In 2011, the world will emit more than 35 billion tons of carbon dioxide. Every day of the year, almost a hundred million tons will be released into the atmosphere. Every second more than a thousand tons – two million pounds – of carbon dioxide is emitted from power plants, cars, trucks, ships, planes, factories, and farms around the world. The average citizen of the world will account for the release of four and a half tons – 9,000 pounds – of CO₂ this year. The average American will be responsible for four times as much, almost 18 tons, or 36,000 pounds of carbon dioxide this year, roughly a hundred pounds of carbon dioxide emissions for every day of the year.

While humans emit far less carbon dioxide than nature, the amount we emit exceeds the capacity of plants and oceans to absorb on top of the amount they’re already absorbing from natural sources. As a result, most of the carbon dioxide we emit remains in the atmosphere. Year over year, the atmospheric concentration of CO₂ creeps up. It will rise only half a percent in 2011, a seemingly tiny change. Yet tiny changes add up. Over the 50 years since 1960, the amount of carbon dioxide in the atmosphere has risen nearly 25%. Since the start of the industrial revolution it has risen by 45%, putting it at a level not seen in millions of years.

On current course and speed, by 2050 atmospheric CO₂ levels will rise by another third from their already record high levels, making CO₂ twice as plentiful in the atmosphere than at any point during the lifetime of our species.

Without reversal or mitigation, the continued pumping of CO₂ into our atmosphere will eventually warm the planet to the extent that catastrophic changes ensue. The only serious debate at this point is just how quickly those catastrophic changes will occur, and which regions will see them in what forms.

To avoid those changes, we need to keep the level of CO₂ and other greenhouse gasses in our atmosphere at a manageable level. It’s unlikely this can be above 450 parts per million in the atmosphere. To stabilize at those levels, carbon dioxide emissions in 2050 will need to be less than half of what they are today, and less than one quarter of the levels they’re on track for if we continue with business as usual. Compare the bottom blue line in the graphic below, which depicts the necessary levels of carbon dioxide in the atmosphere and carbon emissions to achieve them, with the top red line, which depicts something close to business as usual. (Note that in the bottom graph, emissions are listed in billions of tons of carbon rather than billions of tons of CO₂. Multiply tons of carbon by 3.67 to get tons of CO₂.)

We hear a lot today about ways to achieve lower emissions and thus lower CO₂ concentrations in the atmosphere – more efficient cars, green energy sources like solar and wind, changes in lifestyle, and so on. Another option is to take specific steps to remove carbon dioxide from the atmosphere, either by removing it from the exhaust of power plants and other sources, or by scrubbing it out of the atmosphere later. Is it possible to capture enough CO₂ in this way to make a difference? What would it take? Should we even pursue this path, or is it a distraction from cutting carbon dioxide emissions other ways?

Why capturing carbon is a good idea

The best way to keep carbon dioxide levels from rising in the atmosphere would be to simply never emit carbon dioxide in the first place. An ounce of prevention is indeed more valuable than an ounce of cure. Unfortunately to completely eliminate carbon emissions we would need to go to 100% non-CO₂ emitting sources of electrical power – solar, wind, hydro, and nuclear – and simultaneously convert all transportation to either electric vehicles (powered by zero-carbon electrical sources) or entirely fueled by next generation biofuels. To understand that, let’s look at the two most plentiful sources of carbon emissions: electricity generation and transportation.

Electrical generation is the number one source of carbon emissions, making up roughly 40% of carbon dioxide emissions on the planet, most of that from the burning of coal. Most electricity on the planet is used to heat and cool buildings. Green building standards could cut electrical bills, but the lifetimes of buildings are long, and getting owners to retrofit is difficult. The other way to address carbon emissions from this sector is to switch to low-carbon ways of generating electrical power.

As I’ve posted about previously, the cost of solar power is dropping exponentially, and will cross below the price of coal-fueled electricity by 2020. Unfortunately, solar suffers from intermittent supply. At night and on cloudy days, the available electricity drops. Solar power plant manufacturers are working on solar power storage systems to offset this problem, but today the leading edge is to provide 6 hours of storage, enough to make it through the evening television hours, but not enough to provide power 24/7 or to make up for cloudy days or weeks. Energy storage also adds to the cost of electricity, since the storage systems have to be built and paid for. Wind power, far less abundant than solar and far more stagnant in price, suffers similar and even larger problems of intermittent supply. The result is that, until and unless we have breakthroughs in power storage, solar and wind will top out at between a third and a half of the planet’s electrical power needs.

Transportation is the second largest source of greenhouse gas emissions on the planet, accounting for around a third of all greenhouse gasses humans produce. Transportation can be made greener by increasing fuel efficiency of vehicles through technologies like hybrid drive systems, regenerative braking, and lighter and more aerodynamic chassis. Yet these changes affect mostly in-city passenger driving. They have far less effect on cross country transportation on trucks (where cargo makes up more of the weight and traffic patterns are less stop-and-go) and almost no impact on air travel. New aircraft design concepts could cut air travel fuel usage by half, but it will take decades to turn those concepts into production aircraft, and more decades to replace the aircraft already in use.

Electric vehicles charged with electricity from low-carbon sources would do better, but electrical vehicles suffer from the very low power densities of batteries when compared to hydrocarbon fuels (as much as a factor of ten lower) and resulting in heavy vehicles with short ranges. In addition, until night time power is low carbon, charging an electrical vehicle at night, in most places, will essentially be an exercise in burning coal. And while electric motors are more efficient than internal combustion engines, electric cars charged by coal-fueled power plants will still result in net carbon emissions.

The one major hope for transportation to become green is the development of next generation biofuels. Biofuels help with carbon emissions because growing the feed-crops for them extracts carbon dioxide from the atmosphere. While that carbon dioxide is released again when the fuel is burnt, it’s an almost net-zero cycle, unlike the burning of fossil fuels that have been in the ground for tens of millions of years.

Unfortunately, current biofuels crops including corn, switchgrass, and oil-seed rape produce less than half a watt of energy per square meter and compete with food crops. They are both too low in power density and too adverse for world food prices to be practical as large-scale replacements for petroleum products. We can effectively rule those out from having a large effect. Next generation biofuels, including genetically modified algae that can grow on salt water (and thus not compete with food crops) and capture as much as 5 watts per square meter are more promising. However, they have yet to be proven.

If we assume that automotive fleets go up in efficiency, that aircraft go up in efficiency somewhat, and that some biofuels come online, we can perhaps look forward to a reduction in transport emissions of about half over the next thirty or forty years, about the same as we see for electrical generation. That, combined with an increase in solar and wind, leaves about half of the world’s carbon emissions in 2050 still being emitted. It would effectively keep emissions steady with today. That’s insufficient. It would leaves us still walking down the path to catastrophe at today’s rate. Something more is needed.

In that context, it makes sense to talk about capturing carbon dioxide, above and beyond the proposals to reduce its emissions above, and storing it someplace safely out of the atmosphere.

How do we capture and store carbon dioxide?

Broadly speaking, there are two types of carbon capture systems, though there are many possible ways to build systems of each type. The first sort of system is focused on capturing carbon dioxide from power plants where fuel is being turned into electricity. This is commonly referred to as Carbon Capture and Storage or CCS. In principle it could reduce the carbon emissions of coal-powered electrical plants by 90%. It cannot, however, offset the carbon emissions from transportation or other smaller sources such as farming and deforestation.

To tackle those emissions, another form of carbon capture called Carbon Dioxide Air Capture or Carbon Dioxide Removal (CDR) has been proposed. CDR devices could exist anywhere, not just near power plants, and capture carbon dioxide from the very dilute concentrations it exists in atmospherically.

Both forms of carbon capture rely on storage of the carbon dioxide. To store carbon dioxide, it must first be compressed into a liquid, then piped or shipped to an appropriate location, and finally injected into suitable geological formations kilometers below the surface of the earth. There the CO₂ will remain for at least thousands of years, if not far longer.

Both forms of carbon capture require energy as well. Carbon capture at coal-powered electrical plants has the advantage of having the carbon dioxide available at extremely high densities and potentially being able to take advantage of waste heat from the plant. Even so, energy is required. At minimum, 70 kilowatt hours of energy is required to compress a ton of CO₂ from a gas into a liquid. Additional energy is then required to pipe it to a suitable storage location, and then to pump it into a reservoir kilometers below the surface of the earth.

Capturing carbon dioxide away from power plants, from normal atmospheric air, requires even more energy. The basic physics tells us that at minimum an extra 130 kilowatt hours of energy is required to capture carbon dioxide from normal atmosphere, even before spending the energy to compress it into a liquid or pump it into the ground.

We might think that the fact that additional energy is required to capture carbon dioxide means that it’s a losing proposition. After all, that energy itself will result in more carbon emissions. Fortunately, even if we use the dirtiest fossil fuel – coal – the additional energy required emits far less new carbon dioxide than we capture. At theoretical best efficiency, capturing CO₂ from coal power plants would emit less than one ton of new CO₂ per ten tons captured. Capturing CO₂ from thin air – and using coal to power the process – would emit a best case of two tons of CO₂ for every ten tons captured. Seen another way, the best possible net capture efficiencies when the process is powered by coal are 91% and 83%, respectively.

Powering carbon capture devices by sources other than coal would be far better. CDR – capturing CO₂ from normal atmospheric air – could be powered by hydro-electric, wind, or solar power, at locations and times when that power is the cheapest and most plentiful.

Capturing carbon requires more than just energy, of course. It requires investment in the physical infrastructure to capture the carbon, to compress it, to transport it to the right site, and to pump it incredibly deeply into the ground. It requires manpower to do these things, and to maintain monitoring of the sites to ensure that sequestration has been done properly and that unexpected leaks don’t arise.

All together, the pieces of carbon sequestration add up to a noticeable cost. How much cost? A recent study at Harvard’s Kennedy School of Management reviewed all previous work on cost estimation of CCS at coal power plants, and determined that the long term cost would be somewhere between $35 and $70 dollars per ton of carbon dioxide captured and stored. The costs would start much higher for the first plants, as high as $150 per ton of CO₂ captured and stored, but would drop rapidly as more plants were built and the industry scaled.

Fewer cost estimates are available for carbon capture from general atmosphere, but a number of private companies are now at work in the field, and the estimates they’ve discussed fall in roughly the same range – $100 per ton of CO₂ in the early stages, dropping to perhaps $30 to $50 per ton of CO₂ as the technology is scaled.

If we could achieve a cost of $50 per ton of CO₂, what would that do to energy prices? Every $10 per ton of CO₂ increases the cost of electricity by 1 cent per kilowatt hour, and increases the cost of gasoline by 10 cents per gallon. So a $50 per ton cost to capture CO₂ would, if applied back to the cost of CO₂ emissions, raise electricity prices by 5 cents per kilowatt hour and raise gasoline prices by 50 cents per gallon. That is not a bad price for avoiding catastrophic changes to the planet.

Scale of the challenges

Yet carbon capture technology is not without its problems. There are concerns that injecting high quantities of liquid CO₂ near fault lines that are under tension could trigger earthquakes years ahead of when they would normally occur. At least one recent study has also shown that there is a risk of sequestered carbon contaminating drinking water.

The biggest technical challenge is sheer scale. Carbon dioxide compresses to a liquid about half as dense as water. A barrel of liquid CO₂ weighs 70 kilograms or 160 lbs. To capture all 35 billion tons of CO₂ the world will emit in 2011, we would produce nearly 470 billion barrels of liquid carbon dioxide, or roughly 67 barrels per person alive on Earth. That quantity is more than 17 times the total number of barrels of oil the petroleum industry pumps out of the ground each year.

Fortunately, while the volume is vast, geological structures exist to store this much. The Intergovernmental Panel on Climate Change estimates that geological structures away from fault lines and drinking water could store at least 1.1 trillion tons of CO₂, and possibly as much as ten times that. A report by the Global Energy Technology Strategy Platform group at Batelle found geological capacity to store roughly a staggering 10 trillion tons of CO₂ safely.

At the high end, that would provide storage to sequester more than 200 years worth of CO₂ emissions. Even if we limit our estimates to existing oil and natural gas fields alone, structures whose capacities we’re more certain of, we could store around 900 billion tons of CO₂, or enough to keep atmospheric carbon concentrations below 450ppm for the rest of this century. These fields have long term stability demonstrated by the fact that they have held oil and natural gas deposits for millions of years. The carbon they’d sop up would give us significant time to keep working on improvements to zero-carbon power and transport technologies without exacerbating climate change.

The challenge is less in the storage capacity and more in the pumping and transportation capacity. To make a significant dent with carbon capture, we would need to create a pumping and piping infrastructure with a capacity more than ten times that of the current oil industry. That is a major undertaking. It’s well within our capabilities, but not without substantial cost. At the same time, there may be no route to a climatically stable world that avoids this.

How to make it happen

A number of carbon capture and storage pilot programs are underway today, but the technology is very much still in an experimental phase. If concerns about drinking water and seismic activity can be addressed – which the IPCC and EPA both believe – How do we turn carbon capture from a science project into a reality?

My firm belief is that the best way to turn any dirty industry into a clean industry is to make it profitable for companies in the industry to do so. Or, to put it another way, the way to encourage change is to make it too costly to remain dirty for any company to want to do so.

This is not meant in any way to be punitive. The coal and oil industries have reached the scale they have and the emissions they have because consumers have demanded more and more energy, and because the industries have not been told to eliminate their carbon dioxide output. It makes no sense to blame industry when consumers and legislators have worked together to create a landscape in which their current actions are the most sensible ones. To change the actions of energy companies, we need to change the landscape.

The best way to go about doing this is to place a price on carbon. Pumping carbon dioxide into the atmosphere, where it causes long term damage to a planet shared by all, should be something one needs to pay for. The price paid should be at least commensurate to the cost of undoing any harm. On the flip side, efforts that remove a pollutant from the atmosphere should be rewarded at the same rate.

We’ve seen estimates of cost of mature carbon capture systems that range from $35 – $70 / ton, and of the very first systems at around $150 / ton. Where should we set the price?

I would propose a price that starts at zero but ratchets up progressively to $100 / ton (in today’s dollars), at an automatic increment of $5 / ton each year. $100 / ton gives buffer room over the current price estimates for carbon capture and storage. This allows for some flexibility if cost estimates turn out to be too low. On the other hand, if those estimates are accurate, or if the cost of sequestering a ton of carbon turns out to be anywhere under the $100 / ton carbon price we would set, then it would be cheaper for power plants to adopt carbon capture technologies than to pay the carbon price. In the worst case, if the full carbon price is paid, the cost of coal electricity, 20 years from now, would be 10 cents higher per kilowatt hour. If capture costs end up at $50 / ton (the midpoint of estimates), then the cost of coal electricity, 20 years from now, would be 5 cents higher per kilowatt hour.

The gradual and predictable increase in the carbon price would soften the immediate economic shock of it, while giving both consumers and corporations clarity about the future and the ability to plan logically for it. A price even 20 years in the future would push utilities to start planning now for how to retrofit existing power plants and build new ones in ways that minimize carbon emissions.

Paradoxically, a carbon price would also slow the rise of oil and other fossil fuel prices, by encouraging conservation now and thus reducing demand.

A carbon price of this size would have other beneficial effects outside of carbon capture. It would make solar, wind, and nuclear power more attractive on a price basis. Over 20 years it would raise the price of gasoline by $1 / gallon, less than the difference in prices between the US and Europe, but enough to make electric cars, hybrids, and new, more fuel-efficient aircraft designs all more attractive as well.

Perhaps most importantly, a carbon price would create a gold rush of carbon harvesters working to pull carbon dioxide out of the atmosphere. Whoever could get their cost of capturing and sequestering carbon dioxide down the lowest would reap the largest profits per ton of carbon captured, driving innovation in ways to capture carbon at ever cheaper prices. A carbon price would align incentives, making it in the best interests of corporations and entrepreneurs to lower the amount of carbon in the atmosphere. That’s something we should all be excited about.

In summary

Carbon capture and storage technology isn’t a solution to our climate problems on its own. There are unknowns and challenges of scale that need to be addressed. Possible locations for carbon sequestration aren’t infinite in size. They will eventually fill up. But carbon capture can be done, and can be done at massive scale, and at a price that would not destroy our economy. Doing so would give us more time to find ways to switch to inherently zero-carbon methods of powering our civilization and fueling our vehicles. As a complement to efficiency, green energy, and other ways to reduce our carbon emissions, capturing and storing carbon dioxide from our power plans and our atmosphere would be an extremely powerful tool. The best way to encourage carbon capture and storage turns out to be the best way to encourage efficiency, green energy, and other approaches to reducing the carbon in our atmosphere: put a price on carbon emissions.

Sources and further reading

IPCC, "IPCC Special Report on Carbon Dioxide Capture and Storage," prepared by Working Group III of the Intergovernmental Panel on Climate Change [Metz, B., O. Davidson, H. C. de Coninck, M. Loos, and L. A. Meyer (eds.)]. Cambridge University Press, Cambridge, U.K. and New York, 442 pp., 2010.

International Energy Agency, "Carbon Capture and Storage: Progress and Next Steps," 2010.

Global CCS Institute, "The global status of CCS: 2010", Canberra.

Al-Juaied, Mohammed A and Whitmore, Adam, "Realistic Costs of Carbon Capture" Discussion Paper 2009-08, Cambridge, Mass.: Belfer Center for Science and International Affairs, July 2009.

JJ Dooley, RT Dahowski, CL Davidson, MA Wise, N Gupta, SH Kim, EL Malone, "Carbon Dioxide Capture and Geologic Storage", Batelle Global Energy Technology Strategy Program, April 2006.

"Capturing Carbon Dioxide From Air", US National Energy Technology Laboratory, Klaus S. Lackner, Patrick Grimes, Hans-J. Ziock,

The sun strikes every square meter of our planet with more than 1,360 watts of power. Half of that energy is absorbed by the atmosphere or reflected back into space. Seven hundred watts of power, on average, reaches Earth’s surface. Summed across the half of the Earth that the sun is shining on, that is 89 petawatts of power. By comparison, all of human civilization uses around 15 terrawatts of power, or one six-thousandth as much. In 14 and a half seconds, the sun provides as much energy to Earth as humanity uses in a day.

The numbers are staggering and surprising. In 88 minutes, the sun provides 470 exajoules of energy, as much energy as humanity consumes in a year. In 112 hours — less than five days — it provides 36 zettajoules of energy – as much energy as is contained in all proven reserves of oil, coal, and natural gas on this planet.

If humanity could capture one tenth of one percent of the solar energy striking the Earth — one part in one thousand — we would have access to six times as much energy as we consume in all forms today, with almost no greenhouse gas emissions. At the current rate of energy consumption increase — about 1 percent per year — we will not be using that much energy for another 180 years.

It’s small wonder, then, that scientists and entrepreneurs alike are investing in solar energy technologies to capture some of the abundant power around us. Yet solar power is still a minuscule fraction of all power generation capacity on the planet. There is at most 30 gigawatts of solar generating capacity deployed today, or about 0.2 percent of all energy production. Up until now, while solar energy has been abundant, the systems to capture it have been expensive and inefficient.

That is changing. Over the last 30 years, researchers have watched as the price of capturing solar energy has dropped exponentially. There’s now frequent talk of a “Moore’s law” in solar energy. In computing, Moore’s law dictates that the number of components that can be placed on a chip doubles every 18 months. More practically speaking, the amount of computing power you can buy for a dollar has roughly doubled every 18 months, for decades. That’s the reason that the phone in your pocket has thousands of times as much memory and ten times as much processing power as a famed Cray 1 supercomputer, while weighing ounces compared to the Cray’s 10,000-pound bulk, fitting in your pocket rather than a large room, and costing tens or hundreds of dollars rather than tens of millions.

If similar dynamics worked in solar power technology, then we would eventually have the solar equivalent of an iPhone — incredibly cheap, mass distributed energy technology that was many times more effective than the giant and centralized technologies it was born from.

So is there such a phenomenon? The National Renewable Energy Laboratory of the U.S. Department of Energy has watched solar photovoltaic price trends since 1980. They’ve seen the price per Watt of solar modules (not counting installation) drop from $22 dollars in 1980 down to under $3 today.

Is this really an exponential curve? And is it continuing to drop at the same rate, or is it leveling off in recent years? To know if a process is exponential, we plot it on a log scale.

And indeed, it follows a nearly straight line on a log scale. Some years the price changes more than others. Averaged over 30 years, the trend is for an annual 7 percent reduction in the dollars per watt of solar photovoltaic cells. While in the earlier part of this decade prices flattened for a few years, the sharp decline in 2009 made up for that and put the price reduction back on track. Data from 2010 (not included above) shows at least a 30 percent further price reduction, putting solar prices ahead of this trend.

If we look at this another way, in terms of the amount of power we can get for $100, we see a continual rise on a log scale.

What’s driving these changes? There are two factors. First, solar cell manufacturers are learning — much as computer chip manufacturers keep learning — how to reduce the cost to fabricate solar.

Second, the efficiency of solar cells — the fraction of the sun’s energy that strikes them that they capture — is continually improving. In the lab, researchers have achieved solar efficiencies of as high as 41 percent, an unheard of efficiency 30 years ago. Inexpensive thin-film methods have achieved laboratory efficiencies as high as 20 percent, still twice as high as most of the solar systems in deployment today.

What do these trends mean for the future? If the 7 percent decline in costs continues (and 2010 and 2011 both look likely to beat that number), then in 20 years the cost per watt of PV cells will be just over $0.50.

Indications are that the projections above are actually too conservative. First Solar corporation has announced internal production costs (though not consumer prices) of $0.75 per watt, and expects to hit $0.50 per watt in production cost in 2016. If they hit their estimates, they’ll be beating the trend above by a considerable margin.

What does the continual reduction in solar price per watt mean for electricity prices and carbon emissions? Historically, the cost of PV modules (what we’ve been using above) is about half the total installed cost of systems. The rest of the cost is installation. Fortunately, installation costs have also dropped at a similar pace to module costs. If we look at the price of electricity from solar systems in the U.S. and scale it for reductions in module cost, we get this:

The cost of solar, in the average location in the U.S., will cross the current average retail electricity price of $0.12 per kilowatt hour in around 2020, or 9 years from now. In fact, given that retail electricity prices are currently rising by a few percent per year, prices will probably cross earlier, around 2018 for the country as a whole, and as early as 2015 for the sunniest parts of America.

10 years later, in 2030, solar electricity is likely to cost half what coal electricity does today. Solar capacity is being built out at an exponential pace already. When the prices become so much more favorable than those of alternate energy sources, that pace will only accelerate.

We should always be careful of extrapolating trends out, of course. Natural processes have limits. Phenomena that look exponential eventually level off or become linear at a certain point. Yet physicists and engineers in the solar world are optimistic about their roadmaps for the coming decade. The cheapest solar modules, not yet on the market, have manufacturing costs under $1 per watt, making them contenders — when they reach the market — for breaking the $0.12 per Kwh mark.

The exponential trend in solar watts per dollar has been going on for at least 31 years now. If it continues for another 8-10, which looks extremely likely, we’ll have a power source which is as cheap as coal for electricity, with virtually no carbon emissions. If it continues for 20 years, which is also well within the realm of scientific and technical possibility, then we’ll have a green power source that is half the price of coal for electricity.

That’s good news for the world.

Photo: Evening sun by dingbat2005, on Flickr

Sources and further reading:

• Key World Energy Statistics 2010, International Energy Agency
• Tracking the Sun III: The Installed Cost of Photovoltaics in the U.S. from 1998-2009, Barbose, G., N. Darghouth, R. Wiser., LBNL-4121E, December 2010
• 2008 Solar Technologies Market Report: January 2010, (2010). 131 pp. NREL Report TP-6A2-46025; DOE/GO-102010-2867