Intro to Carbon Removal

     Natural CO2 removal by rock weathering is about 1 billion tons (GT or PgC) per year.  That speeds up with higher temperatures, but not enough to fully offset larger carbon pulses (e.g. from permafrost and subsea methane hydrates.  Still, it is a roughly fixed amount, while the size of pulses is variable.

     A pulse of 1 billion tons  is comparable to natural removal rates, so the % remaining after a few years is low.  On the other hand, a pulse (e.g., human emissions since 1900) is far more than the natural removal rate, so most of it will remain in the air after many years. 

     Map below is from NASA, May 2011 - in metric tonnes of carbon per hectare.  About 250 hectares per square mile.  Total Biomass Carbon is apparently for above-ground carbon only.   Mg = tonnes.

     Permafrost covers 1/6 to 1/4 of northern hemisphere land surface, 15-25 million square kilometers.  Its top 10 feet hold ~ 1.9 trillion tons of carbon.  That's 1,200 tonnes (Mg) / hectare (120 kg/sq m, 12 g/sq cm) —  which is 6-7 times what NASA shows for tropical rainforests.  Mongabay et al. estimate 168-420 (µ 302) Mg / hectare for various tropical rainforests, a bit more than NASA.

     Below is world soil organic carbon density (tons / hectare, as above).   by USDA: Natural Resource Conservation Service, 2000.

     Soil carbon densities for most of Russia, Siberia, Europe, Alaska, and Canada exceed total tropical ones for soil + trees.

Scientists Have Long Feared Soil Climate ‘Feedback’.  Now It’s Happening. 1116 - more than just permafrost soils

Global CO2 Emissions Still Accelerating, even as Human Ones Level Off 0516        The increase in CO2 levels in the air is accelerating, even as human CO2 emissions from fossil fuels are leveling off.
    This suggests some combination of fading carbon sinks in the ocean and the land biosphere, with rising emissions from natural sources - permafrost, forest fires, methane hydrates, and peat fires - as well as agriculture and land use changes.

Carbon Dioxide Removal and Reliable Sequestration - NAS 0215 - PDF, 155 pp

     Soil lost 230 GT of carbon over the past 10,000 years, due to land conversion and land use.  This compares to 365 GT added from burning fossil fuels.  Feasible soil carbon sequestration, by moving carbon from the air back into soil, is estimated (to 2065) at 30-50 GT of carbon.
    Other major methods to sequester significant amounts of carbon include accelerated rock weathering (almost as much potential), ocean iron fertilization (with 90-300 GT potential), bio-energy with carbon capture (100-1,000 GT), direct air capture, and sequestration for the 2 capture-only methods.  Cost estimates range from $1 to $1,000 per tonne of CO2 removed, depending on the method, with wide estimates for each method.

Section Map: Reversing Climate Change