The Soil Carbon Sponge

Road to Resilience


Walter Jehne, an internationally known Australian soil microbiologist and climate scientist has some potentially very good theories about how we can mitigate global warming, solve the water crisis, grow abundant food, and create wildlife habitat.  I know that is a big promise, and I can imagine those skeptical eyebrows rising as you read this.  It involves soil regeneration, which I’ve already talked a lot about, but the focus is not so much on carbon sequestration as it is on reestablishing the water cycle (how the water goes from the Earth’s surface into the atmosphere and back to the Earth again).  This gets a little wonky but it is well worth slogging through it.

You are probably already aware that our atmosphere now holds over 400 parts per million (ppm) of CO2.  There are 44,000 ppm of water vapor (a hundred times more).  The power of water vapor to absorb and transfer heat is 20 times higher than CO2, molecule for molecule, although the major role of CO2 is primarily as a reflector rather than an absorber. Plants cool the Earth by transpiring water into the atmosphere.  Much like an air conditioner, the process of evaporation absorbs a large quantity of heat from the planet surface.  As the water vapor rises into the atmosphere it cools and condenses, thereby releasing the heat in the upper atmosphere.  Most of that heat dissipates out into  space although some provides the energy for storms.

To transpire water vapor, plants and soil need to have sufficient water.  In order to store sufficient water, there needs to be a soil carbon sponge.  The soil carbon sponge is created in healthy soils.  When soil is undisturbed, kept covered with plants, receives sufficient water, and has sufficient microbiota, it will sequester carbon.  When all of this comes together, a wonderful thing happens:  the minerals in the soil and the carbon are bound with 1-3% organic matter by the microbiota to create a living community that is 60% air.  This is the soil carbon sponge!

Soils that have been plowed, cleared, and compacted in the usual manner of today, even if organic, may only absorb a half inch of water an hour.  This means that most rainfall will run off into streams.  Especially today with the larger storms we are getting, this leads to catastrophic flooding, soil and crop loss, and pollution in downstream waters.  The soil carbon sponge can absorb 6-7 inches of rain per hour (see Gabe Brown) and often eliminates flooding.  That water is then not only available to the plant and the farm, but also feeds the aquifer and will transpire into the atmosphere via foliage to cool the planet.

Arid places become more arid because the dry soil temperature is always much warmer than plant covered healthy soil.  This creates a high-pressure dome in the atmosphere that does not allow low-pressure, moisture-laden air to enter.  Urban areas add to this effect.  This is the situation in the Central Valley of California, which has been in drought for years now.  Before the soil was exposed and depleted, this area received regular rainfall from nearby marine air masses.

Another problem is getting the water vapor that is in the air to condense and fall again as rain.  There are many areas in the tropics that are in drought even though they are continually in a water vapor haze.  The water is actually condensed into micro droplets, but there has to be sufficient hydroscopic precipitation nuclei for a thousand water droplets to come together to create a raindrop big enough to fall from the sky.  Ice crystals serve this purpose in high latitudes and altitudes. Salt can serve this purpose over the ocean, but over land the best is a hydroscopic bacterium.  These bacteria are produced near the stomata (pores) of leaves.  When the leaves transpire, the bacteria rise into the air with the water vapor.  Thus, forests create their own rainfall!  In the Amazon it is almost like clockwork—every day at about the same time in the afternoon the rain falls.  Per area, much more water evaporates from vegetation than from water surfaces because of the much larger surface area provided by leaves.  So, this is another reason to keep the ground covered with plants.  It is also a very hopeful sign for arid and desert areas.

What I love about this idea is that weather and climate might be a truly biological phenomenon, not a non-living mechanical function.  By merely focusing on healthy soil, more carbon gets sequestered (proven), more rainfall is produced, more water saved in the soil (proven), more food is grown, wildlife habitat explodes, and the earth is cooled.  And all of it is self-renewing, inexpensive, low tech, and anybody can do it at whatever scale because mostly it just requires a little muscle and human care and diligence.  There are probably a few more positive aspects, but no negatives as far as I can see.  It’s still a theory but there is no downside to proceeding with this right away in our own backyards.