LOS ANGELES, CA (Catholic Online) - We can't help but transport ourselves back through time into the shoes or sandals of some ancient denizen of a once vast and unrivalled city whose society is slowly descending into chaos and wonder, "how did this happen to us?"

Today, the collapse of modern civilization is the stuff of science fiction and horror, and as far from reality as any Hollywood blockbuster or the latest zombie thriller. Yet, history is a great predictor of the future and according to history, we are also doomed. 

But why, and how, remain the questions. 

If the answers lie in history, then it pays to delve as deeply as possible to find the facts. Over the past century, a small army of scholars has labored from one generation to the next to decipher what happened to one of the world's largest, most advanced civilizations, and why they disappeared into the sands of time. 

The civilization in question is the Indus or Harappan civilization, and you probably heard about them in world history class - but only for the space of a paragraph and then never again. In fact, the Harappan civilization seems to have been forgotten by all but legend until the 1920's, when the first major archeological discoveries of the civilization were made. 

What we have learned since is that between 3,900 and 3,000 years ago, this sophisticated urban civilization melted away into the countryside leaving behind ruins and legend. The people of the Indus valley constructed meticulously laid out cities with blocks built in grids so perfect, a modern urban planner would be envious. Plumbing existed, even back then, an invention that would not be rediscovered until the height of Roman civilization -- a thousand years later. 

The society appears to have been remarkably egalitarian, there are no palaces or mansions among the ruins to show where rich and powerful people may have once lived. 

According to the few stories that exist and the ancient harbors since uncovered, the Harappan people traded with Mesopotamia, were renowned for their arts, and developed a system of writing that remains unintelligible to us today, but surely provided the underpinning of the vast merchant society that once flourished there. 

So how did such a great and advanced civilization collapse? Why did they not instead conquer the world and spread their culture across the continents of Asia, Europe, and Africa?

The answer does not lie so much with the people as with climate. According to new research, climate change appears to have dried the rivers and streams the Harappan people relied on for survival. 

Despite the modern debate that rages in a few circles, climate change is a fact. But climate change isn't new - it's as old as the planet. In fact, climate change may be responsible for the collapse of nearly every ancient civilization the world has seen.

Researchers used satellite data to ascertain the course of ancient rivers through the region. They also collected sediment samples from inland to the shore. They assessed what crops were grown at various times. Armed with a wealth of data, scientists were able to reconstruct changes to the regional landscape and weather like investigators working a crime scene. 

What they found, specifically, was that the Earth received less solar energy that it had when the civilization flourished. Less solar energy meant fewer monsoonal showers to fill the rivers and lakes of the Indus valley. Increasingly deprived, over centuries, the people slowly relocated to the east where the land would not support large sprawling cities. Instead, they settled into villages and small farming communities, many that exist to this day. 

So the people did not disappear mysteriously, or lose their lives in some violent cataclysm, but rather the civilization died gradually, over a millennia until there was nothing left but legend and ruins to remind people that it once stood. 

The story of the Harrappan collapse is actually typical. Historians can point to the prehistoric civilizations of Africa, that once rose when the Sahara was small and lush green forests gave way to savannahs that swept the continent. It happened more recently to the Maya, whose civilization collapsed after a prolonged period of drought reduced crop yields and food supplies for millions of people over hundreds of years.

These collapses are not sudden. Instead, they are always gradual, taking place over centuries. The frontiers creep back in as resources and other outside influences push civilization back into the corners it came from. 

Like the Maya, the descendents of the Harrappan civilization live today, populating the lands of India and those further East, but they have little remembrance of their ancestors who were once the most sophisticated and advanced people on Earth.

Likewise, as great and complex as we believe our civilization to be, our descendents may have very little to remember us by, except our ruins. Global climate change will eventually alter American civilization in ways that we can hardly imagine. And much like our Harappan ancestors our descendants may one day stand amid the ruins of our crumbling cities and ask, "how did this happen to us?" 

© 2012, Distributed by NEWS CONSORTIUM.