The word "sustainable" has just become part of our regular lingo. It's sometimes used in place of other words about the environment. As a result, we often don't pay attention to it. We should pause and consider what it means--if something is not sustainable, that means--by definition--that it cannot continue perpetually. The system will eventually run out and run dry.
If topsoil is eroding faster than it is being created, then once fertile farmlands will turn into deserts. Many of our utilities are powered by coal, but we know that coal will run out and contribute to greenhouse gases and toxic air pollution in the meantime. If we cut down trees to burn wood but do not replant new ones, we will eventually run out of trees--or at the very least, reduce how much forestland covers the earth which reduces habitat for wildlife. There are limits on these resources.
It does not require climate scientists showing mathematically sophisticated computer models to see these examples. If we accidentally dump motor oil into our morning coffee, we have polluted it. Our delicious morning brew is now undrinkable. We are simply saying that this is happening on a larger scale. For example, pollution from burning coal ruins river and lakes, and makes them unfit for swimming, fishing and for drinking water.
The earth does not have an infinite ability to process all these toxins at the rate we are producing them. Humans have historically assumed that the earth was a "bottomless pit" that could absorb whatever trash we threw her way. We now realize that just as we can pollute small areas, like our cup of coffee, we can also ruin large areas, too--up to an including the whole climate of the planet.
Some people accuse the pope of being hopelessly unrealistic, but it's the opposite: The current system is unrealistic. We have gotten accustomed to something that is doomed from the start. The good news is that we do not have to live this way. It is going to take all hands on deck--from individual households to companies to governments. We can all evaluate how our decisions affect the common home we all share.
Imagine I am going to take a car ride from Columbus, OH, to my office in Fairfield, OH. Google Maps says it is a 133 mile trip one-way. I have this wonderful car--a gift from my father. It is not mine, though. It's for my children, but I am free to use it until they are old enough. However, I only have enough gasoline to go 100 miles. In addition, the oil has begun leaking out.
Nobody in their right mind would support my decision to do this. If for some reason the oil does not completely leak out and burn out the engine, ruining the car and risking a highway accident, then the gasoline will eventually run out. The math is easy: Attempting to go 133 miles when there is only enough fuel for 100 makes no sense. This is compounded because I know there's an oil leak. What a sad way to treat a gift that is really meant for my children to enjoy.
This is an analogy of what we are doing with the climate. We are on an unsustainable, suicide mission. On the surface, the car looks fine. It runs well enough. However, looks can be deceiving. Sometimes you need to pop open the hood to get the real story. By the time the car shows symptoms of the problems, it may be too late. But if you are really honest with yourself--you don't really have to look under the hood. You already knew the math doesn't work.
The same is true for the earth. If you take a look outside your bedroom window, you may see birds flying, trees thriving and green grasses. There may be hot days and cold days. It takes a scientist to pop open the proverbial hood to give us another look--bee populations (who do critical pollinating) are dying off. Most rainwater has pesticide residue in it. Virtually all seafood has some level of toxins in them. Plastics are being absorbed into the food chain. The earth's temperature has been steadily rising since the industrial revolution began and the forests and grasslands to neutralize these greenhouse gases are being chopped down. Everything may start to look more bleak.
We can treat the earth with the same common courtesy we would treat a neighbor: If you borrow their car, you return it--with a full tank of gas. If you make a mess, you clean it up. If you damage something, you fix it or replace it. It's just responsible stewardship.