by Chris Woodford. Last updated: August 16, 2022.
Things aren't looking good for the climate. Every day brings a new story of
melting glaciers or disappearing species, rising temperatures and a
bleak future. It's a good 20 years since world leaders first began to
contemplate the thorny issue of global warming and, in that time,
they've made precious little headway in solving the problem. There's
been some progress, for sure: businesses now talk about carbon trading
and well-meaning people buy recycled goods and
flights—but still humankind seems locked on collision course with a
rapidly changing climate. That's why some scientists are talking
about the need for more radical action to stop a planetary
catastrophe. It's known as geoengineering—and the basic idea
is to make compensatory changes to Earth's climate to reverse the damage people
have already done. What do they have in mind? Does it have any
hope of working? And what if it goes wrong?
Photo: Can humans put right the damage they've done to the planet? Or will
geoengineering cause more problems than it solves. Composite image by Explainthatstuff.com, including a photo of
Earth from NASA on the Commons.
What is geoengineering?
Most of the world's scientists now agree: climate change is real and
happening fast. Over the last century, Earth's
surface temperature has risen by close to 1°C and the
current scientific prediction
by the IPCC (Intergovernmental Panel on Climate Change)
is that, by 2100, temperatures will rise by
another 0.3–1.7°C (with severe cuts in emissions)
or 2.6–4.8°C (with high growth in emissions).
Earth itself is under no threat from global warming: the planet will
continue to exist whatever we do to it. What is
in danger is life on Earth—human life and that of millions of other
species—which is finely tuned to the climate. The risk is that
Earth's climate will be knocked out of balance to such an extent that
life, as we know it, becomes impossible to sustain.
“The globally averaged combined land and ocean surface
temperature data as calculated by a linear trend, show a warming
of 0.85 [0.65 to 1.06] °C, over the period 1880–2012...”
Ch2, Climate Change 2013: The Physical Science Basis,
Geoengineering (literally "Earth-engineering") is the currently fashionable term
for making large-scale interventions in how the planet works to slow down or reverse the effects of climate change. In
theory, the word "geoengineering" could be used to describe
almost any large-scale scheme for tackling climate change. For
example, if millions of people in China all planted a tree on the
same day to capture carbon dioxide (CO2) from the atmosphere, that might
alter the planet enough to be considered geoengineering. If everyone
switched to using recycled paper simultaneously, that could (indirectly) be
construed as geoengineering too—since it would drastically reduce
the number of trees being felled. And building thousands of new wind
farms (or even nuclear power plants) could also be described as
geoengineering of a kind.
Generally, though, it's clearer to define geoengineering in a more specific way.
In this article, we'll say that geoengineering means any attempt
to rebalance Earth's climate through direct, large-scale, human
change to the planet's land, oceans, or atmosphere.
Climate change is being caused by the greenhouse effect (a buildup of carbon
dioxide and other gases in the atmosphere leading to increased temperatures on Earth,
similar to what happens in a greenhouse), so there are broadly two
different kinds of geoengineering solution.
The first is to try to cool the planet by reducing the amount of incoming solar energy. The
second is to remove some of the atmospheric carbon dioxide and lock
it away where (we hope) it won't cause problems. Let's consider each of
these in turn.
Reducing solar radiation
We're all familiar with the way the Sun powers our lives: the weather and
seasons change the amount of sunlight we receive from day to day and
month to month. If Earth's problem is that it's receiving too much solar
radiation, could the solution simply be to block out a fraction of the sunlight,
just as greenhouse owners do with whitewash and blinds—just as we
all do with sunscreens and sunblocks? Various schemes have been
proposed for doing this.
Spectacular volcanic eruptions (such as those from Mount St Helens in 1980 and
Mount Pinatubo in 1991) can significantly reduce how much sunlight
reaches Earth. Eruptions reduce incoming solar radiation
by firing sulfur dioxide gas into the atmosphere. Once there, it reacts with
water vapor to make droplets of sulfuric acid that scatter sunlight
back into space like billions of tiny mirrors.
Photo: Global cooling? The eruption of Mount St Helens on May 18, 1980 flung a column of gas some 24km (15 miles) into the atmosphere, though
much of the force blew sideways. The
volcanic eruption at Mount Pinatubo in the Philippines in 1991 produced about 0.6°C (1°F) of global cooling lasting over 15 months.
Photo of Mount St Helens courtesy of US Geological Survey.
Could people tackle climate change by attempting something similar? We
wouldn't need to explode volcanoes—just pump sulfur dioxide high
into the atmosphere. One of the first people to propose this was
Soviet climatologist Mikhail Budyko.
American Earth scientist and
Wallace Broecker took up the idea in the 1980s when he
suggested a fleet of about 700 Jumbo Jets could be hired to release roughly as
much sulfur dioxide into the atmosphere each year as the Mount
Pinatubo explosion. The
sulfur-screen idea was revived once again in 2006 by
Nobel-Prize-winning scientist Paul Crutzen.
Artwork: Could a thin blanket of sulfur dioxide (orange) reflect
unwanted solar radiation back into space?
Would it work? Some have suggested it would be hugely expensive—tens of
billions of dollars have been mentioned. Ken Caldeira of the Carnegie Institution Department of Global Ecology at
Stanford University, California has crunched the numbers and suggests enough sulfur could be added
to the atmosphere "with a single fire hose," suspended from
balloons, for a relatively modest outlay of $100 million per year.
But he's quick to point out that the cost is less of an issue
than the risk of other problems like massive air pollution or destroying the ozone layer—effectively swapping one catastrophic problem for another.
For some, those problems could turn out as bad as climate change:
some years ago, scientists suggested that the great droughts that plagued Africa in the 1970s and 1980s may have been caused by sulfate pollution produced in
Europe and the United States.
Another question is how long we'd need to "inject" sulfur into the atmosphere.
One study, by climate scientist Victor Brovkin and colleagues, suggested the process "would have to continue for thousands of years until CO2 was removed from the atmosphere".
Mirrors in space
Nothing says we have to use sulfur-based chemicals in Earth's atmosphere to
reduce the incoming solar radiation. Why not do the same job with
some kind of mirror—a giant, metal sun-bloc—further out in space?
It's a breathtaking suggestion, but how realistic is it? Considering how taxing space
scientists have found it to construct the International Space Station
(ISS), you might wonder how they could possibly contemplate an engineering project
vastly bigger in scale. And that's no exaggeration. Rough figures
mentioned by some geoengineers suggest we'd need a mirror the size of Greenland!
Unlike some geoengineering proposals, solar mirrors could be removed relatively quickly if they didn't work or
if they cooled Earth too quickly. So, for example, former US Presidential candidate Andrew Yang (reported by Gizmodo)
has suggested: "If you find that it's effective, then great or if you find that is useless, then you don't use it but then there's no harm done." That, of course, is debatable: what if it had to be removed because it caused actual harm to the climate—such as floods or droughts or something less obviously predictable, such as a
massive increase in malaria?
Artwork: Could a giant, distant mirror bounce precisely the right amount of sunlight away from Earth under remote, computer control?
Perhaps enough light could be reflected without using a giant mirror? In the late 1990s, atomic
scientist Edward Teller and his colleagues
a kind of reflective mesh around Earth.
More recently, Roger Angel of the University of Arizona has
proposed using a trillion or so ultra-thin mirrors, roughly 60cm (2ft) across, to form
an artificial space cloud about twice the width of Earth. Launched by
some kind of space elevator or projectile system, they'd be held in
place by a kind of gravitational tug-of-war between Earth and the
Sun. Rough costings suggest the plan would be prohibitively expensive—anything from hundreds of billions to hundreds of trillions
of dollars. Then again, how much is climate change going to cost us over the coming centuries, in dollars or human life? No-one knows whether hundreds of trillions of dollars could be a cheap alternative.
Clouds naturally reflect sunlight back into space, so why not simply try to
increase Earth's cloud cover? There have been many attempts to engineer the weather with
so-called "cloud-seeding" experiments since the 1940s, a few decades after the invention of airplanes made such things feasible. But geoengineering would need cloud-seeding
on a far bigger scale than planes could manage.
A related idea, known as marine cloud brightening (MCB), proposed by John Latham, aims to make clouds more reflective by adding (for example) tiny particles of sea salt. While some geoengineering solutions would cool the whole planet, MCB could be used over much more targeted areas, such as the polar regions, which are at particular risk from climate change.
How would it work in practice? Some years ago, Stephen Salter and John Latham proposed launching a
huge flotilla of cloudseeders: around 1500 remote-controlled boats that would automatically pipe water up from the oceans and spray it into the
atmosphere. Quite what effect this would have, no-one knows. Being
Earth-based, a system like this would be relatively easy to set up
and control and much cheaper than space mirrors. But how long would
the clouds last? And could we cause as much damage in the short-term
as we try to offset in the long-term if all those extra clouds bring
about sudden disastrous floods or droughts?
Many people would consider ideas like this beyond the realms of practicality;
at the very least, much more research is clearly needed. A 2015 National Research Council report into the various sunlight blocking ("albedo modification") technologies concluded that they "would not require major technological innovation to be implemented and are relatively inexpensive," but they could not address damage to Earth caused by climate change, such as acidification of the oceans or desertification, and would need to be "sustained indefinitely." Without emissions reductions, it said any such plans would be "irrational and irresponsible."
Removing carbon dioxide
If all the carbon dioxide we're adding to the atmosphere is the problem,
could the solution simply be to "suck" some of that pesky gas
back down to Earth and store it underground or in the very deep ocean
where it'll do less damage? A whole other set of geoengineering
schemes have been proposed that start from this assumption.
Carbon capture and storage (CCS)
Those who want us to carry on using fossil fuels have proposed power plants
that don't pump carbon dioxide into the air. Instead, they'd have
modified smokestacks (chimneys) with built-in "scrubbers," which would trap the waste carbon
dioxide gas and turn it into a highly compressed liquid that could be
stored safely out of the way. You'll hear this idea referred to as
capture and storage (CCS) or sequestration. It sounds good in theory,
but it doesn't solve our immediate problem: even if we drastically
reduce Earth's carbon dioxide emissions, there's so much CO2 in
the atmosphere already that global temperatures are likely to carry on rising
for centuries, while sea-level rises could continue for millennia
(see this graph from the IPCC—the
scientific body that coordinates publication of world climate change research).
Artwork: Carbon capture and storage plants would stop carbon dioxide entering the atmosphere from burned fuel. If we ran them on low- or zero- carbon
biomass, they would effectively work like "emissions vacuums," removing some CO2 from the atmosphere. This technology is called
bio-energy with carbon capture and storage (BECCS). (For clarity, the carbon dioxide is shown here as a
grey cloud but it is, of course, invisible.)
So what we'd actually need to do is not merely stop further emissions but
remove some of the carbon dioxide that's already there. Massive
reforestation of Earth would be one option, but it would take time.
One scientist, Klaus Lackner of Columbia University, has
proposed creating artificial trees that would each capture a ton of carbon
dioxide per day in an absorbent resin. The CO2 would then be removed with
steam and turned into a liquid, which could either be used industrially or pumped deep underground for indefinite
storage. Similar prototype projects are now running in Iceland and Australia.
Like other plants, phytoplankton (tiny plants that float near the ocean
surface) absorb carbon dioxide from the atmosphere when they grow. In
theory, fertilizing the oceans could massively increase the amount of
phytoplankton and significantly reduce carbon dioxide emissions. That
idea was originally proposed in 1989 by ocean scientist
of Moss Landing Marine Laboratories. Martin's so-called "iron hypothesis" suggested
adding iron to the oceans would stimulate plankton growth and carbon dioxide uptake.
When the plankton died, they'd fall to the seabed taking with them the
carbon they'd absorbed—effectively removing it from atmospheric circulation. Few
actually been carried out, although a highly controversial test in the Pacific Ocean in 2012 appeared
to confirm the principle.
Despite this, a recent study by MIT suggests plankton would make much less impact on global warming than Martin
supposed. Another problem is that blooms of plankton could massively increase the acidity of the oceans, drastically harming the marine
Artwork: Iron seeding adds large amounts of iron (brown) to the ocean, which encourages the
growth of plankton (green). As the plankton grow, they suck carbon dioxide from the air.
The highly respected but maverick climate scientist
James Lovelock often
turned his attention to geoengineering. Widely credited with
helping to alert the world to the issue of climate change, Lovelock
argued that current attempts to reduce carbon dioxide emissions are
trivial compared to the scale of the cutbacks actually
required. He proposed a number of different geoengineering
solutions to climate change including, in 2007, a
giant vertical pipes that bob up and down in the ocean. Each pipe
would be 100–200m (339–660ft) long and would have a valve either at the
top or the bottom. As it moved downward, cold water would rush in at
the bottom. When it bobbed back up again, the cold water would spill
out at the top, so the pipes would work like a pump continuously conveying
cold water from the deep ocean to the surface. Since cold water is biologically more productive than
warm water, adding more cold water to the ocean surface would
stimulate algal growth in a similar way to adding iron—but with the
same potential drawback: considerable acidification of the oceans.
Apart from this, no-one knows what effect such dramatic intervention would have on the
huge ocean currents that play such a key part in the world's weather.
One of the simplest and currently most fashionable geoengineering proposals
is based on a practice used by ancient Amazonian Indians. The basic
idea is to cook waste agricultural products (plant stems, stalks, and
roots) to make a form of charcoal called
biochar, and then simply bury it, taking the carbon it contains out of circulation.
James Lovelock supported the idea in principle and
Craig Sams, the founder of Green and Black's chocolate, has also worked on the idea. But creating huge biochar
plantations could prove even more disruptive than the current rush
for biofuels, as British environmentalist
George Monbiot has argued: "We would either have to replace all the world's crops
with biomass plantations, causing instant global famine, or we would
have to double the cropped area of the planet, trashing most of its
remaining natural habitats."
Photo: Biochar pellets: The practice of slashing and burning rainforests has added
hugely to global warming by destroying a major carbon "sink." Some people think making biochar, a carbon-rich charcoal (wood burned with reduced oxygen), and then storing it underground will help to
reverse the damage.
The 2015 National Research Council report into carbon dioxide removal and sequestration found present schemes completely unviable: both too puny to make a difference and more expensive than replacing fossil fuels with renewables or other forms of low-carbon energy. Although it supported proven ideas like reforestation and low-till agriculture, it highlighted the high risk and unpredictability of iron-seeding; the large land-take needed for schemes based on storing carbon dioxide as biomass (such as biochar); and the unproven nature of technologies for pulling carbon dioxide directly from the air. Nevertheless, it wisely called for much more research.
Thinking the unthinkable?
Cost and scientific feasibility are certainly important when we contemplate
whether geoengineering schemes are worth pursuing, but there are
political, ethical, legal, and other dimensions to the debate as
The biggest objection to geoengineering is that its vast effects could be
impossible to predict. People already speak of climate change as a
kind of giant experiment with the future. But what if we really did
start tinkering with the climate? What if we corrected the immediate
problem of global warming... but then over-corrected so much that we
risked another Ice Age? James Lovelock warns that geoengineering
could mean managing Earth's climate forever: "Are we sufficiently
talented to take on what might become the onerous permanent task of
keeping the Earth in homeostasis?"
Permanent really is a long time. Once we started injecting aerosols into the atmosphere, we'd have to
continue for hundreds or even thousands of years to prevent global warming from recurring.
On the other hand, it's clear that some of our initial, tentative attempts at geoengineering have actually
been successful. Take the Montreal Protocol, for example: cutting ozone-depleting chemicals has brought
about a recovery of the ozone layer, which is expected to be all but complete
by around 2060–2080. Everyone agrees that geoengineering should be a last resort, but the time may well come for last resorts.
The question of whether we try geoengineering may have to be reframed: can we afford not to undertake geoengineering if the climate nudges toward a point of no return?
Photo: If climate change pushes hundreds of millions of people to the brink of
survival, could geoengineering become morally unavoidable? Photo by Ernest Scott courtesy of US Navy and Wikimedia Commons.
Wise use of money?
But even if we could accept geoengineering in principle, the huge investment most
of these schemes need is likely to put them well beyond contention, at least for the time being.
For just a fraction of the outlay of a space mirror system, we could
develop clean renewable energy on Earth. Why not spend the same money
solving the energy crisis once and for all rather than trying to mitigate its effects? If we could wean
ourselves off fossil fuels entirely in the next few decades, humankind could conceivably live sustainably on Earth for the rest of
its history. Isn't that worth a shot first?
That's certainly how most environmentalists would see the issue. With
Earth's wellbeing at the center of their moral compass, they
generally find geoengineering unethical and disturbing. They argue
that geoengineering makes our wasteful, polluting, resource-depleting
ways here on Earth seem perfectly acceptable. Earth's looming climate
crisis offers the impetus to clean up our act once and for all.
Environmentalists would question the morality of tinkering with the planet's climate
when it could have drastic implications on billions of people's lives
for decades, centuries, or even millennia. Then again, one could argue that people
have been geoengineering the climate since the start of the
Industrial Revolution—that was what caused our problems in the first
place. Does this mean direct climate engineering should be firmly
embraced (because, in a sense we're doing it already) or avoided at
all costs (because it got us into the mess to start with)?
Political feasibility is another objection. Earth's 200-plus nations have found it remarkably difficult to agree on
even modest cuts to their carbon dioxide emissions, so how could they
possibly agree on geoengineering? Different schemes are bound to
affect different countries and continents to different extents;
cloud-seeding, for example, could lead to benefits in one country at
the expense of floods or droughts elsewhere. How could countries hope
to agree on schemes so huge and controversial? Could world wars break
out over attempts by one or more countries to impose geoengineering
solutions on others? Or could the threat of imminent climate catastrophe
finally bring the world together?
Is there an alternative?
Only one thing seems certain about Earth's future: its absolute uncertainty.
According to some scientists,
we may already have passed the "tipping point"—the point of no
return, where Earth's climate becomes progressively hotter until we
reach the point where life becomes impossible. Or maybe we do still
have time to cut carbon dioxide emissions and revolutionize energy
use to make human life truly sustainable. Arguably, we're already
geoengineering the climate and we have a duty to reverse the damage
we've done. Most scientists agree that we're a long way from needing
to fire aerosols into the atmosphere or launch huge space mirrors. But
while geoengineering was considered science fiction only a few years
ago, it's now being talked about with increasing seriousness.
Wallace Broecker calls it an "intelligent insurance policy" for if and when climate crisis
heads towards planetary catastrophe.
In 2021, a landmark report by the US National Academies of Sciences, Engineering, and Medicine
argued that, "given the urgency of the risks posed by climate change, the US should pursue a research program for solar geoengineering", although it stressed that "geoengineering is not a substitute for reducing greenhouse gas emissions."
Prudent scientists are beginning to see we may have to start thinking the unthinkable to avoid the unavoidable.