claim: The trend in CO2 at Mauna Loa is practically identical to the global trend because CO2 mixes well throughout the atmosphere.
evidence: Because of the way air is trapped in ice (pores in the ice close off slowly to form bubbles deep within the firn) and the time period represented in each ice sample analyzed, these figures represent averages of atmospheric concentrations of up to a few centuries rather than annual or decadal levels.