Reduce, remove, reflect — the three Rs that could limit global warming

Since 2019, the UK has been committed to the target of net zero greenhouse gas emissions by 2050. Legally binding net zero targets form the basis for national efforts to meet the international goals of limiting global warming to “well below 2°C above pre-industrial levels” and ideally to 1.5°C.

These goals, launched in 2015 as part of the UN’s Paris agreement, set the stage for climate action in a warming world. Much like the “reduce-reuse-recycle” sustainability initiative, various climate actions fit within three Rs — reduce, remove and reflect. These actions were the subject of a recent debate in the UK parliament.

My colleagues and I have reviewed how these three Rs differ in scope, scale and state of knowledge. Our analysis reveals that a range of climate interventions may complement intensified mitigation efforts (to reduce greenhouse gas emissions), but more research is urgently needed.

Reducing greenhouse gas emissions is at centre stage. This is non-negotiable. Emissions reduction must be deep, rapid and sustained if we are to limit global warming to less than 2°C. These drastic cuts demand an ensemble cast, players from all sectors, from energy to agriculture. The energy to power modern society accounts for almost 75% of our greenhouse gas emissions.

We need a prop change at centre stage: an energy transition from fossil fuels to renewables. This requires electrification and energy efficiency measures — both are central to managing the growth in energy demand sustainably.

At stage right, greenhouse gas removal offers a supporting role by removing historical emissions and offsetting residual emissions from sectors lagging behind in the energy transition (such as shipping and aviation). A number of academics have stressed that a range of removal methods is required to achieve net zero emissions and halt the rise in global temperature.

Conventional carbon removal methods, such as forestation or the restoration of peatlands and wetlands, are vital. However, due to resource constraints (such as land and water security) and ecosystem impacts of global warming, we need to scale new methods rapidly to meet Paris agreement targets. These include ways to capture and store carbon on land and at sea.

Novel methods have many challenges, however, related to their effectiveness (including storage durability and permanence), unintended environmental consequences, economic costs and demands on natural resources. The challenges constraining the scale-up of novel removal methods must be addressed if we are to achieve net zero and halt global warming.

The consequences of climate change are outpacing efforts to abate it. With each year, the likelihood of exceeding 1.5 and 2°C warming increases, posing major risks to society and Earth’s ecosystems. That’s why the third R — reflect — needs to be assessed.

Read more: UK funds controversial climate-cooling research

Sunlight reflection methods have been in the wings on stage left. In the context of limiting global warming to 1.5°C, they have been considered feasible in theory, but fraught with challenges in practice. As the chance of exceeding 1.5°C in the coming years increases, this form of climate intervention needs further consideration. Experts brought together by the UN Environment Programme have concluded that, although this intervention is “not a substitute for mitigation”, it is “the only option that could cool the planet within years”.

The most studied methods to reflect sunlight are called stratospheric aerosol injection and marine cloud brightening. These methods mimic natural processes that cool the earth by reflecting sunlight, be it through the release of reflective aerosols into the upper atmosphere, or the addition of droplet-forming salt crystals into marine clouds in the lower atmosphere.

Read more: Five geoengineering trials the UK is funding to combat global warming

Sunlight reflection methods pose immense challenges with respect to research, ethics and governance. There are many scientific uncertainties about how these interventions will influence the climate. There is also no global regulatory framework in place. Any legislation needs to be based on scientific evidence and informed decisions.

Shining the spotlight

Meeting climate goals requires an ensemble cast performing actions across the warming world stage. Emissions reduction is indispensable and should remain centre stage in climate policy. Climate interventions at stage right and left — in the form of greenhouse gas removal and sunlight reflection — need responsible and responsive direction. Their risks and benefits need to be assessed.

Before curtains are drawn, let’s make sure every climate action — reduce, remove and reflect — gets a fair hearing.

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Dante McGrath does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.