Carbon sequestration

Carbon sequestration is not something we can reliably accomplish in an energy-efficient manner with clear and permanent results. Many otherwise valuable carbon sequestration opportunities are decreasingly powerful due to the effects of global warming. Smil
Pumping CO2 into basalt might have a small effect, but it’s unproven. The idea is to trap CO2 in basalt layers beneath the Indian Ocean and/or the Juan de Fuca tectonic plate. This method, if functional, could only trap 4% of American CO2 emissions — so, this would be less effective than raising car emissions standards Smil, 87
Marchetti's solution: switch to natural gas, refine it in nuclear reactors, reinject gas fields with CO2. Not proven commercially viable, would require lots of infrastructure specifically hundreds of new specialized nuclear reactors and pipelines for transporting CO2 Smil, 86
Exposed peridotite (minable in the Omani desert) naturally absorbs CO2 Smil, 87

Sequestration in forests

Sequestering the amount of carbon we emit in trees would require truly enormous new forests. Planting mixed forests sequesters carbon at the rate over which the trees mature — so in 10–80 years after planting (depending on the type of tree), the tree must continue to live to hold carbon, but it does not offset new emissions Smil, 82.
Offsetting just 10% of 2005 carbon would require a planting as big as the combined forests of North America and Russia, or a ~15% increase in tropical forests Smil, 82.
Sequestration of forests fluctuates to the extent that some years forests can produce more carbon than they sequester Smil, 80
Tropical forests' carbon impacts will change in the near future mostly due to deforestation, but many other forests will be limited by water and soil nutrient availability esp by nitrogen. We will also have more carbon releasing wildfires due to longer droughts from global warming Smil, 82

There is opportunity to sequester carbon in soil, but global warming makes this type of sequestration uncertain in the long term. Soil stores about 4 times the carbon that is stored in land plants Smil, 83.
Tropospheric ozone levels are increasing and can reduce plant productivity thus slowing soil sequestration. Uncertainty caused by global warming means we can’t know whether soil will net store carbon from plants or net emit it from decomposition Smil, 83
Soil carbon is currently at half of preagricultural levels (because of intensive farming practices), so there is opportunity to store much more carbon in soil while also improving soil productivity Smil, 83

Biochar could improve carbon sequestration in soil, but there are logistical and environmental challenges. Soil with biochar stores 2.5x carbon as soil of the same type without it Smil, 83.
There is currently no supply chain set up to source waste biomass Smil, 84.
Biochar can provide only a small piece of the solution; 900 million tons of straw (the total amount produced by affluent counties) turned into biochar (ignoring the logistical and application challenges) would sequester only 2.5% of the CO2 emitted globally in 2005 Smil, 84.
Biochar integration currently requires tillage of the land (which can be environmentally destructive) Smil, 84.
Potential sources of biomass to pyrolize into biochar include crop residues and forestry waste. However, both of those include yet-unsolved logistical challenges, and might be environmentally destructive to collect Smil, 84

We don’t have an infrastructure to capture, move, and sequester carbon. Most sequestration solutions depend on pipelines of CO2 and other infrastructure that we don’t have and which will take time to build Smil, 87
Carbon sequestration through direct intervention by humans has unknown long-term effects. We don’t know what effect there is in injecting CO2 into underground structures. Sudden, catastrophic events might include earthquakes which rupture reservoirs and emit the CO2 gas directly back into the environment. Slow, long-term effects could include chemical reactions between stored CO2 and surrounding groundwater; some evidence suggests that this could result in heavy metals in drinking water reservoirs Smil, 94
Oil and gas companies already use CO2 pipelines and injections to harvest oil, so there is strong technical feasibility for transport and underground injection of CO2. We could even use the existing infrastructure, as a profit incentive for oil & gas companies. Is there a way to use this infrastructure and build additional for carbon sequestration? Smil, 90

[aggarwal]: https://www.sciencedirect.com/science/article/pii/S1040619013001917 "Aggarwal, Sonia and Harvey, Hal. 'Rethinking Energy Policy to Deliver a Clean Energy Future.' Energy Innovation, 2013."