South Africa is under pressure to ratify the COP21 climate change Paris agreement before the end of the year, as doing so could make it binding on a potential Donald Trump-led US.

"If the Paris agreement is in force, legally the US can't get out of it," Environmental Affairs Deputy Director-General Alf Wills, told MPs on Wednesday.

"We are the controlling vote if you want, of keeping the Americans in the agreement."

Wills was addressing Parliament's portfolio committee on environmental affairs about the landmark agreement, which 195 countries signed in Paris last April.

The countries agreed to reduce the increase in global temperature to less than 2 °C. They included the US and China.

US presidential candidate Trump said in May that he would immediately repeal President Barack Obama's executive order were he to win the US elections and assume office on January 20, potentially scuppering the Paris agreement.

South Africa had been identified as the swing vote, the last country that could push the agreement over the 55% threshold before next year, Wills said.

The agreement needed 55 countries (or 55% of global carbon emissions) to ratify it in their domestic Parliaments in order for it to take full binding effect.

Thus far, 29 countries had ratified it, accounting for 39% of all carbon emissions.

However, the likely candidates to sign before the end of the year, such as Australia, would only take the figure to around 53.8%.

South Africa's global carbon emission contribution was relatively low, around 1.4%, but it made South Africa the swing vote, Wills said.

'RATIFICATION UNDER WAY'

"The agreement comes into full effect after 30 days of passing the 55 trigger," Wills said.

"The new president of the US takes office on January 20, 2017, so those countries must ratify before December 20."

Climate change deputy director Judy Beaumont told the committee that the country was on course for ratifying the agreement.

Consultations had already taken place. Cabinet would analyse a socioeconomic assessment before it would be tabled in Parliament.

She said South Africa's energy sector was the largest contributor of greenhouse gas emissions in the country. It contributed 42% to the total, and included state-owned enterprises such as Eskom.

UN Secretary General Ban Ki-Moon would host an event in New York next week, to encourage the remaining signatories to ratify the deal in their domestic Parliaments before the end of the year.