Climate Intervention: What is It, and Why are Scientists Researching It?

What is climate change?

It has been well established by peer-reviewed scientific research that Earth is warming at an accelerated rate, and that this warming is due to human activities (IPCC, 2022) (see the latest International Panel on Climate Change (IPCC) report for the science: AR6 Climate Change 2022: Impacts, Adaptation and Vulnerability — IPCC. The IPCC reports are a synthesis of international research efforts on the climate and climate change, and the impacts of climate change on humans and ecosystems.). The accelerated rate of warming we have experienced over approximately the last 200 years is unprecedented: while Earth has historically undergone periods of warming and cooling, it had previously taken thousands of years to achieve the change in temperatures we have observed over the past 100 years (Pörtner et al., 2022).

"Anthropogenic", or human induced, global warming is attributed largely to the release of gases (known as "greenhouse gases") by the burning of fossil fuels. Examples of fossil fuels include oil, coal, and natural gas, which are used to drive cars, heat homes, and more. Without delving too far into the science, greenhouse gases hold heat in the atmosphere, so the more greenhouse gases that are released (or "emitted") into the atmosphere, the hotter it gets. Though decades of evidence have supported the idea that greenhouse gas emissions contribute to global warming, our dependence on fossil fuels continues. It would be naive to suggest that cutting our emissions is a straightforward process, yet our ability to combat climate change depends on it. Global temperatures are steadily rising, and according to the European Copernicus climate service, 2024 was the first year to reach the "threshold" temperature of 1.5ºC (see Figure 1 below). In the next section, the importance of 1.5ºC is discussed.

Figure 1. Global annual average air temperatures from 1940 to 2024 compared to pre-industrial (1850-1900) temperatures (Poynting et al., 2025).

Keeping global warming within 1.5ºC- why is it important and is it possible?

In 2015, the Paris Agreement was adopted at the United Nations Climate Change Conference, with the overarching goal to limit “the increase in the global average temperature to well below 2°C above pre-industrial levels” and pursue efforts “to limit the temperature increase to 1.5°C above pre-industrial levels.” While this temperature increase is based on longer decadal averages of temperature that have not yet been breached, Figure 1 suggests we are reaching dangerous levels of warming. But why is 1.5ºC an important threshold?

The IPCC (Special Report on Global Warming of 1.5 ºC) shows that by limiting warming to 1.5°C, we may reduce the impacts of climate change on human and natural systems. We are not free from impacts at 1.5°C; compared to pre-industrial levels, we may see a warming of regional extreme temperatures, changes in heavy precipitation, and increased intensity and/or frequency of drought. Certainly, we have already observed some of these changes. For example, in spring 2024, Latin America experienced extreme heat and prolonged drought which led to severe water shortages and the death of dozens of people. Many other countries worldwide experienced similar tragedies. Unfortunately, if we allow temperatures to rise to 2°C compared to 1.5°C, there is an increase in the risk of:

1. Drought and precipitation deficit in some regions, increasing water stress and scarcity,

2. Heavy precipitation associated with tropical cyclones, leading to flood hazard,

3. Sea level rise: 2°C of global warming leads to 0.1 m more sea level rise than 1.5°C. This translates to up to 10 million people exposed to related risks (i.e., displacement from their homes),

4. Impacts on biodiversity and ecosystems, such as species loss and extinction, which disrupts the natural order of ecosystems and the food chain,

5. Elevated ocean temperatures and associated increases in ocean acidity, which impacts marine biodiversity, fisheries, and ecosystems,

6. Poverty and disadvantage in some populations,

7. Exposure to pollutants such as ozone and vector-borne diseases, posing risk to human health,

8. Food insecurity,

9. And economic stress.

This list is not exhaustive but highlights some of the key concerning risks.

As Figure 1 points out, the year 2024 reached the threshold of 1.5°C, a result of still high greenhouse gas emissions. It is becoming less and less likely that we will be able to keep warming at or below 1.5°C, with some scientists suggesting that we may be past the point of limiting warming. Although there are some climate change impacts that may be irreversible in the long-term, such as sea level rise, we still hold the power of minimizing the impacts of climate change. Now more than ever, it is critical that we mitigate our emissions, but the reality is, time is running out. How can we buy ourselves enough time to address one of the most pressing problems of the present day?

Climate intervention

As mentioned previously, we must dedicate ourselves to mitigating our greenhouse gas emissions to eventually reach net-zero, which is seemingly impossible in a fossil fuel driven world. Greenhouse gas emissions continue to increase (see Figure 2 below), a scary fact to consider when also considering 2024 was the hottest recorded year to date.

Figure 2. Greenhouse Gas emissions, 1850-2023 (Ritchie et al., 2020).

We need more time, or some way, to potentially remove the gases we are emitting from the atmosphere. At the very least, a comprehensive risk-management strategy is needed. A part of this strategy is climate intervention, which is defined as "deliberate intervention in the planetary environment of a nature and scale intended to counteract anthropogenic climate change and its impacts" (Secretariat of the Convention on Biological Diversity, 2012). Climate intervention strategies include carbon dioxide removal (CDR) and solar radiation management. Each are described below.

1. Carbon Dioxide Removal (CDR)

Carbon dioxide is the most important greenhouse gas, so its removal from the atmosphere would aid in reducing global warming. CDR methods exist at a range of methods and scales. One method is afforestation, where a forest is planted as a way of creating a carbon sink because trees take in carbon dioxide. Afforestation is an effective and natural way to address climate change, but the scale needed to offset global warming is very large; the addition of one trillion new trees could absorb one-third of anthropogenic carbon dioxide emissions (Jean-Francois Bastin et al., 2019). Depending on how the forest is planted, this could translate to 3,000 to 6,000 hectares of land.

Direct air capture is a technology which aims to capture carbon dioxide from the atmosphere. While this technology exists, it remains costly and energy intensive, so more research is needed before it is deployed at a large scale. Ocean capture, where the carbon is instead captured from the ocean, is still in the developmental research stage.

There are also ways of adjusting farming practices to increase the amount of carbon stored in soil, but managing soil for carbon at a large scale would be difficult, and more research is needed to account for the variability of natural systems. Another method that is not straightforward in implementation is biomass carbon removal and storage (BiCRS), where the biomass from algae or plants is used to remove and store carbon dioxide from the air. While promising, BiCRS could also compete for land with food crops or ecosystems, so implementation of BiCRS would need to be strategic. Finally, carbon mineralization, which is also termed "enhanced weathering," is where minerals naturally react with carbon dioxide, removing carbon dioxide from the atmosphere by turning it from a gas into a solid. Scientists are researching a way to speed up the carbon mineralization process, but this is still in development.

2. Solar Radiation Modification

Solar radiation modification is the idea of increasing Earth's reflective properties (or "albedo") in an attempt to offset global warming. There are a range of strategies underneath the solar radiation modification umbrella. The least climate-altering strategy is surface albedo enhancement, which increases the reflectivity of surfaces on Earth. An example of this has already been implemented in some places in the form of white roofs, or "cool roofs" (see Figure 3 below), which also have the co-benefit of reducing building temperatures.

Figure 3. The left: a regular roof, which absorbs heat and minimally reflects sunlight. The right: a white roof, which absorbs some heat, but largely reflects sunlight away.

White roofs have been implemented in several cities across the US; in fact, Los Angeles, Houston, Austin, Toronto, Miami Beach, Atlanta, Denver, Chicago, and New York have building codes requiring cool roof technology.

Other solar radiation modification strategies are more controversial as they require a more deliberate modification to the climate and expensive, large-scale deployment. It is important to note, however, that these strategies are only the subject of scientific research, contained to computer models and small-scale experiments in the natural world that would not impact anyone. It is unknown still whether these strategies are viable options to offset global warming, and scientists researching these topics do not hope they are necessary. Ideally, we are able to reduce our greenhouse gas emissions to address the root cause of climate change, and this should be at the forefront of our focus. Yet, research on solar radiation modification strategies is paramount because, if they become necessary to deploy in the future, we must fully understand the feasibility and potential consequences of each strategy, and comprehensive research takes time. The two solar radiation modification strategies that have gotten the most attention are stratospheric aerosol injection (SAI) and marine cloud brightening (MCB).

Solar aerosol injection (SAI) involves injection of small reflective particles ("aerosols") or a "precursor gas" (a gas that turns into aerosol via chemical reactions) into the stratosphere (10-16km above the Earth's surface) to increase reflectivity. Proposed aerosol species include sulfate (the most popular), calcium carbonate, and diamond dust. The idea of SAI stems from the cooling effect of large volcanic eruptions, which naturally inject sulfate aerosol into the stratosphere. A recent observation of this phenomenon was in 1991, when the Mt. Pinatubo eruption resulted in a 0.5°C global cooling effect over the following year. Rather than a volcano, however, SAI proposes to have planes flying across the globe, injecting sulfate aerosol directly into the stratosphere.

Marine cloud brightening (MCB) also involves the injection of small aerosols, specifically sea salt aerosols, into marine clouds, which brightens the clouds (making them more reflective). A real-life analog of MCB are clouds formed over ship tracks (see Figure 4 below) due to the ships' exhaust containing tiny sulfate aerosol particles, which act as seeds for moisture to condense onto (many seeds = cloud formation).

Figure 4. Clouds formed over ship tracks in the Pacific Ocean south of Alaska (credit: NASA's TERRA satellite; image by Robert Simmon and Jesse Allen).

These ship track clouds are highly reflective, producing a local cooling effect by reflecting sunlight away, rather than letting it heat the surface. The idea for MCB is the same: the sea salt aerosol act as seeds, either brightening already formed marine clouds or creating new ones. Research using computer climate models suggests that large scale deployment of MCB could produce the same cooling effect as ship track clouds, but enough to cool the globe and potentially offset global warming. The technology for such deployment, however, is still the subject of research.

Considerations for Climate Intervention Research

As someone who is a part of climate intervention research, I understand the importance of research within the field but also acknowledge the serious considerations that should be taken. Here, I will discuss a few.

Climate intervention, especially strategies aimed at deliberately altering the climate (like SAI and MCB), is a scary concept for people outside of the field. There is a lot of misinformation spread about what climate intervention is, why scientists are conducting the research, and whether or not there is a political agenda to the research, which also contributes to fear and misunderstanding. I can only speak for myself, but I think a majority of other scientists would agree that we are involved in the research because we are passionate about the subject matter and concerned for the health and safety of the public (which includes ourselves). Education to the public and transparent (as well as open access) research is key to ensuring the public is engaged in the conversation as well. Scientists do not want to be the only ones in on the discussion.

As mentioned previously, climate intervention is the subject of ongoing research, so there are no current plans to deploy these strategies at wide scale. In fact, this simply would not be possible. (1) Much of the technology does not exist. (2) We are still understanding crucial components of the climate system, such as interactions between aerosols and clouds. Research into climate intervention actually aids in bettering our understanding, and there is international effort to improve our climate models, but our gaps in knowledge remain the subject of substantial scientific effort. (3) Solar radiation modification poses a risk as there is currently no international governing body who would oversee its implementation. There are continued efforts to bring international bodies together to discuss climate change (such as the UN Climate Change Conference Convention of the Parties, "COP," which is a yearly meeting), but additional efforts in discussing a comprehensive risk-management strategy which includes climate intervention are crucial moving forward.

References

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