Using Climate Science on Earth to Understand Planets Beyond Earth

Anthony Del Genio started out his career expecting to become first an engineer and then a geophysicist. He was in graduate school at UCLA and had been prepared by previous mentors to enter the geophysics field. But a 1973 department-wide test focused on seismology, rather than fields that he understood better, and his days as a geophysicist were suddenly over. Fortunately, one of his professors saw that he had done very well in the planetary atmospheres and geophysical fluid dynamics sections of the exams, and suggested a change in focus.

That turned out to be a good thing for Del Genio, for the field of climate modeling, and for NASA. Because for the next four decades-plus, Del Genio has been an important figure in the field of climate science — first modeling cloud behavior and climate dynamics on Earth with ever more sophisticated atmospheric general circulation models (GCMs), and then beginning to do the same on planets beyond Earth.

His entry into the world of Venus, Saturn, Titan and distant exoplanets beyond is how I met Tony in 2015. At the same time that Many Worlds began as a column, Del Genio was named one of the founding leaders of the Nexus for Exoplanet System Science (NExSS) — the pioneering, interdisciplinary NASA initiative to bring together scientists working in the field of planetary habitability. (NExSS also supports this column.)

Del Genio is a hard-driving scientist, but also has a self-deprecating and big-picture, poetic side. This came across at our first diner breakfast together on Manhattan’s Upper West Side (where GISS is located), and was highlighted in a piece that Del Genio just wrote for a new series initiated by the American Geophysical Union (AGU), Perspectives of Earth and Space Scientists. In that series, scientists are asked to look back on their careers and write about their science and journeys. Del Genio’s perspective is the first in this series, and I will reprint most of its bottom half because I found it so informative and interesting.

But first, a quote from Del Genio’s piece that sets the stage: “The beauty of science, if we are patient, is that nature reveals its secrets little by little, slowly enough to keep us pressing forward for more but fast enough for us not to despair.”

The Perspectives excerpt begins when Del Genio is an established expert on the climate modelling of our planet, with a strong desire to use that knowledge in the burgeoning field of exoplanets:

“At the 1984 NASA Division for Planetary Sciences meeting in Hawaii, I was introduced to Clyde Tombaugh, the discoverer of Pluto. This was one of the great thrills of my professional life—a once‐in‐a‐lifetime chance to meet someone who had discovered a planet. Of course, I was wrong on two fronts: Pluto is no longer a planet (which I hope is a temporary state of affairs—to a climate scientist, any relatively spherical object that can retain a nonnegligible atmosphere qualifies as a planet, even Titan), and little did I know then that I would later meet many people who had discovered planets.

I had long been fascinated by the idea of life elsewhere in the universe—initially after being introduced to the Drake equation (a probabilistic equation for the number of technologically developed civilizations in the universe) by Shklovskii and Sagan’s Intelligent Life in the Universe (Shklovskii & Sagan, 1966) and later by Stephen Dole’s Habitable Planets for Man (Dole, 1964). I regarded these only as amusing thought experiments, though, so I took little notice when Wolszczan and Frail (1992) announced the first confirmed detection of planets orbiting a pulsar—not candidates for life by any means.

Exoplanet detections accelerated through the 1990s and 2000s, though, and with the advent of the Kepler mission and observations by ground‐based telescopes, we entered the age of actual, rather than imagined, rocky exoplanets with solid surfaces orbiting main sequence stars. To my great surprise, the question of whether we might someday discover life elsewhere had become real.

My GISS colleague Nancy Kiang and NASA Goddard colleague Shawn Domagal‐Goldman were already thinking about this as members of the Virtual Planetary Laboratory team (of the University of Washington) that had been considering spectral signatures of life on other planets (“biosignatures”) for some time. They sensed that the time might be right for 3‐D climate models to be applied to the emerging problem of exoplanet habitability.

They secured a bit of internal funding from Goddard to put together a small team, including me, to begin to generalize the GISSGCM to simulate planets other than Earth. (Columbia Astronomy colleagues Caleb Scharf and Kristen Menou and several of us at GISS had been unsuccessful at securing funding for this a decade earlier despite good reviews of our proposals.) In 2013 our group submitted a major proposal to NASA that included planetary scientists, astrophysicists, paleoclimate scientists, and several hybrid Earth climate‐planetary scientists (e.g., me) to address questions about the characteristics that might make a planet conducive to life.

As luck would have it, NASA Astrobiology Program Manager Mary Voytek had been thinking for several years about new ways to break down the “stovepipes” that separated research in NASA’s Astrophysics, Planetary Science, Heliophysics, and Earth Science Divisions. Our proposal was selected, and Mary made us a founding member of a “research coordination network” (a concept borrowed from the National Science Foundation) that was given the name the Nexus for Exoplanet System Science (NExSS) https://nexss.info).

Totaling 18 teams in its first iteration, and now up to 34, NExSS’ mandate is to bring together researchers in the four NASA science divisions to accelerate progress in the search for life elsewhere. In addition, I was asked, along with astrophysicists Natalie Batalha, the Kepler project scientist, and Dawn Gelino, now Deputy Director of the NASA Exoplanet Science Institute, to serve as co‐leads of NExSS, along with Shawn and NASA management postdoctoral fellow Andrew Rushby (named by Mary the “Jedi Council”). (Vikki Meadows, PI of the Virtual Planetary Laboratory, has recently joined the Jedi.) My double life as an Earth scientist and a planetary scientist had suddenly become marketable.

What can an Earth scientist tell an astrophysicist that would be useful?

Exoplanet astronomers are continually searching for an “Earth twin”—a planet similar to ours that would be a good candidate to host life. The real question though is how different a planet can be from Earth and still maintain liquid water on its surface, where it, and the life that it might support, could be detected from light years away.

Put another way: What determines the surface temperature of a planet whose atmosphere contains different amounts of greenhouse gases, receives a different amount of sunlight, and so forth, than present‐day Earth does? This is actually the same question of forcings and feedbacks that I have studied for decades to understand 21st Century anthropogenic climate change but taken to extremes. Not surprisingly, then, what are some of the biggest uncertainties in assessing exoplanet habitability? Cloud (and water vapor and lapse rate and sea ice) feedbacks!

Led by my GISS colleague Mike Way and with contributions from many others in the GISS Earth GCM group (Way et al., 2017), a generalized planetary version of the GISSGCM has been developed.We have used it to explore the possibility that ancient Venus under the faint young Sun may have been habitable (Way et al., 2016); to understand the processes that put excessive water vapor into the stratosphere as incident stellar flux increases, a precursor to the eventual loss of a planet’s oceans (Fujii et al., 2017); to determine how the thermal inertia and heat transport of a dynamic ocean might render a planet continuously habitable in the face of oscillations in planet eccentricity (Way & Georgakarakos, 2017); to examine scenarios for a possible habitable climate on the known exoplanet closest to Earth (Del Genio et al., 2019); to understand the transport of volatiles to permanently shadowed polar regions early in the moon’s history (Aleinov et al., 2019); to predict the planetary albedos and surface temperatures of exoplanets from sparse available information using Earth climate concepts (Del Genio, Kiang, et al., 2019); and to understand how high obliquity allows weakly illuminated planets to remain habitable (Colose et al., 2019).

On Earth we are now considered to be in a new epoch, the Anthropocene, in which humankind has become a leading order influence on the planet—in effect, turning Earth into a slightly different planet. In the new era of exoplanet science, formerly uncertain terms in the Drake equation such as the fraction of stars with planets are now observationally constrained—for example, most stars have planets! One of the biggest remaining uncertainties in the equation is the average lifetime of a technological civilization before it destroys itself or consumes all its energy sources.

This is what thinking about other planets in addition to the Earth does. It takes one from wondering what the impacts of anthropogenic greenhouse gas increases will do to sea level, to extreme temperatures, to hurricane intensities, to regional drought in our lifetimes, and ups the ante to the larger question of whether in the long run our civilization will eventually figure things out and learn to sustain itself, or perish.

As I near the end of my career, this opportunity to reflect upon it has made me more aware of lessons I have learned (mostly unintentionally) along the way:

1.) Serendipity can have a great deal to do with the progression of a career.
Many of us may have agonized about the direction we should follow in our careers when we were in school—I certainly did. My career has been anything but a straight line determined by my initial choices. Rather, it has been defined by a combination of failures, being in the right place at the right time, and openness to go in new directions. I have experienced one of the most remarkable periods in the history of science. I entered science about a decade after launch of the first Earth‐orbiting weather satellites and the first successful spacecraft missions to other planets, and I have witnessed visits to every planet in the solar system. I have been in science during the period of humanity’s awakening about anthropogenic climate change (unfortunate for humanity but a tremendous stimulus for more deeply understanding our own planet). Finally, I have seen the universe unveiled as the home of thousands (at least) of known planets orbiting other stars, and I was able to be a contributor to one of the earliest groups thinking about how to determine which of these might be good candidates to harbor life. My career has clearly been shaped by these external events.

2,) Science is usually a team sport.
The media tend to portray science using the paradigm of the heroic lone scientist, usually out in the field, gathering data, and experiencing that “eureka!” moment that immediately overturns an existing science paradigm. Perhaps that is sometimes true, but it has not been my own experience. Almost all my published papers were joint efforts with colleagues whose technical expertise and scientific insight complement my own. I hope that this essay is a suitable way to express my gratitude for how I have benefited from their talents. Some of my papers arose from data collected (by others) during field experiments, but most were modeling, theory, or remote sensing data analyses. And in fields as complex as the climates of Earth and other planets, paradigm overturning is usually a slow motion process—several of my more successful papers have been more highly cited in recent years than in the years that followed their publication.

3.) Cross‐discipline research has made me a better scientist.
I am often asked, “How does studying other planets help you understand Earth?” Although there are a few examples (Kahn, 1989), in general, the best way to understand Earth is to study Earth. The real value of studying both Earth and other planets is the perspective it has provided me on both. A foundation in Earth science helps one interpret observations of other planets, since much of the well‐explored physics of our own atmosphere can be applied to other planets. There are baroclinic eddies on Mars and Saturn, lightning storms due to water condensation on Jupiter and Saturn (and methane convective storms on Titan), and so on. But the relatively poorly observed planets of our Solar System and barely observed rocky exoplanets force us to ask basic, global questions and put our own planet in a larger context. In Earth science, we got caught up in the details so much a couple of decades ago that we largely stopped asking basic questions. In recent years, though, climate change has taught us that we do not understand Earth as well as we may have thought, and some scientists have begun once again to ask basic questions of our planet. These papers and others like them have effectively taken a planetary perspective on our own planet, to the betterment of our field. Exoplanet science has taken things a step further by placing the “small” number of planets in our solar system into the context of thousands of other planets. Given that large a sample, seemingly simple questions such as what determines whether a planet even has an atmosphere turn out to be much more fascinating than anticipated (e.g., Zahnle & Catling, 2017). Conversely, the history of habitability in our own solar system provides insights into processes that may be in play on exoplanets that we as yet know little about (Del Genio et al., 2019). This cross‐discipline fertilization is a trend I hope will continue.

I do not like the idea of starting a book and not getting to read the final chapter. At this stage in my life, though, I have to accept that the questions of the ultimate fate of our society, and the discovery of life elsewhere in the universe (a matter of when, not if, I am certain), may or may not be answered while I am still around to experience them. But to have the chance to live a life in scientific research during a time that saw the beginning of human awareness about both the effect we have on our own planet and the likelihood of alien biospheres, along with the creation of tools to begin to understand them and great colleagues with whom to share the journey, is consolation enough. Still … wouldn’t it be great to get to read the final chapter?”