We know that Earth’s temperature is increasing. We have looked to the sun, to natural cycles, and to other components of the natural world to see if these can explain our present warming— and they cannot.

We do know that human production and atmospheric levels of heat-trapping greenhouse gases have been increasing over the last century. Chemistry 101, that’s been around since the 1700s, tells us that these gases trap heat. And so it is only logical to suspect that these gases may be holding heat inside our atmosphere that would otherwise escape to space.

So the next place to look is right here at home. Is there a logical chain of evidence to connect human production of heat- trapping gases to the changes we have seen taking place around the world? Can a “human fingerprint” be lifted from the evidence surrounding these recent changes? Are we indeed responsible?

The vast majority of scientists say yes. Today, all major scientific organizations in the world are virtually certain that most of the change observed over the last fifty years has our fingerprints all over it. And here is why.

The heat- trapping properties of greenhouse gases have never been in dispute. This can be easily verified in any chemistry laboratory with equipment that has been around for hundreds of years. We know what these gases do.

We can easily compare how much carbon-producing coal, gas, and oil we have burned over the last 150 years to what has happened with carbon dioxide levels in the atmosphere. And what we see is that changes in atmospheric levels of carbon dioxide began with the Industrial revolution.

This figure compares our production of carbon dioxide from burning coal, gas and oil with carbon dioxide levels in the atmosphere:

Production of carbon dioxide from burning coal, oil, and natural gas has increased exponentially since the beginning of the Industrial Era, causing atmospheric levels of carbon dioxide (CO2) to rise from 285 to 385 parts per million.

Here's where you can get the data for atmospheric carbon dioxide (also here), and carbon dioxide emissions.

It would be easy to prove that the increase in greenhouse gases from industrialization caused our temperature change if we just had a second earth. We’d need one identical to our own, except with no people on it. Then we’d be able to observe what conditions were like on that second earth without any changes in greenhouse gas levels. We could then compare one planet to the other to verify the effects that people and industrialization were having on the planet.

But, of course, that’s not possible.

Instead, scientists create a series of virtual earths, complex models of Earth’s climate system. These models can be used to figure out what might be causing the unusual temperature increases we’ve seen. These models also help us determine what to expect in the future.

Contrary to popular belief, these models are not based on simple statistical correlations between temperature and greenhouse gases. No, figuring out what is causing this warming is much more complex than drawing a line graph showing temperature and carbon dioxide both going up. After all, that could just be coincidence. Instead, these models are based on fundamental physical principles— conservation of energy and momentum, heat, and pressure. Dividing the world up into finer and finer grids, climate models today now take into account all the major factors known to affect climate. These include changes in energy from the sun; volcanic eruptions; the effects of different types of clouds and particles in the atmosphere; the influence of forests and fields, water and ice at the earth’s surface; heat transport by ocean currents deep below the surface; and, of course, how humans affect the environment.

That’s not to say that climate models are a perfect representation of our planet. Of course not! And those who build and run even the most sophisticated models in the world would be the first to say so! To perfectly simulate the earth, we’d need to know everything about every process, large or small, that occurs on our planet. Our models are limited by our knowledge. There are many things that we don’t understand and haven’t measured. But climate models continue to surprise us with their abilities, as scientists use the models to simulate the past.

This is how scientists answer the question, “How do we know we are causing the warming?” Using these models, they can create that “earth with no people.” This is a virtual world that changes only because of natural causes: volcanic eruptions, changes in the amount of energy the earth is receiving from the sun, natural variability of the climate system, and many other factors. Computer models are run from the mid- 1800s to the present. Then the average temperatures of this virtual “earth with no people” can be compared to the real, observed temperatures of the Earth over that same time period.

The same models can then be used to create an “earth with people.” This time, the virtual world is permitted to change in response to both natural and human- induced increases in greenhouse gas levels. We can compare the temperature of these two different “earths” to the actual, observed temperature of Earth over that same time period (see Figure 12 in the center insert). On the graph, the red line shows Earth’s actual, observed temperatures over the last century. The blue line represents an “earth without people” and shows the average of the temperatures predicted by a number of different climate models. The black line represents a simulated “earth with people” and shows the average temperature predicted by the twenty- five models.

There’s not much difference between the three colored lines for the first half of the 1900s. This tells us that temperature changes over this time were likely driven by a combination of human and natural factors. However, for the last half of the century, we see a very different picture. Over the land, over the ocean, and over every major continent in the world, there is no way to explain the temperature increases that have been observed if we leave humans out of the equation. In fact, for much of the world, temperatures should actually be decreasing if natural causes were the only influence on climate.

The observed increase in greenhouse gas levels, due to human production, is the only explanation we can find to account for what has happened to our world. We’ve dusted for fingerprints. There’s only one likely suspect remaining.

It’s us.

Here’s a figure that compares the real world temperature to the “earth with no people” and the “earth with people”:

Climate models are the most helpful and scientifically reliable tools available to examine causes of climate change. We can compare observed global temperatures (black line) with climate model simulations of temperature that include human production of heat-trapping gases (red line), and those that do not include human influence (blue line). Only when climate model simulations include human influence are they able to account for the global temperature increase since 1960.

Here’s where you can get the observed global temperatures and climate model simulations (warning: registration and valid research agenda is required).

Want more? Here's what the Intergovernmental Panel on Climate Change has to say about attributing recent warming to human activity [pdf].