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FormalPara Kassandra

I am here with Professor Laura Kay, Professor of Physics and Astronomy at Barnard College and author of 21st Century Astronomy.Footnote 1 Professor Kay, you teach an introductory astronomy course called Life in the Universe. Can you give me a brief description of this course?

FormalPara Laura

This course explores how scientists think about life in the universe. Currently, we do not have data on life anywhere other than on Earth, so this class is more about how scientists approach the question of “life in the universe” than about what is actually out there. Right now, we just do not know. The class is interdisciplinary: it uses astronomy, physics, Earth science, chemistry, and biology to look at both how scientists are working together on this problem and how they are thinking about this question. But there’s no clear answer, and I tell that to my students on the first day. We do not know if there’s any life out there. Life in the Universe is a class that’s about the process of how scientists approach this question.

FormalPara Kassandra

I understand that you also talk to your students about climate change within this course. How do you fit discussions about climate change into a course that, while interdisciplinary, does fall within the Astronomy department?

FormalPara Laura

When students come into my classroom, they have already heard a lot about climate change. Some students have studied it in high school, and all of them have heard about it on the news or within a political context. I started including it in this course because climate change is an essential part of the story of why human life evolved on Earth instead of elsewhere in our Solar System. But, because climate change is such a politicized and familiar topic to my students, I aim to approach it from a different angle. Instead of focusing solely on the current state of climate change on Earth, I emphasize to my students that climate change is not just about planet Earth and not just about our current time. Earth has evolved and changed as a planet, as have the other planets in our Solar System.

FormalPara Kassandra

How do you expand your student’s understanding of climate change beyond our current Earth context?

FormalPara Laura

I discuss with my students the history of the discovery of contemporary climate change on Earth. This includes looking back to the beginnings of the industrial revolution when people first discovered that carbon dioxide emissions could change the atmosphere. Some of the earliest work was done in the mid-1800s by Eunice Foote and John Tydon, who thought about how atmospheres can block radiation. In the 1890s, the Swedish physical chemist Svante Arrhenius calculated how carbon dioxide emissions could lead to global warming. He won the Nobel Prize for his work in 1903. The point is, the idea that carbon dioxide and other greenhouse gases affect our atmosphere is not a new concept.

Beyond Earth, astronomers have studied climate on other planets, specifically Mars and Venus. Studies of these planets have greatly informed our understanding of climate on Earth, making climate change discussions incredibly relevant to an astronomy course curriculum.

FormalPara Kassandra

How could studies of Mars and Venus, planets which differ so dramatically from Earth, provide insight into climate change on our planet?

FormalPara Laura

Mars and Venus are the two planets closest in distance to the Earth, and they give us examples of large-scale global cooling and global warming, respectively. Studies of Mars allowed astronomers to witness global cooling in real-time. Mars has intense dust storms that occur periodically, which astronomers have observed through telescopes for 200 years. When NASA missions began sending space probes to Mars in the 1960s, scientists could, for the first time, measure Mars’s temperature during these dust storms. They found that, as dust quickly covered Mars, the planet’s temperature decreased.

Mars’s global cooling scenario led to two strong scientific theories about the climate on Earth. The first of these theories concerns the death of the dinosaurs. Scientists now think their extinctions were caused by the impact of a huge asteroid, or possibly a volcanic eruption (or a combination of the two), either of which would have sent enormous amounts of dust into Earth’s atmosphere. Similar to the dust storms on Mars, Earth’s temperature would have decreased as dust blocked the sunlight. This sudden decrease in temperature would have killed off the food chain; plants would have died, small animals would have died, and then the dinosaurs, which required large amounts of food to support their massive bodies, would have eventually starved. The second theory to result from the studies of Mars, nuclear winter, also became popular in the 1980s. The theory of nuclear winter proposed that if a nuclear war were to occur, the excess of fire and smoke would block the sunlight and lower the temperature on Earth.

On the other hand, studies of Venus provided insight into global warming. The Soviet Union, which focused its space exploration on Venus, began sending space probes to Venus in the 1960s. Prior to these missions, many people had considered Venus a possible ‘sister planet’ to Earth. Venus is both the closest planet to Earth and the most similar to Earth in size, and science fiction of the time was filled with fantasies about humans or other life there. These fantasies ended as Soviet missions reported that temperatures on Venus were around nine hundred degrees Fahrenheit. (The missions ended quickly because the probes burned up.) At first, Venus’ high temperatures puzzled astronomers. Venus is closer to the Sun than Earth, but not close enough to explain the vast difference between Earth’s liveable temperatures and the incredible heat of Venus. Researchers in the early 1970s came up with a possible explanation: the Runaway Greenhouse Effect. The Runaway Greenhouse Effect proposed that the abundant carbon dioxide in Venus’s atmosphere trapped radiation from the Sun, causing the temperature of the planet to increase. Any water that may have been on Venus’s surface eventually evaporated. Then, the water molecules would have been split by ultraviolet radiation from the sun, with the hydrogen escaping to space.

In the 1970s, astronomers at NASA started building computer models to understand what happened on Venus, and they then applied the basics of these models to Earth’s climate. These `general circulation models’ or GCMs are used to reproduce or explain climate trends we’ve seen in the past, what is seen now, and possibly predict what will happen in the future. When similar models were applied to Earth, with increased carbon dioxide levels beginning during industrialization in the 1800s, they found that these models could explain the changing temperatures on Earth. Over the years, models of Earth’s global warming have become increasingly sophisticated, but the earlier models originated through the study of Venus.

FormalPara Kassandra

If carbon dioxide in Venus’s atmosphere caused such a dramatic increase in temperatures, why didn’t Earth also experience this runaway greenhouse effect?

FormalPara Laura

In the past, Mars, Venus, and Earth had liquid water on their surfaces, and they all started out with a primarily carbon dioxide atmosphere. But all three planets evolved. Venus is closer to the Sun than Earth, so it was always warmer. On Earth, life dramatically impacted the evolution of Earth’s atmosphere and climate. Photosynthesis, first by bacteria and later by plants, fundamentally altered Earth’s atmosphere by producing oxygen which, over time, gave us the oxygen atmosphere that we have now. The existence of an oxygen atmosphere then allowed more complex life to form. Life changed Earth’s atmosphere, and Earth’s atmosphere changed life. Mars and Venus have 95% carbon dioxide atmospheres, whereas Earth’s atmosphere is 78% nitrogen, 21% oxygen, and only 0.04% carbon dioxide. In addition, the atmosphere of Venus is about 100 times thicker than Earth’s (Mars’ atmosphere is 100 times thinner), so the greenhouse effect is much more substantial on Venus.

FormalPara Kassandra

How did early climate change on Earth differ from the current climate changes that we see?

FormalPara Laura

When we look at historical records of carbon dioxide levels, which we can obtain by studying tree rings and ice cores (See Baxi et al., this volume, chapter 25), we see that carbon dioxide levels have varied over time. By studying ice cores in Antarctica, scientists can measure the amount of carbon dioxide going back 800,000 years. By looking at different chemicals and isotopes, they can see that temperature and carbon dioxide levels have fluctuated together. When the temperature was higher, the carbon dioxide was higher, and when the temperature was lower, the carbon dioxide was lower. Multiple factors contribute to these changing levels, including Milankovitch Cycles, which are the cyclical changes in Earth’s axial tilt and orbit. These changes in tilt, orbital ellipticity, and precession occur over tens of thousands of years and appear to align fairly well with Earth Ice Ages and the periods in between the Ice Ages.

The ice core measurements, which Al Gore’s film, An Inconvenient Truth (Paramount Picture Corporation 2006), made famous, shocked many people in the 1990s. They saw that the cycles of changing temperature and carbon dioxide levels go together naturally, but over tens of thousands of years, resulting in slow environmental changes. The changes we are seeing on Earth now are over the last 150–200 years since Industrialization. Scientists are nervous about our current rate of climate change because Earth does not have time to respond to these faster changes naturally.

FormalPara Kassandra

Do other planets offer a possible refuge from climate change and environmental destruction on Earth?

FormalPara Laura

Well, I say no. In Life in the Universe, I emphasize that humans have evolved on Earth for Earth’s specific environment, and Earth has evolved with us. Humans cannot pack up and go to Venus; for example, it is nine hundred degrees Fahrenheit there. While we may no longer have fantasies about going to Venus, there are still plans to build bases on the Moon, which has no atmosphere, has light gravity, and requires humans to bring all supplies with them. And, there are still fantasies about humans escaping Earth and colonizing Mars. On Mars, the Sun would be very dim, and it is quite cold. The atmosphere is very thin and is not breathable, so you would have to wear spacesuits the whole time or live in a pressurized dome. It is unknown if food could be grown under Mars’s weak sunlight, which means we do not know whether Mars could sustain humans. The gravity on Mars is somewhat stronger than on the Moon, yet still only forty percent the strength of Earth’s gravity, meaning that living on Mars would cause human muscles to weaken dramatically. Even if Mars was somehow able to provide a sustainable environment for humans, it’s doubtful that humans would be able to readapt to the higher gravity back on Earth. I do think some people will make short visits to Mars, and maybe small numbers will live there, but I doubt that millions (or billions) of people will relocate from Earth. And, of course, there would be serious questions about who gets to escape Earth to live on the Moon or Mars.

Outside of our Solar System, since 1995, astronomers have found thousands of planets orbiting other stars in our Galaxy. However, we have not collected enough data yet to know if any have the kind of oxygen atmosphere free from poisonous gases that human life requires. Many of these planets are likely balls of gas like Jupiter or Saturn, but some are solid like Earth. Most of these planets are not at a distance from their star that suggests they could have temperatures that permit liquid water on their surface, and Earth life needs water. Upcoming space telescopes may identify distant planets with temperatures and atmospheres that are more Earth-like, but at present, we have no technology that will let us travel to them.

As an aside, I will add that culturally, science fiction has influenced many people’s perceptions of other planets. Many blockbuster science fiction movies, television shows, and books show people going from planet to planet, where each planet has Earth-like gravity and temperatures and a breathable atmosphere (and everyone speaks English). These media make it seem as though there are a plethora of planets for humans to choose from; so that if Earth ‘goes bad,’ we can just go somewhere else. As far as astronomers can tell, realistically, that’s not what we actually see out there, at least not yet.

FormalPara Kassandra

What do you hope to accomplish by expanding your students’ understandings of climate change beyond Earth and beyond our current time period?

FormalPara Laura

Well, professors always aim to get students to think beyond the present moment and think about history. So I do want students to understand that Earth’s climate has indeed changed throughout history. We have all seen the ‘Ice Age’ movies, but the contemporary climate changes are also different in many important ways. Additionally, I hope to get students to think about scientific methodology rather than political frameworks. For example, we talk in class about how we can measure the temperatures of planets in our solar system and make estimates of the temperatures of planets in systems across the Galaxy. Some climate change doubters argue that it is difficult to measure temperatures on Earth, but if we can measure temperatures on Mars, of course, we can measure the temperature of Earth.

I also hope to make my students think about climate change in a universal context rather than getting bogged down in bickering about the details of whether it is real or what to do about it (see Pfirman & Winckler, this volume, chapter 19). Instead, I want them to think about how there are many planets out in space, but it is not known if any of them are suitable for us, given how life has evolved on Earth. On the one hand, studying other planets lets us see the processes that lead to changes in atmospheres, so we can help understand how they happen on Earth. On the other hand, studies of other planets show that there is not a place to relocate to. We need to care about what is happening on Earth.

FormalPara Kassandra

Do you see teaching climate change as a justice issue?

FormalPara Laura

It is certainly an environmental justice issue to keep Earth habitable for humans! There is no way we can relocate everyone to another planet in the foreseeable future. You may have heard the activist slogan, `There is no Planet B.’ I don’t spend much time in class debating the merits of taxing carbon emissions or carbon capture like an economist might. Instead, I focus on my area: how astronomers think about planets, which extends to how astronomers think about a planet like Earth. General education science classes are possibly the only science class that a college student will take, and Life in the Universe may be a student’s only opportunity to hear about climate change from a scientific perspective. Climate change discussions should be included in any general science class when possible, and professors of the sciences at Barnard try to put in the most relevant angle to their department. Climate change is a multidisciplinary problem. It needs an interdisciplinary approach, and the more people thinking about the problem, the better.