The Faint Young Sun Paradox

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Hopefully, I’ve convinced you that ongoing changes in the Sun’s output are relatively small (at least during human history). But before we move on, I want to point out something strange: scientists have discovered that when Earth first formed about 4.5 billion years ago, the Sun was much fainter than it is today. In fact, astrophysical models tell us that the Sun’s energy output was about 30% lower back then!

Check out the graph below. It shows the time-series of luminosity (i.e., emitted energy), radius, and temperature of the Sun over the past 4.6 billion years of its life—and it projects another 7 or so billion years into the future. The values on the y-axis are all normalized to present-day values, meaning a radius of 1.2 would mean the Sun is 20% larger than it is today. I want to emphasize that the x-axis is in billions of years, so all of human history is constrained to that small point where all three lines cross 1.0. (None of us will be around—barring some serious medical breakthroughs—to experience much beyond that!) 

If you’ve taken an astronomy class, you might have learned that the Sun is continually growing larger (as this graph shows!). But for this class, we’re just focused on energy, so pay attention to the luminosity (the red curve). For the first two billion years of Earth’s life (note, Earth formed about 60 million years after the Sun’s birth), the energy the Sun output was less than 80% of what it is today. This means early Earth should have been much colder—so cold, in fact, that it should have been frozen solid. Yet, we know from the proxy records we’ve discussed that liquid water existed on the surface, and life was already starting to take hold. In fact, things were quite warm! 

This puzzle is known as the Faint Young Sun Paradox—how did Earth stay warm enough to support life when the Sun wasn’t as bright? Astronomers Carl Sagan (you have probably heard of him, or at least heard of "The Pale Blue Dot") and George Mullen first raised this question in 1972. So, what could have kept Earth warm enough to sustain liquid water and life? Scientists aren’t 100% sure, but the most widely accepted theory involves greenhouse gases. 

When Sagan and Mullen first introduced the Faint Young Sun Paradox, they suggested that high concentrations of ammonia gas (NH₃) could have been responsible for keeping Earth warm. We haven’t talked much about ammonia because—well—there isn’t much of it in the atmosphere today! But it is an effective greenhouse gas, meaning it can trap heat in the atmosphere, much like carbon dioxide (CO₂). However, there’s a catch: ammonia is easily destroyed by sunlight. Once exposed to ultraviolet radiation, it breaks down into nitrogen (N₂) and hydrogen (H₂) gases, which don’t trap heat as effectively. Although Sagan later suggested that a photochemical haze might have shielded ammonia from destruction, research eventually showed that this idea wasn’t plausible—the haze itself would have cooled Earth’s surface, counteracting any warming from ammonia. I mention this to highlight that even the most brilliant scientists aren’t always right! 

Greenhouse gases to the rescue?

So, if ammonia couldn’t do the job, what could? Most scientists now agree the answer is carbon dioxide. Remember when we first introduced the greenhouse effect? We explained that carbon dioxide is a potent greenhouse gas, and we’ll dive deeper into that next lesson. For now, the key takeaway is simple: more greenhouse gases = a warmer planet! CO₂ levels in Earth’s early atmosphere were likely much higher than they are today. 

Scientists have used models to estimate how much CO₂ would have been needed to keep Earth warm enough for liquid water during the Faint Young Sun period. These models suggest CO₂ concentrations could have been up to 1,000 times higher than present-day levels. Trust me, that’s a lot of carbon dioxide! This “supercharged” greenhouse effect would have compensated for the Sun’s dimmer output, allowing Earth to remain warm enough for water—and life—to exist. 

The figure below shows this tradeoff over time. In the early part of Earth’s history (on the left side of the graph), solar energy (brown curve) was much lower, but CO₂ concentrations (blue curve) were sky-high. As we move forward in time (left to right), the Sun’s brightness increased while CO₂ levels dropped—without this balance, Earth would have become too hot for life as we know it. 

But carbon dioxide wasn’t the only greenhouse gas at play. There’s also evidence that methane (CH₄), another potent greenhouse gas we’ll discuss soon, played a crucial role. Early Earth was home to a variety of microbes—single-celled organisms—that produced methane as a byproduct of their metabolism. These microbes were anaerobic, meaning they didn’t require oxygen to survive. In the oxygen-poor atmosphere of the time, they thrived, and their methane emissions could have significantly contributed to warming the planet. 

In the early 2010s, scientists analyzing ancient marine sediments found clues suggesting methane worked alongside CO₂ to keep early Earth warm. By pulling out ocean sediment cores, they discovered certain iron-rich minerals that coexisted during the early part of Earth’s history. This hinted at a balance between CO₂ and hydrogen (H₂) in the atmosphere, with methane playing a key role. Methane is far more effective at trapping heat than CO₂, so even small amounts could have had a big impact on global temperatures. 

Why is this important? 

The Faint Young Sun Paradox doesn’t just teach us about Earth’s distant past—it also highlights the incredible power of greenhouse gases. Even small changes in CO₂ and methane levels can dramatically impact the planet’s temperature. In other words, the lesson of the Faint Young Sun Paradox is clear: greenhouse gases matter—a lot! In fact, our planet has fundamentally relied on the balance between energy from the Sun and the Earth’s atmosphere to sustain life. If we could snap our fingers and put as much CO₂ into the atmosphere as there was during the early Earth’s history, we’d fry like an egg (well, eggs wouldn’t exist, I suppose…) 

 William Shakespeare once wrote in The Tempest, “What’s past is prologue.” He used the quote in the context of fate, suggesting that everything leading up to the present shapes what happens next. But we can also apply this to climate: history sets the stage for our present and future. Understanding the role of greenhouse gases in Earth’s climate system, both past and present, is key to anticipating the future of our planet.