About four and a half billion years ago as our solar system was beginning to come together in
a quiet little corner of the milky way galaxy two large clumps of coalescing
rock were developing relatively close to each other and at more or less the same rate. One
of them was a planet we now call Venus and the other one became the place we humans call home.
Earth and Venus have similar size and mass and both planets are volcanically active
which means from time to time vast quantities of subterranean carbon dioxide gets spewed out into
the atmosphere. Over geological time scales that CO2 built up and today it makes up 96%
of the atmosphere on Venus keeping that planet at a nice toasty temperature of 475 degrees Celsius.
We're not 475 degrees Celsius here on earth though are we, so apart from being a bit
further away from the sun, what lucky break did we get that Venus didn't. Well photosynthesis was
pretty handy for a start. Nature's own miracle combination of sunlight and carbon dioxide
provided the energy for the astonishing explosion of life on earth that we enjoy today. But while
all that lovely stuff was going on there was a much less visible process taking place as well.
Atmospheric carbon dioxide was reacting with minerals on land and on the seabed
in a process known as Chemical Weathering to form carbonate-rich rocks like limestone which
over hundreds of millions of years locked up the vast majority of the warming gas,
bringing our planet back down to what was a nice survivable average temperature of about
15 degrees Celsius, until about 200 years ago when we humans started burning fossil fuels and
belching massive quantities of CO2 back out into the atmosphere again. So now we've got a climate
crisis on our hands and scientists all over the world are frantically trying to find ways to drag
our species out of the smelly brown stuff and back onto a path towards a more sustainable existence.
And it turns out one of the potential solutions may be a super accelerated
version of nature's own chemical weathering process. So how are we going to do that then?
Hello and welcome to Just Have a Think. We really do use an awful lot of stuff to keep our modern
world going. Probably way more than most of us realize, as this very enlightening new book by
Bill Gates's favourite author Vaclav Smil has been reminding me. Quite a lot of that stuff
comes from blowing up large chunks of mountains and countryside to liberate rocks from which we
can extract the metals and minerals we all rely on in our daily lives. The by-product of that process
is an awful lot of rock dust that has to be carefully managed at great expense to the mine
operators. Some of the more enterprising of those operators have been quite cleverly marketing their
waste product to keen gardeners around the world for several years now, hailing it as some kind of
miracle nutrient additive that can immediately make soils more fertile and productive. It's an
extremely expensive way to improve your vegetable patch though! While it certainly won't do you any
harm you'd probably be better off spending your money on a trailer full of well-rotted
cow manure from your local farm. But there is a growing body of evidence suggesting that employing
very large scale use of rock dust on agricultural land could make a huge contribution to the carbon
dioxide removal or CDR challenge that we face in the 21st century. This research paper published in
July 2020 by a team from the Leberhulme Centre for Climate Mitigation at Sheffield University
outlines the size of the task. It points out that even the most ambitious greenhouse gas emissions
phase-out plans of our global leaders failed to achieve the targets set out in the 2015 Paris
agreement for limiting global warming unless a whole bunch of atmospheric CDR happens as well.
And the numbers are eye watering. We're talking about removing 10 billion metric
tons of CO2 from the air every single year based on current emission reduction policies.
Now we might get serious and actually start reducing our emissions in a more meaningful way
in the coming years, although there's absolutely no sign whatsoever of that happening anytime soon,
and even if we did employ every technological and social measure
to radically cut down our greenhouse gas emissions immediately, we'd still need to be removing about
2.5 billion metric tons a year every year through most of the rest of this century.
The best we can manage at the moment with current technologies like
direct air capture or DACCS is about 10,000 tons a year according to this very recent analysis.
And here's another interesting little stat from an Australian number crunching consultancy
called Keynumbers. They calculated that in 2020 the world used 462 exajoules of energy
from fossil fuels which resulted in 32 billion tons of carbon dioxide emissions. Capturing all
that CO2 using DACCS, say Keynumbers, would require 448 exajoules of energy. You see the
problem there? So barring a miracle we're going to need something else to get us out of the doodoo
and that's where chemical weathering might just play a very important role. Nature's version works
like this. Atmospheric CO2 gets dissolved in rain to make a dilute solution of carbonic acid
which is the same stuff we drink in fizzy soda. As the rain falls onto rocks like basalt which have
a high content of silicate minerals those minerals react with the carbonic acid to form water-soluble
ions like calcium and bicarbonate. Those ions get into waterways via groundwater runoff
and eventually travel out to the oceans where marine animals use them to build their shells.
As those animals die they fall to the ocean floor and their shells eventually turn into limestone
and other carbonate rocks and over geological time scales that rock gets buried in the earth's crust
locking the carbon content up for millions of years. The trouble is the naturally occurring
version of the process takes centuries to work so it's not going to help us in the next 30 years.
But by crushing silica mineral rocks like basalt into a fine powder the surface area available
to react with the carbonic acid is massively increased which in turn accelerates the speed
of the reaction. It's a process the science bods call enhanced silicate rock weathering or ERW and
it was initially conceived as an effective way to draw down carbon into the soils of tropical
rainforests because the process works particularly well in warmer more humid environments.
But as the team at the Leverhulme Centre for Climate Mitigation continued their studies
they realized there was a really significant opportunity to combine ERW with existing cropland
soil management processes in major agricultural economies where farmers already use industrial
scale machinery to spread fertilisers and other chemicals on the land. Those spreaders could
easily be utilized for the distribution of crushed basalt at virtually zero extra cost of the farmer.
There'd be some extra side benefits as well. When crushed basalt is added to soil
it gradually reacts to produce magnesium, calcium and small amounts of zinc and copper, all of which
are essential nutrients that plants need for healthy growth. So over time the farmers would
get healthier soil and the world would get a much needed drawdown of atmospheric carbon dioxide.
And as those carbon rich rocks eventually seeped back into the ocean they'd help reduce the
acidity of the ocean water creating a healthier environment for sea shells and corals to grow.
The research team calculated that if the process was implemented to its full potential it could
be removing as much as 2 billion metric tonnes of carbon dioxide every year by 2050. That's
equivalent to the combined annual emissions of Germany and Japan. Most of that would need to come
from the world's largest agricultural producers and highest greenhouse gas emitters, China and
the USA, but there's so much of this crushed rock building up as a waste product all over the world
that many other countries could easily get involved as well. A more recent paper
by the same Leverhulme team focused on the potential of ERW here in the United Kingdom.
That paper found that the process could be delivering as much as 30 million metric
tons of carbon dioxide removal per year in the UK by 2050 which equates to about 45%
of the overall carbon removal required for this country to meet its net zero emissions target.
What's not to like then? Let's crack on immediately eh? Well, as usual it's not
quite that simple. There are a couple of slightly wrinkly caveats that our governments will need to
consider before flying headlong into this kind of globally ambitious initiative. First of all
it'll obviously require a great deal of energy to transport vast quantities of crushed basalt
from mines to agricultural areas where it will be applied and today that energy mostly comes
from fossil fuels so we need to be sure that the atmospheric carbon dioxide removed by the process
wasn't getting cancelled out by the CO2 emitted from production and transportation. Secondly,
although adding crushed rock to agricultural land would gradually enhance the quality of the soil
it wouldn't happen overnight and even though the spreading equipment already exists handling the
product and managing the supply chain would be an additional logistical cost to individual farmers
who are already struggling to survive on razor thin margins so convincing them to add an extra
process to their operation just so they could feel like they were contributing to climate mitigation
might be a tough ask. There will most likely need to be some pretty robust government-funded
carbon credit incentive schemes in place to make it worthwhile for farmers to adopt these new
practices. The Leverhulme team did crunch the numbers though for different countries around
the world and they reckon that China, the USA, India and Brazil have the greatest potential.
After all the mining and transport costs are factored in the study found that enhanced rock
weathering in those countries would come in at between $80 and $180 per tonne of CO2 removed.
That's certainly very competitive with direct air carbon capture which is currently costing
as much as six hundred dollars per ton. And the team argues that a major benefit of using crushed
basalt in this way is the very fact that it can be used on the same land as agriculture
rather than competing with it for space as many other climate mitigation projects do.
Just like most other climate mitigation technologies currently in development the big
question is whether enhanced rock weathering can genuinely be scaled up in a real world scenario.
If it can though then the benefits in soil health, reduced ocean acidification,
and carbon dioxide removal could be massive. Let me know what you think in the comments
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