Climate engineering is the deliberate modification of the climate to temporarily offset global warming, preventing some of the worst effects of climate change while society ramps up greenhouse gas mitigation and carbon dioxide removal efforts. What are the risks of doing climate engineering versus not doing it? How do we know? Are there certain climate objectives that climate engineering can meet, and are there objectives it can’t?
What is it?
Figure caption: Some of the effects of stratospheric sulfate aerosol geoengineering. From Kravitz and MacMartin (2020). https://www.nature.com/articles/s43017-019-0004-7
Geoengineering describes a set of technologies designed to temporarily, deliberately modify the climate, buying time for society to ramp up greenhouse gas mitigation and carbon dioxide removal efforts. Some of the most commonly discussed technologies include:Stratospheric sulfate aerosols. Large volcanic eruptions inject, among many things, sulfur into the atmosphere. This sulfur forms highly reflective microscopic droplets (aerosols) that reflect a portion of sunlight back to space, cooling the planet for 1-3 years. If stratospheric injections could be sustained, the planet could stay cooler.
Marine cloud brightening. The ocean is very dark and absorbs a lot of energy from the sun. Thick blankets of low clouds can be very bright, and when they are over the ocean, much of that solar energy that would have been absorbed by the ocean is instead reflected upward. Under the right conditions, if certain aerosols (like sea salt) are injected into those marine low clouds, the clouds can become brighter. If this could be done reliably over a large enough portion of the ocean, then the planet could be made cooler.
Cirrus thinning. Unlike low clouds, which overall cool the planet, cirrus (high clouds made of supercooled water and ice) trap heat that normally would have radiated out to space. The net effect is a warming of the planet. If cirrus clouds could be seeded with the right type of particles, the ice crystals in those clouds would become larger and fall out faster, making the cirrus thinner, and allowing more heat to escape to space.
Why work on it?
Figure caption: Schematic of some of the proposed ideas for geoengineering. From Irvine et al. (2016).
Greenhouse gas emissions mitigation is the only permanent solution to climate change. However, it has thus far proven to be slow and expensive. In the meantime, the effects of climate change are being felt throughout the world, and they will only get worse as more greenhouse gases are emitted. People are discussing geoengineering as a potential way of quickly, temporarily offsetting climate change, but there is much we don't know. It is important to understand what geoengineering can and cannot do, so that we know whether it can help the climate problem, but also so we don't rely on it to do things it can't. Also, we need to know the side effects.
Ultimately the decision about whether to use geoengineering will be made by our politicians. My goal is to provide as much information as possible so those decision makers can make the best informed decisions possible.
What do we do?
Figure caption: Climate model output from the Geoengineering Large Ensemble (GLENS), a community resource demonstrating geoengineering to meet multiple simultaneous objectives in a climate model using control theory.
We use climate models to understand geoengineering, with a focus on mission-driven research: what are the uncertainties with the greatest consequences, and can those uncertainties be reduced? Some of our research areas and activities include:
Multi-model intercomparisons: We compare the results of geoengineering simulations conducted in multiple models, often as part of the Geoengineering Model Intercomparison Project (GeoMIP), to understand where models agree and where they disagree on the projected effects of different kinds of geoengineering.
Feedback and design. One of the questions that comes up a lot is, "If we do geoengineering, what will happen?" That is a difficult question to answer because geoengineering will do many different things, depending on how it's done. This design perspective, combined with control theory to meet objectives in the presence of uncertainty, is critical for decision making: given a set of political or socioeconomic outcomes, can geoengineering meet them?
Building communities. We collaborate with the DECIMALS Fund, an effort designed to build geoengineering research capacity in developing countries so that international climate discussions and negotiations involving geoengineering can include local expertise focused on key issues for developing countries.
Many of these activities are done in collaboration with the Cornell Climate Engineering Research Program.