I bought a textbook called Principles of Planetary Climate, by Raymond Pierrehumbert. It seems pretty interesting and I am going to write some sparse notes on the chapters as I go through. It's like live-blogging. I am live-blogging a textbook. I hope this is informative, but mostly it is to help me digest the material.
Table of Contents
- Close to Home
- Into Deepest Time: Faint Young Sun and Habitability of the Earth
- Goldilocks in Space: Earth, Mars, and Venus
- Other Solar System Planets and Satellites
- Farther Afield: Extrasolar Planets
- Digression: About Climate Proxies
- The Proterozoic Climate Revisited: Snowball Earth
- The Hothouse/Icehouse Dichotomy
- Pleistocene Glacial-Interglacial Cycles
- Holocene Climate Variation
- Back to Home: Global Warming
- The Fate of the Earth and the Lifetime of Biospheres
This chapter will mostly be pointing out interesting things and then assuring me that everything will be explained later in the book. "We hope not to be too Earth-centric," he says. This is promising.
This is a chapter that is mainly simple questions with interesting answers. There are lots of questions we can ask about how the Earth works just based in our everyday experience. For example, what causes the seasons? Why is it warmer sometimes than others? Does the Sun warm the Earth? What cools the Earth? If nothing cools the Earth, how do we avoid just getting hotter and hotter? and other basic questions that people have been asking for millenia.
There's a quick detour into different temperature scales (Celsius vs. Kelvin vs. Fahrenheit) and talks about their advantages and disadvantages.
What's the deal with air? How does it affect the climate of a planet? The Moon doesn't have air, how hot/cold does it get? Very. We know this because we can look at the heat radiating from it. This also answers our earlier question about what cools the Earth.
Now we can think about how the existence of an atmosphere and its composition affects the cooling of a planet, and here is the first mention of greenhouse gases.
The Solar System is constantly changing. There's a quick rundown of Earth's formation and early years. The big point is that liquid water has been widespread on Earth pretty consistently for at least the last 3 billion years, and sporadically for even longer.
This is kind of surprising! 3 billion years is a long time and, in fact, the sun was about 20% dimmmer back then. This should make the Earth too cold to have liquid water on the surface. There must have been incredible amounts of greenhouse gases early on to keep the Earth warm. But that atmosphere would be stiflingly hot today, so somewhere in there the atmospheric composition has undergone massive changes.
Historically we have often viewed Venus as a pleasant and habitable place not unlike Earth, but as we now know it's... not. We also know that Mars is really cold.
So we see that we are kind of in between two extremes. The big question here is: what "went wrong" on Venus and Mars, and how did we somehow avoid both fates?
Some interesting, smaller questions that might help us answer the big one include:
- Venus has a very new surface - by this I mean that there are not a lot of craters around. "The crust may have been engulfed and resurfaced as recently as 500 million years ago," says the book. Why did this happen on Venus but not on Earth?
- We see that there was once liquid on Mars, a long time ago. But this was when the Sun was dimmer. So there must have been a very dense, greenhousey atmosphere. What happened to it? We'll touch on this later, apparently.
We also talk briefly about Venus' runaway greenhouse effect here. I am glad it did not occur here, though I am interested in the specifics of why not. Pierrehumbert keeps bringing up how cool theories come from combining different bits of basic science in novel ways.
Let's talk about gas giants! Apparently the atmospheric flow really affects their thermal structure, which could be pretty cool later on. Long-lived storms like the Great Red Spot are also interesting. Unfortunately, there isn't going to be much fluid dynamics in this book, though I guess I can finally finish reading The Life of a Leaf to get some intuition for this stuff.
I had learned of Uranus and Neptune as "gas giants" but here he reclassifies them as "ice giants" because of their composition. They're made of an "ice mantle", which is both (a) a great name for a Magic card and (b) "actually a hot, slushy mixture" more like a "water-ammonia ocean."
Here's a cool way of thinking of these four planets: on each of these, there is a temperature gradient from cold to hot as you go deeper into the atmosphere. Thus there is a band of altitude where the temperature is appropriate for liquid water. So life could presumably exist there. Pierrehumbert points out that, after all, life originated in the oceans here on Earth.
Now we talk about Titan and Europa. Titan with its methane oceans, full hydrological cycle, and dense nitrogen atmosphere (how is that even still around?); Europa with its ice crust and liquid water mantle. My favorite line of this book so far is: "At the cold temperatures of Europa's surface, as on Titan, water ice is basically a rock, just as sand can be considered an 'ice' of SiO2 on Earth." Really blowing my Earth-centric mind.
And of course, he ends this section with this: "these moons are, as has been said, 'always icy, never dull.'"
There's a bit about exoplanet detection and stellar evolution here. Basically we are looking for planets orbiting relatively stable stars at appropriate orbits. Turns out we haven't found any at the time of this book's publishing, but we are really churning out the exoplanet discoveries nowadays so who knows? In any case these planets present interesting new climates to study, and new edge cases for us to think about our theories with. Exoplanets are just really far away and so it's pretty hard to get details about them (and it's incredible that we've learned as much as we have).
So we haven't really done a great job, as a species, of recording the climate history of the Earth. Especially since we haven't been around for pretty much all of it. But the Earth itself, and other lifeforms that have been on it, have been shaped by the climate, and we can use such climate proxies to infer some details of the climate in the distant past. And of course people are figuring out new ways to do this as I write.
For example, certain animals are cold-blooded, so they can't live in cold climates, so if we find them in the fossil record at a time we know that it can't have been that cold, then and there. Or some animals use different substances to make their shells based on the temperature and we can analyze that. Of course these assume that the biology of these animals is not too different from the ones we see today, which admittedly is a little sketchy.
We can also look at sediments - if we see rocks that have deformed what was once the ocean floor, we know that they were probably carried out onto the open ocean by ice floes. That tells us something about the temperature at that time as well.
We can even look at the chemical composition of the ocean that is preserved in the ocean floor - but we need to be careful to avoid being misled by samples that have been altered by later contact with the ocean.
We can also look at differences between isotopes to see some interesting effects. For example, lighter isotopes tend to be more reactive as they have higher velocity at the same temperature. They also evaporate and leave the atmosphere more easily. There are also subtle effects in that some reactions prefer different isotopes based on the temperature, and, as such, we can infer the temperature at the time the reactions occured in a sample by looking at the isotopic distribution.
For example, photosynthesis likes lighter carbon, so the carbon in plants is lighter than the carbon in the air (all this is by very small amounts, like 0.25%). Carbon participates in "the carbon cycle," which involves carbon being outgassed from the Earth and then buried again somehow.
We know that carbon is outgassed from the Earth with a specific isotope fraction. We also know that carbon is basically either buried as organic matter or as a carbonate, so if we look at the isotope fractions of both organic matter and a carbonate at a certain time we can figure out the isotope fraction of the air at the time too!
This lets us find anomalies known as "carbon isotope excursions", which tend to correspond to times when the carbon cycle was disturbed somehow.
The amount of ice and the ocean temperature also affect the isotope fraction of ocean water, which is preserved in certain fossils, so we can use these fossils to determine those aspects about our water supply in the distant past. Unfortunately it is difficult to tell the effect of ice and the effect of water temperature apart, so there is some sleuthing involved here. Turns out that the fossils we use are easily distinguishable between the ones that live on the surface and the ones that live on the sea floor. The sea floor has a much more stable temperature than the surface, so we can compare the readings from the two different types of fossils to get a better idea of the true temperature. Additionally, there are times during which, by other means, we can determine that there was little to no ice coverage. Then any isotope fraction effects must be due to temperature.
Ok! Now that we know a bit more about how we know about the past, we can get back to talking about the past.
This picture from Wikipedia represents the major bits of geological time that will be important.
A short aside on continents! We haven't talked about them yet. Pierrehumbert says "continents consist of light material that has floated to the outer portions of the solid Earth and beeen incorporated into the crust." I'd learned that the crust was floating on top of the mantle in Magic School Bus, but had never realized that meant that the mantle was denser than the crust.
While they are floating around on the outside of the Earth the continents are also affecting the climate. They are the primary place where weatherable rocks are weathered, regulating atmospheric CO2; they affect the shape of the oceans and how they transport heat across the Earth; and they allow for new forms of life which affect the climate in myriad ways.
We only really know what the continents looked like starting from the end of the Proterozoic (still about 600 Mya!).
During the Proterozoic, the atmosphere and oceans became more oxygenated. Life and the climate had to adjust to accomodate this. This oxygenation occured in spurts, and by the end of the Proterozoic the atmosphere was probably about as oxygen-rich as the one today.
By looking at climate proxies we find there is lots of activity at the beginning and end of the Proterozoic (the Paleoproterozoic and Neoproterozoic, respectively). However, the billion years between these two are known as "the most boring period in Earth history." Pierrehumbert calls it the Big Yawn. If we knew more about it, we would probably be able to find all sorts of interesting things going on, but as it stands, we have no evidence of excitement during this period. No glaciation, no anything. Eukaryotic life does appear at the very beginning of this but don't seem to have caused a splash - maybe they were the cause of this incredibly long period of stability.
In any case, at the beginning of the Proterozoic there was a Great Oxidation Event. There's tons of evidence. For example, banded iron formations, which require low oxygen levels, abruptly stop occurring at the beginning of the Proterozoic. We see huge carbon excursions, indicating a huge transition in the carbon cycle, such as a large increased proportion of organic carbon burial. It looks like the oxygen levels skyrocketed and then crashed again as the banded iron formations returned. We see several periods of glaciation, one of which (called the Makganyene) sees glaciers even in the tropics.
By 1.7 Bya we see that the atmosphere has about 10% as much oxygen as it does now. Before the Proterozoic it was closer to 0.001%, so 10% is pretty high. This number stays constant throughout the Big Yawn, until 700 Mya.
The Neoproterozoic brings back the excitement of the Paleoproterozoic, with carbon excursions, glaciations, and more. There's a mysterious carbon isotope excursion that is not associated with any "Snowball event." It's called the "Shuram excursion," and during this time the carbon in the carbonates (which tends to be heavy, remember, to offset the light organic carbon) was 1.2% lighter than our standard measurement. In a typical Snowball event we see the carbonates at maybe -0.5%, so this is a Big Deal. We have no idea what happened - the best guess is that there was "a transient reorganization of the carbon cycle, in which a large isotopically light pool of suspended organic carbon in the ocean is oxidized and deposited as carbonate."
Lots of stuff is going on with the oxygen, too. It's hard to tell exactly, but what we do know for sure is that by the end of the Neoproterozoic oxygen levels everywhere, down to the bottom of the ocean, were similar to present levels.
Pierrehumbert: "A very Big Question is why all this excitement suddenly resumed after a billion years of stasis."
There are also lots of Big Questions specifically about Snowball events, where the Earth is frozen over. How do they start? After starting how do they end again? Is it guaranteed that they do end, or have we just been lucky with them so far?
Though we are used to our current ice caps, we should recognize that for significant parts of Earth's history, the poles were warm enough that the poles were not frozen. About 50 million years ago, for example, we use oxygen isotope information to calculate that, even at the poles, the coldest temperature was around 15 C or 60 F. So the poles were ice-free, even in the winters. This sort of climate is known as a "hothouse climate."
A few million years before this point there was a sudden dip in heavy oxygen - this indicates a spike in temperature of about 4 C. It's called the PETM (Paleocene-Eocene Thermal Maximum) and took about 10,000 years to spike and 200,000 years to subside again. We're not sure what caused it and have seen nothing else as drastic as it. For some context consider that between 1920 and 2005 we have increased the Earth's surface temperature by almost 1 C. That's less than 100 years, or 1% of the time it took the PETM to set in.
The opposite, a climate where there is ice at the poles, is known as an "icehouse climate." We are in one of these now.
Some things to look forward to: we'll be looking at the science of the dinosaur asteroid's effect on climate, and at different possible theories of the PETM. Eventually.
Once we hit the Phanerozoic, 600 Mya, we have a decent idea of what the continent(s) looked like. Here is a cool site where you can look at maps of what the Earth looked like back then. It's probable that this changing geography influenced climate. Surely having no land at the poles makes it difficult for glaciers to exist. On the other hand, we get land at the poles long before we get ice caps and significant glaciers, so this is not the only factor in icehouse/hothouse transitions. CO2 is a likely suspect. There's a lot to say about hothouses and icehouses, and their role in the evolution of the biosphere, but in a recurring theme, we'll touch on it later as we've learned more of the science.
It is important to remember that "icehouse" is not the same as an "ice age." An icehouse climate is simply a climate where we have permanent ice at one or both poles for millions of years. An ice age is a surge in ice sheet volume during an icehouse climate. The time between ice ages is called an interglacial, even though glaciers and ice caps still exist.
We start to see these ice ages about 4 million years ago. I think the reason we see ice ages only recently is only because we don't have enough data from earlier to see these relatively quick fluctuations. By about 2 Mya the ice ages have settled into a rhythm, about one every 40,000 years. About 800,000 years ago, the ice ages slow down - we only get one every hundred thousand years. We also see that the ice builds up slowly but disappears quickly.
We don't have a perfect answer for why these ice ages have such periodicity, and why the periodicity would change. We do think, however, that the interplay between our (slightly) eccentric orbit and our axial tilt is important. Right now, for example, winter for the Northern Hemisphere is when Earth is closest to the sun - so the winters are slightly warmer than they would be if the opposite were true. More on this stuff later (in this article, even!). All this was first formulated by a guy called Milankovic, and there is still active research on how these Milankovic cycles affect climate on Earth. We can even use Milankovic cycles to think about Mars, but we have so little paleoclimate data for Mars that we can't really make any detailed conclusions.
Much of what we know about ice ages comes from ice cores in the Antarctic and Greenland. We can look at the isotopes of water, as well as the composition of air trapped in ice bubbles. We see that the CO2 fluctuations sync up well with the temperature fluctuations. We're not sure why there is such a CO2 cycle, and if it causes or is caused by the ice ages, or if there is a causal relationship at all. But we do know that CO2 is a greenhouse gas and does warm the Earth, and a decrease in CO2 will decrease the temperature.
In the Greenland ice cores we see that there's been significant temperature variation on the scale of about 1000 years throughout the past 100,000 years. This activity, called "millenial variability," is especially prominent between 60,000 and 10,000 years ago. As we come out of the most recent ice age, we see an abrupt warming event (called the Bolling warm period), followed by an abrupt cooling event (called the Younger Dryas), and then - stability. Ten thousand years of relatively stable climate, known as the Holocene. You can see these millenial variations below:
Of course, the Holocene has been very short, so crazy climate shenanigans have had no time to happen yet. But the suspicious lack of millenial variation is worth investigating more, and there are still points of interest in the Holocene.
Let's go back to Milankovic cycles, or, more broadly, the precession cycles. The tilt-based seasons and the distance-based seasons line up approximately once every 22,000 years. Currently, northern winter is when the Earth is farthest from the sun, and vice versa. This gives the northern hemisphere cooler summers and warmer winters. 11,000 years ago it was exactly opposite. However, for some reason, the warmest northern summers occurred about seven thousand years ago, a delay of some 4 thousand years. This time is called the "altithermal." During this time summer temperatures were 2-4 C warmer than they are now. The details of how the altithermal affected climate are active subjects of research now.
The precessional cycle, perhaps unsurprisingly, affected precipitation patterns in the lower latitudes. The Sahara was wet (well, wet enough to be savannah grassland) from about 14,500 years ago to 4700 years ago. There is also some evidence that mountain glaciers are also related to the precessional cycle - the glaciers on Kilimanjaro were laid down when the Sahara was wet, for example.
There is also the Little Ice Age, which was a time from about 1500-1800 during which temperate climes in the northern hemisphere were cooled by about 0.5 C. We think this is because of a transient dimming of the Sun, but there are many other factors that may be involved. Because this is such a subtle effect, the principles behind it are harder to coax out.
So we know that CO2 has has a big effect on the climate throughout Earth's history. We also know that life has had an impact on the carbon cycle. Much organic carbon has been sequestered without oxidizing over the past billions of years. Now we are digging some specific forms of this up and oxidizing millions of years' worth of sequestered carbon in a few centuries.
We are substantially affecting the carbon cycle. Don't let anyone tell you otherwise. We output 8 gigatonnes of carbon in 2005, and this rate has been growing since then. There were about 600 gigatonnes in the atmosphere in pre-industrial times, so that doesn't sound like too much. But this means that at 2005 levels, we would be doubling the amount of carbon in the atmosphere in 75 years.
Plants worldwide "fix" about 50 gigatonnes of carbon each year, but only 0.4 gigatonnes per year are truly sequestered, with the rest being recycled into the air by bacteria. The natural outgassing of CO2 leads to about 0.1 gigatonnes per year. Our output of 8 gigatonnes per year completely dwarfs these fluxes.
The oceans have acted as a buffer for us, absorbing about half of the carbon we have emitted to date. The rate at which this happens, however, depends on the mixing between the upper and deep ocean. If we stopped burning fossil fuels today, it would take about 600 years for the ocean to absorb 80% of the excess CO2 in the atmosphere. Even with the oceans taking up carbon as quickly as possible we have enough fossil fuels to get the atmosphere up to 6 times the pre-industrial carbon dioxide concentration.
This leads to many questions: how much will the Earth warm exactly? Where will this warming happen? How does this affect sea levels? Rain? Civilization? The basic science behind these is well-developed, but the details and granularity we need to answer these questions requires a deeper understanding.
People started thinking about the science of global warming in the 19th century, but we didn't really have the data until the 70's and 80's to verify that, indeed, atmospheric CO2 was rising and the global temperature with it.
Between 1920 and 2005 the temperature has gone up 1 C. There was a pause from about 1940 to 1970 because burning coal releases sulfur compounds which reflect the sun, but then this mitigating effect was overpowered by increased CO2 and thus we see more temperature rise.
Other complicating factors include the behavior of water vapor, another strong greenhouse gas, in a warmer atmosphere, and the combined warming/cooling effect of clouds. Clouds both insulate the Earth and reflect sunlight, and the extent to which these balance each other out is determined by the type of cloud. There are many other complications as well in the problem of determining exactly what happens to the Earth at +2 C or +4 C.
We do know that it is something of geologic proportion happening on a human time-scale. In the distant future climate scientists will be left to puzzle over this abrupt spike in atmospheric carbon.
No place can be habitable forever. Eventually the Sun will become a red giant and engulf the Earth. Long before that it will be too bright for our weathering process to compensate and we'll run into a runaway greenhouse effect. Even before then, to maintain a habitable temperature we'd need incredibly low amounts of atmospheric CO2, which cripples photosynthesis.
Alternatively, we could run out of CO2 outgassing from the Earth, and as we bury it all in the crust the Earth will freeze over.
And that's why I'm reading this book, because I want to learn more about this crazy world we live in and maybe figure out ways to keep it healthy.