(Bloomberg) -- U.S. companies with contracts to buy only renewable power are still relying heavily on fossil fuels because of a lack of clean-energy supplies, a Columbia University study has shown.

American firms have to draw between 20% and 50% of their electricity from regional electric grids even when they’ve signed deals to purchase on renewable power, the university’s Center on Global Energy Policy said in a report.

It shows there’s a mismatch that undermines the climate credentials of U.S. businesses, the report concluded.

“Commitments to buy 100% renewable electricity may not equate to a company actually reducing its power carbon footprint to zero,” said the study’s authors, Melissa Lott and Bruce Phillips.

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Alphabet Inc., Walmart Inc., Nike Inc. and other companies have pledged to purchase power only from renewable sources, in a strategy that is focused in long-term power purchase agreements. The target of corporate buyers is reducing emissions and hedging against power price fluctuations. But the study raises an alert that this strategy might not be enough to look good regarding the fight against climate change.

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