A most troubling aspect of the global climate problem is the carbon dioxide (CO2) emitted by petroleum consumption. Oil is still the world's largest source of energy, though it now runs second to coal as a direct source of anthropogenic CO2 emissions. Oil is a liquid and is by far the main feedstock for liquid fuels, i.e., hydrocarbon energy carriers such as gasoline, diesel, jet fuel, bunker oil and so on.
Combustion of any liquid hydrocarbon fuel directly releases an amount of CO2 that varies only a little depending on the fuel. This is true for most carbon based liquids, not just strictly hydrocarbons. For example, burning ethanol directly releases only about 2% less CO2 than gasoline for a given amount of energy provided.
Therefore, in terms of direct emissions, changing the form of the liquid fuel, e.g., from gasoline to ethanol or from diesel to biodiesel, makes no appreciable difference in the amount of CO2 released to the atmosphere.
Therefore, as long as society burns liquid fuels -- and we now consume enough of them to make oil the world's number one source of commercial energy -- we have what can be called a "liquid carbon" problem. I'll leave to others the tasks of thinking about how to replace liquid fuels, for example, with electric cars or hydrogen-powered vehicles, using energy carriers that don't contain carbon atoms. Meanwhile, society has a huge liquid carbon problem to solve.