This is the fifth in a series of reposts from gregladen.com on global warming.
During the 1970s and 80s, creationists had a long list of reasons to doubt evolution, and every one of those reasons was wrong. But they had so many reasons, and it was so hard to keep track of them all, each with various versions, that a creationist that was trying to not live a lie could convince themselves that they had an honest dispute with evolutionary biology. But if you sat down and looked at every detail, "creation science" could be shown to be nothing more than a big bag of falsehoods. So to continue to be a creationist you had to be willing to live that lie.
Then intelligent design creationism came along. IDC does not require that you have a long list of lies. Instead, you have one single great big lie that can't be disproved (and is utterly unrelated to science, and in fact, can't be proven, either).
This is a little bit like what happens to some Global Warming Deniers. One could question the source of CO2, the relationship between CO2 and warming, one could blame cows for releasing methane, and so on and so forth. I've always found it amusing (and annoying at the same time, of course) to watch GWD's struggle through all of these little nitpicky falsehoods, losing ground all the time, then suddenly discover that the earth's climate has changed very dramatically in the past ... before humans existed, or before human impacts were as profound as they are today. Past "natural" climate change is the intelligent design creationism of global warming denial. Since such dramatic change has happened in the past, how could a little bit of global warming today matter?
Well, it does. Why it matters will be the subject of a different posting, and I've written about this before on this web site. For now, I simply want to outline the nature, magnitude, and causes of past climate change, to provide a framework of discussion.
This is the stuff that, to me, is really interesting, because of my interest in evolution in general and human evolution in particular. Evolutionary change happens in relation to climate change. That's not the only cause of evolutionary change, but it is a big one, maybe the big one.
In order to really understand this, it is necessary to have a reasonably good understanding of how climate works, and that would require an entire blog epic to do justice. But I can give you a basic idea.
The earth is a big round ball that the sun shines on. Because the earth is a ball, the sun's energy is unevenly distributed across the earth at any given moment. I'm not talking about the fact that the big ball is spinning (giving us night and day) but rather the fact that the sun is putting a lot more energy near the equator than it provides at the poles.
Heat has the property of dissipation, or spreading. So if the equator is warmer than the poles, the heat is going to spread from the equator towards the poles. This is going to happen in ocean currents and air currents. But these currents are not simple or continuous. They are very complex inherently, but made even more complex by the fact that the earth is covered by a quirky, and over the long term changing, pattern of continents ... breaking up the oceans ... and land masses with big changes in elevations (mountains) breaking up the air currents.
This complexity provides our global weather patterns. Removal or addition of a mountain range, the opening or closing up of an ocean, or the linkage of continents formerly separated, all change the fundamental way in which climate is manifest.
In the meantime, there is variation in how much overall solar energy reaches the earth. Over many tens of millions of years, there is probably not too much average change in how much energy the sun puts out. There were big changes at the beginning of the solar system, and there will be at least one big change in the future (when the sun explodes) but for the period of humans, or the longer period of mammals, and for longer periods, there is not a lot of change on average. There are short term changes, but remarkably they have not been linked to climate changes despite Herculean efforts to do so.
What does change over the time period and scale we are concerned with is the exact distance at a given moment between the earth and the sun, and the pattern of this distance over the course of the year, in the context of the fact that the earth is slightly tilted in relation to the sun.
The tilt gives us seasonality. Most American adults, when polled, are found to incorrectly believe that the march of seasons is caused by changes over the year in the distance between the earth and the sun. To be fair, people are given a multiple choice question with this as one of the options, and it may seem reasonable once presented (though with a little thought it is impossible). Just to be clear, seasonality results from the fact that the earth maintains a tilt as it orbits around the sun, so that at one point in the year, the north pole is tilted away from the sun, and at the opposite time of year the north pole is tilted towards the sun. Towards the sun = warmer = summer, away from the sun = cooler = winter. The opposite is happening, of course, in the southern hemisphere.
But what if the tilt increased? Would that give us "more" seasonality? Of course it would. The degree of difference between a typical summer day and a typical winter day would increase. If the tilt was reduced, this difference would decrease. This means, among other things, that the "hotness" of the northern summer would be less than average if the tilt was minimal.
Imagine a very very snowy winter. The big giant snow piles in the Shopping Mall parking lot are huge. The more snow that fell during the winter, in a place like Minneapolis or Edmundton where there are few if any days of melting over most of the winter, would take a very long time to melt. If this snowy winter happened while the earth's tilt was extreme, then a very hot summer would come along and melt the snow quickly. But if it happened during a period when the earth's tilt was minimal, the summer would not be so hot and the snow pile would last much longer.
The glaciers of future ice ages will probably be born in the Shopping Mall Parking Lot.
The earth's orbit is not a circle, but an ellipse, and since it is an ellipse, the sun sits at one of the two foci that the ellipse has. This means that the distance between the earth and the sun does change over the course of the year. This is not what causes seasonality and it is independent of seasonality. If the ellipse was very elongated, and the earth had no tilt, this ellipticity would be the cause of seasonality, but not with opposite seasons in the northern vs. southern hemisphere.
So given that there is an ellipse, what would happen if the ellipse was very elongated? That would mean that there would be several weeks out of the year that the earth is way far from the sun. If the ellipse was an almost perfect circle, there would not be any such weeks.
What about the direction of the earth's tilt in relationship to the ellipse of the Earth's orbit? If the North Pole was tilted towards the sun when it was at the closest point (on the ellipse) to the sun, that would be different than if it was pointed away from the sun, or at a right angle, on that day.
So we have three conditions ... tilt, eccentricity, and this relationship between solstice and ellipse, which we'll call precession of the equinox. We can see how they might matter to the pattern of hot and cold over the year, and we can imagine how the pattern of hot and cold over the year might cause the giant snow pile at the mall to melt more quickly or to melt more slowly.
And we have the prospect of altering these conditions ... of these conditions changing for some reason.
It turns out that these three conditions do change over time. The pattern of change is called Milankovitch Oribtal Geometry, and the changes for each condition, being cyclical (periodic), are called Milankovitch Cycles. They are called this because a guy named Milankovitch thought them up a long time ago. Nobody believed him until much later in time (beginning in the late 1960s or early 1970s, but not really believed by everyone until about 1984/5). Here are the Milankovitch cycles summarized:
Eccentricity ... the change in the degree to which the Earth's orbit is an ellipse ... runs form most elliptical to least elliptical over a period of 100,000 years. Now, if you were a giant (a really big giant) standing 10 or so Astronomical Units from the sun and looking at the earth's orbit, you would not see this difference. It is not that dramatic. But it is enough to change the distribution of the energy from the sun over the course of the year.
Axial Tilt ... how much the Earth is tilted ... from about 21.5 degrees of tilt to about 24.5 degrees of tilt ... runs over a period of about 41,000 years. Again, this is not a lot of difference in tilt but it is enough to matter. By the way, this tilt has nothing to do with magnetic orientation. (OK, it probably does in some way, but the magnetic declination and this tilt, as numbers, are not to be considered as related.)
Precession of the equinox ... the changing relationship between the Earth's tilt and the ellipse ... changes over a period of about 23,000 years.
Most of the earth's land mass is in the northern hemisphere. If you want to get a real rush, compare a view of the earth from the north pole with a map of the earth from the south pole, and look at the difference in how much land/water there is in the northern vs. southern hemispheres. Pretty cool.
It is probably the case that glaciers are somewhat self-regulating ... more glacier means more albedo (reflection of solar energy) and thus the big giant heater of the earth's surface is not as effective when there is a lot of glacier or snow cover. You don't get a lot of snow cover on the ocean, compared to land. So it might be the case that a very snowy winter, where the snow lasts a long time, in the northern hemisphere, can cause a cooling of the earth's atmosphere. Maybe this happens a few years in a row, and this causes the oceans to cool down and thus absorb more atmospheric Carbon Dioxide. This reduces the greenhouse effect and further cools the earth. So the snow stays around even longer, and we start to get parking lot piles that last until August. Then a big giant volcano happens to go off, and the aerosols from the volcano further cool the atmosphere. And so on.
And then there is an Ice Age.
There are many possible scenarios for the onset of an ice age (or the stopping of an ice age) but most of them involve the northern hemisphere, because of the land masses there and the interaction between ocean currents, major waves of air movement, and the continents and their mountains.
In these scenarios, you need to have the northern summer be a cool and wimpy summer. So you get, through precession, the summer to occur when the earth is way out on the ellipse. You get, through reduced tilt, less seasonality and thus an even wimpier summer. And you get, by having more of an ellipse than average, even reduced solar energy during this summer.
In other words ... and this is the key point ... when the three Milankovitch factors line up to cause a chilly June, year after year, conditions are ripe for the onset of an Ice Age. Most likely, this in and of itself is insufficient to actually get an ice age. But these conditions would be prevalent for centuries, and a lot of things can happened over that course of time. The Atlantic Conveyor, a major ocean current that moves southern heat to the Northern Hemisphere, could turn off (like it did in that movie). Or the continental wave of airflow over the northern hemisphere could shift to the south of where it usually is, causing very severe winters and cool summers, or a couple of big giant volcanoes can go off over a short period of time.
The graph shows a proxyindicator for Earth's climate over the last several million years. This is from a paper on African climate change during the Pliocene and Pleistocene, and it happens to be a good graphical representation of what I want to show you. This is a "Delta-18" curve. This shows the ratio of two stable isotopes of Oxygen, Oxygen-16 and Oxygen-18, in the worlds' oceans, as determined by sampling the exoskeletons of little critters (like plankton) that lived in the ocean, died, and who's remains were deposited at the bottom of the sea.
Ice is made from atmospheric water vapor, and atmospheric water vapor is "isotopically light." So if you have a certain ratio of Oxygen 16 vs. 18 in the ocean, you get more of the O-16 in vapor (compared to the ocean) because the O-18 bearing water molecules are less likely to evaporate. (They are heavy. "Uffda, I can't evaporate" they say...). Therefore, and if you don't know this already, be prepared to learn one of the coolest facts you'll ever hear ... the glaciers trap light oxygen, causing the oceans to become isotopically heavy. Therefore, (still holding your breath?) by measuring the istopic ratio of the ocean we can estimate with precision how much ice, at any one point in time, is trapped in the Earth's glaciers. And this can be done for any given moment in time in the past, as long as we can get a good sample of those critters who's bodies blanket the depth of the sea.
So this squiggle is showing you the shift back and forth between ice ages and interglacials. Or more exactly for the last 2 million years of time, this is ice ages and interglacials. Before that I'm not sure if you would call the cool periods full blown ice ages, but they are cool periods. Back and forth, at a very high rate, the climate changes. (As Yoda would say).
The left side of the graph is cooler, the right side of the graph is warmer. You have to look really close to see that we are now in a warm period, and frankly, the data on this graph are not fine enough in resolution to show the last few thousand years accurately.
The big picture that this graphs shows us is a general cooling of the earth since the late Miocene. Note that one particular historical moment is noted on this graph: The approximate time of the closure of the Panama Land Bridge. This meant that ocean currents became more restricted in their circulation around two major continents: North America and South America. As a rule, when you close off ocean circulation you get less heat moving from the equators to the poles (remember back at the beginning of this post?). Restricting circulation in this way seems to have fundamentally changed the nature of the Earth's climate pattern from one in which there were cool vs. warm periods to one in which there were Ice Ages and Interglacials.
You can easily see on this graph that the closure of the Panama Land Bridge did not bring on a sudden change. We actually think this was merely the first in a series of events that has led to the formation of a full-on ice age pattern. But here is something that is really important. Of the many mass extinction events recorded in Earth history (like when a giant rock hit the earth and killed off the dinosaurs), one of these events was a spectacular period of climate change seemingly associated with the closure of the Panama Land Bridge. This is important because even though the land bridge stayed closed, the extinction event stopped. Do you see the significance of this? What caused the extinction was not so much the fact that the world was different, but rather, the fact that the world changed abruptly. Subsequently, there was a settling down, or adaptation, of various systems. This is living (and dead?) proof of one of the most fundamental laws of nature:
All change is bad.
(Unless otherwise compensated for).
Source of graphic:
deMenocal, Peter B. 2005. African climate change and faunal evolution during the Pliocene-Pleistocene. Earth and Planetary Science Letters, 220:3-24.