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Geoengineering is ‘no substitute’ for cutting emissions, new studies show

Posted on 11 August 2015 by Guest Author

This is a re-post from Robert McSweeney at Carbon Brief

Attempts to limit climate change by removing carbon dioxide directly from the atmosphere would not prevent the irreversible damage to the oceans, according to a new study.

While a second study finds that brightening clouds to reflect more of the Sun's radiation could help boost crop yields in parts of China and Africa.

Speaking to Carbon Brief, authors from both studies highlight the importance of reducing carbon emissions now, rather than trying to engineer the climate later.


Geoengineering is the deliberate large-scale intervention into the Earth's climate system to try and limit human-caused climate change, and it can be divided into two main methods.

Removing carbon dioxide from the atmosphere, often described as Carbon Dioxide Removal (CDR), is one approach. The other is reflecting some sunlight away from the Earth before it can be trapped by greenhouse gases, commonly known as Solar Radiation Management (SRM).

The two new studies explore the implications of each of these methods, and the results are decidedly mixed.

Ocean acidification

We start with the oceans. About a quarter of the carbon dioxide emitted by human activity is taken up by the world's oceans. There it reacts with water to form carbonic acid, reducing the pH level and making the oceans less alkaline.

This process is known as ocean acidification, and it can have serious implications for marine life, says Sabine Mathesius from the Helmholtz Centre for Ocean Research in Kiel in Germany. She explains to Carbon Brief:

"The resulting increase in the ocean's acidity disturbs important biological processes, like the build-up of calcium carbonate shells. If ocean acidification continues at the current rate, many species at the bottom of the food chain, as well as corals, could face extinction. In consequence, species that depend on them, including fish and ultimately humans, would be affected too."

Mathesius is the lead author of a study, published in Nature Climate Change, which investigates whether CDR could help stave off ocean acidification. Her results suggest that continuing to emit carbon dioxide in the hope of being able to remove it from the atmosphere later could consign our oceans to changes that are irreversible on human timescales.

Carbon dioxide removal

The study focuses on two scenarios, or representative concentration pathways, for how we deal with rising carbon emissions.

RCP8.5 is the highest of the emissions scenarios used by the Intergovernmental Panel on Climate Change (IPCC), and is described in this paper as "business-as-usual". While RCP2.6is the lowest IPCC scenario, where emissions are curbed to keep global average temperature rise to within 2C above pre-industrial levels.

The researchers simulated applying three different levels of CDR to the RCP8.5 high emissions scenario: none ("CDR0"), five billion tonnes per year ("CDR5") and 25bn tonnes per year ("CDR25").

The paper notes that CDR25 is "probably unfeasible" as it is way beyond the potential of technologies that currently exist. Even CDR5 would required "gigantic efforts", but is technically possible, says Mathesius. Five billion tonnes of carbon is equivalent to 18.3bn tonnes of carbon dioxide - around half of global annual emissions in the present day.

The chart below shows what happens to total atmospheric carbon dioxide concentrations if these scenarios were played out over the centuries ahead. Only using CDR25 (orange and dashed blue lines) does carbon dioxide levels return to anything close to the RCP2.6 low emissions scenario (green dashed line), but not for at least two centuries.

Mathesius Et Al 2015 Fig1bProjections out to year 2700 of global atmospheric carbon dioxide concentrations under RCP8.5 with no CDR (black line), CDR5 from 2250 (red), CDR25 from 2250 (orange), CDR5 from 2050 (purple dashed), and CDR25 from 2150 (blue dashed). RCP2.6 shown as a green dashed line.

Irreversible changes

The charts below show what these scenarios of carbon emissions and CDR could mean for ocean acidification. The pH of the oceans drops in all scenarios, but recovers most quickly in the RCP2.6 scenario and the "unfeasible" CDR25 option.

Mathesius Et Al 2015 Fig2adChange in globally-averaged pH for the entire ocean (left-hand chart) and surface ocean (right-hand chart). Legend matches earlier chart. Source: Mathesius et al. (2015)

Acidification occurs first in the surface waters and then spreads to the rest of the ocean over time, explains Mathesius:

"The fastest alterations occur at the surface, where the ocean is directly exposed to the rising carbon dioxide concentration and increasing temperature of the atmosphere. Within decades to centuries, these altered water masses are transported by the large-scale ocean circulation into great depths."

When and if atmospheric carbon dioxide levels do fall, the oceans eventually release the gas back into the atmosphere, says Mathesius. But the vast conveyor belt of the oceans means it would take much longer for the oceans to recover as well, she says:

"Once in the deep ocean, the acidified water is not in touch with the atmosphere anymore. Even if we could re-establish the atmospheric carbon dioxide concentration, the deep ocean would remain acidified for many centuries."

This would be too late for the marine species that would have long become extinct, she adds.

So while CDR could potentially help reduce net carbon emissions, it should only be in combination with ambitious climate policy, concludes Mathesius:

"Our study shows that CDR has the ability to reduce atmospheric carbon dioxide and its impacts for the oceans to some extent - but not nearly enough to counteract the impacts of a business-as-usual emissions scenario."

Marine cloud brightening

The second study, published in Environmental Research Letters, considers how SRM over the oceans could boost crops yields and reduce harvest failures in northeastern China and West Africa.

Marine cloud brightening (MCB) is potentially a way of "seeding" clouds to reduce incoming solar radiation in the tropics, says lead author Dr Ben Parkes, formerly of the Climate Impacts Group at the University of Leeds, and now at the University of Pierre and Marie Curie in Paris. He explains to Carbon Brief.

"By increasing the cloud droplet number in the clouds, they become brighter and reflect away more incoming solar radiation."

Parkes and his colleagues ran simulations of a "very mild form" of climate change where carbon emissions rise by 1% a year until atmospheric concentrations reach double pre-industrial levels (560 parts per million, ppm). They then added in a simulation where clouds over 3.3% of the world's ocean were brightened - predominantly over the Pacific and South Atlantic.

Using a crop model they compared how well spring wheat in China and groundnuts in West Africa would grow under these scenarios when compared to a third "control" scenario that holds atmospheric carbon dioxide at 440ppm.

The results show crop yields are higher under the climate change scenario, and greater still with MCB implemented. The yields increase as a result of lower temperatures, increased rainfall and the boost to crop growth caused by higher levels of carbon dioxide in the air, says Parkes.

The study found mixed impacts on the number of crop failures. Under climate change, the number of failures in groundnut crops in West Africa were lower than the control scenario, and were reduced further with MCB. For spring wheat in China, crop failures under climate change were higher than the control, though MCB scenarios tempered this increase. Generally, MCB was more effective at reducing "mild" crop failures than "severe" ones, the study finds.

Parkes also notes that although geoengineering could bring some benefit to these regions, the impacts may differ elsewhere:

"MCB changes the precipitation rates globally and in regions of reduced precipitation - notably the Amazon but also South East Asia - we would expect an increase in crop failures. This is something that would require further investigation."

Wheat Field Shutterstock

Fields of wheat in China. Credit: ssguy | Shutterstock

Treating the symptoms, not the illness

Brightening clouds over the oceans can only be a temporary way of offsetting the impacts of climate change, warns Parkes:

"MCB doesn't undo climate change but instead mitigates some of the effects; in a medical metaphor, it treats the symptoms not the illness."

And if climate change is more severe than the mild scenario they tested, geoengineering efforts would be less effective in preventing crop failures, Parkes adds:

"If [the increase in emissions] were to continue, they would eventually have a greater warming effect than the cooling effect from the geoengineering. This is why all solar radiation management schemes are a temporary solution while a transfer to a lower emission economy takes place."

This is a conclusion shared by Dr Richard Matear and Dr Andrew Lenton from the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia. Writing in an accompanying News & Views article to the CDR paper, they say that cutting emissions now is a much safer option than relying on geoengineering:

"[The research] demonstrates that proposed technological solutions, like CDR, to the problems of global warming and ocean acidification are no substitute for reducing carbon emissions, which remains the safest and most reliable path for avoiding dangerous climate change."


Mathesius, S. et al. (2015) Long-term response of oceans to carbon dioxide removal from the atmosphere, Nature Climate Change, doi:10.1038/nclimate2729

Matear, R. & Lenton, A. (2015) Restoration of the oceans, Nature Climate Change,doi:10.1038/nclimate2737

Parkes, B. et al. (2015) Crop failure rates in a geoengineered climate: impact of climate change and marine cloud brightening, Environmental Research Letters, doi:10.1088/1748-9326/10/8/084003

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Comments 1 to 9:

  1. Lets see ... at $600/tonne (ref), CDR25 would cost $1.5x1013/year, and CDR5 would be "only" $3x1012/year.

    I hope there are some big cost reductions coming in CDR.

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  2. A question that should be asked more often in the climate debate is: who is going to pay for this ? I even think it is a kind of denial that this question so seldom pops up. Perhaps it is until now even the greatest feat of fossil fuel companies that they have succeeded in suppressing that question. The answer should actually be: the polluter will pay, no matter what. If we choose to tackle the climate problem: the polluter will pay to remove the CO2 from the atmosphere. If we choose not to do anything about climate change: the polluter will pay for all catastrophes caused by his actions in the next centuries. (Before I am attacked from all sides let me assure you: I don’t think there is a REAL choice. But fossil fuel companies should realize that those are the 2 options they are facing).

    Let’s calculate the carbon tax necessary to cover the costs of removing all emitted CO2 from the atmosphere.
    A car using 5 liter diesel fuel per 100 km emits 132 g CO2 per km.
    At the current price of 1,1 €/liter diesel fuel (current price in Belgium)
    the driving costs are: 5/100*1.1 = 0.055 € per km
    So, how much more expensive would diesel fuel become if we had to pay for the removal of the CO2 from the atmosphere (taking into account the estimate of 600 $, or 500 € per ton CO2 ?)
    Extra cost : 500* 132/1000000 = 0.066 € per km.
    So the price of diesel fuel would more than double. And the price of most, if not all consumer products would go up as well.
    Still, I think it makes sense to introduce a worldwide carbon tax.

    After all, what is the alternative ? Taking the money necessary to fight climate change straight from the taxpayers ? Trying to sell 25 billion tonnes of captured CO2 to greenhouse owners and carbonated beverages producers ?

    Of course, we don’t want to destroy the economy. So the carbon tax should start low and increase gradually year after year. This gives both companies and people the time to adapt. Luckily the purpose of the carbon tax is not really to fund an expensive technology to remove CO2 from the air (although it can be used for that purpose). The actual goal is to level the playing field between fossil fuel and renewables and speed up the development and adoption of low-carbon or zero-carbon technologies.

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  3. Dear no2 comment,

      Do you even know what sort of legalised behemoth fossil fuels are? These are legalised entities: without governments they don't exist and you actually think they have a birthright to extract sovereignised resource?


     (Do you believe in pirates or something???)

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  4. Dear no2: can I join your pirate ship that won't last very long: please?

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  5. @ #2: I'm not entirely convinced that end user payments - or any intermediary ones, for that matter - are wholly effective in the long run. We have become so used to present levels of energy use that increasing the cost rapidly leads to increases in incomes to compensate, bringing us back to square one.

    One thing that is rarely hammered home in this whole "debate" with the man in the street (who is not particularly well informed) is that there is to all intents and purposes a certain amount of carbon in/on the planet. Over millenia prior to the industrial revolution and after billions of years of evolution this amount of carbon reached a balance in the atmosphere. What has happened since the 1800s is that hundreds of millions of years' worth of carbon has been released from that locked up underground in the blink of an eye, altering the entire balanced system.

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  6. The trouble with comment no.2 is that there is no recognition of what the word 'cost' means. Governments release a money supply that allows costs to be recognised by the exchange of said money supply.

    Governments also release carbon emissions by allowing certain types of economic exchange to be realised with the aformentioned,and now famous, money supply. In the end Governments are responsible and that means us if we do infact live in representative democracy...

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  7. A third alternative is to remove heat from the earth- air- ocean system; what I call benign atmospheric engineering, BAE. A forth is perhaps "all of the above"; what if a BAE system did all of the above, removed CO2, reduced insolation, and increase long wave radiation?
    Realistically, IPCC procedures guaranty underestimation of the climate crisis. What I understand is that their prediction for sea level rise went from 12 inches to a yard within a decade because we underestimate when and how CO2, CH4, and other GHG releases from permafrost and ocean strata will effect climate, because most models don’t incorporate such feedback loops effectively. Realistic climate change seems to be much worse and sooner than expected and planned for.

    So… we must develop tools with which to manage and adjust climate. Further, natural variations in climate may be costly to civilization, absent CO2 based energy systems, so again the same conclusion. By avoiding the discussion, our good intentions blind us to necessary potential solutions. Further, although I don’t study ocean acidification, it may be another huge feedback loop. This again leads to the same BAE development conclusion.
    Although it would be best to reduce our C-foot print, it has not happened, and it is unlikely to happen on time. Alternatively, we can develop BAE, but we must be open the idea and its development. Advanced designs can be integrated into food and energy production so that by providing for anthropogenic needs we reduce the anthropomorphic load on the planet and biome balances. But, our minds must be open to BAE.

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  8. @Bozzza #3: what exactly do you propose ? To force fossil fuel companies to give back all profits they ever made during their existence , and to put all CO2 that was ever emitted due to their activity back into the ground because you question their right to extract natural resources from the soil ?

    Or do you propose to nationalize all existing fossil fuel reserves (and you ask if *I* believe in pirates ???)

    I agree with you that a company shouldn’t have the right to extract natural resources, just because they were the first to stake the claim. The natural resources of a country should be under democratic control. But this isn’t the case and it is not going to change.

    Either we can have such unrealistic demands that they will never become reality, or we try to find a practical way out, preferably without destroying the economy.

    My proposal: “the polluter pays” is a principle that can be enforced legally and has been enforced legally in the past. From the point in time when science comes to a conclusion, for instance about the danger of asbestos, a company can be held legally responsible for the damage they are causing (it doesn’t matter if they claim they weren’t aware of the science, they should have known).

    I think there is no case in which the scientific consensus has been so formally and comprehensively documented as for antropogenic climate change. Any fossil fuel company continuing the business as usual after this consensus was published can therefore be sued. I would use this as a big stick to get fossil fuel companies to agree to a carbon tax. The same is true for governments: the science is clear, the government’s task to protect the public against internal and external threats is also clear. They should either take measures to protect the public or face legal charges.

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  9. @Wol: you would be right if there were no alternatives to fossil fuel for energy production.

    A carbon tax would not make energy in general more expensive. It would make fossil fuel based energy more expensive. This would have the following consequences:

    • it will motivate manufacturers to produce more energy-efficient appliances, vehicles etc.
    • it will speed up the replacement of fossil fuel based power plants by renewable energy.
    • it will make it more interesting for investors to invest in low carbon or zero carbon technologies.
    • the most polluting fuels (f.i. tar sands) with the highest CO2 emission per barrel produced would become the most expensive, and be prized out of the market

    Of course, this doesn’t happen overnight. It takes time. That’s why a carbon tax should start low and increase gradually.

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