An international treaty to reduce CO2 emissions: Group of answer choices Can be verified only by keeping track of atmospheric concentrations of CO2. Cannot be verified, because there is no way to tell whether some countries are cheating. Can be verified through a combination of economic and geophysical data, including the pattern of CO2 concentration and isotopic compositions in space and time. Cannot be verified because people hide their CO2 emissions with giant smokestacks. Can be verified only by keeping track of economic data