Half of it! eek

According to the CIA the worlds GDP is 54.62 trillion. Therefore the below clearly off the wall plan would cost approximately half of the worlds GDP. Bummer. 

https://www.cia.gov/library/publications/the-world-factbook/print/xx.html

Carbon sequestration algae

27 500 000 000 / 1 200 000 000

= 23 x worlds military budget.....

okay what is the worlds GDP?

It's 1.2 trillion. 

http://www.globalissues.org/article/75/world-military-spending

economic issues

Okay. I think i read $50 a Tonne.

So 50 000 000 hectares x 110 tonnes per hectare + 50 per tonne = 27 500 000 000 million dollars per year.....

What is world expenditure on arms?

50 000 000 hectares

Only half the size of south Australia (approx 100 000 sq kilometres)

So not that much really....

now I know I didn't calculate ongoing emissions. But there are alot of other techniques. We don't have to rely on just sequestering algae..

There are a "few" logistical/technical/economic issues with producing that much algae. But there is alot of non-ariable land out there... and if your just dumping it in the ocean the algae could be transported as sludge.

I wonder what the economic issues are?

50 million hectares

That is the same amount as currently planted with GM crops! 

www.europabio.org/articles/article_111_EN.pdf

South Australia is 100 000 000 square kilometers, and mostly desert. So if you grow algae and pump into into the ocean sediments you can sequester the amount of CO2 that humans produced historically in 100 years.

And hooray for that.

(they maybe a few other consideration outside of land....) 

Tonnes per hectare

using that CSIRO paper mentioned ealier algal ponds produce 110t/ha/yr

Okay. Lets aim to have all historical human emissions out of the atmosphere in 100 years (ignoring ongoing emissions at this stage; eggh)

So 780 billion tonnes X 1/1.5 = 520 billion tonnes

520 billion / 110  (tonnes/hectare) x 100 (years)= 50 000 000 hectares. 

Alot....

how much?

Algae sequestration

I have been looking around for how much CO2 algae sequestors per unit.

Best info so far is 1.25-1.75 tonnes of CO2 per of tonne of algae. Which is good. Less mass of algae than for pure CO2 (presumably because photosynthesis produces oxygen and stores carbon).

So we need to bury 1.00/1.5 (middle) X 780 X 10^15 = 520 billion tonnes.

How about in the sediments at the bottom of the ocean? plenty of room!

How much CO₂ can be utilized by growing 1 tonne of algae?

This ultimately depends on the algae strain and available nutrients, however, 1 tonne of algae can sequester about 1.5-1.75 tonnes of CO_2. CO₂ consumption is based on the overall lipid-protein-carbohydrates balance of the final algae. CO₂ incorporation is ultimately based upon the overall algae composition that results in the macromolecule composition of the harvested algae. These macromoleculesmolecules are between 40 and 80% carbon with an average composition of 50% carbon.

http://www.dynamicbiogenics.com/page1.html

How to make algal ponds

http://www.csiro.au/files/files/poit.pdf

de-acidifying the ocean?

The water treatment facilities worldwide "would remove hydrochloric acid from the ocean by electrolysis and neutralize the acid through reactions with silicate minerals or rocks," according to a statement from the American Chemical Society. "The reaction increases the alkalinity of the ocean and its ability to absorb carbon dioxide from the atmosphere. The process is similar to the natural weathering reactions that occur among silicate rocks but works at a much faster rate." 

http://news.mongabay.com/2007/1120-geoengineering.html

But I have to ask again. How much of these rocks are available? It would want to be a lot!

how to sequestor carbon?

Well if we had to trap it using chemical weathering (the most stable way) it would probably weigh a lot more

Using this ideal case, one ton of serpentine can dispose of approximately one-half ton of CO2. Reaction 3 illustrates the transformation of forsterite, which is the end member of the common silicate mineral olivine. One ton of olivine can dispose of approximately two-thirds of a ton of CO2. Again, the reaction is exothermic and releases 90 KJ/mole of CO2

and there wouldn't be enough raw material available to do it....

Vast Capacity - Raw materials for binding the CO2 exist in vast quantities across the 

globe.  Readily accessible deposits exist in quantities that far exceed even the most 

optimistic estimate of coal reserves (~10,000 × 109 tons) (5). 

 (http://www.netl.doe.gov/publications/proceedings/01/carbon_seq/6c1.pdf). 

not to mention mining all that rock would chop up the environment fairly badly (on the scale that coal is now)

What mass of CO2 have humans put into the atmosphere?

Wikipedia: Carbon dioxide [has] an average concentration of 385 ppm by volume or 582 ppm by mass. The mass of the Earths atmosphere is 5.14x10^18kg, so the total mass of carbon dioxide is 3.0x10^15kg (3,000 gigatonnes).

From various sources I know that pre-industrial CO2 was around 285 ppm by volume. The ratio won't change if I use volume rather than mass, so;

(100/385) X 3000 x 10^15 = 779 X 10^15kg.

If you read this, and no-one does, please correct the above in the comments

Cheers

Sam.