Gord Miller, the province's independent environmental commissioner, used his newly released annual report to say the government is relying too heavily on natural gas generation.
Miller's warning follows last week's announcement by the provincial government that Ontario is replacing its controversial feed-in tariff programme for large wind and other renewable projects with a competitive procurement process.
The size and timing of this depends on the results of its current long-term energy plan (LTEP).
Ontario's dependence on gas is likely to increase as a large chunk of its nuclear generating fleet is shut down for refurbishment at the end of this decade.
"This increased use of natural gas could lead to a rise in greenhouse gas emissions of between 4 and 16 Mt, depending on factors such as the existing generation capacity and weather," said Miller.