1recently, humans have become more aware of how their actions impact earth. governments, organizations, and individuals try to improve the quality of earth’s air and water by changing the way they do things. one area in which great improvements for the environment have been made is building and construction. this process, often called green building, is taking off in our country and around the world. the word green is used describe things that are good for nature because green plants can be a symbol of nature.

the author most likely italicized the word green in the first paragraph

I don’t know what you’re asking here. By italicise do you mean emphasise?

If that’s what you mean, then the author is definitely emphasising the word green in that paragraph. They used the word green to represent nature because nature is mostly green. Green is often used in recycling symbols to demonstrate how recycling can help the environment. So green reflects or symbolises nature and taking care of the environment.
These green buildings the author discussed, are buildings with a lot of trees and plants in them (I’m just guessing here). Green buildings reflect a more non-waste living and promote more natural aspects to the modern world.

Not sure if this helped...