# Contents

## The idea

Carbon negative energy refers to any form of usable energy whose production reduces the amount of carbon dioxide in the Earth’s atmosphere.

If you can think of other sources of carbon negative energy, enter information about them here! — John Baez

## Biofuel versus biochar

Photosynthesis can take carbon out of the atmosphere and provide useful fuel. If all the carbon taken out is returned to the atmosphere when this fuel is burnt, this process is roughly carbon-neutral. This is an oversimplification of the actual story. On the one hand, it takes energy to grow and process vegetation into usable fuel; if this energy comes from fossil fuels there may be a net gain of CO2 in the atmosphere. On the other hand, not all the carbon is returned to the atmosphere: some stays in the ground, at least for a while, in the form of roots, humus, etcetera.

We need detailed figures here! What is the overall carbon footprint of biofuels? People have done research on this already, and the results should probably go in a page on biofuel. — John Baez

Ignoring these subtleties, we may very roughly say that fossil fuels are carbon-positive while biofuels are, at best, approximately carbon-neutral. On the other hand, biochar offers the possibility of strongly carbon-negative energy production: significant long-term reduction in atmospheric CO2 combined with the production of significant amounts of energy. The idea here is to create some usable fuel from plant material, while also creating large amounts of charcoal which can be buried to sequester carbon. Unlike rotting vegetable matter, buried charcoal can sequester carbon for centuries or even millennia.

For more, see biochar.

## Olivine weathering

One of the main long-term mechanism that removes carbon dioxide from the ocean and atmosphere is the weathering of rocks. During major periods of tectonic uplift in the Earth’s past, huge slabs of rock rich in the mineral olivine (mostly peridotite) were pushed up through the Earth’s crust, with some of it being exposed at the surface. The resulting chemical weathering caused or contributed to a significant drop in CO2 levels leading to global cooling.

We can do this ourselves: we can dig up olivine, grind it into powder and spread it around. It then ‘weathers’ by reacting with carbon dioxide as follows:

2CO2 + MgSiO4 $\to$ 2MgCO3 + SiO2

or in words:

carbon dioxide + olivine = dolomite + silica

A related reaction is:

Mg2SiO4 + 4 CO2 + 4 H2O $\to$ 2 Mg2+ + 4 HCO3- + H4SiO4

So, 140 grams of olivine will sequester 176 grams of CO2, with the help of 72 grams of water, i.e. rain or seawater.

These reactions are exothermic but slow. They dosn’t seem practical today, but they may be one day. Some work on this has been done by Olaf Schuiling, a professor in geochemistry at the University of Utrecht: