the production of carbon dioxide is widely held to contribute to social / environmental problems such as global warming. This carbon pollution is a negative externality. It is a cost imposed on the whole of society and not just the individual who consumes a certain product. e.g. if you drive a car, the external costs are felt by everyone else.
Because certain carbon intensive industries create negative externalities, the social cost of production is greater than the private cost.
In a free market, these negative externalities are not included in the price leading to overconsumption and social inefficiency. We can say there is a missing market, because the external cost of carbon emission is ignored.