We just looked at hydrogen gas, so let’s continue with the gas theme and look at carbon dioxide (CO2). As we have seen it is essential to life as part of the carbon cycle. Plants can not grow without CO2.
CO2, along with CH4 are simple carbon molecules. Carbon has the ability to have 4 direct bonds for each carbon atom. For CH4 these are single bonds with hydrogen. For CO2 they are double bonds with oxygen.
CO2 is a stable molecule which has zero calorific content (it does not burn / provide energy). It has industrial use in fire extinguishers, as dry ice (it has no liquid phase at normal pressure), and in carbonated drinks. It can be manufactured, but in Australia is likely to come from gas wells. In South Australia, the Caroline 1 well has produced CO2 commercially for 3 decades. It was discovered while drilling for natural gas in the SE and has been a major CO2 supplier since. You have probably consumed some of this CO2!
CO2’s biggest claim to fame in recent years is that it is a major greenhouse gas.
It is an inescapable fact of physics that various gases in the atmosphere help warm the planet. If they did not, the Earth would be a very cold place. Solar radiation passes through the Earth’s atmosphere and warms the Earth. Heat is radiated from this heated Earth and is absorbed by greenhouse gases and radiated once more. This is called the Greenhouse Effect (even though greenhouses don’t work that way!).
Even though the basic physics is simple , the process quickly becomes complicated on the Earth’s surface. For a start there are many greenhouse gases. Water vapour is the most critical, and the most controversial. For many years it was thought that the large amount of water vapour in the atmosphere would swamp any effect of the small amount of CO2. Other greenhouse gases would also have an effect. These include methane, CFCs, nitrous oxides, sulphur dioxide (reverse effect) and particulate matter (reverse effect).
In addition there are many possible reasons for fluctuations in the amount of light / heat reaching the earth’s surface. These include solar output (tied to sunspot activity), planetary cycles (Milankovitch cycles), albedo (earth’s ability to reflect solar radiation), volcanic activity etc.
It is the effect of all these together that create the arguments over what, if anything is warming the planet. All told, the current evidence is that CO2 levels are rising and the planet’s atmosphere and oceans are warming. How these are measured is another story! In addition human activity (anthropogenic) is undoubtedly adding CO2 to the atmosphere. We will get to all these issues over time.
The idea of a greenhouse effect started when scientists in the early 1900s were trying to show that low levels of CO2 in the atmosphere caused the Ice Ages. They did some straightforward calculations to show a temperature drop would occur with less CO2 ( before the controversies with water vapour etc) and it was likely that low CO2 had caused the Ice Ages. In more recent times it was found that, in addition to CO2 levels planetary cycles were a major influence on the development of the Ice Ages. These include precession, eccentricity and axial tilt and are cycles of the order of 20,000 to 100,000 years.
We now have estimates of global sea levels, temperatures and CO2 levels extending through geological history. Sea levels fluctuate on a regular basis (over millennia) by many metres, with sea level in the last Ice Age about 120m lower than present. The last Ice Age ended about 12000 years ago, with a rapid rise in sea level flooding coastal settlements and burying ancient reefs along the way. In contrast, about 55 million years ago we had a very warm period called the Paleocene / Eocene Thermal maximum. During this period CO2 levels were approximately double today’s level of 400 ppm in the atmosphere and plant and animal life flourished (no humans around then) and sea levels were significantly higher (no ice caps).
Since that time, Australia moved far enough north to isolate Antarctica which subsequently became perpetually covered in ice about 35 million years ago. Carbon was being buried very quickly during the warm period and CO2 in the atmosphere was steadily decreasing. These two effects likely caused Antarctica to become ice bound continually since then.
As the amount of CO2 decreased steadily from the Eocene, levels reached very low concentrations about 2 million years ago, allowing Milankovitch cycles to come into play. We entered the current Ice Ages at this time with long periods where the poles of the planet were ice bound and ice extended across the US, Europe and northern Asia. We are still technically within this Ice Age period (currently in an interglacial), although with current CO2 levels and warming we are very unlikely to head back into an Ice Age any time soon.
Humanity was nearly wiped out a few times during heavy Ice Age times, but flourished in the warm interglacials. Modern humans developed rapidly after the end of the last Ice Age. We may have flourished too much, as current warming is now a major threat. A major problem with current warming is the speed at which it is happening.
Carbon is recycled on a daily basis with plants converting CO2 to carbohydrates, then decay into hydrocarbons. As we have seen there are also cycles over geologic ages. During warm periods plants and animals flourish with vast amounts of carbon stored at the surface. Living (organic) material decays and is buried under the ground as carbon (coal) and hydrocarbons. At sea, massive reefs develop tying carbon up in calcium CARBONates. These are also buried over time, or sometimes uplifted and eroded. Carbonate cliffs around the world’s oceans are testament to higher sea levels and vast Eocene aged (and younger) carbonate reefs.
Methane, originally buried under the oceans escapes upward and is trapped in massive stores of frozen methane (clathrates) at the sea bottom in various parts of the world.
On the other side of the cycle, decomposition of rocks creates CO2 and volcanoes continually spew CO2 into the atmosphere. This is supplemented by hydrocarbons naturally leaking from underground reservoirs and also by human activity.
There is no steady state in this carbon cycle, no ideal or NATURAL level. The Earth and its geology and biology are constantly changing. Between 1645 and 1715 the Earth went through a mini Ice Age which coincided with a period of low sunspot activity (and low solar output). This was a period of intense hardship with famine and disease prevalent. Previous mini Ice Ages had similar problems. Whatever we can do about climate, one thing is certain – it will never be constant.
CO2 is dissolved to some extent in the ocean causing a more acid environment. Nutrients from farming practices etc add to this process. The ocean has a PH of around 8.1 (down from a pre industrial reading of 8.2) and so is not acid (a pH of less than 7). Acidification is problematic in that the ocean chemistry will change, affecting living beings and reefs. Paradoxically carbonate ions are less available and reef building organisms have a hard time.
Mass extinctions occurred during the Eocene Thermal Maximum in deep sea, shelled vertebrates. Reefs have proliferated during Cretaceous and Tertiary Geologic periods. This shows their potential resilience and ability to withstand change. We are entering new territory today due to the very rapid increase in dissolved CO2 in our oceans and the differences in our current ocean chemistry from that in the past. Marine ecosystems will change – we are just not sure how!