This is the abbreviation of Million Tonnes of carbon dioxide equivalent. 1 tonne is equal to 1,000 kilograms.
This unit allows us to compare the potential warming impact of an emission of:
one greenhouse gas (like nitrous oxide)
an emission of the same amount of carbon dioxide.
Example: The global warming potential for nitrous oxide over 100 years is 298. This means that:
emissions of one million tonnes of nitrous oxide
is equivalent to
emissions of 298 million tonnes of carbon dioxide (298 Mt CO2eq).