In my previous post, I provided an “exam” that I had made up in an attempt to inject a bit of humor onto this blog. In the process, however, I received some serious questions concerning one of the figures I had used. Therefore, I am now going to use this “teachable moment” for providing an explanation of that figure, which is shown again below. In order to digest what is about to follow, a few brain cells will have to be temporaritly dedicated to the task. Because of the central importance of this figure, however, I can assure anyone who has some interest in the science of climate change that they will feel rewarded for their efforts. This figure really does provide the essence of what we are up against.
This figure shows the concentration of carbon dioxide thought to have been present in our background atmosphere over the last 800,000 years. All but the most recent of these measurements came from an “ice core record” provided by the EPICA Dome C research station located on a remote polar plateaus of Antarctica, about 1,300 miles from the South Pole and managed by a consortium of European countries. The CO2 trapped in the air pockets of that very long ice core are assumed to reflect the CO2 content of the Earth’s atmosphere at the time when the snowflakes that led to that bit of ice were first formed and settled to the ground. The age of each cross section in the ice core can be determined by counting the number of visible “rings” from the top of the core. These rings are formed by the increased transport of dust to the Antarctic continent during every summer season.
The reason for including this figure in the exam referred to above was to show the extremely sharp rise in CO2 observed during the Industrial Age at the very right edge of the graph where CO2 rises sharply from 280 ppm to 400 ppm. Nevertheless, most of the questions I received concerned the rest of the graph in which the CO2 level is shown to have risen and fallen several times between about 180 and 280 ppm over the 800,000 year time span. The bottoms and tops of these oscillations are known to be associated with the glacial and the interglacial periods, respectively, of the last 800,000 years with the last interglacial period in which we now live (called the Holocene) starting only about 12,000 years ago. The cause of these oscillations in and out of glacial periods is now well-known and will be explained in the remainder of this post.
A quick summary: These oscillations between glacial and interglacial periods are caused by three factors. One is the exact position and orientation of the Earth as it rotates around the Sun. Another is the decrease or increase in glaciation on the Earth as it either warms or cools. The third is the emission or absorption of carbon dioxide as the Earth either warms or cools. For more details of each of these factors, read on.
Over the last 50 million years, the Earth has been cooling and changing from the hot “water world” it used to be with sea levels about 70 meters higher than today. Due to plate tectonics, the continents of the world have also been drifting northward. India, for example, was an island in the Southern Hemisphere (SH) 60 million years ago and drifted northward until it rammed into the continent of Eurasia in the northern hemisphere (NH). As a result of this drift, a majority of the Earth’s total land mass today is in the NH with oceanic surfaces more prevalent in the SH. As we will see, this fact, along with the detailed way in which our planet rotates about the Sun combine to cause the glacial / interglacial oscillations shown in the first figure. So how exactly does our planet rotate about the Sun? That question will be addressed next.
The path of the Earth’s rotation about the Sun is defined roughly by a circle which the Earth traverses once each year. Simultaneously, the Earth spins about its rotational axis once each day. If that was all there was to the Earth’s orbit, however, there would have been be no glacial to interglacial transitions and the Earth would have had the same climate year after year during the last million years. The fact is, however, the detailed motions of the Earth relative to the Sun also include three minor twists and turns that are caused by the gravitation pull of the other planets in our solar system. These minor perturbations cause the natural climate changes we are trying to understand here and are called the Milankovitch cycles in honor of the Serbian mathematician who correctly predicted them back in the 1920’s.
One of these minor perturbations is to the orbit taken by the Earth. Its nearly circular path actually changes continuously between circular and slightly elliptical. The other two variations concern the rotational axis of the Earth. The angle of its tilt relative to the Sun continuously changes by a few degrees and it also wobbles (precesses) continuously. For more details concerning the Milankovitch cycles, I will refer you to Wikipedia. For our purposes here we only need to know that these subtle changes in our planet’s motions create continuous alterations in how our Northern and Southern Hemisphere is irradiated by the Sun. For example, whenever these minor orbital effects cause the north pole to be pointed more directly towards the Sun, the summer season then experienced in the NH will, of course, be somewhat warmer than usual. More on the importance of the NH’s summer later.
Next, we need to think about the extent of glaciation that exists throughout the Earth at any point in time. This has a large effect on the Earth’s temperature because incoming radiation from the Sun is effectively reflected back into space if that radiation strikes a snow or ice covered surface and tends to be absorbed if it strikes either the ground or the surfaces of the oceans. We call the fraction of sunlight reflected the “albedo” of the Earth. The total albedo of the Earth in its present condition, for example, is about 0.30 indicating that 30% of incoming solar light is reflected back to outer space. Therefore, increased glaciation of the Earth will increases the reflection of incoming sunlight and this, of course, will cause the Earth’s temperature to decrease.
Next, note that because the NH has much more land mass than the SH, the total changes in the extent of glaciation on Earth at any point in time will depend largely on what’s happening the NH – where between glacial and interglacial periods the southern extent of glaciation over North America, for example, changes from the position of Kansas in the USA to the Arctic coast line of northern Canada. Similarly large changes in glaciation also occur over the continents of Europe and Asia during a glacial to interglacial transition. The SH, by comparison, does not have such vast land masses over which large changes in glaciation can occur. None of Australia is never covered with glaciers and all of Antarctica is always covered with glaciers. Therefore, there is relatively little change in the extent of glaciation over those two continents of the SH and neither one contributes significantly to the the Earth’s total changes in albedo as it moves between glacial and interglacial periods. Thus, it is in the NH where large changes in glaciation and changes in the reflection of incoming sunlight can occur.
If you have followed the sequence of thoughts provided so far, you will now understand why we can declare the following simple rule of thumb: the direction of glacial / interglacial changes is determined largely by how “nice” the summers are in the NH. If by changes in the Milankovitch cycles, the NH is being provided “good” (that is, warmer) summers relative to the average, then the glaciers of the NH will retreat northward. On the North American continent, the southern edge of glaciation might thereby begin retreat from Kansas to Minnesota and through Canada and up to the Arctic Ocean. These changes in glaciation would then have a large effect on the amount of solar radiation that “sticks” to the Earth versus that which just “bounces off”. Thus, as more sunlight sticks as the glacier recedes, the Earth gets progressively warmer. Thus, a relatively small change in the Milankovitch cycles is greatly amplified by its effect on the glaciers of the NH. Conversely, when the Milankovitch cycles change so as to cause the NH to have cooler than average summers, the opposite events occur. The glaciers of the NH then begin grow and advance southward causing increased sunlight reflection and continuously lower temperatures..
OK, but we have not yet explained the changes in CO2 concentration observed over the last 800,000 years as shown in the first figure and the answer to this question also brings us to the third major influence on the Earth’s temperature. When the Earth is warming as it comes out of a glacial period, the oceans begin to warm as well, of course, and as they do they release a large amount of CO2 into the atmosphere. This release of CO2 occurs several hundred years after the initial period of warming caused by changes in the Milinkovic cycles and the Earth’s albedo. But from that point forward, the increasing level of CO2 provides a third driving force for continued warming during the following 5,000 years required to reach the next interglacial period.
An increasing level of CO2 provides additional warming because CO2 is a Greenhouse Gas (GHG) providing a “blanket” of thermal insulation between the surfaces of the Earth and outer space. The GHGs manage to do this by absorbing infrared (heat) radiation emitted by the Earth and its lower atmosphere and then reemit IR radiation in all directions including back down towards the Earth’s surface. Without the GHGs the major portion of the IR omitted by the surface would simply pass into outer space thereby creating a colder world. The amount of warming thereby caused by the continuously increasing CO2 level is then further amplified by causing increased vaporization of water, as well, which is ubiquitous throughout most of planet and is, itself, a powerful GHG.
As an addition to the above explanations, I will provide another figure showing ice core records obtained again form the Dome C site and also from the Vostok site some 600 miles away which is managed by Russia and provides the same type of information going back about 450,000 years.
This figure shows the temperature deduced from these two ice cores and in the bottom row shows the relative amount of glacial ice thought to be on the planet at all times over the last 450,000 years (deduced from separate geological measurements).
There are several points worth noting in this figure. One is that the temperatures deduced over time at the two different sites are in excellent agreement. Since these sites on the Antarctic plateau share the same background atmosphere, this result would be expected and lends credibility to the ice core analysis technology Another point of interest is that the variations in temperature with time closely match the variations of the CO2 level shown in the first figure – as would be expected if the causes of the glacial / interglacial oscillations provided above are valid. And finally, extent of glaciation or “ice volume” over the entire Earth does, indeed, correlate well with the rise and fall of temperature and CO2 levels, as would be expected if the explanations for the glacial / interglacial cycles provided here is correct.
That will have to do it for now in explaining the first figure shown above and the reason why we have had both glacial and interglacial periods over the 800,000 years. For more details and addition information, I can refer you to a more complete “short course” on the subject of climate change offered on my main web site, ericgrimsrud.com. Just go there and hit the short course tab.
Finishing with the BIG QUESTION: in our modern Industrial Age how can we expect to get away with increasing our atmospheric CO2 to levels so much higher than have seen in more than a million years? The answer to that question is clearly not yet known by anyone who takes the science seriously. We are presently doing an experiment on the only planet we have and most of our scientists who know a great deal about this think we might very likely be headed out of our present interglacial period in a direction that is opposite of that from which we came. In that case, we will be saying goodbye to mankind’s beloved Holocene and hello to a hotter and unknown period to be increasingly called the Anthropocene – whatever that place turns out to be.