Removing atmospheric Carbon Dioxide

Some hair brained suggestions have been made for removing Carbon dioxide from the air or even from the smoke stack of coal fired power stations.  Suggestions have also been made about putting little mirrors in the Le grange point between us and the sun to cool off the earth.  Just imagine how much fun this would be  at the next economic crisis when funds are cut for  constantly  renewing these mirrors and we already are up at, say, 500ppm carbon dioxide. not to mention a likely reduction in photosynthesis from the shading which would itself, cause a reduction in the uptake of carbon from the air.

I have read that in order to remove carbon dioxide from the stack of a coal fired power station (or one growing biomass) we would need to use an extra 30% more power.  In other words you would have to burn 30% more coal.  I wonder if you would also have to burn 30% of the 30% which equals 9% more coal to sequester the  30% extra coal you burnt.  Then you would have to burn 30% of the 9% which equals another 2.7% to take up this extra 9% worth of carbon dioxide.  Lets go one more.  30% of the 2.7% is 0.81%.  So far we are up to a total of 42.51 percent more coal burnt in order to sequester the carbon dioxide produced.  Sorry I'm being facetious.  It could be that the boffins already calculated this sequence and it came to 30% overall.  Perhaps they will put in wind turbines to generate the power to remove the carbon from the smoke stacks!!!! (instead of simply using the power from wind turbines to replace the use of fossil fuels)

It is axiomatic that we have to first stop putting more sequestered carbon into the atmosphere.  Just imagine the stupidity of trying to pull Carbon dioxide out of the atmosphere while coal fired power stations are pouring more into the air.  Not rocket science - right??

However this blog is about measures we can take to remove Carbon dioxide from the air.


Let's get real about this.  Has anyone noticed that the level of Carbon dioxide goes up and down by 7ppm during the year or more accurately, 8 up and 6 down.  Natural processes are far stronger than anything we are likely to come up with.  How about if we could get the system to go up 6 and down 8.  Let's see what natural processes we could encourage.

And while we are at it, the world production of Carbon dioxide from fossil fuel in 2008, which is the latest figures I can find, was 2.988 x 10¹³ kg.  The mass of the earth's atmosphere is about 5 x 10¹⁸kg.  Dividing one by the other and we see that we are putting enough Carbon dioxide into the atmosphere to raise the level of Carbon dioxide by 5.97ppm each year.  In actual fact the net rise in Carbon dioxide is around 2.5ppm per year (and rising).  So about 3ppm is being taken up somewhere.  If we stopped the use of all fossil fuel, this net uptake of about 3ppm would not stop.  At least at first, we could expect Carbon dioxide to decrease at about 3ppm per year.  However, we could even do better than this.

There are a lot of carbon sinks we could encourage. 


Amount of CO₂ in the atmosphere
The figure of 5 x 10¹⁸kg of atmosphere was from wikipedia.  Here is a calculation that comes up with a slightly different figure.
*The pressure on each square metre of land at sea level is 10.3 tons.  In other words, the column of air above each square meter weighs this amount.
*The area of a sphere is 4πr² so the area of the earth with a radius of 6371km is 4π x 6371² = 5.10 x 10⁸ square kilometers.
*There are 10⁶ square meters in a square km so the area of the earth is 5.10 x 10¹⁴ square meters
  *5.10 x 10¹⁴ square meters times 10.3tons per square meter equals 5.25 x 10¹⁵ tons of atmosphere## (= 5.25 x 10¹⁸kg

*The Carbon dioxide concentration is just about to reach 400ppm so I will use this figure.  Note that this is the parts per million by volume.  To calculate the weight we must multiply by 44/29.  44 is the molecular weight of CO2 and 29 is approximately the molar weight of air.  The weight of Carbon dioxide in the atmosphere is therefore 5.25 x 10¹⁵ x 400 / 10⁶  x44/29 = 3.186 x 10¹²tons of CO₂ in the atmosphere.

Now that we have a handle on the size of the problem, let's look at how we could allow Gaia to remove the Carbon from the air.

1/  Stop the Production of Palm Oil and clear fell logging and let Open fields revert to jungle
A mature jungle, by definition, does not produce any net oxygen or remove any carbon from the atmosphere.  The rate of trees falling and rotting is equal to the rate of photosynthesis.  (that is the meaning of mature - or at least one definition of the term).  In the tropics where the soil is above about 25degrees C, humus doesn't accumulate so mature tropical jungles, while they sequester (hold) a lot of Carbon,  do not remove any net Carbon  from the atmosphere.

A growing tropical jungle is a different beast all together.  You only have to look at what happens when a forest giant falls in a mature jungle.  The little saplings which have been stunted from a lack of light shoot up at an astounding rate. Pretty soon that part of the forest is impenetrable.  The trees compete and eventually a few are left and they contain huge amounts of wood with its sequestered carbon.  As a tree continue to grow, it continue to sequester more carbon dioxide and this continues until it dies and returns its carbon to the atmosphere.

Let's say a certain type of wood is 50% water*.  That is to say it is half wood, half water.  So in 100kg of freshly cut wood you have 50kg of actual wood.   About 50% of dry wood is carbon.  Carbon has an atomic weight of 12.  Oxygen, 16.  So CO2 has a molecular weight of 44.  Every kg of carbon sequestered in wood represents 1kg x 44/12 equals 3 and 2/3 kg of carbon dioxide removed from the atmosphere.  To grow  100kg of wet wood the tree has actually removed 183kg of carbon dioxide from the air.

* note that in the above link, they use the amount of water divided by the amount of dry wood.  I think this is a little confusing as they get 100% moisture or even more, depending on the species of tree.  I think the amount of water divided by the freshly cut wood is less confusing.

So if we stop producing palm oil and let the jungle take over again, a huge amount of Carbon will be sequestered from the air.

2/  On any land where logging is practiced, build the logged wood into long term structures and replant.
If you look at the above link under the words "50% of dry wood is Carbon" you will see a calculation that for Douglas Fir on the coast of BC with a 70 year rotation Assuming you build the lumber into long lasting structures.  Such  a forest will result in the removal of 5 tons of CO₂ per hectare per year.  This assumes that only the milled timber goes into long term structures and doesn't assume any use for the waste wood such as paper, press board or charcoal for soil improvement and sequestration so it is a very conservative estimation.  There are 100 hectares in a square kilometer so each square kilometer planted in Douglas fir on the coast of BC would remove 500 tons of CO2 per year.  The results are so variable for different areas and different species that I am not even going to try to estimate how much CO2 could be removed from the atmosphere by  logging and using the wood for long lasting structures.  However, using just this approximation you can see that it is substantial.  Pyrolyze all the waste wood making cooking gas, gasoline, diesel and air line fuel and you displace oil extraction.  Incorporate the charcoal into agricultural soils and you sequester considerable carbon, long-term in the soil.

3/  Turn wood waste into charcoal and use in tropical soils
Humus does not accumulate in tropical soils the way it does in the soil under temperate forests.  However, it has been found that charcoal can replace humus in tropical soils.  It is stable and serves the same purpose of storing nutrients and releasing them to plants.  This is called Terra Preta and it has been found in certain areas in the jungle where generations of people have incorporated charred organic material into the soil that they use for growing crops.  Tropical soils are very poor for agriculture partially due to their lack of ability to store nutrients.  Add charcoal to these soils and they are markedly improved.

4/  Stop,,,,, Completely Stop the Harvest of Whales
Many species of whales feed at depth and poop on the surface.  This has been termed the Whale Pump and in pre-hunting times must have brought mega quantities of nutrients up into the photic zone. Whales also take nutrients from polar waters to oligotrophic* tropical waters where they go to give birth.  While many species of whale do not feed in the birthing areas, they feed their babies who poop nutrients into the nutrient poor tropical waters.  The phytoplankton gets  nitrates, phosphates and all sorts of other 'ates' from this rich source of manure and absorb carbonate from the water to build their bodies.

* Nutrient poor.

The Carbon gradient from the air to the water is therefore increased and the sea water can absorb more Carbon dioxide from the air.  It is estimated that about half of the Carbon dioxide we have produced has been absorbed by the oceans.  If this was not so, we would be approaching an atmospheric concentration of around 550 ppm now instead of 400ppm.  As with any reaction, as it proceeds it slows down.  At some point, the oceans will be saturated with respect to carbon dioxide and will cease to absorb any more.  At that point, other things being equal, our 2-3ppm yearly increase in Carbon dioxide will jump to 4 to 6ppm.

Long before that happens, though, the oceans as we know them will be dead.  Already there are indications that Pteropods, a swimming snail that serves the same function in the food chain as krill, are having trouble forming their shells because of ocean acidity.  Note here that if we restore the whale pump, not only will the oceans  be able to take more carbon dioxide out of the oceans  but the danger to the ocean food chains will also be reduced.  It will also increase the amount of fish we can take sustainably from the oceans.

5/ Put Half of the Oceans off Limits for Fishing.
Our catches of fish are pitiful compared to what they once were*.  We have destroyed so many populations that it is amazing that the oceans still function.  The amounts of carbon stored in the fish, invertebrates plankton and so forth must have been huge.  We have fished out the oceans, eaten the fish, pooped out the residue and released all this Carbon dioxide into the air.  Let the fish stocks recover and they will once more hold mega quantities of carbon.

*Read the book Sea of Slaughter by Farley Mowat to get an idea of just what we have destroyed.

Even better, have you ever seen recreational fishermen, line fishing just on the borders of the tiny marine reserves  we have set aside. The catches there are great as adult fish from the reserves look for new sources of food outside the reserves.  Imagine what the fishing would be like if we set half of our areas aside as no fishing zones.  There would no longer be any need for FADs, drift nets, bottom trawls or purse seines.  The fishing would be so great that only hook and line methods would be necessary*.  We not only sequester carbon but improve our fisheries at the same time.

*Mowat's book again.


6/ Protect our Corals
Sea level is going up at about 3mm per year.  No matter what we do, it won't slow down any time soon.  There will be an overshoot even if we stop all carbon emissions tomorrow.  Over the whole transition from a glacial, 20,000 years ago to our present Holocene interglacial sea level rose at about 6mm per year although there were intervals in which the rate rose to about 56mm per year.  Coral skeletons are CaCO3 and are a tad over 60% carbon dioxide as are the shells of mollusks (oyster reefs) and any other structure made from Calcium carbonate.  As the sea level rises, the constraint of the surface is removed and corals can grow upward.  If our corals are healthy, they will absorb large quantities of carbon dioxide as they grow upward.  If we stop acidifying and warming our oceans and take a few other measures to re-establish the health of our coral reefs such as not fishing certain species,  corals will help us get rid of atmospheric CO2.


7/ Let Grasslands Recover
Many new civilization mine their dirt until there is nothing left and the civilization collapses. Most of the carbon which had been stored in the soils goes into the atmosphere.  At the very least we have to adopt farming practices that stop this process.  Even better would be if we could restore the environment that existed, for instance, on the great plains of North America.  The plants of grasslands are mostly under ground.  This an adaptation to fire.  Grass fires are intense but if short duration and the roots and stems of the grass remains to sprout leaves at the next season.  However, we don't want fires and there is a far better option.    Have a look at this Ted Talk by Allan Savory,  Better still, read The Omnivore's Dilemma, by Michael Pollan starting at chapter 10.  Also read Growing a Revolution by David R Montgomery.   By the time you have read both of these you should be convinced that there is far more our farmers can do despite their protestations that they are doing all that is possible.   And they will have a far more fulfilling farming experience and an improved bottom line.



8/ Reflood Bogs
Bogs, or wetlands as they are often called sequester carbon at a great rate.  This is especially so if the bottom of the bog is anaerobic.  Cellulose, which is 50% Carbon is refractory under anaerobic conditions,  The Hula in Israel is a good example.  It is a wetland in the rift valley upstream of  the Kineret (sea of Galilee).  The Israelis drained it and turned it into farmland.  The peat which had accumulated over Milena started to oxidize and release nutrients and carbon .  It polluted the Kinerit from which Israel draws her water.  A few decades ago, Israel realized the problem and re flooded the Hula.  Now it once more sequesters carbon and cleans water flowing through it to the Kineret 

9/ Put Nutrients back on to the land
The Chinese have managed to keep an agricultural civilization going on the same piece of land for over 5000 years.  She did this by recycling all animal and human wastes back on to the land.  The flush toilet is going to be China's undoing unless they have systems to cycle the nutrients from sewage plants back on to the land.  This sort of fertilizer has the added advantage of containing much organic carbon so it feeds the micro-organisms of the soil.  Think of the plains of Africa or North America in their pristine state.  Every bit of waste, every body went back into the soil.  The Indians of the great planes even put the bodies of their dead on platforms for the birds and insects to return to the Great Spirit.  We have depleted the carbon content of our soils.  Restoring the system would pull even more Carbon out of the atmosphere.


10/  Allow Beavers to Repopulate Every Stream Possible
Beavers have a number of effects with respect to carbon sequestration.
1) by raising the water table around their dams, Beavers increase the growth of all the vegetation.
2) by capturing the spent salmon after they have spawned, Beavers hold a valuable source of nutrients which came up from the sea.  These nutrients are cycled away from the dam in the droppings of all the animals that get some of their food from the beaver pond and its immediate surroundings.  Plant growth including forests is stimulated, sequestering more carbon.
3) by burying cellulostic material,  Beaver dams settle out silt from the water and capture 'bed-load'.  All the bits of cellulose and even their lodges and dams are eventually buried and become a deep carbon rich deposit.  When agricultural man found this rich bottom land, he drained it and mined it with his crops much as was done in the Hula.  The more Beaver dams we can allow to flourish, the more carbon we will remove from the atmosphere

11/ Protect Boreal forests
The tree line is moving northward with climate change.  This mimics what happened when the continental glaciers left the land.  Forests reestablished and much carbon was sequestered.  The forests are going to creep northward.  We must just let them do so without hindrance.

Final Note
Most of the systems above involve getting nutrients back into natural systems and then protecting them so that they can build up their biomasses and lock up carbon dioxide.  With a population that is already decreasing in many of the countries of the world and the means available to assist countries that haven't reached this favourable situation, we should soon be able to return land to nature*.  Most important, though, is that we cease to use fossil fuels.  Besides they are far to valuable to burn.

*See the TED talk by Monbiot  on re-wilding.

carbon sinks

Glaciated periods (glacials) end with a sharp increase in Carbon dioxide.  It looks most likely, as odd as it seems, that the rise in CO2 is the result rather than the cause of the melting.  In a couple of former blogs, I suggested why this might happen.  The release of CO2 then accelerates melting, in a feed-back loop.  Over the subsequent  years of the interglacial, carbon sinks continually reduce the level of atmospheric Carbon dioxide.  When Carbon dioxide levels fall sufficiently, snow begins to accumulate at high latitudes and high altitudes in the summer and we edge into another glacial.  I'd like to try to catalog as many potential sinks as I can think of, regardless of their perceived importance.  Others can work on working out which ones are significant and which not on the road to the next glacial.

Note that you can reverse each of these arguments to see what we are doing to increase atmospheric Carbon dioxide.

Corals
Corals and all other marine organisms that have a Calcium carbonate shell are sinks for carbon dioxide.  Every molecule of CaCO3 contains one molecule of Carbon dioxide (60.6% by weight).   In addition to the building of reefs by oysters and corals, some phytoplankton have calcium carbonate shells.  There are globigerina, a type of foraminifera, pteropods, a swimming snail and so forth.  When these die, they sink to the bottom of the ocean and form calcareous oozes.

 Curiously, at great depth (below about 4500m) the calcium carbonate dissolves so these oozes only form in medium deep water.  The white cliffs of Dover are such deposits and give a visual indication of the extent of carbon sequestration in the form of calcium carbonate that phytoplankton can effect.

Note: In an item I just saw on Discovery Channel (Feb20,2012) it appears that corals are much more common in the deep ocean than previously believed and even form quite large "hills" of Calcium carbonate in some locations.   This sink would presumably stop if the ocean overturn ceased and the deep ocean became anaerobic but as long as the system is working, deep water corals are another significant sink.

Question:::  What happens when this water which is enriched in dissolved Calcium Carbonate upwells into  shallower water???


Of special interest with regard to corals in the photic zone is that they are limited in their growth by the surface of the oceans.  If you have visited coral reefs you may have seen some of the brain corals which show concentric rings on their tops.  These rings are caused by  very low tides which have killed off the top of the coral.  As the diameter increases a new ring is formed.  Rather than recording yearly growth, these rings record king tides.  Corals don't grow in air.  Following the end of the last (and every other) glacial (ice period), the sea rose.  At the maximum extent of the last glacial the sea was about 120m below its present level. The last glacial lasted for about 125,000 years since the end of the Eemian interglacial.  With various smallish ups and downs, the sea level steadily dropped to this lowest point of 120m below its present level.  Coral reefs, of course, died as they were exposed and then a hundred thousands  years of waves pounded on the shore, washing the corals away, leaving cuts into the land.  Around 20,000 years ago the ice began to melt but the melt only really got under way about 11,000 years ago.  Sea level rose  quickly (in geological terms) to its present level.   During part of the transition to our present Holothurian interglacial, sea level occasionally was rising as much as 56mm/year.  The over all rise during the melt was about 6mm per year, twice the present rise.  For reference, at present (2012), the rate of sea level rise is about 2.8mm.yr.   



In tropical areas, corals grew to fill this 120m gap up to the surface of the ocean, an over all growth rate of 6mm per year.  It is estimated that the total area of coral reefs, world wide is 284,300 Square Kilometers.  This figure times 1,000,000square meters per square km times 120 meters depth times 2.5sg (specific gravity) for limestone times 60.6% Carbon dioxide in limestone  divided by 1billion gives a figure of 5,174 gigatons of Carbon Dioxide sequestered in corals since the end of the last ice period. To put this into perspective the total Carbon dioxide in the atmosphere today is about 3000 gigatons.  Corals which have grown since the ice age have sequestered about 1.6 times as much carbon dioxide as the Carbon dioxide at present in the atmosphere.  As sea level rises, we have a potential carbon sink in corals as long as we don't kill them with acidification or thermal shock. massive coral bleaching could, of course,  shut down this carbon sink.

Ocean Overturn
At the poles, sea ice freezes out fresh water ice leaving salty brine behind.  This brine is heavier than open ocean sea water and hence sinks.  As it moves across the ocean bottom, it picks up nutrients from the mineralization of the constant rain of organic material from the surface.  Where wind conditions are suitable, surface water is pushed away from the shoreline to be replaced by this upwelling, nutrient rich water.  As this water comes into the photic zone, productivity is immense.  The ocean off Peru is an example and the Anchovi produced there provides a significant percent of the fish meal for world animal production.  The Atlantic overturn is another example with the Gulf Stream being the surface manifestation of this overturn.  The shut down of such ocean systems would cause Carbon dioxide to rise more quickly in the atmosphere

Swamps
If the water in a swamp is sufficiently stagnant and there is sufficient organic loading to make the bottom of the swamp anaerobic, then all cellulose that falls into it is preserved.  This includes all plant material and probably explains the formation of many of our coal measures.   If the area is sinking, the organic material is buried, heated, gives up its volatile fraction and what remains is mostly carbon with some mineral material which forms the ash when coal is burnt.  This process removes carbon from the atmosphere and also contributes to sending us toward another ice age.  

Growing Forests
A mature tropical forest, at first glance, would seem to be a huge sink for carbon dioxide.  After all, the rate of photosynthesis is huge.  This is an illusion.   Once a tropical forest has truly reached maturity, the rate at which vegetation dies and is oxidized equals photosynthesis.  Mature tropical forests, however, do represent a large sequestering of Carbon as long as they remain untouched but they have no net effect on removing more carbon from the atmosphere.

The carbon content of organic material, wood included, is about 50% of the dry weight of that material.  If an plant is 80% water, 20% dry matter, then it's carbon content is about 10% (of its wet weight). The cutting down of the forests of the world has released a large amount of carbon into the atmosphere and this carbon would be rapidly taken up by letting the forests grow again.  In this way, tropical forests could be a huge carbon sink if allowed to regrow.  The only way a mature tropical forest will continue to sequester carbon is if it contains swamps as detailed above.  Of course another way is to selectively log tropical forests (at a rate that doesn't imperil their survival) and to use the wood in long lasting buildings, furniture etc.  New trees grow where the old ones are harvested and a mature tropical forest then becomes a carbon sink.

New forests are a whole different ball game.  At the maximum extent of the last period of glaciation ('glacial' as opposed to 'interglacial'), ice, estimated to have reached a depth of 3km, covered most of Canada and a strip of America down to and extending beyond New York.  Much of Europe, Germany, Poland and  Russia and the UK were covered.  Smaller ice sheets were found on high land right down to the equator (Mt Kilimanjaro, for instance).  When the continental glaciers melted all the scraped-clean land was open for colonization (primary and secondary succession).  Much of this land ended up clad in forests of giant trees with their sequestered carbon.  As the tree line moves north with global warming, we have another potential carbon sink.

While we are at it, temperate forests are a different situation from tropical forests.  When Tropical forests are clear felled, it is found that the soil is thin and is exhausted very quickly.  This is due to the characteristics of humus.  Humus is the final break-down product of organic material and contains a lot of carbon.  Amongst it's other characteristics, it holds nutrients in a form that can be used by plants.  Above about 25 degrees C, Humus breaks down.  

In temperate forests, the humus builds and builds, depositing more and more organic material in the ground.  When temperate forests are clear felled, agriculture can carry on for far longer than in the tropics, before the soil is exhausted. Temperate forests can continue to sequester carbon even after they are mature (with respect to the total mass of live vegetation they contain). The re-establishment of temperate forests not only would sequester considerable carbon but continue to sequester carbon when the forest is mature.  It is a bit rich for northern hemisphere countries to complain about the destruction of tropical forest when they have decimated their own forests.

Permafrost
During the summer, the top foot or two of permafrost melts and a range of dwarf, ground hugging trees, lichens and mosses grow.  In the winter this freezes.  Each year another small layer is added and the layer of organic material deepens.  Permafrost areas are carbon sinks.  Of course, the flip side is that thawing them with global warming is a source of carbon. It is estimated that the carbon stored in permafrost today is greater than all the carbon of all living things and is twice the carbon in the atmosphere.     If the layer is deep enough (about 300m) the pressure is great enough that a portion of this carbon is in the form of methane clathrate which only needs a little warming to be released. In addition, any geological methane seeping up under permafrost can also be stored as clathrate as it comes into contact with moisture.  The depth of the frozen soil is not the only factor for the creation of methane clathrate.  The frozen permafrost can act as an impermeable layer, like the lid of a pressure cooker and clathrate can form at shallower depths.

It is interesting to note (and counter-intuitive) that by covering an area of permafrost with a deep layer of insulating  ice during a glacial, the permafrost will be melted by the heat coming up from the earth.  This deep organic soil would break down anaerobically giving up methane.  This would likely collect at the bottom of the ice sheet as a methane clathrate, ready to be suddenly released when the ice sheet melted.  Of course, the clathrate already stored in the permafrost would give up it's methane too which would seep up and be stored in the ice sheet. This may be part of the explanation of how Carbon dioxide rises so quickly at the end of glacials.  Released methane trapped under the ice oxidizes rapidly into Carbon dioxide and shows up in bubbles in the ice cores from Greenland and Antarctica as Carbon dioxide rather than as Methane.

Note that the Firn layer (permeable top of an ice sheet) is about 70m and gas can diffuse between this layer and the atmosphere so the signal of sudden methane or for that matter Carbon Dioxide would be smeared out throughout the layers.

Grasslands
Native grasslands have most of their biomass underground.  This is an evolutionary adaptation to fire.  Grass fires are intense but short and the heat doesn't penetrate far underground.  With the introduction of nutrients from wind-imported-dust, grasslands in their natural state continue to grow upward and add more and more organic material.  Organic material is ultimately stored in deep rich humus containing soil.  This rich accumulation of nutrients was mined by farmers planting wheat and other crops, especially in the great plains of America, and the carbon returned to the atmosphere.  In pre-agricultural times, grasslands were carbon sinks. Now they are sources of carbon.  If returned to their native state, they once more would sequester carbon.  Some research suggests that the north American Buffalo can produce more meat per hectare on native North American prairie than our much vaunted beef cattle.  At the same time they allow the sequestration of carbon to once more occur in the grasslands. Have a look at this TED talk on how to restore our grasslands.  Direct seed drilling without plowing has also been shown to increase the Carbon content of soils.

Calcareous Oozes 
Many plankton animals such as foramanifera have calcareous tests.  These sink  form layers of calcium rich deposits on the ocean bottom up to about 4500m.  Below this depth, calcium carbonate is soluble.  These oozes get buried and are carried toward subduction zones by the ocean bottom conveyor system.  Over geological time, these deposits of lime are recycled by volcanoes as Carbon dioxide. 60.6% of calcium carbonate is Carbon dioxide. When you think of deposits like the chalk cliffs of Dover it is apparent that calcareous deposits are a large sink for Carbon dioxide.

Silicate rocks
As erosion or volcanic activity exposes new silicate rocks to the atmosphere, they are attacked by Carbon dioxide, producing carbonates.  This is also a sink for Carbon dioxide but on a geological time scale.

Cement
As cement is produced a large amount of CO2 is released.  As it cures the Carbon dioxide is re-absorbed.  All cement structures around the world are slowly absorbing some of the Carbon dioxide that was released when they were produced.

 Carbon sinks are slow but inevitable.  As they proceed, the Carbon dioxide content of the air decreases, the climate cools until snow can begin to accumulate over the summer.  It would appear that with the amount of sequestered carbon we have introduced into the atmosphere by burning fossil fuels, the next descent into a glacial  will be much delayed.  It is interesting to think that we will have used up the world supply of sequestered carbon in the blink of an eye.  We would expect the present interglacial age to last another 10 or 20 thousand years.  With our Carbon output, let's say we have pushed the end of the present interglacial out to 30,000 years.  If we still exist then as a species, which seems unlikely, our descendants will be faced with the onset of a glacial age with no large resources of sequestered carbon left to counter it.