Geoengineering the ocean could help slow climate change

Out in the open ocean, a large vessel casts out several tonnes of iron sulphate flakes to the water. Several days later, large pools of blue-green pigments—phytoplankton blooms—are visible at the ocean’s surface from satellites orbiting Earth.

Phytoplankton bloom in the Barents Sea, 2010. An international consortium of scientists is now investigating the potential of ocean negative emissions technologies, including iron fertilisation, in support of the Paris Climate Agreement. Wikimedia/NASA image acquired August 31, 2010

Phytoplankton work hard for several days to draw down carbon dioxide (CO₂) from the atmosphere, and eventually die, sinking to the bottom of the ocean and sequestering carbon in the process. The vessel moves on to its next site to lock up another mass of atmospheric CO₂.

Despite sounding like a science fiction blockbuster, it could be a glimpse at a not-so-distant future. An international consortium of scientists, including CSIRO, is embarking on a project to investigate the potential of ocean negative emissions technologies (NETs) as part of a global attempt to mitigate climate change.

Ocean NETs: From fringe to frontier science

It’s not exactly the future that scientists expected.

Geoengineering technologies, including ones like ocean-based iron fertilisation, were up until the last decade or so considered ‘fringe’ science: either critically under-explored, or regarded as a last resort measure for climate change mitigation.

In the 2015 Paris Agreement, most countries resolved to limit climate warming to below 2 degrees Celsius.

A subsequent report by the Intergovernmental Panel on Climate Change (IPCC) recognised that emissions reductions alone would not be enough to reach this goal. Countries will also need to employ NETs to pull gigatonnes of CO₂ out of the atmosphere each year.

“The reality is that carbon dioxide is continuing to accumulate in the atmosphere, despite some temporary slow-down of global carbon emissions during the COVID-19 pandemic,” explains Dr Andrew Lenton, leader of the Earth Systems portfolio in the CSIRO Climate Science Centre.

“Reducing greenhouse gas emissions is obviously the main goal, but it’s become clear that we will need to do much more if we are to stabilise or reduce global temperatures,” says Dr Lenton.

While land-based NETs like carbon capture and storage have received much of the attention to date, a new project being led out of Germany’s GEOMAR is investigating whether ocean NETs have potential to work in isolation, or complement other land-based NETs.

The project, funded through the EU, will contribute to major international and national assessments of possible climate mitigation options.

Closing in on knowledge gaps

Coastal ‘blue carbon’ sinks like tidal marshes, mangroves and seagrass also sequester carbon, and will be investigated as another potential ocean NET. Image: pxhere

The ocean has huge potential to help mop up CO₂ that is otherwise lost to the atmosphere, currently taking up about a third of anthropogenic (human-caused) emissions.

Iron fertilisation, alongside ocean alkalinity enhancement, blue carbon sink enhancement and artificial upwellings or downwellings are some of the proposed scenarios that the research network will be exploring as they seek to close in on knowledge gaps.

Take iron fertilisation for example. When ocean conditions are right (large cold ocean upwellings bring nutrient-rich water to the surface), phytoplankton blooms (microscopic marine algae) will develop naturally.

But human intervention in nature is another matter, and unexpected consequences can result. One example is that by artificially inducing phytoplankton blooms through iron fertilisation, we could cause deoxygenated ocean zones, which directly impact marine life.

“Geoengineering has undoubtedly received a lot of negative commentary to date for this very reason,” says Dr Lenton.

A key aspect of research into ocean NETs is to investigate how natural systems will actually respond to the engineered changes. That includes any detrimental impacts, environmental co-benefits and climate mitigation potential. This knowledge will be used to shape economic policies, governance decisions and public acceptance around the technologies.

There are also big knowledge gaps around the social, economic and jurisdictional implementation – in other words, the impact of NETs on society.

That’s why GEOMAR is taking a transdisciplinary approach, bringing together social and political scientists, economists and legal scholars with climate scientists. The project will investigate socio-institutional barriers, like the public acceptance of new technologies, to help build a clear institutional framework for implementation.

From back of envelope to definitive calculations

Currently the known impact of individual NETs is through ‘back of the envelope’ type calculations. In other words, they’re far from proven.

Dr Lenton says too little is known around ocean NETs just yet to make definitive assessments around their utility.

“Many ocean NETs are only in very early conceptual or theoretical stage, whereas some, like iron fertilisation, have undergone small-scale field experiments,” says Dr Lenton.

For ocean NETs to progress towards pilot projects, and eventual deployment, we will need a better handle on the uncertainties and feedbacks within the Earth system.

This is where CSIRO comes in to the GEOMAR project: Dr Lenton will be using modelling to provide some more definitive numbers around ocean NETs at scale. He’ll be utilising the Australian Community Climate and Earth Simulator (ACCESS) to test and monitor the global carbon system response to ocean NETs.

Group Leader at CSIRO’s Climate Science Centre, Dr Rachel Law, has worked on ACCESS for over 10 years. She explains the role of models in exploring these various scenarios: “As we go into the future, if we want to stabilise Earth’s temperatures, we’re going to need negative emissions. And that means we’re going to need to find ways to get carbon out of the atmosphere.”

“Modelling allows you to test sensitivities and answer questions like “if I could employ the technology across this much of the world, or in these specific oceans, how much carbon could we take out?” says Dr Law.

Understanding the ocean and Earth system response to NETs

Many parts of the ocean are already experiencing the acute impacts of climate change. This includes acidification and warming, most prominently witnessed in consecutive mass bleaching events on the Great Barrier Reef. Marine heatwaves that occurred in the Tasman Sea have also been attributed to climate change.

The extent of a marine heatwave that occurred off the East Coast of Tasmania in mid-February 2016. A recent study led by IMAS found that human induced climate change was responsible for a marine heatwave off Tasmania’s east coast in the summer of 2015/16. Figure: IMAS

Modelling that accounts for current ocean observations (pH, salinity, and temperature), can help with assessing the benefits and risks of various scenarios against each other.

One of the other key benefits of ACCESS is that it’s a coupled Earth system model. It can simulate the components of the climate system as well as how they interact: ocean, atmosphere, sea ice, land surface, carbon cycle, atmospheric chemistry and aerosols.

This means it’s equipped to simulate the carbon cycle and the feedbacks in the climate system. It will help scientists understand so-called ‘tipping points’. That’s where you nudge the system so far that it actually moves into a different state that is impossible to recover from.

Exploring the future of our planet

Dr Law explains that some of the complexity of the project lies in the natural variability in the climate system. This will require running the model many times with different initial conditions in order to detect the signal from the change that the scientist has made.

The model simulations run for weeks on high-performance computers operated by the National Computational Infrastructure (NCI).

Dr Law says that while models like ACCESS have their limitations, they’re the only tool we currently have to explore these ‘what if’ questions on a global scale.

“We’ve invested quite a lot in getting a version of ACCESS that has the carbon land and ocean cycle in there. It’s come out of CSIRO’s work in collaboration with the Bureau of Meteorology, Australian universities and international collaborators.

“It’s great to have a tool to be able to answer some questions around critical areas. Specifically, our future carbon pathway of the planet.”