How We Know Humans Are Causing Global Warming

Even the most hard-core global warming deniers are unable to deny that the earth is warming. Now they claim that it is not due to human activities. But we know that the observed rise in CO2 IS due to human activities. How? In the same way that cycling officials knew that biker Floyd Landis doped with synthetic testosterone while winning the 2006 Tour de France.

With serial liar and doper Lance Armstrong retired and most of the other top riders expelled for illegal drug use, Landis had become one of the favorites to win the tour that year. He was leading until, in stage 16, he dropped to eleventh place. Then, just as his chances of winning seemed slim, Landis won the next stage going away and went on to ride the Champs-Élysées in the winner’s yellow jersey.

A few days later, Landis’s team announced he had failed a test for banned steroids, including testosterone. Landis appealed the ban, raised an estimated $1M for his defense, and wrote a 300-page book titled, “Positively False: the Real Story of how I won the Tour de France.”

After years of denial, in 2010 Landis reversed himself and admitted that from 2002 through 2006 he had used a grab-bag of banned substances and methods. Why did he finally have to give up his denial? Because the carbon isotope test nailed him. People lie, but isotopes do not.

Testosterone is mostly carbon. Synthetic testosterone is made entirely from plants, which have a different carbon isotope ratio than our environment overall. The carbon in Landis’s body revealed the presence of plant carbon, proving beyond reasonable doubt that he had doped with synthetic testosterone.

So how do scientists use the method to confirm that humans are causing global warming? Since 1800, CO2 in the earth’s atmosphere has risen 40%. The obvious source of the added carbon is the 330 billion tons that we know fossil fuel combustion has added to the atmosphere since the Industrial Revolution. Yet global warming deniers deny the obvious cause-and-effect connection. Well then, let’s prove it.

First, coal, oil, and natural gas also come from plants and have the distinctive carbon isotope ratio of plants. As CO2 in the atmosphere has built up steadily, the isotopic composition of carbon in the atmosphere, indicated by "Del C13," has shifted just as steadily in the direction of plant carbon, as shown in the chart below.

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The increase in carbon emissions (black line) vs. the relative abundance of C13 (red line). The red scale is negative and gets more negative upward. The more carbon we emit, the more the carbon in the atmosphere shifts in the direction of plant carbon.

So we know the added carbon in the atmosphere is coming from plants, but which plants? To answer that question, we use another isotope of carbon: C14. it is radioactive and dies away to undetectable levels in 50,000 years or so. Fossil fuels, being millions of years old, have no C14 left. Adding ancient carbon should have lowered the proportion of C14 in the atmosphere—and it has, as the charts below show.

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This chart, based on measurements of C14 from tree rings, shows how its relative abundance among the carbon isotopes was declining with the advance of the Industrial Revolution until about 1950, when it suddenly shot up. The reasons is because C14 is a byproduct of atomic bomb explosions.

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This chart picks up the story after 1950. C14 continued to build up until the ban on above-ground atomic bomb tests went into effect in the 1960s. Then C14 began to decline again.The only conclusion from these two slides is that if we take away the C14 produced by atomic explosions, some process has been diluting the carbon in the atmosphere with C14-free carbon. That means ancient carbon, and the only known source of ancient carbon in the amounts required to explain the data is fossil fuel combustion.

The two animated charts below show how closely the isotope trends follow the overall amount of carbon dioxide emitted. Download the charts here or try that link if they do not work in Firefox.

Humans are causing global warming. Case closed.