Producing 2 TW of wind power would require the operation of 2 million state-of-the-art wind turbines, starting today,” Lewis has written. Wind power could potentially meet 10 percent of the globe’s energy needs, if used exhaustively on all the suitable land around the globe.

Nuclear fission is the only carbon-free technology that can be developed to scale to generate 10 terawatts or more. But the public needs to understand what it means to “build to scale,” Lewis said.

In short, to generate 10 TW, the world would need 10,000 reactors, he said.

Meanwhile, there’s enough uranium to power those 10,000 plants for only 10 years, he said. Another solution is to use plutonium and reprocess it – but that of course carries proliferation concerns.

Solar power is enticing because it’s clearly the single largest source of power we know. More energy from the sun hits the Earth in one hour than all of the energy consumed on the planet in an entire year.

The land demands for current solar energy technology, if we build to scale, are enormous. Lewis likes to display a map of the United States showing a box, covering a large swath of the Midwest, to illustrate the land area required to produce 3 terawatts from a solar energy farm operating at 10 percent efficiency. The map is published in his 2007 paper: “Powering the Planet.”

In reality, the area covered would be widely distributed across the nation, but it would still cover 1.7 percent of U.S. land, Lewis said.

The economics of scaling up solar energy also doesn’t currently add up, Lewis said. If solar voltaics installations grow at a rate of 33 percent a year for the next 30 years, all the energy generated by a single year’s new batch of solar cells would be needed to manufacture the following year’s batch.

“So, you haven’t offset a watt of carbon-free power,” Lewis said.

Of course, the world’s nations aren’t going to choose one source of carbon-neutral energy to meet the global terawatt challenge. But by examining the realities of scaling up each source, Lewis offered a sobering reality check of the challenges ahead.

Toward the end of his presentation, Lewis summed up the biggest energy challenges the globe faces.

Perhaps the biggest, as he phrased it, is: “Storage, storage, storage.”

“We really don’t know how to store massive quantities of electricity well,” he said.

Summarizing the view of the National Academies of Science, he said: “We need to think about ways to cross the storage chasm to bring on large fractions of intermittent renewables with 99 percent reliability or more in our energy system.”

Just for batteries in electrified vehicles, some revolutionary thinking in materials science will be needed to replace expensive material such as platinum and ruthenium with cheap metal such as iron. “We don’t know how to do this at all,” he said.

As he concluded, Lewis pointed to improving energy efficiency as an obvious and concurrent task as the world moves toward a carbon-neutral future.

And the world has three “big cards” to play: developing and scaling-up carbon capture and storage technologies for coal-fired power plants, developing and scaling up nuclear power that moves beyond “once-through” technologies, and finding ways to better harness the sun’s energy.

“Some combination of these things has to happen in this vision if we’re going to get to zero sometime by 2050,” he said.

Additional details about Nathanial Lewis and his research are available online here and here.

Bruce Lieberman

Bruce Lieberman, a long-time journalist, has covered climate change science, policy, and politics for nearly two decades. A newspaper reporter for 20 years, Bruce worked for The San Diego Union-Tribune...