In spring 2022, scientists began testing an extreme approach to tackling the climate crisis, dumping iron filings, rice husks and fertiliser into the Arabian Sea to stimulate phytoplankton growth capable of absorbing CO2 and storing it in the ocean. Unconventional approaches to tackling climate change such as these have been on the rise over the last year. For example, geoengineering trials are underway to cover large areas of sea ice with reflective materials to bounce light and heat back into the atmosphere to slow down melting.

Such methods are being described by proponents as "pragmatic insurance policies" that could "quickly counteract the climate’s warming". But these interventions could yield unpredictable consequences. For example, in theory the ocean’s ability to absorb CO2 could be increased by adding ground-up rocks, and a similar approach could also be deployed in soil using basalt. But the financial and environmental costs of mining and grinding these rocks, plus the unknown effect on marine life, could make the approach unviable.

Ocean fertilisation projects, such as the iron filings trial in the Arabian Sea, have the potential to increase the amount of CO2 captured by the sea but could also cause vast algal blooms that harm other aquatic life.

Meanwhile, scientists working at the frontiers of synthetic biology are seeking to change the genetic code of plants so that they can absorb more CO2 from the atmosphere. Scientists are engineering algae that can convert carbon dioxide into fuels such as ethanol or even plastic replacements. And bacteria is being created that absorbs atmospheric methane, another greenhouse gas. However, altering genes in this way could cause unintended consequences such as invasive species and the spreading of antibiotic resistance.

Questions also abound as to whether these interventions will be sufficiently developed in time to counter climate change. Worse, an excess of hype around these innovations may even pave the way for carbon-intensive industries to continue polluting, in the hope that damage can be reversed in years to come.

Despite the risks, we may end up in a position where we need to entertain the full spectrum of interventions, if the current trend of climate inaction from governments and industry continues. This is because we not only need to rapidly reduce greenhouse gas emissions, but also remove increasing amounts of atmospheric CO2 from historic emissions and harder-to-decarbonise industries. The Intergovernmental Panel on Climate Change has suggested that the world needs to remove up to 1000 gigatonnes of CO2 by the end of the century to limit warming to 1.5°C above pre-industrial levels. The UK government has recognised this as part of its net-zero strategy and set an ambition to capture five megatonnes of CO2 each year by 2030.

As eye-catching as more novel strategies are, there are some established approaches to removing CO2 from our atmosphere. Nature-based approaches, such as reforestation, ecosystem restoration and improved agricultural practices, capture atmospheric carbon and store it in plants and soil – a process known as carbon sequestration. But there’s a chance that nature-based approaches could only succeed in removing half the CO2 we’d need to sequester, depending on the speed and scale at which we can cut emissions.

Given the limits of nature-based techniques, scientists have developed technologies for carbon capture and storage. There are two methods that already feature as part of the UK government’s net-zero strategy. Bioenergy carbon capture and storage (BECCS) involves growing crops that sequester carbon, burning them to produce energy and then using carbon capture and storage technology to sequester the carbon produced (although finding suitable land to grow the crops can be a challenge). Direct air carbon capture and storage (DACCS) uses technology to extract carbon out of the air before storing it underground, but requires careful management to ensure that energy-hungry carbon capture systems don’t produce more carbon than they sequester. There’s also the potential for disruption to ecosystems and local communities.

It is clear that human interventions in natural systems come with inherent risks. Reducing emissions is, in theory at least, an easier, cheaper, safer and fairer path. Yet the slow progress on this front means that we may need to use interventions more radical than nature-based solutions to sequester enough carbon to keep warming below 1.5°C. In the short-term, it seems most likely that policymakers will remain focused on approaches that are closer to deployment such as BECCS and DACCS. However, it may be that as the existential risk to our species grows, we are increasingly willing to entertain more extreme efforts. This may mean co-opting living organisms and ecosystems, with their inherent capacity to sequester carbon, as a last resort to undo the damage caused by humans.