Climate change through geological time


During this course we’ll examine the problem of climate change through geologic time, as revealed in the field of paleoclimate. We’ll look at records like this that span, in this case, the history of the planet. Going back about 4 and 1/2 billion years. On this particular diagram, we see a proxy, shown here by this black curve, for the volume of ice on the planet. Going forward from 4.6 billion years ago to the present here, notice that this time scale is not linear. For example, the first five billion years of Earth’s history is compressed into this region of the diagram. One sees that the volume of ice on the planet has varied greatly through its history. There were some early glaciations, for example, quite a few of them, centered around 550 million years ago here. Some very interesting times, for example, here during which 90 percent of the marine life died, and there was a high volume of ice on the planet. And on the other hand, a very warm period from 80 million years ago, or so, spanning to 40 million years. After which time, the volume of ice on the planet slowly increased.

These fluctuations, you see at the very end of the record, are the fluctuations associated with the great glacial interglacial cycles of the last three million years. Why did the climate vary this way? How come they were glaciations early in the Earth’s history? A period of almost no ice, and then back to an ice climate. The Earth’s climate history abounds with interesting problems and paradoxes. One of the most well known is called the Faint Young Sun Paradox. Basically, a question of why wasn’t the early Earth frozen. Here we see a graph going back to the beginning of the Earth, that shows an estimate of the solar output as a fraction of today’s value. So this ratio is one today, but it was only about, a little bit more than 0.7 in the beginning of the Earth’s history. And what you see here is an estimate from very elementary climate model considerations of the Earth’s surface temperature. Making certain assumptions, like the composition of the atmosphere was constant through this time.

Which is almost, certainly wasn’t. But according to this calculation, before roughly 2 billion years ago, the Earth’s main surface temperature was below freezing. And certainly, three or four billion years ago, the Earth’s surface temperature should have been so cold that there would have been no liquid water on the planet. Yet, geological evidence is very clear on the point that there was plenty of liquid water early in Earth’s history. So why was it that, in spite of this lower solar output, the Earth’s climate was not so cold? There’s evidence that the Earth went through a cycle of extreme climate swings, centered about 550 million years ago. These climate swings may have been so extreme, that there were periods of time when the Earth was essentially a snowball covered with ice, reflecting all incoming sunlight. But alternating between this, and states, were the Earth was completely free of ice. So the snowball Earth landscape might have looked something like this. Whereas, the hothouse Earth might have looked something like this, even at the poles. The Earth’s climate has gone through these extreme swings.

Why did this happen, and why was it focused about 550 million years ago? We’ll also talk about the problem of the Earth’s very warm climates. For example, during the Cretaceous and the Eocene, there’s very little evidence of ice at either of the poles. And the mean temperature in the Arctic could have been as warm as 20 degrees C in the annual average. We have fossil remains of reptiles, like this beast here, in places like Greenland. Where they certainly don’t exist today. What explains the warmth of this period of Earth’s history? We’ll also talk about the use of proxies for making deductions about the Earth’s temperature and other properties, such as, the volume of ice on the planet, going back in time.

This shows a proxy that consists of the ratio of two isotopes of oxygen in deep sea marine organisms, or their fossils, going back 65 million years. This is a good proxy for the volume of ice on the planet and also for the temperature of polar oceans. And shows that the temperature, or inverse ice volume, reached a maximum in the early Eocene period, about 50 million years ago. And broadly speaking, has been on the decline since then. Although, with interruptions. For example, there’s evidence that about 25 million years ago, there was an abrupt thawing of ice in Antarctica. And then it became reglaciated about 15 million years ago. Since that time it’s been a steady downward slide. But with these large oscillations toward the end of the record, which we know as the great glacial interglacial cycles. Why did the climate behave this way? Notice, as well, there is a spike in temperature somewhat more than 55 million years ago, known as the Paleocene-Eocene Thermal Maximum, where the temperatures really got hot, really fast.

What caused this to happen? Can we understand that? Now, if we focus on a much more recent period of the Earth’s history, a tiny sliver of the graph you just saw, we get marvelous records of the Earth’s climate behavior from ice cores in the Arctic and in the Antarctic. Here, for example, is a record of temperature constructed from oxygen isotope proxies going back 450,000 years in Vostok, in Antarctica, as well as a proxy measure of global ice volume. You can see that all three of these records co-vary so that ice volume, and the proxy for temperature co-varied with spikes in temperature occurring roughly every 100,000 years in between which the climate was quite cold. This is when the great ice sheets covered parts of North America and Europe, for example. Whereas right at the moment, we’re living in a uniquely stable and warm period of time, called the Holocene, for the last 10,000 years, or so, there have been other warm interglacials like here, here, and here. Why did the climate vary in this almost cyclical way? And what does that imply about the climate of the future? Here is a reconstruction of what the ice cover might have looked like at the peak of the last glacial period about 18,000 years ago when there were huge sheets of ice covering much of North America and Eurasia and bits of South America and Australia as well.

So New York and Boston were probably under a mile or two of ice 18,000 years ago. 18,000 years is the blink of an eye, geologically. How could this happen? When will it happen again? Accompanying these changes in ice volume are very large changes in sea levels. This is a reconstruction of sea levels around the world, going back 24,000 years. So if we go back then, we see that sea level, relative today, was about 120 meters, almost four hundred feet lower. At the end of the last glacial period, sea levels began to rise as the ice melted. They rose 120 meters, stabilizing at their current values, about 7,000 years ago. You’ll see that the last 7,000 years have been spectacularly stable. Both in sea level, and actually in temperature, as well. It is not an accident that this is the period over which human civilization has thought to have developed. Now if we focus on the last 2,000 years, we get a record that looks like this. What you see on this graph is the temperature, relative to its 20th century average, reconstructed by various different research groups, using different proxies.

So these different colored curves represent different estimates. And, naturally because their proxies, and they are not perfect measurements of temperature, they disagree with each other. Nevertheless, they show a similar pattern. A gradual warming over the first 1,000 years of the record, to a maximum called the Medieval Warm Period. This is the time when Greenland, for example, was briefly settled. Followed by a decline in temperature to a broad minimum, over the period between about 1600 and 1850. This is known as the Little Ice Age. After that, temperatures began to rise quite rapidly, as revealed, both by the proxy records, and by actual measurements with thermometers; the instrumental record. And you can see the temperature here, in 2004, was quite a bit larger than what we have seen over the last 2,000 years according to these proxies. Why did the temperature vary this way? Why the temperature increase for the first 1,000 years, then start decline again, and then suddenly go up towards the end of the record? Here’s a reconstruction over the same period of time of temperatures in the Arctic. Once again, we see reconstructions from various proxies, including ice cores and tree rings in blue.

With uncertainty represented by the gray shading. Where as this red line here, at the end, shows the instrumental record. And we see a similar pattern, although not much evidence of the Medieval Warm Period in this record. But a gradual descent of temperature reaching a minimum around the year 1800. And then rising quite rapidly after that. What explains this temperature record? Now, if we focus on the instrumental record, which dates back only about 150 years. Here’s a graph showing the annual mean of temperature, according to several different groups, relative to its average over the period 1961 to 1990. You can see that despite some small disagreements among these groups estimates of the temperature, they basically show that the temperature was fairly level until about 1921, when it started to go up. Reaching a maximum in the 1940s. But then declining slightly into the ’60s and ’70s, and then rising quite rapidly afterwards. This is an interesting variation of climate, as reflected by temperature.

And we like to know why this is happening. And will it continue to happen? .