In Johannesburg, South Africa, the South West Township (Soweto) was one of several areas established early in the last century for blacks to live away from the white ruling minority. On the hills surrounding Soweto, the company that provides South Africa's electricity, Eskom, built a large coal-burning power plant. The electricity was for white areas of Johannesburg, miles away from the plant, and the residents of Soweto had no electrical connections. The major long-term global problem with using coal to generate electricity is the ________.