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Some greenhouse gases (GHG), such as industrial halocarbons, are only made by humans, and thus their presence in the atmosphere can only be explained by human activity. For naturally occurring GHG, several independent lines of evidence make it crystal clear that they are increasing because of human activities.
Unstable Gas Levels
First, CO2, methane, and nitrous oxide concentrations were stable for thousands of years. Suddenly, they began to rise like a rocket around 200 years ago, about the time that humans began to engage in very large-scale agriculture and industry (see figure).
Only Human-Generated Gases are Increasing
Second, scientists and economists have developed estimates of all the natural and human GHG sources. When they add them up, only the human contributions are increasing. In fact, the amount of human-made GHG in the budget are more than enough to explain the rise in concentrations, which means that natural processes are absorbing the excess amount, keeping GHG concentrations from rising even more.
For CO2, the most important human-produced GHG, scientists can tell from chemical measurements of the atmosphere that the additional CO2 is from:
– combustion (i.e. burning fossil fuels) because the amount of oxygen in the atmosphere is decreasing in direct proportion to the rise in CO2;
– a prehistoric (fossil) source because the amount of radioactive carbon in the atmosphere has been decreasing over the past century;
– from plants (i.e. ancient trees that became coal and oil) rather than a geological source (i.e. volcanoes).
Together, all of these independent lines of evidence leave no doubt that GHG concentrations are increasing because of human activities.
Source: Center for Climate and Energy Solutions