how do environments change over time, and what causes them to change?
There are a lot of factors that contribute to environmental changes. The first critical concept is to accept that our environment is not static – it is always changing. The idea that climate change can be stopped is kind of like thinking that the Earth can be stopped – so that it stays in the same place in its orbit. It just ain’t gonna happen.
So, now that we have accepted that climate is variable instead of static, why does it change. The whole concept of “climate” encompasses a lot of different components – some of them are fairly stable, like the amount of sunlight that hits the earth, and some are a lot more variable like the detailed make-up of the atmosphere. Lets start with some of the more stable ones.
The amount of sunlight that hits the earth is pretty constant, but even that has some variability. As the sun’s activity changes, the amount of energy (light) that it emits also changes. Every 11 years, there is an increase in sunspot activity, which means that there is a change in the amount of energy that the sun emits, which (of course) changes the amount of energy that hits the earth. The amount of the suns energy that is absorbed by the earth is also variable – a lot of the energy is reflected back out into space. The exact amount is determined by the reflectivity of the earth. The amount of energy that is reflected back out determined the albedo, or how bright the earth looks from space. Once again, this number is fairly constant, but still has variability. An increase in cloud cover, snow cover, or pretty much anything that changes the color of the surface of the earth will change the albedo, thus changing the amount of energy that the earth absorbs from the sun.
Now, lets look at some of the more variable components. The detailed make-up of the atmosphere plays a huge role in climate. Small changes in the amounts of certain elements or compounds can have huge effects on the ability of the atmosphere to retain energy. As the suns energy enters the earths atmosphere, it is either reflected back into space, absorbed by the atmosphere, or passes through the atmosphere to reach the surface of the earth. The atmosphere can only hold so much energy – it is stored as wind, heat, and in a whole series of complex molecules that hold onto the energy or use it to form new compounds. As the chemical composition of the atmosphere changes, its capability to hold and store energy also changes, which in turn changes the temperature of the atmosphere.
So, how does the composition of the atmosphere change? Lots of ways. As the biota (all of the living things put together) on the earth changes, so does what they dump into the air. There was no free oxygen in the air until green plants started dumping it into the air (oxygen is a waste product of plant metabolism). Because the biota of the earth is also always changing, the content of the atmosphere is too.
Finally, that light from the sun can reach the surface of the earth. When it hits the ground (or a tree, or a building, or whatever), it has a chance to be reflected back through the atmosphere into space, or it can be absorbed by whatever it hits. If it is absorbed, it will either warm up what it hits, or (in the case of green plants) be used to drive metabolism. As the surface of the earth changes, what the sunlight hits will change, which will change the chances of it being absorbed or reflected. A good man-made example is paving – roads, parking lots, etc. Most paving is black, and is really good at absorbing the sun’s energy. So if, for example, we plot a great big parking lot down in the middle of Alaska, in the winter that parking lot will be a whole lot warmer than the surrounding snow because it will reflect a lot more light than the white snow surface does. One parking lot in the middle of Alaska won’t make much difference to the world environment, but a big area of pavement like any large city can (and does) make a difference. When you add up all the surface area of the world that is paved, you get a pretty good amount of extra energy being absorbed.
A natural (in some cases) case of surface change that effects the absorption of sunlight is changes in forest and plains. Large areas of wilderness undergo natural cycles between being grasslands/plains and forests. Forests tend to be darker colored, and are thus better at trapping the energy from the sun. As these natural cycles take place, they can drive climate change – but the relationship is complex – they not only drive climate change, but they are effected by climate change.
Finally, there can be sudden huge events that can trigger climate change. There are two excellent examples of this that we know about. The first is the asteroid that hit the earth and “killed off all the dinosaurs”. Is wasn’t the asteroid that killed the dinosaurs, it was the climate change that the impact caused. Similarly, when Krakatoa erupted, there was a “year of no summer”, and the climactic effects of the eruption lasted for decades. The reasons for the two changes in climate are the same: the event did the same major thing: they dumped a whole slew of dust into the air.
This dust changed the way that sunlight interacted with the earths atmosphere. A lot more light was reflected back out into space, and the light that wasn’t reflected was a lot more likely to be absorbed into the atmosphere. This means that the overall amount of energy that is trapped by the atmosphere/earth together is lowered (because more is reflected). IN the case of the asteroid, this was enough of a change to trigger a major long-term climate change. In the case of krakatoa, the change was much more short-lived, but still noticeable. Another effect of this is that all of the plant life on the surface that is relying on the sun’s energy to stay alive suddenly are starving – not as much sunlight is getting to them. Lots of plants die off. But wait: those plants are critical for keeping the oxygen in the air, so when they die, the balance is knocked even further out of whack. Then because there is less oxygen, the atmospheres ability to hold energy changes, and so on and so on.
Fortunately, the system tends to be self correcting – when it gets knocked out of whack, it self-corrects and swings back the other way – if it is too cold, the system reacts by becoming more efficient at trapping energy from the sun. But then it gets too warm, so it self-corrects again. Things keep self correcting back and forth in smaller and smaller swings until something else comes along and knocks it all out of whack again.