Article Derived From Transcript of YouTube Video: How Aerosols Brighten Clouds — and Cool the Planet | Sarah J. Doherty | TED

Transcript of YouTube Video: How Aerosols Brighten Clouds — and Cool the Planet | Sarah J. Doherty | TED

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Climate scientist discusses the unintended consequences of reducing particulate pollution, revealing that aerosols, while harmful to human health, also cool the climate. The speaker explores the complexities of aerosol-cloud interactions and their impact on climate warming. They introduce the concept of marine cloud brightening as a potential method to mitigate climate change, emphasizing the need for better data and models to understand this process fully. The scientist also highlights the importance of international collaboration and transparency in climate research, particularly involving marginalized communities most affected by climate change.

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Aerosols and Their Impact on Climate

So, I am a climate scientist. You might expect me to discuss the various ways we are making the climate warmer. However, today, I want to share a different story with you, one about unintended consequences.

For many, it's easy to overlook that alongside emitting greenhouse gases, humans have been adding a significant amount of particulate pollution to the atmosphere. These tiny particles, known as aerosols by scientists, are responsible for the premature death of between four and ten million people annually worldwide, posing a major public health crisis. Efforts to clean up these emissions are underway and are to be applauded.

The Unintended Consequence of Reducing Aerosols

But here's the twist: the unintended consequence of reducing these aerosols might actually be accelerating climate warming. Most of these aerosols have a cooling effect on the climate. Throughout my career, I have studied how aerosols absorb sunlight in the atmosphere and increase the reflection of sunlight away from our planet. They scatter sunlight back to space and, when mixed into clouds, can make them brighter or more reflective. Both effects serve to cool the climate by reducing the amount of sunlight available to heat the Earth's surface.

We estimate that human-caused aerosols are currently cooling the climate by about half a degree Celsius. Without their climate effects, we would already be experiencing significantly worse climate impacts than we are now facing.

The Dilemma of Aerosol Reduction and Climate Cooling

The dilemma is that as we clean the air for human health, we are also reducing the concentration of these aerosols in the atmosphere, thereby removing a source of climate cooling. Unlike greenhouse gases, which continue to warm for decades to centuries, aerosols only last about a week in the atmosphere, and their cooling effect dissipates almost immediately after we stop emitting them.

There is also uncertainty in our estimates. While we believe aerosols cool the climate by about half a degree Celsius, the actual effect could be smaller or larger. It's possible that they are cooling the climate by up to almost a full degree Celsius. This uncertainty means we don't know how much climate warming they will unmask as we continue to clean the air.

Understanding Aerosol-Cloud Interactions and Uncertainties

Let's delve deeper into how aerosols cool the climate and why these effects are so uncertain. Aerosols primarily cool the climate by increasing the reflection of sunlight from clouds. This increase in cloud brightness due to aerosols is not usually visible because clouds naturally vary greatly in their brightness. However, there is a striking example called "ship tracks," where satellite images off the west coast of North America show lines of clouds that are brighter and more reflective than the surrounding clouds.

To understand this, it's important to know that cloud droplets always form on an aerosol. Over the ocean, there are fewer aerosols in the atmosphere, resulting in clouds with a small number of larger droplets. When a ship adds aerosols to the atmosphere and the clouds, the water is distributed over these aerosols, creating a cloud with a larger number of smaller droplets. This change in droplet size increases the cloud's reflectivity.

This isn't just happening where ship emissions mix into clouds; it's happening over broad regions of the planet where pollution aerosols mix into clouds. While this example clearly shows how pollution aerosols can make clouds more reflective, it doesn't always occur. The reasons are, as scientists often say, "It's complicated" and "It depends."

Clouds are incredibly complex and constantly evolving. When aerosols are added to clouds, they don't just change droplet size; they can also change how clouds evolve, affecting their brightness. Depending on atmospheric conditions, clouds can become more or less reflective with the addition of aerosols, or they might not change at all. However, under the right conditions, aerosol additions to clouds can significantly brighten them.

Mimicking the Effect of Pollution Aerosols for Climate Mitigation

This raises an interesting question: Could we rapidly reduce climate warming by mimicking the effect that pollution aerosols have on clouds, but by adding natural aerosols instead of pollution? Specifically, by adding sea salt aerosol to clouds over the ocean, where sea salt already acts as a seed for cloud droplet formation?

We began studying this problem using computer models. When we add tiny sea salt aerosols to clouds over the ocean in global climate models, we find that brightening just a fraction of the clouds over the ocean does indeed rapidly and significantly reduce climate warming from greenhouse gases. The models suggest it's possible, but they lack the ability to resolve all the detailed interactions between aerosols and clouds. They can't tell us how much cloud brightening is possible or where.

To address this, we need to turn to models that cover more localized areas of the globe but include many more details about aerosols, clouds, and their interactions. What we really need is better real-world data to test and inform these models that we use to study marine cloud brightening.

The Challenge of Controlled Aerosol Studies

The most uncertain aspects of marine cloud brightening's potential have to do with how small-scale air motions in clouds, over just a few square kilometers, respond to the addition of aerosols. Being able to systematically study how clouds respond to aerosols, like a single plume of aerosols over a small area of clouds, could greatly improve these climate models.

Today, I want to tell you about a powerful approach our team is developing to do just that. The approach involves adding a single plume of sea salt aerosols to a small area of clouds over the ocean and observing how those clouds respond. Essentially, creating a single clean ship track.

The observations for studies like this would resemble those we've conducted for decades to study how pollution aerosols affect clouds. Research aircraft equipped with specialized instruments can measure in great detail the atmospheric conditions, the aerosols, the clouds, and how they all vary.

The difference between past studies and these new controlled aerosol studies is that we can compare clouds with different aerosol concentrations but that are otherwise the same. This allows us to quantify where changes in cloud reflectivity are actually caused by the aerosols, rather than varying due to other factors.

Developing a New Instrument for Cloud Aerosol Research

Generating the sea salt aerosol plume with the right characteristics for these controlled aerosol studies is a significant technological challenge. The aerosols need to be just right. To date, no one has demonstrated the ability to generate both the size and quantity of aerosols needed to consistently and appreciably brighten marine clouds for these studies.

As a climate scientist striving to better understand how aerosols affect clouds and climate, I am thrilled to be part of a team developing a new instrument to meet this challenge. Our new cloud aerosol research instrument is specifically designed to generate a very large number of very tiny sea salt aerosols. These aerosols are about one-thousandth the width of a human hair, which is the ideal size for marine cloud brightening.

First Scientific Studies with the New Instrument

I am also excited to announce that we have just started our first scientific studies with this instrument. Two weeks ago, we set up our new Coastal Atmospheric Aerosol Research and Engagement (CAARE) Facility on the flight deck of the USS Hornet Sea, Air and Space Museum in Alameda, California.

On the Hornet, we are making observations at multiple locations along the flight deck of the sea salt aerosol plume generated with our new instrument. These measurements will allow us to study how the aerosol evolves as it's transported towards clouds and whether this instrument is delivering the right aerosol with the right characteristics for use in later studies at sea, such as the single plume experiment and how clouds respond.

Openness and International Collaboration in Climate Research

We've set up this study specifically at a museum to make it easily accessible to the public, educators, and other researchers. We consider this level of openness to be a crucial part of our program. We hope that the work at the CAARE research facility can be the start of broader international engagement in this research, particularly by our colleagues in historically marginalized communities who are most vulnerable to climate change. Their direct engagement in this research is absolutely critical for equitable and informed discussions about whether we would ever use marine cloud brightening to cool the climate as a way of addressing climate risks.

Marine Cloud Brightening: Not a Solution, but a Potential Treatment

Let me be clear: marine cloud brightening will not reverse the effects of greenhouse gases. This is not a solution to the climate crisis. I cannot stress that enough. However, marine cloud brightening might be a way of treating the main symptom of the problem, which is too much heat in the atmosphere and ocean.

We believe that the world needs the best information possible to decide whether approaches like marine cloud brightening might be a component of how we chart a safer course into a future with a rapidly and dangerously warming climate. It's also critical that we better understand the evolving role of aerosols in climate change and the climate system if we don't want to be flying blind into the coming decades of climate change.

Invitation to Join the Research Effort

I hope I have left you as excited as I am about these new capabilities we're developing to study these crucial questions. And I invite you all to join us at our new CAARE research facility. Thank you. (Applause)


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