Stalled efforts to expand companies’ use of carbon credits to offset greenhouse-gas emissions are raising the prospect that some will backtrack or abandon targets to shrink their carbon footprint.

Since 2015, when governments agreed in Paris to try to keep the world from warming more than 1.5 degrees Celsius (2.7 degrees Fahrenheit), more than half of the world’s largest 2,000 publicly listed companies have announced targets to cut their emissions to zero on a net basis by 2050.

But environmental advocates are expressing concerns that companies are falling behind on those targets. Companies are in turn complaining that clean technologies are not being rolled out on time and that government policies are not doing enough to support the transition away from fossil fuels.

The Science-Based Targets initiative (SBTi), however, dealt a blow late last month to efforts to expand the use of offsets. The non-profit, which audits companies’ emissions targets, said its research found that carbon offsets are largely ineffective in reducing emissions because their climate benefits cannot always be verified. It delayed a final decision to 2025 on whether to allow companies to use offsets to meet emissions targets.