Ecology

13.       Ecology

The earth’s climate is a highly complex system, as we are increasingly coming to understand in this era of global climate change.

For one thing, there are ice ages, and within each ice age, alternating cold and warm glacials and interglacials brought about by a number of factors including three cyclic changes in the earth’s orbit.

Firstly, there is the eccentricity cycle of about 93,000 years, secondly there is the change in the tilt of the Earth's equatorial plane roughly every 41,000 years, and thirdly there is the 26,000 year precession cycle.

These cycles do not (for once) involve feedback, but there are a huge number of feedback mechanisms which do influence the earth’s climate and ecology.

Just to give one example, there has been considerable interest of late in the possibility that marine plankton may contribute to global climate control by the production of DMS (Dimethylsulphide), which can influence cloud formation and hence sunlight falling on the oceans (and the plankton).

We are still learning about these mechanisms, and all of them are not fully understood. We do, however, know enough to be confident that increased carbon emission from human activities are responsible for increasing global warming.

Fires and Oxygen

I have already talked about fires in Thermal Runaway, but now I want to take that to the global picture.

One of the facts that I recall best from James Lovelock’s book Gaia was the remarkable control over the earth’s oxygen level. It has remained at a very constant level, as a percentage of the earth’s atmosphere, for millions of years. How is this so? Well, as Lovelock so nicely describes, there are feedback mechanisms that keep it there.

It comes, at least to those of us in temperate climates, as something of a surprise to realise how many thunderstorms there are in the world. I have seen different estimates of 1000 and 10,000 present at any one time. Anyway, its a lot. And these storms start a lot of fires, of course many of them are never seen by many of us, as they are deep in forests. The likelihood of lightning starting a fire depends on two things, how dry is the wood, and how much oxygen there is in the atmosphere.

If the oxygen level were to rise by even a small amount, then a great many more fires would start. These fires would consume more oxygen, but also reduce the oxygen producing plants, and thus reduce the level of oxygen. The sensitivity to the oxygen level is acute; a 1% rise in oxygen level increases the probability of starting a fire by 70%. If the oxygen level were 25%, even damp grass would ignite. Conversely, at the current oxygen level, even a 20% moisture content reduces the probability of ignition to only 1%.

The whole control mechanism of oxygen level in the atmosphere is very complex and not yet fully understood, involving the oxidation of rocks and animal life (which consume oxygen),  plants, bacteria and sea life (which produce it). Burial of carbon and its release through the production of methane comes into it too. In fact, forest fires are a pretty small part of the equation, which makes it convenient that the level is kept where it is.

It seems a reasonable assumption that, if you find any variable in a complex dynamic system that is staying at a constant level, then there must be some negative feedback mechanics that are keeping it in place. In a modification of the ‘If it ain’t broke, don’t fix it’ slogan, I guess you could say ‘If it don’t move, it must be fixed’.

Forests and Deserts

Another feedback loop involves forests and deserts. Forests do not reflect a lot of sunlight, and so absorb a lot of heat. This leads to higher evaporation, cloud formation and rainfall. The rain enables forest growth to be sustained. If the forest is reduced from some cause (logging, clearance, insects, fires), then the amount of rainfall is reduced, which can lead to an increasing loss of forest and a process of desertification.

Forests also are involved in global climate change because they are a major carbon store. Deforestation from burning leads to more carbon in the atmosphere, which leads to global warming, which may also lead to disruption of tropical rainforests. This is yet another potentially dangerous feedback loop, with global warming and loss of forest chasing each other in a vicious spiral.

Its actually more complex, because deforestation can also involve changing patterns of land use, such as cattle ranching, which itself can lead to increased methane in the atmosphere.

These global feedback mechanisms can unfortunately be very sensitive things. It has been recently estimated (well, using a model) that the Sahara desert could be eliminated by planting vegetation to just above 18 degrees north of the present rain forests. A small distance less than that would result in the vegetation dying back to desert again.

Clouds

Now this is where you are going to have to remember that stuff about the water cycle that you learnt in school – come, on you must have done. My kids did it about three or four times.

Water falls as rain, then evaporates with heat from the sun, forms clouds, clouds make rain, OK?

Clouds reduce the amount of heat from the sun reaching the earths surface. So an increase in clouds could reduce the surface temperature of the earth. This reduction would reduce the amount of water evaporation, and so reduce the amount of clouds. Likewise a reduction in clouds would lead to an increase in earth temperature, and so more evaporation, and more clouds. This is our old friend negative feedback keeping things nice and stable – we hope.

As usual, the earth is a bit more complex than that, and we also need to consider things like ocean currents and wind systems which move the heat round a bit.

Snow

Snow reflects sunlight pretty well, so if snow starts to melt in one of the snow covered regions, more sunlight is absorbed because the ground is now darker. This causes the ground to heat up and melt more snow in a postive feedback loop. Remember, positive feedback – bad.

Once again, this process also works in reverse, so an increase in snow can lead to the earth to get colder, and colder, and colder. There is currently an idea doing the rounds that once there was a ‘snowball earth’. Actually, it would have been more of an iceball, but that doesn’t sound so good. The idea is that the earth went through a bad positive feedback loop that ended up with the whole earth covered in ice.

The idea was originally rejected, becase it seemed there was no way of escaping from this rather unpleasant state (temperature even at the equator would have been around -20°C). Now there are suggestions that the process could be reversed, either by a slow process of heat convection, or by a rapid heating up from a volcano blitz. Or maybe something else. The fact is, no one really knows yet if this ever happened, but it makes a good story.

Gaia

As I mentioned earlier, James Lovelock brought all our attention to the many complex ways in which the earth’s systems are kept in balance in his book Gaia, which has now become a byword for the earth’s eco system.

Something as big and complex as the earth has got to have a lot of feedback going on to keep things in balance. Lovelock pointed out that the fact that the earth was not in the state of balance of a lump of rock was sufficient evidence that life existed. If there was no life on earth, the oxygen level would very rapidly fall to zero, as oxygen is a highly reactive element that would combine with others until there was none left. Just think of how quickly iron can rust if unprotected. In fact, oxygen was effectively poisonous to early life forms; we can live in it because we have complex mechanisms to stop us becoming rusty (like skin).

I already mentioned oxygen balance, but the whole atmosphere is kept in a remarkably steady balance, with levels of gases from Nitrogen, Nitrous Oxide,  Ozone and Carbon Dioxide all being maintained. Whoops, did I say Carbon Dioxide? Well, of course that opens up the whole greenhouse gas, global warming Pandoras box. The danger here is that the earth’s feedback mechanisms cannot cope with the rapid increase in Carbon in the atmosphere that has been caused by man’s activities.