Course Introduction

y the end of the 20th century the human population of Earth equaled 6 billion. Or at least it probably did! Why can't we say for certain? The problem is that the number of human inhabitants is not known with much precision - some countries don't take a population census at all, some don’t publish their census figures, some are known to be unreliable (states in conflict or with suppressed minorities); even in western countries the census is probably only good to about 2% -- meaning the population could be 2% bigger or 2% smaller than the published figures. Today the human population of our planet is:

6.1 billion +/- 250 million.

That's about 4% total uncertainty. 250 million is about the population of the U.S., so we know the human population of the world to no better than the population of this country.

Why does this matter to a course about the Earth?  There are two very important reasons:

The shear number of humans on Earth, with their intelligence, their industry, their drive to improve themselves (and their moral drive to improve the situation of others), has caused humankind to become "geological" in scope and magnitude. The actions of humans have become an integral, interacting, component of Earth's complex web of systems. And as population grows humanity will, if we have not already, come to be the dominant component of Earth's many systems.

21st Century Earth Science is rapidly moving toward the study of interactions between human and physical systems with the objective of understanding them "well enough" that we might be able to predict their future state with useful skill.

But first; we must have an understanding of Earth's physical systems. How well do we know our planet? How well can we see its future? What are the limits to inference about our planet and its future? Can we consider such questions absent any consideration of the human contribution first, then convolve this information with information from social and behavioral sciences to understand how humans might live well on a changing planet?

That is the basic concept for this course. First; we need to understand our planet.

Here's one way to think about what we will try to achieve in our lectures.  Imagine you are a visitor from another world and you have come to Earth for one semester. You have been given the mission of learning enough to be able to make the entry under "Planet Earth" in the encyclopedia of your home world. What would you write? This is by no means a trivial question. You don't have much time and it is immediately apparent on arrival that the Earth is a large and complex object. Solids, liquids and gases are present and all are in some sort of motion, even the solid part, and there are distinct interactions between these components. There are numerous life forms and it is apparent that one species - humans - has come to dominate the earth to such an extent that it can alter the earth's solid, liquid and gaseous systems. Some of the humans seem to be worried about the way they are changing these systems; others don't. Somehow the basics of this planet have to be described usefully, without an undue amount of precis.

The task of the visitor and the objective of this course are fairly similar. I will try to impart some of the essentials of what we know about our planet. To do this I will use some of the critical results of modern Earth science research, placed in a background of some historic studies of the Earth. I will emphasize physical aspects of the Earth -- Geophysics -- rather than the chemical aspects that will be described by Professor Langmuir in his lectures.

In addition to the simple transfer of information (me describing to you some things that we know about the Earth) I will explain how we have come to know these things. Remember that most of the Earth is fundamentally inaccessible. Except for the surface layers we walk on, and some layers that have been exposed by tectonic motions from original depths of maybe 10 kilometers, the great majority of the Earth cannot be examined first hand. In this way the deep Earth is like outer space or the inner atom; we are required to infer what is going on in the Earth's deep internal processes from expressions of those processes on the surface, a very great distance from where they are acting. Just how this inference is done is an underlying theme of these lectures.

This inference has limits which means that the knowledge we have always involves some measure of uncertainty. Almost nothing we know about the Earth is known with certainty. This is not unique to Earth science, of course, and actually applies to all of physical science. We are also going to discuss those limits and uncertainties, even though, in most college Earth science texts, such limits are almost never discussed. One of the most important reasons for attempting to understand limits and uncertainties is because they fundamentally govern our ability to predict the future behavior of the Earth; to say when an earthquake might take place, a hurricane might make landfall, or a dramatic shift might take place in Earth's climate.

If we can predict we can anticipate, and if we can anticipate we can prepare and perhaps mitigate some of the harmful affects of Earth’s behavior on humankind, and be wise about humankind’s behavior with regard to the Earth.