So people say of evolution and climate change, but what does “theory” really mean?

“It’s just a theory” is an easy phrase, often used to dismiss scientific assertions with which one is uncomfortable. Evolution and climate change are two areas that quickly come to mind.

Actually, the word “theory” is used to denote a whole range of related concepts. So let’s start with much less controversial example, gravitation.

When a cup slips out of your hand in the kitchen, it falls reliably down to the floor (often breaking in the process). We are so accustomed to this quite amazing phenomenon that we take it for granted.

Yet two of the greatest physicists in history, Newton and Einstein, have theories of gravitation named after them. And the theories are quite different.

Why do we need theories about something that is so evidently true? Well, in the first place, the observation that things fall is not comprehensive. How fast do things fall? Do they do so everywhere, or only in our corner of the universe? Newton’s theory of gravitation holds that any two objects are attracted to one another according to precise mathematical rules. When one object is small (an apple) and one is large (the earth) attraction moves the apple a lot and the earth only a little. So the apple seems to fall. Planets are in orbit because the gravitational pull towards the sun constantly deflects straight-line motion into a circular path. The same is true for the earth and the moon. So Newton gave us quantitative descriptions of gravity, from dropped cups to heavenly bodies.

We verify the observation that things fall a hundred times a day. Newton’s theory provides exact predictions of motions that have been checked for centuries by astronomers and others. Newton’s theory is actually a fact.

Where does Einstein come in? He gave us a mental picture of how gravity works: that objects—masses—distort space as a bowling ball distorts a trampoline, and that the process of falling is simply rolling down the very hill that the mass created in space. Einstein also realized that objects that move very rapidly—close to the velocity of light—become more massive. In the low-speed world of Newton, masses never changed. So Einstein showed us a new mental picture, and also that Newton’s theory was valid in our everyday world, but that it had to be modified for objects at very high speeds. Einstein’s theories have also been shown to agree very closely with experiments. For example, particles in atom smashers circulate at speeds near that of light and become predictably more massive as they do.

Theories start with well-established observations, and reinforce them with quantitative descriptions, new insights and understanding, and extensions to realms beyond the original observations. Fully established theories become as much facts as the observations that gave rise to them.

All of us agree on the observations and the theories of gravitation. The Flat Earth Society has effectively disappeared. But what about a much more controversial topic, like evolution? Here too there are key observations like the “falling cup.”

First, the earth is very old. This is less straightforward than the falling cup, although a trip down the Grand Canyon showing dramatic geological layering is pretty convincing. But the age of the earth comes from measurements of the decay products of radioactive materials, and these decays have been measured with impressive accuracy. The current best estimate of the earth’s age is about 4.5 billion years.

Second, records of life have been found that span most of this period. These range from tiny bacteria-like creatures appearing about 3.6 billion years ago to the first mammals about 300 million years ago. A wide variety of techniques has been used to detect and measure the age of these life forms.

Third, the DNA of organisms that are alive today both confirms the two observations noted above and makes them more nuanced and richer. The differences between human and chimpanzee DNA are small, while the differences between human DNA and that in bacteria are dramatically larger. Because DNA changes accumulate over time, the changes can be used to project into the past to look for common ancestors. The observations based on DNA similarities match very well with those from rocks and fossils.

Unlike gravity, of course, the discussion of evolution is clouded by religious beliefs that seem to conflict with the observations. Genesis and Darwin are not easily reconciled, and divine intervention and scientific observation represent very different concepts. Humans are very good at compartmentalizing their beliefs, accepting the science that develops a drug or creates wi-fi, yet denying the science that explains our origins billions of years ago. However, hidden assumptions about the continuity of time lie behind all observations. As you read this page, it could be that the universe was just created, and that we came nicely equipped with memories and histories.

What Darwin did was to provide a framework to understand how life unfolded through a process of descent with modification, mediated by the influence of natural selection to weed out the least fit: evolution. Darwin of course supported his theory with huge numbers of observations, recorded on his famous trips.

Climate change differs from evolution and gravitation because it leads to calls for social or governmental action—evolution has not led to demands that we restrict our driving or that we levy carbon taxes.

What is really going on? The story is messier than those above.

Most scientists studying climate change agree that:

• The earth’s surface has gotten significantly and steadily warmer over the last century.

• Human activity causes much of this by generating heat-trapping carbon dioxide (CO2).

• Continuation of global warming will cause severe and destructive climate change, with drastic consequences for human habitation.

Very few seriously dispute the first conclusion. Air temperatures are without doubt on an upward trend, even though there are a lot of small peaks and valleys in the graphs. A more stable source of temperature information is the oceans, where the heat deposited has been going up steadily and ominously.

The second point is more controversial but is backed both by “falling cup” observations and computer models. The earth is truly a giant greenhouse, and carbon dioxide has to have the same effect on the large scale as it does on the small. In addition, historical records of atmospheric CO2 levels have been obtained from air bubbles trapped in glaciers thousands of years ago. These levels parallel the comings and goings of glaciation.

Clearly there are residual issues of whether CO2 causes warming, is merely associated with warming, or both. Are there factors we have missed, or factors like variation in the sun’s warming, that could be contributing in ways that we do not assess correctly? The full story is nuanced and complex.

The last conclusion belongs to the realm of prediction. It is based on extrapolations of existing trends and sophisticated computer simulations. It also represents the interface of science and society and government. What do we believe? How do we take action? If you are serious about thinking through these issues, these are the two places that need the work. Unfortunately, the impartial information you need is not so easy to come by.

Eaton Lattman, PhD, is director of the Hauptman Woodward Research Institute.This column is one of an occasional series exploring news about science and technology in and around Buffalo.