What is marine cloudbrightening?
MCB is a technique to brighten marine clouds. Brighter
(whiter) marine clouds mirror more sunlight back into space than dark ones. Therefore, if we could
brighten marine clouds on a global scale just a little, we could cool the Earth by up to 1 C, refreeze
the Arctic which is heating up 3 to 5 times faster than the rest of the planet, and roll back climate
change, at least temporarily. By cooling down the areas below the clouds, MCB could bring relief to
ecosystems under heat stress, such as the Great Barrier Reef.
MCB is potentially our most effective way to adapt to global warming.
Cloud brightening happens because smaller water droplets reflect
more light than bigger water droplets: the Twomey effect. This is visualized in the image of the two jars:
the jar on the right contains smaller glass spheres than the one on the left and therefore
appears brighter.
Now cloud water droplets grow on tiny dust particles: aerosols. When aerosols act as
nucleation sites for droplet formation, they are called Cloud Condensation Nuclei (CCNs). The
more CCNs in the clouds, the larger the number of water droplets growing on them. This larger
number of droplets will compete for the available water vapour, and thus the droplets will
remain smaller than in the case with only a few CCNs.
This larger number of smaller droplets will brighten the
clouds. Whiter clouds reflect more solar radiation back into space and therefore provide
cooling!
Satellite images of ships’ exhausts in which soot and sulfur act as CCNs drive home the point.
Even under a cloud deck, these exhausts lead to bright lines in satellite images (see the image
opposite). Bright! Means more reflected light! But soot and sulfur is hardly an attractive CCN. What about
sea salt nano particles generated at sea, as suggested by Latham in 1990? The concept of Marine
Cloud Brightening was born.
How doesMCB work?
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Choose a step below to find out more
Issue #01
Droplet distribution
MCB’s biggest challenge is the droplet distribution. For MCB to work efficiently, droplet generators must produce “Goldilocks” sea water droplets: not too small, not too big. In fact, in a narrow bell curve between 200 nanometre and 800 nanometre in diameter (the latter is the size of SARS-CoV-2 virus). Smaller droplets might dissipate clouds, larger droplets might even cause Marine Cloud Darkening and we don’t want either to happen! When evaporating during their rise into the clouds, Goldilocks droplets will yield sea salt CCNs in the ideal range between 50 nanometre and 200 nanometre in diameter. These CCNs will produce the desired Twomey effect (see the slide below).
Issue #02
Droplet production
A further challenge of MCB is that droplet generators must produce the Goldilocks droplets in the range of 1016 – 1017 droplets per… second (!) That droplet generating process must be energy efficient too...
Issue #03
Sea water filtration
Filtration of sea water when using sub-micron droplet generators is a significant challenge.
Issue #04
Clean stratocumulus formations
MCB works best under low lying cloud carpets like stratocumulus formations (1-2km above sea level). These clouds should preferably be free of contaminants so that the sea salt CCNs do not have to compete with other anthropogenic aerosols. If clouds are already polluted, MCB will be less effective. Fortunately, there are huge carpets of such marine clouds hanging over the West coast of the large continents.
Issue #05
Caution vs Urgency.
An isolated study (J. Haywood et al.) suggests that "MCB off the coast of Namibia could induce drought over Brazil." This deserves to be mentioned and is the reason why the RAF actively sponsors research into MCB's side effects. However, MCB would not be deployed solely off Namibia's coast. A global deployment could significantly cool the planet, offering relief and benefits, even to areas like Brazil currently suffering from unprecedented floods (May 2024). While caution is necessary, we are about to breach tipping points. There is no time to lose. Doing nothing is not an option. We must fast-track MCB research.
Issue #06
Cirrus cloud contamination
As discussed, MCB relies on marine convection currents to carry the sea salt CCNs into stratocumulus formations, typically 1-2km above sea level. The danger that these CCNs end up in high altitude cirrus cloud formations, typically at a height between 5km and 12km above sea level, is therefore negligible. Cirrus clouds, as well as aviation contrails, are suspected to contribute to global warming. We certainly don’t want to brighten that lot!
Issue #07
MCB is a temporary solution.
MCB’s cooling properties are subtle. If adopted, MCB will only be able to temporarily refreeze the Arctic and reverse global warming. Without effective Reduction (of GHG emissions) and Removing (GHGs from the atmosphere), Refreezing will eventually fail. However, MCB is now more urgent than ever! Reduce and Remove are so slow off the mark that we might soon breach several dreaded tipping points (read more on tipping points here). Beyond them, there is no return to a balmy planet. With all the consequences for humanity as we know it.
Click the arrows below to view each issue faced
What are thebenefits of MCB?
The localised approach allows for greater control and minimises the risk of unintended consequences both at a local level and on a global scale.
Calculations show that the annual amount of targeted sea salt CCNs necessary to obtain effective cooling is less than what a vigorous North Atlantic gales produces in a couple of days!
MCB only happens when marine clouds are seeded with sea salt CCNs. If seeding is switched off, MCB will not last for more than a few days i.e., the lifetime of marine clouds. Consequently, MCB’s effects are fully reversible within a matter of days.
Apart from the stratocumulus cloud carpets mentioned above, MCB can target vulnerable ecosystems that are under severe heat stress. These include coral reefs, overheating sea currents, summer cloud formations over the Arctic, coastal areas etc… Used for these purposes, MCB provides both local relief and global cooling all at once.
Hurricane formation is intensified by elevated sea-surface temperature. MCB could provide local sea surface cooling in hurricane-prone areas in order to mitigate the intensity of hurricanes (typhoons). Again, in such case, MCB provides both local relief and global cooling all at once.
A growing body of scientists fear that we have already breached several tipping points, amongst other the melting of the Greenland ice cap and the extinction of coral reefs. If such breaches are left to progress, they become unstoppable and irreversible due climate change’s profuse and powerful positive feedback loops. In other words: once tipping points are breached, there is no way back. Tipping points are the scariest aspect of climate change. By providing global cooling of up to 1C, MCB puts a temporary break on such catastrophic breaches and keeps tipping points at bay.
MCB benefits from on an extensive understanding of cloud physics and marine ecosystems. Proof of Concept could be obtained latest by the summer of 2026 and large-scale deployment could start by the end of the decade. This could be just in time to avoid the worst of global warming and keep tipping points temporarily at bay.
A. MCB is budgeted to cost abt usd 10 bio in the deployment phase, and yearly maintenance is estimated at usd 1-2 bio/year. Global warming today – let alone tomorrow - already costs in the order of usd 200 bio/yr, not factoring in human suffering. Unfairly, societies which emit the least GHGs, such as in the Sahel, in many areas around the Equator and in most of the Global South are at the greatest risk of global warming. MCB’s global cooling would greatly relieve their distress.
B. By cooling the planet globally by up to 1C, and therefore refreezing the Arctic, MCB buys humanity enough time - maybe 30-40 years – to get rid of anthropogenic GHG-emissions and massively remove GHGs from the atmosphere. MCB enables the 3Rs of the climate repair deal: Reduce, Remove, Refreeze.
What are thechallenges of MCB?
With RAF-funds, universities are cracking the many challenges of MCB!
Currently there is no funding for MCB
Believe it or not: research into MCB gets no funding. Without adequate financial backing, scientific studies exploring the feasibility, effectiveness, and potential risks associated with MCB remain limited. That is why the RAF hopes to attract even more funding for research into MCB.
Challenge #02
Droplet distribution and production
Today, there is no spraying device that can produce the 200 nm to 800 nm Goldilocks droplets which create MCB. Thanks to a RAF donation, researchers at the Centre for Climate Repair (CCR) at Cambridge are already developing no less than four (!) novel droplet generators. All these novel droplet generators promise to produce the Goldilocks droplets in the range of 1015 – 1017 droplets per… second, with a minimal energy input.
Sea water filtration
Filtration of sea water when using sub-micron droplet generators is a significant challenge.
Theoretical Proof of Concept
Computer modelling must first show that MCB can indeed provide the necessary cooling with little or no side-effects. The science is not settled yet. Thanks to a RAF donation, researchers at the Technical University Delft Climate Institute (TUDCI) are making rapid progress, using their large eddy cloud simulation models.
Field trials
By combining the knowledge pools at CCR and TUDCI, researchers might be able to carry out the first “plume” field trials as early as the summer of 2025.
Necessity to obtain societal and political approval
MCB cannot be deployed without broad-based societal and political support. Fortunately, the tide is turning in favour of MCB. Until recently, some scientists considered MCB as a form of irresponsible geoengineering, which it is not. Many are now realizing that MCB might be that rare combination of the least harmful and the most efficient form of climate adaptation. However, society as a whole – including the Global South - must be convinced of the merits of MCB - and its supervisory framework - before any large-scale deployment can be undertaken.
Click the arrows below to view each challenge faced
What has RAF-funded researchachieved so far?
Within 18 months, RAF financial support has allowed CCR and TUDCI to make rapid advances in the field of MCB.
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MCB testing to save the Great Barrier Reef
A. The Southern Cross University has invited CCR to help assess its MCB field trials focused on protecting the Great Barrier Reef from coral bleaching.
B. The first series of tests provided encouraging results, indicating that the practical effectiveness of MCB might even perform better than earlier models predicted.
CCR research into droplet generators
CCR is developing no less than four novel droplet generators to produce the desired
sprays.
One such device – the superheated sprayer - is ready to be upscaled. This comes on top of the
droplet generators being developed at Palo Alto in the USA.
TUDCI research into CCNs
TUDCI is incorporating CCN distributions provided by CCR into its large eddy cloud models to examine if the hoped-for Twomey effect can be modelled.
TUDCI research into MCB side-effects
TUDCI is investigating potential side-effects of MCB. One key claim is that MCB won't significantly change precipitation patterns, but is this really the case? The science behind MCB’s side-effects is still not well understood - but not for long!
Summer of 2026
CCR and TUDCI are ever more confident that they can deliver MCB Proof of Concept by the summer of 2026, backed-up by peer reviewed computer modelling and field trials.
It’s time to Refreezethe Arctic
A nascent body of fact-based, peer reviewed science allows us to have high hopes for MCB.
MCB promises enough global cooling to temporarily halt the loss of the Arctic ice cap, and
it could be available within a short timeframe. Refreezing the Arctic will give us a fair
chance to keep tipping points at bay.
MCB is potentially our most effective way to adapt to global warming.
However, the window of opportunity to deploy MCB at scale is closing fast. Research into MCB
must be fast-tracked with the highest possible priority, whilst society debates its merits
transparently and in good faith.
The RAF is proud to act as a catalyst in the delivery of MCB Proof of Concept.
More Ice, More Life!
References
Awaiting references from RAF
