The opening day of the 229th AAS meeting featured a special panel session, entitled "Geoengineering the Atmosphere to Fight Climate Change: Should Astronomers Worry About It?" The discussion featured presentations by four panelists, as well as questions both from committee members and the audience.

Geoengineering is a large family of potentially useful tools against climate change that might enable humankind to lessen the effects of global warming by intentionally changing the Earth's climate, either locally or globally. Panelist Thomas Ackerman of the University of Washington gave an overview of possible ways to accomplish this.

One way to do it that is often discussed in the popular press is to reduce CO2 via carbon sequestration. This is difficult, however, requiring (for example) immense amounts of additional forests, comparable in area to the continental US. Another possibility is to reduce incoming sunlight, for example, by inserting aerosol particles into the atmosphere.

Importantly, while geoengineering could mitigate the rise in global temperatures, it would not stop it, as no combination of geoengineering methods can address the main driver of climate change: the release of greenhouse gases. Geoengineering also does not address ocean acidification or glacial melt.

One of the main reasons why geoengineering proposals are being considered is because the climate change models considered by the International Panel on Climate Change all require not only bending the arc on greenhouse gas emissions, but also reducing both emissions and the overall atmospheric level of CO2 to stabilize the climate and reverse global warming. Panelist Jane Long of the Breakthrough Institute pointed out that merely reducing atmospheric CO2, for example, would cause global temperatures to stabilize, but at a higher level, thus making the effects of climate change permanent.

Astronomers are concerned about geoengineering, as well as climate change, for a number of reasons. Observatories depend on the climate of their sites, requiring dry, calm and clear weather conditions. Increasing atmospheric aerosols would worsen the cloud cover and transparency at observatories, for example, and if the overall humidity were increased there could also be more storms. However, climate change itself can produce worse effects, changing the climate drastically on a global scale.

David Grinspoon, a panelist and senior scientist at the Planetary Science Institute, pointed out that astronomers should be interested in geoengineering for a number of reasons. First of all, understanding what mechanisms can affect Earth's climate will help us to understand the drivers of climate on other planets.

In addition, we are now at the dawn of a new geologic era: the Anthropocene - an era marked for the first time by intelligent modification of our entire biosphere. This new era, which is the focus of Grinspoon's recent book "Earth in Human Hands: Shaping Our Planet's Future," may be dangerous. These modifications might stabilize Earth's system, but they might destabilize it instead.

Importantly, as Ackerman said, if we want to try to change the climate we have to choose the climate we want - we cannot maintain both global temperature and precipitation. This was echoed by Long, who emphasized that "there is no natural state [for Earth] - it's all dynamic." An equally interesting question, stressed by both Ackerman and Melville Ulmer, a professor of physics and astronomy at Northwestern University, is who gets to decide which climate might be preserved - in other words, what global temperature, what level of increase in cloud cover, and the like.

The discussion included not only a focus on geoengineering itself but also how we need to interact with decision-makers about the subject. Long emphasized that it's important to understand transparency - not just to publish data, but also to engage policy-makers.

At the same time, while there may be a need for geoengineering, now is probably not yet the time, since we don't yet know enough about its effects, stressed Ulmer. This was echoed by Long, who agreed that, on the short term, it's folly to attempt long-scale radiation-modification efforts because we are ignorant of the long-term consequences. Ackerman, however, opined, "I think there aren't as many unintended consequences as you might think."

A constant theme that ran through the session was that while there may be significant adverse effects on astronomy from geoengineering, astronomers have to realize that the global climate is more important. Besides, as Grinspoon mentioned, geoengineering Earth's climate is not fundamentally different from terraforming Mars, Venus, or other worlds. So, by learning how to potentially mitigate the effects of climate change on Earth we are learning about climatology on other worlds as well as how to modify other planets for human habitability.