CO2 refers to carbon dioxide, the most common greenhouse gas emitted by human activities, particularly from burning fossil fuels. CO2e stands for carbon dioxide equivalent and is a metric used to compare the emissions of different greenhouse gases based on their global warming potential (GWP). CO2e accounts for other gases like methane (CH4) and nitrous oxide (N2O), expressing their impact as equivalent to an equivalent amount of CO2, allowing for a unified measure of total greenhouse gas emissions.