evidence: Simple measurements demonstrate that CO2 is a greenhouse gas contrary to claim in online blogs [...] Flawed Reasoning: The fact that CO2 can act as a feedback to climate changes triggered by other factors in no way shows that it isn't a greenhouse gas. [...] First, greenhouse gases are well studied, and their properties are nonnegotiable: They absorb and re-emit longwave radiation, whether they’re in a laboratory setting or in the real atmosphere. To back this up with historical evidence, scientists have known since the 1860s that CO2 is a greenhouse gas and since the 1890s that this will affect the heat budget of the Earth through warming. Even then, these claims were based on empirical evidence, and they’re supported by decades of laboratory research.