A Short Course in Comprehensive Thinking – Part 31 – Ultimate and Proximate Causes

Not all causes are equal.

In order to understand how things work and why events happen, which is the raison d'être of science and much of historical investigation, it can be helpful to distinguish various dynamics and rank them in importance.

On the political-economic front, the ultimate cause of the West’s meddling in and invasions of the Middle East is the oil, and everything else comes in a distant second place, although Western pundits act like the ultimate cause does not exist, in another one of those elephant-in-the-room situations. We just “stumble” over their oil wells, quite accidentally. Wink, nudge. Just who are the pundits deceiving besides themselves?

Another example is the temperature of Earth’s surface, why we have lived in an ice age for millions of years, and how humanity’s activities are warming Earth’s surface. The ultimate cause of Earth’s surface temperature is the Sun. If not for the Sun, Earth’s surface would be solid, with no watery ocean and a scant atmosphere, within three degrees of absolute zero, or about -270^o C, and obviously no life as we know it. The Sun has been slowly growing brighter over its lifetime, there are variations in its output, and the sunspot minimum in what is called the Little Ice Age likely was influenced by reduced solar output. However, the change was less than 1%, and scientists generally consider it unlikely that the Sun’s output has fluctuated significantly over its life so far, so the hot and cold phases of Earth’s history had other variables likely causing the changes.

After that ultimate cause of Earth’s warmth that dwarfs all else, there are proximate causes, and the most important of them is gases in Earth’s atmosphere that trap infrared radiation coming from Earth’s surface, and water and carbon dioxide comprise the primary ones. Water, however, only lasts in the atmosphere for about a week before coming out in precipitation and is unevenly distributed, while carbon dioxide lasts for more than a century and is evenly distributed. Scientists have concluded that carbon-dioxide levels set the atmosphere’s temperature, and its variation over the eons has been the most important determinant, after sunlight, of Earth’s surface temperature.

Earth’s surface has a carbon cycle, in which volcanoes spew carbon dioxide to the atmosphere, sedimentary deposition in the oceans moves carbon to the bottom of the surface’s carbon cycle, and oceanic plates are subducted beneath continental ones as carbon is returned below Earth’s surface. The volcanism that resulted from the formation and breakup of a supercontinent ended an ice age and initiated a hot period on Earth that lasted about 200 million years, when dinosaurs dominated Earth’s landmasses. About 50 million years ago, Earth began cooling off again, and for the past 35 million years, Earth has been in icehouse Earth conditions. A contributor to declining carbon dioxide other than declining volcanism may have been the building of the Himalayas by India’s slamming into Asia, beginning a little less than 50 million years ago. The exposed rock is weathered by rainfall that is slightly acidic, due to absorbed carbon dioxide, but the building of the Himalayas has been accompanied by a release of carbon dioxide from crushed sedimentary rocks that metamorphose. That net impact is still being debated.

Another contributor to the Little Ice Age was the genocides inflicted by the Mongol Hordes and Europe’s conquest of the world, as tens of millions of people died, maybe a quarter of humanity or more, and the regenerating forests absorbed carbon dioxide in doing so, making Earth even colder. During the past four billion years, the heat from Earth’s interior has had negligible impacts on Earth’s surface temperature.

After sunlight and greenhouse gases, the next most important variable of Earth’s surface temperature has been the position of the continents. Antarctica has been near the South Pole for several hundred million years, Earth’s ice ages during that time have always begun there, and the current icehouse Earth period began when ice sheets grew again at Antarctica. The other continents have moved more than Antarctica has, coming together and breaking apart in a cycle that is around 500 million years long. The gradual closing of a seaway that circled the equator contributed to the cooling, and the ice sheets on Greenland began forming 10 million years ago, during another cooling phase. When the gap between North and South America was finally bridged about three million years ago, our current ice age was on its way.

Changes in Earth’s orientation to the Sun likely comprise the next most important variable in our current ice age. Earth does not orbit the Sun in a circle, but in an ellipse, its axis of rotation is at a tilt from the Sun (which is why we have seasons), and its axis of rotation wobbles and the eccentricity of its orbit changes, which the scientist Milanković first described. Those fluctuations are primarily due to the gravitational effects of Earth’s neighboring planets. There are several periodic oscillations regarding Earth’s orientation to the Sun, including those which operate on 100,000, 41,000, and 21,000-year cycles. Over the past million years, ice sheets have grown and retreated roughly on the 100,000-year Milanković cycle, which surprisingly has the weakest effect of those three cycles, while the shorter cycles have also made their impact.

Milanković cycles seem to have caused the advance and retreat of the continental ice sheets that have buried northern North America and Eurasia, and positive-feedback effects have amplified the effect, such as ice reflects more sunlight than bare ground, so when ice sheets form, the sunlight reflection makes Earth even cooler. When the ice sheets melt, the opposite happens, which makes Earth’s surface even warmer. But scientists are always producing alternative hypotheses, and one is that variations in solar output are more influential than Milanković cycles in our ice age.

Another feedback effect is that colder oceans absorb more gases, such as oxygen and carbon dioxide, and when the oceans warm, carbon dioxide is released, which warms Earth even more. This is considered to be a leading proximate cause of ice-sheet fluctuations.

During interglacial warm periods, carbon dioxide and methane are released from Earth’s soil and ocean sediments, which are positive feedbacks that lead to more warming. There are myriad positive and negative feedbacks to the current process of Global Warming, but human activities dominate the proximate causes, and the carbon dioxide from burning Earth’s hydrocarbon deposits trumps everything else; it contributes between 70 and over 100 times to what volcanism does today.

If a contest was held to determine how to best turn Earth from icehouse to greenhouse conditions, the winning entry would be to mine and burn all of Earth’s hydrocarbon deposits, which is exactly what humans are doing. Humanity may achieve it soon, and the last time that Earth went from icehouse to greenhouse conditions, the greatest extinction in the history of complex life happened.

Longer-term trends have also impacted Earth’s surface and its inhabitability. Earth’s carbon cycle has inexorably buried carbon, and the carbon-dioxide content of Earth’s atmosphere has gradually declined over Earth’s history. Life on Earth is being slowly carbon starved, and plants have adapted to it. Around 600 million years from now, carbon-dioxide levels will decline to the point where the photosynthesis processes of most plants, including trees, will no longer be feasible, and forests like today’s will vanish. Even those plants that will survive longer, mainly grasses, which use a more carbon-efficient process today, will eventually die out, and animals will go with them.

The Sun is about 30% brighter today than it was 4.5 billion years ago, soon after it formed. In a billion years, it will be another 10% brighter, and it will evaporate Earth’s ocean around then. Earth’s ocean is thought to lubricate the tectonic plates, so that they can move. The end of the ocean will likely mean the end of tectonic plate activity as we know it (and declining radioactivity in Earth’s interior also slows plate tectonics and reduces volcanism), and Earth will become geologically inert, similar to the Moon and Mars today. By that time, only microscopic organisms will survive on Earth, similar to how it was before the rise of complex life, and they will eventually die out, too, before the Sun grows into a red giant and engulfs Earth.

While those longer-term trends will have ultimate-cause impacts for life on Earth, in the near term, other dynamics predominate, and the greatest is humanity’s mining and burning Earth’s hydrocarbon deposits, to fuel its industrial age. Do we really want to see how this experiment might end? I know how to end it, now.