The Verdict: Science Says Humans Responsible for Increase in CO2, Plain and Simple

With 2012 fast approaching, we are seeing the interconnections of science with politics more and more as presidential and congressional candidates get ready for primary season. Of course, one issue where science and politics overlaps in a major way is climate change. Climate change is largely driven by the concentrations of greenhouse gases, the most notable being carbon dioxide, CO2. There are many who believe that even if CO2 levels have been rising over the past 150 years (which corresponds to when the industrial revolution began), any evidence of global warming and climatic shifts are simply natural cycles, with little or no influence from humans burning fossil fuels.

If there is one thing in the science of climate change that is not in doubt, it is that human beings are largely, if not entirely, responsible for the dramatic increase in carbon dioxide (a leading greenhouse gas) since the second half of the 19th century. Now, THE question that is correct to ask is how do we know this? How do we know that humans burning carbon-based materials like coal, oil, and natural gas has led to increased carbon dioxide levels, and that the increased levels are not due to natural causes?

A series of tests provide the answers to this question. There are multiple, independent tests that all lead to the same conclusion. And what's more, these tests rely on basic, fundamental science principles that are not disputed...yes, there are basic, science facts that the conspiracy believers simply have to ignore if they continue their rhetoric.

A brief summary is provided below, but a better, more detailed explanation is found here.

One way of thinking about human contributions to the rise of CO2 levels in the atmosphere, which have increased from about 280 parts per million (ppm) to 380 ppm in just the past 150 years is human activity, i.e. the industrial revolution. One can calculate the amount of carbon burned over that time span, and one finds that humans have put enough carbon in the atmosphere that could place CO2 at some 500 ppm. So why do we measure only 380 ppm? This is because the earth is a complex system, and there are natural carbon sinks such as forests and the oceans, which absorb CO2 out of the atmosphere. These sinks have been able to absorb some of the excess CO2 humans have put into the atmosphere, but in the end we are putting greenhouse gases into the air at a rate faster than natural sinks can absorb it. We are not in chemical equilibrium, and as we now have more humans on the earth than ever, and burning even more carbon materials at faster rates than any other time in human history, it is a natural prediction that this increase in greenhouse gases will rise and put us in a larger non-equilibrium state with carbon sinks.

There is another way to test whether or not humans and the burning of carbon materials are responsible for the increase in CO2 during the last 150 years. It is the measurement of the relative abundance of carbon-13 to carbon-12. The most abundant carbon is carbon-12...this is the form of carbon we are made of, as well as plants. Carbon-13 is an isotope of carbon, with one extra neutron in the nucleus.

There is a natural concentration of the different isotopes of carbon in the atmosphere that is accurately measured. Scientists use the ratio of C-13/C-12 to quantify this concentration of carbon types. Carbon-14, which is radioactive and used in dating many different types of objects, is much more rare than carbon-12 or -13. So Nature has a basic value for the C-13/C-12 ratio in the atmosphere, that has been effectively constant for hundreds of thousands of years with minor variations due to events such as major volcanic activity. By the way, how do scientists measure this over long time periods? Water, ice, and plants absorb carbon from the atmosphere. Ice core samples from the poles can be dug out and measured and dated. CO2 levels have increased to levels never seen in at least the last 400,000 years, for instance. And this rise has occurred in the last 150 years.

But here is one last aspect of the isotope ratio. The ratio has one value for the atmosphere. But when you burn stuff, there is a significantly different ratio in the CO2 produced from that combustion process! So the study to do, and has been done multiple ways and by independent groups, is to measure the present ratio of Carbon-13/Carbon-12 and compare it to past values. The expectation is that as carbon-based materials are burned, there will be a rise in CO2 (this is simple chemistry that one cannot get around...sorry, no true clean coal exists...if you burn it, CO2 will be produced). but at the same time, the C-13/C-12 ratio should decrease. This is because plants favor absorbing the lighter C-12 from the atmosphere more than C-13. And coal, gas and oil are made from plants that die. Plants, and therefore coal, gas and oil, have a lower C-13/C-12 ratio than the atmosphere.

What is the result of such studies? C-13/C-12 is flat for thousands of years in the atmosphere...natural atmospheric concentrations. But in the last 150 years, CO2 has increased, and C-13/C-12 has decreased, just as predicted. So multiple tests confirm that the burning of carbon-based materials by humans (because there have not been any constantly burning forest fires during this same period, or any other natural process) in the past 150 years has occurred, and this corresponds, over the same time period, to the unprecedented increase in CO2 levels in the atmosphere.

Yes, humans are responsible for at least a significant portion of the increased CO2 levels in the atmosphere. CO2 is a greenhouse gas. CO2 traps heat, and can cause warming of the atmosphere. These are basic facts of chemistry and physics. It is not a hoax. The more difficult problem to solve is the longer-term consequences on the global climate. This is done via computer simulations. The global climate system is an unbelievably complex system, and as climate models improve of time we may have a better grasp of what will eventually happen.

If CO2 levels continue to increase at increasing rates, which will be the case if developing nations like China and India continue to rapidly increase automobile use and coal-burning plant production, and the US does not do anything to decrease its CO2 deployment into the atmosphere, obviously we will continue to see further changes in the natural climatic cycle.

How do we get the general population in tune with the science? When will policymakers accept science facts instead of ignoring them for political gain? And when does it become too late, where even if we cut off all carbon combustion, there will be no turning back the clock on climate change and potential disastrous consequences of high greenhouse levels? That part is debatable; but human responsibility for increased CO2 levels is no longer debatable.