In this series of blog posts, I attempt to give an overall view of the physics/chemistry-based climate science dealing with climate change and today’s global warming. I do so because I can’t find an overall summary such as the one I’m about to try to create. My hope is that readers will understand why this science makes me so alarmed and seemingly so pessimistic. As always, misunderstandings and misstatements are my fault and do not reflect on the science itself.
Let’s begin with sunlight. The sun’s light is always accompanied by warmth/energy. Even on barren Mars, without an atmosphere to “contain” that warmth, temperatures at the surface are 100 degrees F below zero, only part of which is due to the planet’s internal heat. The rest is sunlight striking the surface during daylight and giving off heat that is absorbed and radiated by surface materials.
The actual amount of absorption and heating depends on the “color” of the surface (or, in scientific jargon, the “albedo” of materials). More specifically, think of the color spectrum you were taught: white reflects all (except perhaps infrared) radiation, and absorbs no or very little heat. Black absorbs all (except perhaps ultraviolet) radiation, and therefore absorbs a lot of heat. On the Earth, water is blue and absorbs a moderate amount of heat; ice and snow are white or off-white and absorb very little heat; and soil and rock tend to be brown and black and absorb lots of heat. Likewise, land covered by vegetation tends to be light to dark green and absorbs much more heat than the sand that would otherwise predominate on land – and that doesn’t even consider the effects of photosynthesis.
Finally, the atmosphere of Earth takes reflected light and reflects it back to Earth, adding yet more warmth. The amount reflected depends on the amount of certain elements in the atmosphere.
The warmth of Earth therefore consists of three layers, added over time:
1. Light striking the surface of the planet, heating it to perhaps -100 degrees F;
2. The atmosphere that both reflects some light back to the surface and prevents water from evaporating into space – resulting in oceans that absorb more light/heat, raising the temperature to perhaps -30 degrees F; and
3. Animal and vegetative (plant) matter, both dead and alive, that absorb still more light/heat and raise the global average temperature to 56 degrees F – in a “steady state” climate.
Carbon Dioxide and Other So-Called “Greenhouse Gases”“Greenhouse” is a misleading term for what these gases do, which is to reflect light from other parts of the spectrum than are handled by oxygen and hydrogen (the main components of the atmosphere). However, compared to oxygen and hydrogen, these can vary much more over long periods of time. In the case of carbon, a “steady-state” value is about 250 ppm, but historically carbon has been above 1000 ppm and below 200 ppm at various times.
Carbon is one of six elements in the periodic table that is constantly cycling between the atmosphere and the surface of the planet. Life forms are carbon-based, so animals and vegetables (in the sea as well) constantly add carbon that is either buried as part of the animal/vegetable fossil, or released to the air via breathing or burning (in the case of forests). However, in order for the amount in the atmosphere and the surface of the Earth to roughly balance, carbon must also be absorbed by “weathering”, wind/rain erosion of rocks that exposes new rock with which carbon can combine. These are usually eventually washed down to the oceans, which equalize with the atmosphere by agitation that propels the carbon into the air. In essence, then, over the long run this cycling stabilizes carbon in the atmosphere at about 250 ppm. The “half-life” of carbon in the atmosphere is about 100 years, so if no cycling upwards were going on it would take about 200 years to drain the atmosphere of carbon.
Probably the only other “greenhouse gas” relevant to climate is methane, which is CH4. Without me going into a long discussion of methane, you should know that it, too, has seen a massive upsurge in emissions over the last 165 years due to human emissions. However, the half-life of methane is about 9 years, and the amount of emitted methane needed to have an impact on global temperature comparable to carbon is about 4-10 times what is presently being emitted per year, while the amount unlockable in shallow waters or permafrost in any given 9 years, even at our accelerated pace of warming, is probably less than that. Rather, because methane contains carbon, the long-term worry is the possibility that the methane in permafrost will be converted primarily to carbon, which would add about 0.6C (guesstimate) to hundreds-of-years global warming.
Plate Tectonics and the Milankovitch CycleThere are two main ways that atmospheric carbon can deviate significantly from the norm, absent human intervention. One is predictable, cyclic variation over a period of 100,000 to 200,000 years: the Milankovitch cycle. The other is underwater volcanism that ejects carbon, typically while moving one of the Earth’s plates.
The Milankovitch cycle results from three changes in the Earth’s orbit around the Sun:
1. The Earth wobbles around its axis of spin;
2. The Earth’s orbit at some times of the year takes it closer to the Sun, at others further away;
3. Like a rubber band, the Earth’s yearly orbit sometimes becomes more like a circle, sometimes more like an ellipse.Visualize these in your mind. At about the point where it is winter in the Northern Hemisphere (where most of the land is) and where the other two effects place the Earth farthest from the Sun, an ice age is kick-started. The temperature descends gradually as ice encroaches downwards from the Arctic Ocean over land, changing the albedo in the areas affected. Less carbon is exposed, and so less carbon is emitted into the atmosphere. This continues until the three effects are closest to the Sun, in Northern Hemisphere’s summer, when a pretty rapid rise in temperature and carbon occurs, reaching a steady state that lasts for about 50-100,000 years.
The underwater volcanism effect is much less frequent but can be far more powerful in its effect on atmospheric carbon and global warming. In the most recent example, about 55 million years ago, continuous underwater eruptions in around the plate then near the South Pole sent it steadily northwards to crash into the Eurasia plates, yielding India plus the Himalayas at the point of impact. During this period, which is unlikely to have lasted less than 20,000 years, steady output of carbon into the atmosphere kept the atmospheric carbon above 1000 ppm. The results were high temperatures (about 7 degrees C more than the present time), mass extinctions of land and sea flora and fauna, and very high sea levels – so-called “hell and high water.” Mass extinctions, high temperatures, and high sea levels have also been confirmed for the previous such atmospheric-carbon rise (about 155 million years ago).
Also noteworthy is what happened after the eruptions came to an end. Atmospheric carbon decreased back to its “steady-state” level, but only slowly. In the case of the most recent such episode, atmospheric carbon took 50-54 million years to return to “steady-state”, reaching it only 1-5 million years ago. The reason is that the oceans were in effect saturated with carbon: most of any decrease in atmospheric carbon was offset by fresh contributions from the ocean, while “weathering” that returned the carbon to the planet’s interior worked only slowly to end that saturation.
And another factor worthy of note is the composition of the carbon dioxide. Carbon dioxide put in the air during one of these extraordinary periods is more acid than “normal” CO2. Therefore, the atmosphere and rain are both more acid than in “steady-state” periods.
SummaryThe Earth’s climate can therefore be said to be a process that operates to keep climate relatively stable both in the short (10,000s of years) and long (billions of years) term, but where too large a deviation from atmospheric carbon stability has the opposite effect: it drives and maintains further deviation to a new “semi-steady state” that lingers for a while even when the main impetus for deviation is gone. To cite one example: we are presently in the late stages of the “high-temperature” phase of the Milankovitch cycle; what some scientists call the “Goldilocks” climate (not too hot nor too cold for human purposes). Absent human-caused carbon emissions, we would have expected to see a slow descent into an Ice Age begin in less than 10,000 years.
The critical factor in creating both Milankovitch and plate-tectonic deviations from a “Goldilocks” steady state is atmospheric carbon. In the case of the Milankovitch cycle, increased/decreased sunlight is the initial cause of warming/cooling, followed by a feedback loop between sunlight absorption and carbon emissions. In the case of underwater volcanism, the atmospheric carbon itself is the initial cause of warming, followed by a feedback loop between sunlight absorption and carbon emissions as well as further volcanic carbon injections.
A minor note: Above a certain point, atmospheric carbon would become so prevalent as to drive global temperatures above the boiling point of water. The oceans would then evaporate, and from then on global surface temperatures would be such that acid (from the carbon) rain would simply evaporate before it reaches the surface, and no life apparently could survive. The planet Venus now operates in just such a way. Luckily, even if all fossil-fuel reserves were used, we cannot presently reach that point of atmospheric carbon. However, heat from the sun increases at the rate of 1 degree C every billion years, and therefore, at the earliest, it would be possible for Earth to turn into Venus 900 million years from now.