SCoPEx | Keutsch Research Group

Stratospheric Controlled Perturbation Experiment (SCoPEx)

SCoPEx is a scientific experiment to advance understanding of stratospheric aerosols that could be relevant to solar geoengineering. It aims to reduce the uncertainty around specific science questions by making quantitative measurements of some of the aerosol microphysics and atmospheric chemistry required for estimating the risks and benefits of solar geoengineering in large atmospheric models. SCoPEx will address questions about how particles interact with one another, with the background stratospheric air, and with solar and infrared radiation. Improved understanding of these processes will help answer applied questions such as, is it possible to find aerosols that can reduce or eliminate ozone loss, without increasing other physical risks? 

At the heart of SCoPEx is a propelled scientific balloon that can travel a few meters per second (walking speed) relative to the surrounding air. The propellers serve two functions. First, the propeller wake forms a well mixed volume (roughly 1 km long and 100 meters in diameter) that serves as an experimental ‘beaker’ in which we can add gasses or particles. Second, the propellers allow us to fly the gondola back and forth through the volume to measure the properties of the perturbed air.

The advantage of the SCoPEx propelled balloon is that it allows us to create a small controlled volume of stratospheric air and observe its evolution for (we hope) over 24 hours. Hence the acronym, Stratospheric Controlled Perturbation Experiment. If we used an aircraft instead of a balloon, we would not be able to use such a small perturbed volume nor would we be able to observe it for such long durations. 

SCoPEx builds on four decades of research on the environmental chemistry of the ozone layer in the Anderson/Keith/Keutsch groups. SCoPEx will use or adapt many of the high-performance sensors and flight-system engineering experience developed for this ozone research. Analyzing these experiments will improve our knowledge beyond what is currently available within computer models or is measurable with confidence under laboratory conditions.


This FAQ aims to answer some basic questions about the SCoPEx experiment. We will update this FAQ periodically. For a more in-depth overview, see our 2014 publication.

What is the experiment? We plan to use a high-altitude balloon to lift an instrument package approximately 20 km into the atmosphere. Once it is in place, a very small amount of material (100 g to 1 kg) will be released to create a perturbed air mass roughly one kilometer long and one hundred meters in diameter. We will then use the same balloon to measure resulting changes in the perturbed air mass including changes in aerosol density, atmospheric chemistry, and light scattering.

What material will be released?Initially, we plan to release ice (frozen water) to make sure the instrumentation works properly. Later, we plan to release calcium carbonate, a common mineral dust. We may also release other materials such as sulfates in response to evolving scientific interests.

Do other environmental science experiments release materials outdoors?Yes. A number of environmental science experiments release or have released materials outdoors to create controlled perturbation for the same essential reason as we plan to do in SCoPEx—to directly control an experimental variable, which is crucial to scientific understanding. Examples of experiments include Free-Air Carbon Dioxide Enrichment (FACE) experiments, which release ozone and carbon dioxide (CO2) in the air for long durations to understand the impacts of climate and air pollution on crops and natural ecosystems; or Dispersion of Air Pollution and its Penetration into the Local Environment (DAPPLE) experiments, which have released sulfur hexafluoride (SF6) and perfluoromethylcyclohexane into urban air to study the transport of air pollutants. These experiments differ in various ways, e.g., they have not released material into the stratosphere (the upper atmosphere); they are listed here to merely show that there are environmental science experiments that release materials outdoors.

Is this material dangerous?The test will pose no significant hazard to people or the environment. Calcium carbonate is a nontoxic chemical commonly found in nature, for example as limestone, and sub-micron precipitated calcium carbonate particles like the ones we will uses are a common additive to consumer products such as paper and toothpaste. In general, the amount of materials to be released (less than 1 kilogram for calcium carbonate) will be very small compared to other routine releases of material into the stratosphere by aircraft, rockets, or routine balloon flights. For example, the release of experimental materials will be small compared to the release of the iron filling ballast that are commonly released to control the altitude of stratospheric balloons. Additionally, if we test sulfate in this experiment, the amount we would use would be less than the amount released during a one minute of flight of a typical commercial aircraft. Aircraft release sulfates due to residual sulfur content of aviation fuel. 

Are there other risks?As with any aircraft flight, when flying a balloon there is a possibility of malfunction and risk of falling debris.

What is the status for locating and timing of the experiment?We are exploring possible locations in the United States. We have not scheduled dates for engineering or science flights and we are engaged in a governance process including the appointment of an advisory committee to help us determine when and if it would be appropriate to conduct the experiment. The schedule and flight location also depends on a process that involves engineering development and balloon availability. We will update this page when we determine a definite schedule and location, which will be at least three months before flight. You can request notification by signing up to this email list.

We had originally planned for flight operations to be managed by World View, a high-altitude flight services company based in Tucson, Arizona, but for engineering reasons we will likely use another flight operator.

Why conduct the experiment?This experiment will help us learn more about the efficacy and risks of solar geoengineering. Computer modeling and laboratory work tell us some very useful things about solar geoengineering, but as with all other aspects of environmental science, computer models ultimately rest on observations of the real environment. Measuring the ways that aerosols alter stratospheric chemistry can, for example, improve the ability of global models to predict how large-scale geoengineering could possibly disrupt stratospheric ozone. 

Will SCoPEx test geoengineering itself?
This is an experiment not a test. A test could make sense late in the development of an engineering system when design and development have proceeded far enough that it could be useful to test whether some part of the system works as designed. That's not our goal. This is a science experiment that will (we hope) improve knowledge of some aspects of stratospheric aerosol physics and chemistry relevant to solar geoengineering. This knowledge will improve large-scale models (which are all ultimately dependent on physical observations) that will in turn improve estimates of the overall efficacy and risks of solar geoengineering. This may seem like an idle distinction, but it matters. We are not, for example, testing whether it's possible to scatter sunlight back to space, because there is no meaningful scientific uncertainty about that question.

Will SCoPEx develop hardware for geoengineering deployment?
We are not conducting SCoPEx to develop hardware that can be used for deployment. In fact, this is one of the reasons why we chose to loft the particles using a balloon rather than an aircraft (since aircraft are more likely to be used for deployment). Overall, the purpose of SCoPEx is NOT to advance our understanding of the aircraft or other platforms for deployment of solar geoengineering. It aims to reduce the uncertainty around specific science questions by making quantitative measurements of some of the aerosol microphysics and atmospheric chemistry required for estimating the risks and benefits of solar geoengineering in large atmospheric models.

Who is providing the funding?
Experimental hardware and operations will be funded from internal Harvard research funds provided to Professors David Keith and Frank Keutsch. Additional research funding will be provided by Harvard’s Solar Geoengineering Research Program (SGRP). All donations to SGRP are philanthropic.

How will intellectual property be managed?
We strongly discourage the commercialization of solar geoengineering. David Keith and John Dykema recently authored a blog post on this topic, explaining why they oppose commercial work on solar geoengineering and will not file solar geoengineering patents. Frank Keutsch, David Keith, and John Dykema will not file for patents associated with SCoPEx.

While it technically may be true that Harvard owns intellectual property arising from research conducted using university resources, based on Harvard’s IP Policy and the individual Participation Agreements faculty and researchers sign, as a practical matter the university will not file to protect or enforce intellectual property against the wishes of the contributing faculty member. Moreover, neither SGRP or its donors can make any claim on the intellectual property related to the experiment.

Does SCoPEx violate the Convention on Biological Diversity?
SCoPEx does not violate the Convention on Biological Diversity (CBD). 

The Conference of the Parties to the CBD adopted a decision that includes a section on climate related geoengineering. It states, "that no climate-related geo-engineering activities that may affect biodiversity take place, until there is an adequate scientific basis on which to justify such activities and appropriate consideration of the associated risks for the environment and biodiversity and associated social, economic and cultural impacts, with the exception of small scale scientific research studies that would be conducted in a controlled setting in accordance with Article 3 of the Convention, and only if they are justified by the need to gather specific scientific data and are subject to a thorough prior assessment of the potential impacts on the environment."

SCoPEx would not affect biodiversity because it would pose no significant hazard to people or the environment, as noted above.

How will the experiment be governed? 
Initial oversight of environmental, health, and safety issues will be managed by responsible entities from Harvard University and the balloon flight operator. Scientific peer review and broader research governance matters will be overseen by an independent advisory committee. You can learn more here.

Key Personnel

Developing the balloon system, instrumenting it with sensors engineered to work in the stratosphere, and making and analyzing the observations requires collaboration of aerospace engineers, instrumentation specialists, and atmospheric chemists and modelers. There will, thus, be a growing group of researchers working on the project. Current key personnel include:

Frank KeutschStonington Professor of Engineering and Atmospheric Science

Professor of Chemistry and Chemical Biology
Harvard University
Principal Investigator of SCoPEx

David KeithGordon McKay Professor of Applied Physics, Harvard John A. Paulson School of Engineering and Applied Sciences 

Professor of Public Policy, Harvard Kennedy School

John DykemaProject Scientist, Harvard John A. Paulson School of Engineering and Applied Sciences

Program Manager for SCoPEx

Lizzie BurnsFellow, Harvard John A. Paulson School of Engineering and Applied Sciences

Governance Manager for SCoPEx

Learn More

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