Report on HPAC Meeting with Daniel Harrison on MCB
rob...@rtulip.net
Meeting Report: Dr Daniel Harrison on Marine Cloud Brightening for the Great Barrier Reef, Healthy Planet Action Coalition, 14-15 May 2026
Overview
The meeting featured a presentation by Dr Daniel Harrison, principal investigator for the Cooling and Shading Program of Australia’s Great Barrier Reef Restoration and Adaptation Program and Chief Investigator for the UK ARIA-funded project Marine Cloud Brightening in a Complex World. Dennis Garrity introduced the session as an opportunity for members to hear directly about the technical feasibility, energy requirements, financial costs and scalability of marine cloud brightening.
Dr Harrison presented the Great Barrier Reef work as the world’s most advanced practical field research program into marine cloud brightening. He emphasised that the project is still at an experimental and scientific validation stage, not a deployment stage. The work is testing whether sea-salt aerosols can be generated, lofted into clouds, incorporated into cloud droplets and used to increase cloud albedo in ways that could reduce heat stress on coral reefs.
The climate emergency for the Great Barrier Reef
Dr Harrison opened by describing the severe deterioration of the Great Barrier Reef. He said the reef has experienced six severe mass bleaching events in the last decade, with essentially no part of the reef now untouched by bleaching. These events are driven by rising ocean temperatures and marine heat waves that push corals beyond their thermal tolerance.
He explained that bleaching is not simply a temporary loss of colour. If heat stress persists, corals can die and reef ecosystems can shift into degraded states. Examples include rubble-dominated reefs where dead coral fragments prevent recolonisation and macroalgae-dominated systems where algae take over after coral mortality. These shifts can become difficult or impossible to reverse.
Dr Harrison argued that the reef has now crossed a practical threshold where bleaching can occur in almost any year. He also linked recent global temperature acceleration to the reduction in sulphur emissions from shipping after the 2020 IMO fuel regulations, while noting that the exact attribution remains scientifically uncertain. His view is that reduced ship aerosols have removed a significant inadvertent cloud-brightening effect, including locally over the Great Barrier Reef.
RRAP and the Cooling and Shading Program
The Reef Restoration and Adaptation Program began with a wide assessment of possible interventions to help the reef survive climate change. Dr Harrison said the program initially considered about 140 ideas, narrowed these to 43 and then prioritised options that could be scientifically demonstrated, ecologically effective, socially acceptable, technically feasible and economically viable at scale.
The Cooling and Shading Program focused on engineering methods to reduce bleaching stress by either cooling reef waters or reducing light stress. It involved 17 research organisations and up to about 70 researchers at its peak. Marine cloud brightening emerged as the most promising large-scale cooling approach because of its high energetic leverage.
Why marine cloud brightening is attractive
Dr Harrison explained the appeal of MCB in simple physical terms. Directly pumping cold water onto reefs requires energy inputs of the same order as the cooling achieved. By contrast, MCB uses relatively small amounts of energy to create nano-sized sea-salt aerosols. When these particles reach cloud base and activate as cloud droplets, their volume can grow by up to around 500,000 times, with nature doing much of the work.
This leverage makes MCB attractive for cooling a vast shallow ecosystem such as the Great Barrier Reef. Early modelling suggested that the cumulus-type clouds over the reef, although not the classic marine stratocumulus cloud decks often discussed in global MCB proposals, are still susceptible to brightening. The main mechanism is the Twomey effect, where added aerosols increase the number of smaller droplets in a cloud, making the same cloud brighter rather than creating a fundamentally different cloud.
Modelling results
The modelling work had three main stages.
First, atmospheric modelling indicated that adding sea-salt aerosols could increase the albedo of clouds over the reef. Second, oceanographic and biogeochemical modelling assessed whether such cloud brightening could reduce water temperatures. Dr Harrison said the required forcing for reef-scale cooling is higher than what is usually discussed for global MCB because the effect is concentrated in space and time, mainly during summer marine heat waves. He indicated that cooling reef waters may require forcing closer to 10 W/m² during the relevant period.
Third, ecological modelling examined coral cover under different climate scenarios. Under a low-emissions pathway broadly consistent with the Paris targets, MCB appeared able to help coral cover recover and stabilise. Under a business-as-usual warming scenario, MCB could help for a few decades but would eventually be overwhelmed. This is because the aerosol-cloud response is asymptotic: the first added cloud condensation nuclei have the strongest effect, but each additional increment produces less extra brightening.
The clear conclusion was that MCB is not a silver bullet. It may relieve stress on the reef and buy time, but it must be combined with emissions reduction and other reef interventions.
Field experiments and technical progress
The field program has proceeded cautiously, in close partnership with regulators. Dr Harrison said the regulator explicitly wanted to be involved from the beginning rather than seeing research conducted elsewhere and then presented as ready for reef use. This led to a stepwise program, beginning with small experiments and gradually expanding.
The current system is ship-based and atomises seawater into aerosols. It now produces roughly 10¹⁵ aerosols per second. LIDAR and aircraft measurements show the plume can reach cloud base at about 1,000 metres and affect cloud over an area roughly 10 km by 10 km in experimental conditions.
A major achievement has been the development of an Australian aircraft platform equipped for cloud microphysics, aerosol and meteorological measurements. This has allowed the team to sample seeded and unseeded cloud directly. Dr Harrison reported that the data show a strong Twomey effect, especially in clean background conditions: seeded cloud samples show more numerous smaller droplets than background cloud for the same liquid water content.
Energy requirements and scalability
The strongest practical constraint is the aerosol generation system. Dr Harrison was emphatic that the current equipment is a research platform, not a deployable technology. It is a small industrial plant, requiring extensive safety checks and trained operators. Two shipping containers of compressors are needed to supply the compressed air. The water pump is small; the energy, weight and complexity are dominated by air compression.
The current machine uses roughly 300 kW to produce about 10¹⁵ aerosols per second. Dr Harrison cautioned strongly against simply scaling that figure up to estimate deployment cost, because the current machine was designed to get into the field quickly, not to optimise energy efficiency.
For deployment-scale work, much more efficient nozzle technology is needed. He suggested the program may need about a 30-fold improvement in energy efficiency to cool the Great Barrier Reef effectively. Recent research may already have found a three- to five-fold improvement, with ARIA-funded work aiming for further gains. A practical target is to reach between 10¹⁶ and 10¹⁷ aerosols per second, with the higher figure making implementation cheaper.
Governance, consent and social licence
A major theme was governance. Dr Harrison stressed the program’s commitment to free, prior and informed consent from Traditional Owners before work on Sea Country. He said the team has not conducted a field campaign without Traditional Owner participation.
The program has also invested heavily in community and stakeholder engagement. Dr Harrison described this as genuine two-way engagement rather than public education. Community panels, Traditional Owner groups, tourism stakeholders, regulators and local communities have all influenced where and how the research is conducted.
He argued that this approach has been essential to the program’s success. It has slowed the science in some respects, but without it the science could not have happened. The work is also subject to tight oversight because the Great Barrier Reef is one of the world’s most regulated marine estates. Every field campaign requires permits, risk assessment, reporting and renewed review before scaling up.
Geoengineering framing
In response to Robert Tulip’s question, Dr Harrison said he would not characterise the reef project itself as geoengineering, although it uses geoengineering-relevant technology. He distinguished between intermittent regional use to reduce coral bleaching and a planetary-scale effort to restore global albedo.
He argued that the risk-benefit equation is fundamentally different for a local, time-limited intervention over a small fraction of the globe compared with an attempt to cool the planet. This difference is central to the project’s social acceptability. Lessons from the reef program may inform wider solar radiation modification research, but the reef use-case should be judged on its own purpose, scale, risk and benefit.
Other cooling and shading options
Jonathan Cole asked about floating surface films. Dr Harrison explained that these belong more in the “shading” category than the “cooling” category. Corals bleach as a function of light stress in the presence of heat stress, so reducing light during peak-risk periods can reduce bleaching even if the water itself is not cooled much. Surface films have reportedly achieved about 30% light reduction, but their likely use is local protection of high-value reef sites rather than large-scale water cooling.
John Macdonald asked about nanobubbles and ocean whitening. Dr Harrison said these ideas were within the program’s initial scope but were not prioritised for funding. Reasons included lack of demonstrated capacity to generate enough bubbles at relevant scale and concerns, whether proven or perceived, about possible harm to coral ecosystems. He did not dismiss the idea, but said further feasibility work is needed.
Mike MacCracken asked about meteorological dependence. Dr Harrison said cloud response depends on conditions such as low background aerosol levels, suitable boundary-layer clouds, cloud fraction, cloud thickness and other second-order cloud processes. He believes that, broadly, added aerosols produce net cooling in low clouds, as seen in the ship-emissions record. However, optimisation will require much better understanding of cloud regimes and aerosol size distributions.
Ocean circulation and hot water inflow
Robert Tulip asked whether cooling inside the reef could simply be overwhelmed by hot water entering from the open ocean. Dr Harrison replied that the answer depends on the spatial scale of intervention and water residence time. The Great Barrier Reef lagoon is shallow, averaging about 40 metres, with limited exchange. Modelled cloud brightening takes about four to six weeks to cool, or more precisely to avoid warming relative to the control case. After that, the system reaches a balance between warm water entering, cooled water leaving and continued radiative forcing.
He said a larger area of cloud brightening produces greater cooling because water spends longer within the cooled domain. He also noted that cloud cover, circulation and regional heterogeneity tend to average out more than expected.
Funding and next steps
Dr Harrison said the Reef Restoration and Adaptation Program was due to wrap up on 30 June and that the next phase had not yet received the hoped-for federal budget funding. However, ARIA funding will support further work, particularly on cloud microphysics, second indirect aerosol effects and improved aerosol generation.
A major stakeholder roadshow is planned through northeast Queensland, beginning around August, to communicate results and seek input from communities, Traditional Owners and affected industries. Tourism operators have already been supportive, including practical assistance during field campaigns.
Key implications for HPAC
The meeting showed that marine cloud brightening has moved beyond theory into serious field science. The Great Barrier Reef program has demonstrated aerosol generation, plume lofting, cloud-base interaction and measurable changes in cloud droplet number and size. This is a major advance for sunlight reflection research.
The most important barrier is engineering scale-up, especially the energy efficiency of aerosol generation. Current equipment is not deployable, but the research pathway is clear: improve nozzles, increase aerosol output, reduce energy cost and test cloud response under more varied conditions.
The social licence model is equally important. Dr Harrison’s program provides a practical example of how controversial climate intervention research can proceed through consent, transparency, regulation and community engagement. That model may be as valuable as the technical findings.
For HPAC and related advocacy, the meeting supports three messages: MCB deserves accelerated research, field experiments can be governed responsibly and sunlight reflection should be assessed by use-case rather than dismissed under a broad geoengineering label. The reef program offers a concrete model of cautious, transparent, adaptive experimentation in response to an urgent climate threat.
From: healthy-planet-...@googlegroups.com <healthy-planet-...@googlegroups.com> On Behalf Of rob...@rtulip.net
Sent: Saturday, 16 May 2026 1:30 PM
To: 'Healthy Planet Action Coalition' <healthy-planet-...@googlegroups.com>; 'Daniel Harrison' <Daniel....@scu.edu.au>
Cc: 'Lucinda Shearman' <lucindas...@hotmail.com>
Subject: [HPAC] Recording of HPAC Meeting with Daniel Harrison on MCB
Dear Daniel and all
Here is the temporary link to the raw recording of the HPAC meeting on 14-15 May, including video, audio, transcript and chat.
The meeting starts at timestamp 7:25. The pre-meeting seven minutes will be edited out when it is uploaded to the HPAC website in two weeks.
Many thanks Daniel. I wanted to ask, is there anything we can do to campaign for ongoing funding for your work?
Regards
Robert Tulip
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