Like water on Earth, methane on Titan is a strong greenhouse gas that evaporates and condenses, amplifying climate change and causing strong feedbacks. In studying the climate balance of Titan, Pollack’s group of researchers discovered some interesting and seemingly paradoxical effects. Titan, by our standards, is really cold, at -290 degrees Fahrenheit. Without any methane greenhouse, it would be much colder still, by about 22 degrees. Yet, if we put all that methane into a basic climate model, we find that there should be about twice the level of greenhouse warming that is actually observed. What’s missing from the model?
This question led to the discovery of the “anti-greenhouse effect.”5 It has to do with all that orange organic haze suspended in Titan’s upper atmosphere. It turns out that the passage of radiation through this haze is having an effect exactly opposite from that of a greenhouse gas: it blocks visible light but allows infrared light to pass through. Such a haze will prevent sunlight from warming a planet yet will allow the planet to cool efficiently into space. The effect on Titan’s climate is to negate about half the value of the greenhouse warming caused by methane. The Titan anti-greenhouse effect turns out also to be a pretty good match for what happens to Earth’s climate in the immediate aftermath of a huge asteroid impact or giant volcanic eruption and what would happen to it in the aftermath of a nuclear war. So the term anti-greenhouse effect, first used to describe climate processes on Titan, is now commonly invoked in modern descriptions of nuclear winter and other aspects of climate change on Earth.
Now, when you hear some people advocating or warning against “geoengineering” Earth by spraying sun-blocking aerosols into the upper atmosphere, they are proposing to induce a process that is constantly at work on Titan. I’ll return to the physics, and the wisdom, of such an anti-greenhouse project in chapter 4.
Climate Catastrophes in the Solar System
The interplanetary approach to climate allows us to ask a basic question: why is Earth like this and the other planets so different? The deep-time perspective afforded by planetary exploration lets us pursue this question all the way back to the origin of the planets, and the divergent evolution of their atmospheres and climates.
Spacecraft studies have provided plenty of evidence that the extreme climates we see today on our neighboring planets are completely different from their earliest climates. Venus, Earth, and Mars have each experienced catastrophic climate change, and as is often the case with siblings, their personality differences have become accentuated over time. Now each seems almost a caricature of a different planetary type: one hot and dry, one cold and dry, one water-soaked and thoroughly infested with life.
Climate is complex, messy, and hard to predict, in large part because planetary climate systems are full of feedback mechanisms, which tend to dampen (negative feedback) or amplify (positive feedback) any initial change. Positive feedbacks will destabilize a system, meaning that small changes can grow and knock it into a completely unrecognizable state. Negative feedbacks do the opposite: resist change and add stability.* Examples of both negative and positive feedback can be found everywhere in our daily lives. Just this morning a friend with an infant child joked that he was too tired to remember how to make coffee. This is an example of a negative feedback that in theory (but, fortunately, not in practice) could keep him in a stable state (tired and uncaffeinated) all day long. Now, suppose he made one pot of coffee and started to wake up. If this made him restless, he might get up and make another pot. The more nervous he got, the more coffee he’d make and consume, which would only make him even more hyper. This is an example of positive feedback, which leads to instability and a rapidly changing state. Yet suppose he reached a point where his hands were so jittery that he could no longer operate his coffee machine. Then another negative feedback would kick in, which would stabilize him at the maximum level of caffeination at which he could barely make coffee. No doubt tomorrow morning he’ll again be tired.
Earth’s climate has fluctuated through the ages. Ice ages arrive every few hundred thousand years, and are themselves punctuated by multithousand-year periods of slight warming, called interglacial periods. Right now we’re in one of these interglacials, which started about twelve thousand years ago.
A few times our planet has entered much more prolonged and severe global freeze-overs during which it looked like the ice world of Hoth from Star Wars. In other intervals it has warmed to a globally ice-free, tropical state. Somehow Earth has always bounced back from these extremes. There are many processes that can doom a planet to fire or ice. As we learn about the history of Earth’s fellow travelers, and become more sophisticated about planetary evolution, what really stands out as unusual about Earth is how stable its climate has been. This is especially remarkable given that the Sun has slowly been getting hotter. The amount of energy entering Earth’s atmosphere from the Sun is 1.4 kilowatts per square meter. We call this the “solar constant,” but it is not. The Sun is steadily getting brighter, and has been since its birth 4.6 billion years ago. When our planet was born, our star was only about 70 percent of its current brightness. With so little solar heating, Earth should have frozen solid. Apparently it didn’t, and therein lies a paradox.
There’s something puzzling about early Earth. Climate models show that, all other things being equal, when warmed only by that wimpy young sun, the planet should have been completely iced over. Yet geological evidence (for example, the presence of ancient water-formed sediments) shows us that young Earth was not frozen. This contradiction was first pointed out by Australian geologist Ted Ringwood in 1961, and was discussed in 1972 by Carl Sagan and George Mullen, who named it the “faint young sun paradox” and, with Sagan’s flair for branding and popularizing a topic, brought it to the wide attention of the scientific community.
The obvious way out of the paradox is that “all other things” were not equal: Earth’s atmosphere has evolved, and the greenhouse effect was much greater in the past. The answer may be simply that early Earth more closely resembled its two triplet siblings, Venus and Mars, and had an atmosphere that was loaded with CO2, supporting a global greenhouse strong enough to compensate for the feeble early Sun, keeping Earth’s climate warm, wet, and habitable.
The earliest climate of Earth is still somewhat of a mystery, but it is clear that although the atmosphere has changed radically over time, the climate has generally remained stable, within the right temperature range for liquid water and “life as we know it” over billions of years. How has this happened? It turns out Earth has a natural thermostat, resulting from the strong negative feedback between two major parts of the Earth system: weathering reactions and volcanoes.
Weathering reactions are the way in which carbon atoms are pulled from atmospheric gas molecules to be locked into solid mineral crystals. They happen when rainfall flows over and trickles through silicate rocks on the continents. Atmospheric CO2 dissolved in this water makes carbonic acid, reacts with minerals in the rocks, and eventually ends up in the ocean, where the carbon is deposited in sedimentary carbonate rocks. The process is aided on both ends by various organisms, bacteria that increase the absorption of CO2 in the soil and ocean creatures such as corals that secrete carbonate shells. The net effect is almost alchemical: air turning into rock. Weathering cools the climate by sucking CO2 out of the atmosphere.
Yet carbon doesn’t stay sequestered in carbonate rocks forever. Volcanoes eventually return it to the air as CO2. Over millions of years, in the slow, unceasing roil of plate tectonics, every part of the seafloor is eventually subducted, pulled deep into Earth’s mantle. Carbonate rocks don’t survive under that kind of heat and pressure. The carbon is again turned to gas that, dissolved in fresh magma, returns to the surface. Throughout Earth’s history, volcanoes of every sort (from gently oozing seafloor pillows to wide, gushing basaltic floods to devastating Krakatoan mega-eruptions) have always produced, along with fresh rock and new lands, massive volumes of CO2.
Volcanoes adding carbon
to the atmosphere, and weathering removing it—that’s the cycle. Yet what turns this into a self-regulating thermostat is the fact that the two parts of this cycle respond quite differently to climate changes. Volcanoes barely react to climate. The rate of CO2 production by volcanism is ultimately determined by the interior heat flow of Earth. So volcanoes deliver carbon like the Pony Express: through any kind of weather. When it comes to climate, volcanoes just couldn’t care less. Through ice ages and hothouse eons, Earth exhales volcanic CO2, sometimes stuttering or hiccupping, but never stopping to worry about surface conditions.
By contrast, the weathering reactions removing CO2 from the air are highly sensitive to climate. During hotter epochs, chemical reaction rates speed up and the hydrological cycle accelerates, fueling more rainfall and faster weathering, all of which sucks CO2 from air and into rock. Yet when the continents freeze over, and rock is buried under solid ice, these weathering reactions grind to a halt and the removal of atmospheric carbon ceases. Thus, a prolonged ice age will always be self-limiting because it will eventually cause a buildup of warming CO2, and a hot phase will always bring itself to an end by increasing the rate of weathering, drawing down CO2 and cooling our planet again.
So, we’ve got negative feedback, which stabilizes the climate. Over the ages, the planet’s internally driven volcanic cycle and solar-powered hydrologic cycle conspire to adjust the CO2 level such that surface temperature remains comfortably in the range of stable liquid water. This seems fantastically convenient for us, and for Earth’s water-based biosphere. Also, as the Sun slowly brightens over billions of years, the thermostat responds, lowering the CO2 content of the atmosphere. Earth slowly self-adjusts its level of greenhouse gases to balance against the rising influence of its warming star. Given enough time to respond to any provocation, the climate will return to a stable state.
Only when we compare Earth to other planets do we see how many factors have contributed to creating and maintaining this remarkable life-sustaining planetary thermostat. Venus and Mars, it seems, also started out with warm oceans, and when they were young worlds, a similar carbonate thermostat was likely once operating on all three planetary siblings. Yet on each of our neighbors, for different reasons, the thermostat broke down and the climate veered off toward an uninhabitable state.
Venus shows us what would happen if Earth had been born too close to its star. The essential factor is that water vapor, like CO2, is a strong greenhouse gas. This can cause a positive feedback, which is like having your thermostat wired the wrong way, so that when it gets too hot, the heat turns on. When a planet warms, more water evaporates. The water vapor increases greenhouse heating, which causes still more water to evaporate, which increases heating, ad infinitum. Unchecked, this will lead to a “runaway greenhouse.” Venus, 30 percent closer to the Sun, gets twice as much sunlight as Earth. As the faint young Sun brightened, Venus passed a point where it got too much sunlight and fell unavoidably into this runaway greenhouse state. All the water boiled off and seeped out into space. The volcanoes kept pumping out CO2, and as a result, Venus today has an almost pure CO2 atmosphere that is nearly one hundred times as thick as Earth’s, and it’s hot as hell there: hot enough to cook all forms of earthly life and to fry our spacecraft.
Size Matters
What about Mars? Why did the climate thermostat break down there? Of the three siblings, Mars was the runt and shows us what would have happened to Earth if it had been made too small. The more we study with rovers and orbiters, the more we see signs of an early, more Earth-like time on Mars, with vigorous rainfall, rushing rivers, and wind-lashed lakes under a sky thick enough to be an earthly blue. Mars seems to have had a billion-year spree of warmer, wetter climate and possibly even life. Then it all stopped, and not much has changed since, in more than three billion years. What happened? With only one third of Earth’s gravity, Mars couldn’t hold on to its atmosphere. An early pummeling by asteroids and comets created explosive impacts that repeatedly blasted air off the planet. Held only by the weak gravity, more gas was stripped off by the solar wind, or speeding air molecules simply flew into space. The Martian greenhouse grew ever feebler. Today the thin CO2 atmosphere is so cold that it is partially frozen into polar caps of dry ice. Any water that didn’t get swept into space or destroyed by the ultraviolet light that penetrates the remaining thin air lies frozen in the ground or locked up in the polar caps.
It’s even worse than that: little Mars was always destined to be a frozen desert; the planet is doubly damned by its small size.
As we’ve explored and begun to develop a science of comparative planetology, a key question has been whether other planets also have plate tectonics, with all that it implies on Earth (continual renewing of the land and perpetuation of geochemical cycles with their stabilizing effect on climate and enabling potential for life)? It turns out that, to a large extent, planetary character is determined by size.
So far we have not found another planet with plate tectonics. Mars shows some hints that it may have had such a system early on, during a brief, vigorous phase of internally driven geologic activity. Yet, early on, Mars seized up, its crust thickening, its convective heat engine losing steam and its plates annealing into one solid, unbroken, immobile sphere. This, we’ve come to understand, is to be expected on smaller worlds. They lose heat quickly, and thus don’t maintain the vigorous internal convection that has sustained plate tectonics on Earth. Their internally driven geological activity ceases and they end up covered with craters, largely devoid of active or recent geological features.* When it was still a young world, Mars lost most of the internal heat that once drove massive volcanoes, replenished the air with CO2, and kept the carbonate thermostat working.
Size is key. Below a certain size, worlds will end up like Mars. Larger planets will hold in enough heat so that their deep interiors will remain molten for billions of years, giving them more active geology and younger surface ages. Given this emerging understanding, hopes were high for finding plate tectonics at Venus, which, at 95 percent Earth’s diameter, is nearly a twin in size. Yet, for those interested in the global geology of Venus, the history of exploration has been marked by delayed gratification. The planet is surrounded in a thick acid fog that makes photographing its surface from orbit impossible. Long after we had global geological maps of Mars, for Venus we had only the foggiest, only that fog. This finally changed when orbiting spacecraft were sent up with radar imaging equipment that could see through clouds. The Soviets did this first with Venera 15 and 16, which in 1983 managed to map part of the Northern Hemisphere. Then, in 1990, NASA’s Magellan spacecraft got to Venus and, over its four-year mission, made detailed radar maps of the entire surface. We saw thousands of volcanoes of a bewildering range of types and sizes, steep canyons, vast plains, towering mountains, mysterious river channels apparently carved by lava, and nearly one thousand impact craters scattered around the planet. What we didn’t find was any evidence of plate tectonics. If you look at a global map of Earth at the same level of detail, you can easily see a planetwide network of structures defining the boundaries of tectonic plates: steep trenches where plates are being dragged down into the mantle, and seafloor ridges where plates are growing and spreading. On Magellan’s maps of Venus, we see no global pattern suggesting the push-me-pull-you dynamics of plate tectonics.
If size is key, why did a planet so similar in size to Earth evolve in such a different way, absent the global tectonic system that seems key to the character of our planet? This remains a mystery. The difference is likely related to the extreme dryness of Venus compared to Earth. On Mars, when the geology died it drove the climate to extremes. On Venus it may have been the other way around: I suspect that the extreme climate change that befell Venus at some point in its history doomed plate tectonics and condemned the planet’s surface to remain a lifeless inferno. After Venus lost its oceans to a runaway greenhouse, the interior also would have started to dry out, and this might have shut down pl
ate tectonics. The question has forced a closer look at how and why plate tectonics works on Earth. We’ve learned there are many ways that plate tectonics is aided and lubricated by the presence of our planet’s pervasive hydrosphere. Venus could have started out with Earth-style plate tectonics and then lost its ability to recycle its surface and interior, as it lost its water to a runaway greenhouse, and the interior of the planet was slowly wrung dry. This seems plausible, but without further missions to Venus we can’t really claim to understand the divergent evolution of these sister planets. Also, as I’ll discuss in the next chapter, we are still learning of all the ways the ubiquitous presence of life itself has deeply altered Earth.
Earth in Human Hands Page 6
