Global guidelines are needed to address the inconsistency, KPMG said in its annual Survey of Corporate Responsibility Reporting.
Vincent Neate, KPMG's UK head of climate change and sustainability, said: "There is a clear need for improvement and global reporting guidelines on carbon to help companies improve their reporting and help investors and other groups make informed decisions. Industry bodies, regulators, standard setters, investors and others all have a role to play."
While UK companies have some of the highest disclosure rates because of the country's mandatory reporting, only 83% of the UK companies analysed have targets for reducing their emissions, KPMG said. Of these, only 42% provide enough information to be able to understand how they are progressing against any targets set.
"While it is positive to see the increase in reporting by UK firms; without more information on how targets are being set, and how the company is then tracking against them, it is very difficult for anyone to draw a meaningful conclusion on the company’s performance using the data currently available," Neate said.
European companies are the most likely to report on carbon and provide the best reports. Germany reports are of the highest quality, followed by the UK, KPMG said. Companies in the US and Asia Pacific are least likely to report on carbon.
One in five companies in high carbon sectors, such as mining and chemicals, does not produce carbon reports, KPMG said.
However, including carbon reporting data in annual financial reports is now a "firmly established global trend", KPMG said. "Almost three in five companies do this now, compared with only one in five in 2011", it said.
The Financial Stability Board recently announced that it is setting up a task force on climate-related financial risks. The task force will develop voluntary, consistent climate-related financial risk disclosures for use by companies in providing information to lenders, insurers, investors and other stakeholders, it said.
The Climate Standards Disclosure Board has introduced a voluntary framework to help companies include climate information in their financial reporting.