Can we get to 350 ppm? Yes, we can

In a recent post, I made the optimistic argument that, despite all the obstacles thrown up by rightwing denialism, the world is on track to reduce CO2 emissions to zero by 2050, on a trajectory that would hold atmospheric concentrations of greenhouse gases below 450 ppm. On current models, that gives us a 67 per cent chance of holding the long term increase in global temperatures below 2 degrees. Warming of 2 degrees would not be cataclysmic for humanity as a whole but it would be a disaster for many people and also for vulnerable ecosystems such as coral reefs. That’s why wants to reduce concentrations to 350 ppm from current levels above 400 ppm.

Is that even possible?
And, what would it mean for global warming?

In this post, I’ll argue that the answer to the first question is definitely yes. I’m going to start with the assumption (based on this post) that we can reduce emissions from fossil fuels to zero by 2050, and keep concentrations below 450 ppm at that point. What are the options to reduce concentrations over the following fifty years? In the absence of some new technological fix (not implausible, but there’s nothing in sight as of 2017), there are three main possibilities

* Reducing methane emissions and concentrations. Methane emissions arise mainly from agriculture (paddy rice and ruminants), with some possible addition from fracking. It appears feasible, though not trivial, to greatly reduce these sources at fairly low cost. And because methane has a short residence time, a reduction in emissions will lead fairly rapidly to a reduction in concentrations. The conversions are very tricky, but the radiative forcing associated with methane is currently about 0.5 watts, compared to 1.94 for CO2. So, if methane concentrations were reduced by 40 per cent, that would be equivalent to a 10 per cent reduction in CO2, or about 40 ppm.

* Natural absorption. Only around 50 per cent of the CO2 we emit (the so-called atmospheric fraction) ends up as increase in atmospheric concentrations, with the rest being absorbed by oceans. After that initial addition to sink, CO2 stays in the atmosphere for a long time. However, there is still some additional absorption by sinks. Yale Climate Connections suggests that around 50 per cent is absorbed in 50 years, and around 70 per cent in 100 years. So, by 2100, an additional 20 per cent or so of the CO2 emitted around now will have been absorbed by sinks. A rough estimate would be 0.2*(450-280) or 35 ppm, where 450 is the peak concentration and 280 the stable pre-industrial level. Of course,this is far from an ideal solution, since CO2 contributes to acidification of oceans and therefore to coral reef decline.

* Reforestation and other land use changes. Land use change is currently a big net contributor to global warming, but a systematic program of reforestation could turn this around. The potential has been estimated at 85 ppm.

Against these possibilities, there is currently a net cooling effect, equivalent to around 50 ppm, from aerosols associated with air pollution. Hopefully, pollution will be reduced over the coming century, but that makes the task of stabilizing the climate a bit more difficult.

A question I haven’t yet been able to find a good answer on is: how much warming would a trajectory peaking at 450 ppm and declining to 350 ppm ultimately produce? If anyone can point me to a good source, that would be great.

Finally, at least some of the pollutants we’ve emitted over the past century will, on our current understanding, stay there for hundreds or thousands of years, leading to long term problems of sea level rise. But if we can get to 2100 without destroying the planet through climate change or nuclear warfare, I’m sure our great-grandchildren will work out some way of cleaning up what’s left of our mess.

27 thoughts on “Can we get to 350 ppm? Yes, we can

  1. Cheering.
    1. Switching from smallish net deforestation to large net reafforestation is politically very difficult. See the Amazon, the US Northwest, Scottish grouse moors, illegal logging in the Congo, Indonesian palm oil plantations, etc. Green purists want to see restoration of virgin forest with none of the logging that makes the project economically feasible. Even strong forest regulatory agencies as in France and Finland are subject to political pressures to deviate from sustainability.
    2. Reafforestation can’t be kept up indefinitely. It’s a one-off. In a net zero energy system, that may be enough.
    3. Mineral carbonation of olivine, common in basalt, does work, see the pilot in Iceland. It is on the horizon – just not close. This and other technologies for sequestration deserve much higher research priority than they are getting today.
    4. JQ is an economist. How can large-scale sequestration be funded? It won’t ever become a self-sustaining commercial proposition, as wind and solar have become, and electric vehicles surely will. So it has to be taxes. Current politics suggests there are very strong groups in society who would rather the world ends than pay them.

  2. Agriculture fixes the wildly inaccurate figure of around 10 billion tonnes of CO2 a year. If just 1% of that was sequestered, it would take a long time to make a significant difference.

  3. @1

    1.1 The political difficulties have been overcome in the case of the US Northwest, which is seeing a steady net gain.
    And palm oil is also being constrained. The EU is in the process of banning imports for biofuel, and Malaysia has banned new clearance of forests for palm oil.

    1.2 True, but as stated, if we can get to 350 by 2100, I think our descendants can fix the rest

    1.3 Interesting

    1.4 I’m an economist, but my mother was a demographer. Looking at the age profile of denialists and lukewarmers, I’d say this problem will have solved itself well before 2050.

  4. So John, what you’re saying is that if we bury the deniers today we store the carbon and remove the road blocks to progress? Let’s do it!

  5. James, with regard to your point 2, biomass from afforested areas can be harvested and sequestered so it won’t be a one off. Where there is transportation it can be dumped in the ocean, either in areas of sedimentation or in deep water. In places like Canada, the land of a trillion stupid lakes, it can be dumped in cold water. In other places it can be turned into char, or biochar, as the cool kids call it these days, and used as a soil amendment which can increase the amount of biomass, and thus locked up carbon the land can sustain.

    Where I am we have a railway, we have a road system, we have a port, we have abyssal depths a few hundred kilometers away, and we have vast expanses of pebbles covered with a light coating of dust that could benefit from amendment with biochar. We have farmers who sometimes have crops fail due to lack of rain and who on rare occasions don’t harvest due to low prices. (This normally only happens with perishable stuff, not grains.) We have farmers who have wind breaks that could be harvested for biomass and we have farms that produce a lot of chaff.

    So I think that here there is a price at which agricultural waste would be turned into biochar and added to soil or dumped in a ship and then dumped in the ocean. At a slightly higher price farmers would start specifically growing biomass for sequestration purposes.

    The necessary price appears to be much lower than estimates for sequestering CO2 by non-agricultural methods, except for some reforestation/afforestation. While other methods may turn out to be cost effective, at the moment there are farmers dumping biomass on ships in Australia for under $100 US per tonne of fixed CO2. If that biomass doesn’t have to be edible, they should be willing to do it for considerably less.

    So, grow-harvest-dump, appears to be the cheapest way to sequester CO2. Hopefully we’ll develop cheaper/better methods, but at the moment it’s hard to see how it can be beat on price when it comes to removing CO2 from the atmosphere on a moderate scale.

  6. A good post and discussion. It’s a pity that we seem to be lonely pioneers, at least in the nerdy blogosphere.

    A small shout-out for building in wood. There are wooden pagodas in Japan from the 7th century. Structural timber sequesters carbon for a century. Engineered wood beams and panels give architects great flexibility, and slash the volume of concrete needed for foundations.

  7. ” I’m an economist, but my mother was a demographer. Looking at the age profile of denialists and lukewarmers, I’d say this problem will have solved itself well before 2050.”- worth bottling.

    @John Quiggin

  8. @rdb
    The old Strand/Benford paper relies on evidence, presumptively sound, that current communities of bugs and larger critters on the ocean abyssal plains are not very good at digesting bales of terrestrial crop and wood waste dumped on them to their surprise. Bacteria evolve fast and the genes spread. There must be a risk that faced with gigatonnes of new potential food, they would learn to eat it, as a handy addition to the standard menu of fish shit and dead whales. At that point you would be acidifying the ocean depths, a not entirely sound plan.

    The same Benford who writes hard SF?

  9. Wholesale prices just went negative in the middle of the day in South Australia thanks to good wind and rooftop solar production. And due to the fact the state government now requires 2 large gas generators to operate continuously.

    So if it is taken as given that at times grids in the near future will often have electrical available for times at next to nothing, what’s the best way to use it to remove CO2 or other greenhouse gases from the atmosphere?

    I can’t think of anything very efficient at the moment. The energy could be used to liquefy CO2 so it could be dumped in the ocean depths. As long as it is released at a great enough depth it will be sequestered for at least hundreds of years. Pumping it underground at suitable locations is also an option. CO2 could be cooked out of limestone and sequestered and the resulting lime (calcium oxide) would then absorb atmospheric CO2. With the addition of hydrogen and energy and lots of capital it would also be possible to fix the C in CO2 in plastics that can be stable for at least thousands of years.

    The problem with all these methods is they tend to involve capital that will be sitting idle when energy prices are not low, which tends to make them expensive despite the low cost energy being available at times.

    Surplus renewable energy at night could be use to increase crop growth by providing artificial lighting. The capital cost of this is reasonably low these days but it’s not currently done outside of greenhouses due to the energy cost.

  10. James, provided the water was deep enough, we could bubble CO2 gas directly into it and it would be dissolved at depth and remain sequestered for at least hundreds of years. The ocean has a turn over time of around 10,000 years. The dumping location will affect the sequestration period, with the largest area of ocean upwelling being on the Pacific side of South America.

    And, oh yes, make no mistake, we will be completely destroying the current ecology of hundreds of square kilometers of my old enemy, the abyssal plain. All to save the lives of the surface dwellers. Why? Because it is they who are most likely to destroy my even older and greater enemy – the sun!

    And yeah, I think it is the same Greg Benford.

  11. @Ronald
    The people studying electrolysis are IMHO paying too much attention to energy efficiency, which is irrelevant when the offpeak electricity is free, and not enough to keeping capital costs really, really low. How about a big plastic tank of seawater and a couple of stainless steel electrodes?

  12. We know what happens when billions of tonnes of biomass gets dumped somewhere it can’t burn and sediment can be deposited on it. It becomes coal. While bacteria do appear to eat some of the hydrocarbons in the earth’s crust, because of surface area and exchange problems it’s really hard to chow down on it. Since nothing has come along that chews through the earth’s coal deposits at any significant rate, biomass dumped in large quantities on the deep ocean floor will probably mostly stay where it is long term, although I am sure an interesting ecosystem would develop once the dump site was left undisturbed.

    But there may be a precedent. Evolution of liginase enzymes by fungi could have sped up the rotting of wood and slowed coal formation at the end of the Carboniferous period.

    Despite this, I think a lot of carbon will remain on the ocean floor until it is subducted and eventually out gassed by a volcano. It takes around 200 million years for seafloor to be cycled, so if dumping spots are random we are looking at a sequestration period for a large portion of the carbon averaging around 100 million years.

    Unless of course the reason why fossil fuels aren’t gobbled up is because earth life is currently in a “You can’t get there from here” type situation using evolution. There is not path that can be taken to faster digestion of coal that living organisms can cross. But someone might be able to jump that gap in their bedroom lab. Goodbye coal. Hello Venus mark II.

  13. James, yep, if you are happy with a 25% efficiency, seawater and buckets will do.

  14. Wouldn’t the total methane emissions include a contribution from melting permafrost and seafloor deposits? How would we stop that?

  15. I have doubts over the value of hydrogen as a storage medium. It is easy to make, but storing it and using it can be expensive-ish due to its volume and tendency to damage metals. For storage over a few days something simply made hot, perhaps just rocks, using electric resistance heating and electricity generated from steam when needed. Basically CSP with storage without the CSP. Not a very efficient from of storage but low cost and still more efficient than low capital hydrogen.

    Hydrogen may not turn out to be useless, but unless it finds a role to play, such as perhaps aviation, its use will probably be limited beyond chemical industries stocking up on it when electricity is cheap.

  16. Monsieur Joggles, methane emission from the arctic cannot be stopped, but they can be reduced by stopping, or failing that greatly limiting, human emissions of greenhouse gases. While they are larger than we want to be, methane emissions from thawing permafrost and methane clathrates are currently a small portion of human emissions. Methane emissions from coal mining may be three times higher. Eliminating thermal coal mining will be a big help in reducing methane emissions.

  17. Recently I have been reading about two developments which assuming the claims are accurate could contribute significantly in terms of a achieving sustainable future. The first claim I am highly sceptical of – namely that it would be economic to capture the Co2 from gas powered electricity generation and convert it to carbon nano tubes (CNT) which have many applications and which command a high price. Is this just a ‘plant’ by the fossil fuel industry or does it have real potential? See

    The other is what appears to be a quite visionary project led by NASA scientist Jonathan Trent. The OMEGA Global Initiative. This project proposes large scale Offshore Membrane for Growing Algae

  18. The project pulls together existing and emerging technologies to clean waste water, provide a source of food, provide potable water, wind, wave and solar energy and aquaculture in an integrated system. Worth looking it up on the Internet.

  19. This analysis seems entirely too optimistic. Most estimates of CH4 forcing put it at something like 23 times that of CO2 over a century. Your post does not refer to the decomposing permafrost currently sequestering somewhere between 10 and 100 times the volume of CO2e (in the form of CH4) that humans emitted after 1850. If se don’t prevent permafrost decomposition, we have a much larger problem. Draw down of CO2 plus active geo-engineering to hold the permafrost and its methane stores — and that’s not even considering the need to halt falling albedo.

    Biosequestration of CO2 using algae seems a far more maintainable option than re-afforestation (which is good on other grounds). Algae grows far faster than flora with lignin masses. It’s easy to harvest and would be relatively cheap to dump. It requires little maintenance. We have both stores of nutrient rich water and heavily insolated land. And we have some very deep ocean not that far away. Our aim needs to be to draw down at least those 1500GT of CO2 that was entered the atmosphere since 1850 before 2030, while reducing insolation at both the Arctic and Antarctic where impacts on vegetation could be minimised. Projects like this clearly meet the kinds of tests for carbon offsets.

    That buys us some time to decarbonise our energy systems. Decarbonisation, while essential cannot prevent Charney forcing on the timelines we need. The time when that might have sufficed probably ended around 2000. Now only geo-engineering can foreclose catastrophic consequences, IMO.

  20. John Turner, there are 55 megajoules of energy in one kilogram of natural gas. To turn the carbon in that into nanotubles would require a minimum of perhaps 25 megajoules of energy. If the process if 50% efficient then 50 megajoules out of every 55 in natural gas would be required.

    Worse, we are only likely to burn natural gas occasionally when wind and solar and hydro output isn’t high enough, so the CO2 capture capital would be sitting idle most of the time.

    So this doesn’t pass the laugh test. Energetically it is the same as saying we can turn the carbon dioxide from burning natural gas into coal – except potentially worse.

    Hopefully OMEGA will work out just great, but at the moment it is really what’s called pie in the ocean. Which is not a good thing because it will get soggy. Maybe in the near future will have the technology to keep that ocean pie crisp and tasty, but we’re not there yet and OMEGA may have a hard time beating the competition.

  21. What’s the potential market for carbon nanotubes? Hundreds if tonnes? Thousands? Tens of thousands? Hundreds if thousands? Certainly not millions. Current carbon emissions are in the billions. The scheme cannot possibly have a significant sequestration impact.

  22. According to the internet carbon nantubles are about $280 US a gram. If the cost can be got down to $10 a gram then it would only cost $6.25 million to sequester one tonne of CO2. At that cost the demand for carbon nanotubles could soar to grams per person per year.

  23. Cement making is considered a serious problem when it comes to reducing emissions. But there is one very basic thing that could be done to reduce emissions and that is to stop adding carbon to it. Stable carbon in the form of fly ash from coal power plants is added to cement so it will cure faster. If this isn’t added then concrete will draw CO2 from the air as it cures.

    People will say it is impossible to use slow curing concrete because it cures too slow, but if there was a carbon price then it would be cheaper than fast curing concrete and a miracle would occur as people suddenly realize there are actually plenty of places it can be used. Money is funny like that.

    Fast curing cement could use biochar or ash from burning biomass, potentially from a biomass power plant.

    Thermal energy required to make cement is an issue, but not an insurmountable one. Looking at the 6 US cents a kilowatt-hour concentrated solar thermal power can now apparently cost under good conditions, solar thermal heat may be competitive and if the cost of renewable electricity falls low enough at times that can be used.

    One good thing is the earth should now be past peak cement now that China is pretty well cemented up. The bad news is that since cement isn’t recycled in the way metals are, demand should remain high for a long time as places like India and Nigeria guzzle cement like it’s going out of style.

    Until it’s replaced with carbon nanotubles, of course.

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