I (will) use "sustainable" and "green" frequently in this blog. Even though I am a bit wary of them because they seem to be buzzwords at this point and will no doubt be stale far too soon. However, they concisely convey the idea of intelligent, thoughtful use of resources, a practice that will always be important even if the language changes.
The Earth is a closed system. Nearly. It's an open system but mostly in some trivial or negligible ways. Energy comes in from the sun at a fixed rate and is radiated as thermal energy. And only the occasional small probe clears our gravitational well. All the physical resources we'll ever have are already on the planet and the sun won't get any brighter.
While we haven't run out of land yet and there is still oil in the ground, these resources will eventually run out. The population is growing. Quickly. Energy use is growing. Quickly. People need land to live on. People need food to eat. Farmers need land to grow food. Everyone needs energy. Alternative energy is great. Global warming is a big deal. But even if we somehow manage to cease all CO2 output and undo a century of damage without changing energy usage, we will still have serious problems in the future.
Technology is seen as a cure-all by many. It will, of course, be a huge help. But complex natural systems are not linear. When you push an ecosystem to the brink and it collapses, you can't just back up to the point of collapse and expect everything to go back to normal. In engineering terms, this is called hysteresis. It will be much more cost effective to solve these problems before they assert themselves.
Life on earth is a billion years old because systems evolved to recycle every last scrap of anything. We won't make it another hundred unless we follow suit. It is inevitable that solutions to these problems will be turn up eventually, humans are too resilient to be killed off entirely. We're like roaches in that regard. But unless we are proactive about finding the answers, billions of people will suffer or die before the answers find us.