wood and burying it in a hole in the ground, while, next door, humanity
continues digging up fossil wood and setting fire to it? It’s daft to imagine
creating buried wood at the same time as digging up buried wood. Even
so, let’s work out the land area required to solve the climate problem with

The best plants in Europe capture carbon at a rate of roughly 10 tons
of dry wood per hectare per year – equivalent to about 15 tons of CO2
per hectare per year – so to fix a European’s output of 11 tons of CO2
per year we need 7500 square metres of forest per person. This required
area of 7500 square metres per person is twice the area of Britain per person.
And then you’d have to find somewhere to permanently store 7.5 tons of
wood per person per year! At a density of 500 kg per m3, each person’s
wood would occupy 15 m3 per year. A lifetime’s wood – which, remember,
must be safely stored away and never burned – would occupy 1000 m3.
That’s five times the entire volume of a typical house. If anyone proposes
using trees to undo climate change, they need to realise that country-sized
facilities are required. I don’t see how it could ever work.

C. Enhanced weathering of rocks

Is there a sneaky way to avoid the significant energy cost of the chemical
approach to carbon-sucking? Here is an interesting idea: pulverize rocks
that are capable of absorbing CO2, and leave them in the open air. This
idea can be pitched as the acceleration of a natural geological process. Let
me explain.

Two flows of carbon that I omitted from figure 31.3 are the flow of
carbon from rocks into oceans, associated with the natural weathering
of rocks, and the natural precipitation of carbon into marine sediments,
which eventually turn back into rocks. These flows are relatively small, in-
volving about 0.2 Gt C per year (0.7 Gt CO2 per year). So they are dwarfed
by current human carbon emissions, which are about 40 times bigger. But
the suggestion of enhanced-weathering advocates is that we could fix climate
change by speeding up the rate at which rocks are broken down and
absorb CO2. The appropriate rocks to break down include olivines or mag-
nesium silicate minerals, which are widespread. The idea would be to find
mines in places surrounded by many square kilometres of land on which
crushed rocks could be spread, or perhaps to spread the crushed rocks
directly on the oceans. Either way, the rocks would absorb CO2 and turn
into carbonates and the resulting carbonates would end up being washed
into the oceans. To pulverize the rocks into appropriately small grains
for the reaction with CO2 to take place requires only 0.04 kWh per kg of
sucked CO2
. Hang on, isn’t that smaller than the 0.20 kWh per kg required
by the laws of physics? Yes, but nothing is wrong: the rocks themselves
are the sources of the missing energy. Silicates have higher energy than
carbonates, so the rocks pay the energy cost of sucking the CO2 from thin

1 hectare = 10 000 m2