I recently asked my students this question on a quiz (not in those words) and it got me thinking. It may seem like an easy answer at first, but once you start thinking about it (too much in my case), it gets a bit more complicated.
So let's stick to the basics...
Environmental science lies at the intersection between people and the Earth - and it is not just one science.
Rather, it is an interdisciplinary field involving many of the natural and social sciences. The natural sciences include biology, ecology, and physical sciences such as chemistry. The social sciences include sociology, economics, and political science. Many of the natural sciences overlap each other, as do the social sciences.
The natural sciences tell us how the Earth, or environment, works. The social sciences tell us how people and economies work. Environmental science, therefore, tells us how the environment works, how people work, and more importantly, how we impact the environment and how it impacts us back.
So, what is "the environment"?
The environment can be divided into four main "spheres":
1) Atmosphere (Air)
2) Lithosphere (Land)
3) Hydrosphere (Water)
4) Biosphere (Living Things)
These spheres are constantly exchanging energy and matter with each other, absorbing our pollution, and making life on Earth possible. Every day, each one of us relies on these four spheres to live. No need to get into the science, but no one can last very long without all four.
Given that, environmental science also asks: "How can we can continue to develop and improve our standard of living while preserving the life-sustaining capacity of the Earth for future generations?" This is more often referred to as sustainable development, or sustainability.
The most important thing to keep in mind, however, is that the Earth will be fine. It has been through much worse in its 4.5 billion year history than us measly, little humans. We won't be fine if we don't clean up our act. Rather than saying "save the environment," we should be saying "save ourselves, from ourselves."
I hope this helps. And I hope my students read this!