The as-yet unnamed power station, referred to as "Coal 3" is slated to be built once Medupi and Kusile is completed.
"We have previously observed with appreciation how the department of trade and industry has both supported and committed to the development of the renewable energy industry. But the recent announcement related to coal 3 puts this commitment at risk," said Melita Steele, climate and energy campaigner for Greenpeace Africa.
Scientists have finalised the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report on the likely impact of climate change.
The report says that scientists are more than 95% certain that climate change is man-made.
Greenpeace challenged Minister Rob Davies to reconsider the coal-fired power plant.
"Today, we are handing over an open letter to the Minister, urging the department of trade and industry to reconsider its support of coal 3, which is likely to sabotage the South African economy rather than reignite it," said Steele.
"Building yet another coal-fired power station seems to both ignore the grim science behind potentially catastrophic climate change as outlined by the IPCC, and the opportunities offered by renewable energy. Burning coal to produce electricity is one of the dirtiest and most destructive practices on the planet," she added.
Renewable energy has taken off in SA with several installations due to come online within the next two or three years.
According to a report on the Climate Action Programme website, the investment into renewable energy topped $5.7bn, as measurement by the United Nations Environment Program (Unep).
The environmental organisation may have some allies in its opposition to coal-fired power plants in the Cabinet.
Beeld reported that Science and Technology Minister Derek Hanekom said government would have to rethink its plans for a third mega coal-fired power station.
"Huge amounts of water are used when a coal power plant generates electricity. Poisonous gases released when coal-powered energy is generated induce climate change affecting temperatures and rainfall," he was quoted as saying.
The World Wide Fund for Nature (WWF) said that the Medupi coal fired power station was a relic even before its construction and the country should invest in moving toward efficient energy production.
"While South Africa's Integrated Resource Plan (IRP) remains stuck in an old paradigm of bulkiness and capital intensity, greater effort should be made to reduce demand, avoid as far as possible the building of too many complex power stations and focus on more modular and rapidly deployable solutions such as renewables," Saliem Fakir, Living Planet Unit head at the WWF.
Medupi was meant to come online in 2012, but the project has been hampered by delays and allegations of poor quality construction.
Steele said that by turning to renewable energy solutions, SA could also avoid the additional environmental cost of water and air pollution resulting from a coal-fired power plant.
"It is critical to stop building new coal-fired power stations to safeguard South Africa's water resources, avoid human health impacts and avert some of the worst impacts of climate change. The global energy sector is the largest contributor to human-caused climate change, accounting for two thirds of global emissions."
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