S&P lowered the credit rating for US oil giants ExxonMobil, Chevron and ConocoPhillips on Friday, citing higher risk to the industry due to oil price volatility and climate mitigation policies.

The ratings agency cut the debt grade for the companies by one notch to "AA-" for ExxonMobil and Chevron, and to "A-" for ConocoPhillips, following a sector-wide review last month that showed a somewhat worse risk profile the whole industry.

In similar reports Friday, S&P said the downgrades were warranted despite "significantly" better market for energy producers in 2021 "and beyond" compared with 2020.

S&P said climate change posed a risk for the industry because of the possible loss of market share to renewable energy.

"In our opinion, stricter regulations, substitution and secular shifts in industry supply/demand fundamentals will contribute to a more difficult operating environment for fossil fuel producers and will likely augment the risk of stranded assets and significant asset write-downs," S&P said in its report on Chevron.

The S&P analysis suggests the upside for oil giants once the world recovers from Covid-19 may not be as high as from some past recoveries.

Last year, the five biggest global oil giants -- BP, Chevron, ExxonMobil, Shell and Total -- lost $77 billion due to the industry downturn amid weak demand due to Covid-19.

S&P now rates the oil industry as having "moderately high" risk compared with "intermediate" risk due to recent and potential oil and gas price volatility.

The benchmark WTI has surged in the past three months after plunging in the early weeks of the coronavirus pandemic, closing in on $60 a barrel,

The industry also faces a weaker profitability outlook and "significant challenges and uncertainties engendered by the energy transition, including market declines due to growth of renewables," S&P said in a January report.

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