21 June 2023

Three climate interventions: Reduce, remove, repair

Courtesy Climate Crisis Advisory Group

In September 2022, Stockholm University’s David Armstrong McKay and his colleagues concluded that even global warming of 1-degree Celsius risks triggering some tipping points, just one data point in an alarming mountain of research on tipping points presented in the last year and a half. Clearly, even the current level of warming of around 1.2°C is unacceptably dangerous.

To protect small-island states, the Great Barrier Reef, Antarctica, Greenland, the Amazon — indeed to provide protection for the many places and people we care about — requires returning to a climate similar to the relatively stable Holocene conditions of the last 9000 years and fixed human settlement, during which time carbon dioxide (CO2) levels did not exceed 280 parts per million (ppm) CO2. it also requires preventing a cascade of tipping points in the meanwhile.

For example, in 2022 a group of Australian scientists suggested that from a geologic perspective: “a justifiable aim for a future climate is one akin to pre-industrial conditions.” Other evidence points to the need to return to pre-industrial levels of 280 ppm, for example in relation to the cryosphere.

If this were the goal, activists and policymakers would be advocating a “three levers” approach to reversing global warming: a strategy to rapidly “reduce, remove and repair”. That means:

  1. Reducing emissions to zero at emergency speed;
  2. Removing carbon by drawdown to return atmospheric conditions to the Holocene zone; and 
  3. The urgent research to identify safe interventions that protect and repair vital systems and, in the shorter term, aim to prevent warming reaching a level that triggers a cascade of calamitous tipping points that are irreversible on human timescales.

The harsh reality is that the first two levers by themselves — zero emissions and drawdown — are not sufficient to stop the Earth system charging passed 1.5°C in the next decade or so, regardless of the emissions path, and to significantly higher temperatures —  likely more than 2°C around mid-century — with truly catastrophic impacts for some peoples, regions and natural systems. In essence:

  • Warming to date plus the observed Earth Energy Imbalance (EEI) at the top of the atmosphere adds up to about 2°C for today’s level of greenhouse gases.  And the paleoclimate record suggests the current level of CO2 is sufficient for 3°C or more of warming in the longer term. 
  • Thus a strategy of mitigation (emissions elimination only) will not prevent catastrophic outcomes.
  • A safe-level of greenhouse gases to preserve and restore vital climate systems may require returning to the pre-industrial level of around 280 ppm. Large-scale carbon drawdown is essential in achieving this goal, but this cannot be done at a scale and speed fast enough to prevent more tipping points being activated and the possibility of a cascade of consequences leading to enhanced warming. 

Thus a ‘third lever’ of action is required: the urgent scaling up of research and investigation into an additional range of climate interventions that aims to rapidly cool the planet is required, including solar radiation management (SRM) cooling. If shown to be efficacious, SRM could play a vital role in flattening the warming peak whilst allowing time to achieve zero emissions and carbon drawdown on a path back to a safe, liveable climate.

There is a near-term risk that warming may also trigger the “Hothouse Earth” scenario, in which climate system feedbacks and their mutual interaction drive the Earth System climate to a “point of no return”, whereby further warming would become self-sustaining (that is, without further human-caused perturbations). Scientists have warned that this is possible even in the 1.5–2°C range. 

At 1.5°C, “we're at risk of crossing irreversible thresholds on unique and threatened systems”, says Johan Rockström, director of the Potsdam Institute for Climate Impact Research.

Image credit
As illustrated, even as the world moves to  zero emissions, and CO2 levels start to decrease by natural processes and by CDR, albedo modification can flatten  the level of peak warming — and perhaps help avoid existential climate impacts and extreme damage —  until the other processes fully kick in. 

Climate intervention is the “deliberate large-scale manipulation of the planetary environment to counteract anthropogenic climate change”. Climate interventions, also termed geoengineering, climate restoration and climate repair, fall into two broad categories: CO2 removal and albedo modification.

Carbon dioxide removal

Carbon dioxide removal (CDR) involves taking the excess CO2 out of the atmosphere and the ocean, and storing it safely and securely in biomass, soils or rock formations, or in neutralised or stable forms in the oceans. CDR techniques may be classified as nature-based solutions, and technical solutions, and include: 

Nature-based solutions

  • Ecosystem sequestration, in which ecosystems can be restored or created to use photosynthesis to capture CO2 and store it in biomass, for example in peatlands, terrestrial forests and shallow saltwater ecosystems such as mangroves, kelp and seagrass. Re/forestation and wetlands restoration is well-established, safe and the most cost-efficient CDR option at present.
  • Regenerative land management practices that enhance water retention and soil carbon, which are proven and cost effective, including the use of biochar to store carbon and rejuvenate soils.
  • Marine up-welling, which extends the scale of marine kelp, sea grasses and seaweed farms, offering new carbon sinks, plus production of food for cattle which increases milk yields whilst lowering methane emissions from livestock. Biomass allowed to drift down to the deep ocean would remain for hundreds of years or millenia. 
  • Ocean iron fertilisation, in which fertilising deep ocean areas with light sprinklings of iron dust can generate, in a matter of months, green, plankton-rich forests, accompanied by burgeoning fish stocks and a huge variety of marine wildlife. One result is more organic matter settling in the deep ocean. 
  • Enhanced mineralisation, mimicking a process in nature where silicate rock weathering on land  binds CO2. Crushed carbonate rock can also be exposed to CO2 dissolved in water to create bicarbonate ions that can be stored in the ocean. This has been demonstrated in the laboratory and in small scale field trials, but has yet to be demonstrated at scale.

Technical solutions

  • Negative emissions construction, the increased use of plantation timber (and potentially new forms of concrete and road materials) to store carbon in the built environment.
  • Ocean alkalinization, the adding of alkaline substances — minerals such as olivine, or artificial substances such as lime or some industrial byproducts — to seawater to enhance the ocean’s natural carbon sink by converting dissolved CO2 into stable bicarbonate and carbonate molecules. This idea is at an early stage of development.  
  • Direct chemical capture by machines, to store in geological formations or in immobile form in the ocean or ocean sediments, which is now at the demonstration stage, but cost and energy use are currently prohibitive. 
  • Bioenergy with carbon capture and storage (BECCS), the use of crops to manufacture bioenergy, with underground storage of CO2. This technology is unproven at scale or cost, but a favourite of policymakers and incorporated into Integrated Assessment Models and the Paris Agreement as a means of justifying a longer life for the fossil fuel industry via BECCS “offsets” for continuing carbon pollution.

Some of these techniques have well-known safety profiles and are at a high technical readiness level, whilst others are unproven at scale and cost or speculative, and need technological development, safety testing and suitability evaluation.

Until emissions reach zero, CDR complements decarbonisation in mitigating the rate of increase of CO2. After that point, CDR can reduce the absolute level of CO2, but it is a relatively slow process, and not a primary tool in countering a locked-in temperature rise of 1.5°C by around 2030, and likely warming of 2°C before 2050 unless there is a radical reduction in emissions far beyond current national commitments.

Repair with albedo modification

Albedo modification (AM) is the reflection of more sunlight away from the planet.  Options include: 

  • Enhancing surface reflection with mirrors, such as the Mirrors for Earth's Energy Rebalancing (MEER) project which is at an early stage of research development, but appears to not be bounded by material or energy use constraints. MEER addresses the imminent urgency of climate change due to temperature increase and weather extremes while reshaping energy production and consumption to renewable energy.
  • Marine cloud brightening (MCB) to increase reflectivity, in which saltwater spray is added to the lower atmosphere which has a high water vapour content, making existing clouds whiter (more reflective) or helping new clouds to form in a clear sky. Field research on MCB to protect the Great Barrier Reef is under way, and the Centre for Climate Repair at Cambridge is urgently researching MCB as a means of cooling the Arctic. 
  • Solar radiation management (SRM), more accurately described  as stratospheric aerosol injection (SAI), which increases the amount of stratospheric aerosols to reduce incoming solar radiation. This in a way mimics the cooling effect of major volcanic eruptions that inject sulphur dioxide gas into the stratosphere creating small particles of sulphuric acid that reflect some sunlight back to space;  when Mount Pinatubo in the Philippines exploded in volcanic eruption in 1991, it cooled the planet by 0.6°C for about 15 months due to the particulate matter released.There is strong evidence that SAI, if applied at a scale, could significantly reduce or fully eliminate the earth’s overheating in a relatively short period of time. SAI is the most studied form of albedo modification, with hundreds of published research papers, but with potentially significant risks.
  • Increasing reflection of the terrestrial surface, everything from ice whitening to more reflective human infrastructure such as roofs.
  • Decreasing the amount of high-altitude cirrus clouds to allow more out-going radiation.
  • Space-based methods, which are highly speculative. 

Some of these are at the early stage of research, whilst others have been more extensively researched, such as SAI. All are at a relatively low level of technical readiness. 

Unlike CDR, albedo modification cannot reverse warming by reducing the CO2 level, nor does it have a direct effect on ocean acidification caused by rising levels of CO2. However, as an interim measure, it could, in theory, “reduce some harm done by climate change during the time it takes for societies to implement deep cuts in greenhouse gas emissions while also potentially developing and deploying CDR systems. It could also, in theory, cool the climate quickly and thus prove highly valuable should society at some point face rapid changes in climate that cause unacceptable damage.”

Intervention research and advocacy groups