sentences: gases released during the use of fossil fuels causes global warming
docs: global_warming is heating. hydrofluorocarbon is greenhouse_gas. methane_gas is greenhouse_gas. chlorofluorocarbon is greenhouse_gas. combustion_of_fossil_fuels is common_source_of_air_pollution. greenhouse_gas is gas. worldwide_excess_of_carbon_dioxide has resulted_in_global_warming global_warming is heating_air. greenhouse_gases is causing_world's_temperature_to_rise.