But the greenhouse-gas data companies would have to disclose won’t necessarily be on a like-for-like basis, the details of the SEC proposals show. And only some of the data will have to be verified independently, under the SEC’s plans.

The landmark SEC proposals add a dense layer of climate reporting requirements to the existing bedrock of mandatory financial disclosures.

Companies would have to report on the greenhouse gases created by their operations and use of energy, known as Scope 1 and 2 emissions. Some larger companies would also have to disclose the carbon created by their supply chain and customers. These so-called Scope 3 emissions would have to be reported if they are part of a carbon-cutting target or information a typical shareholder would want to know.

The proposed rules allow companies to choose how they work out the Scope 3 emissions, as long as that methodology is disclosed. That flexibility eases the reporting burden on companies but could allow for some marked differences in approach.

Amazon.com Inc.

reports voluntarily on its Scope 3 emissions, which form part of its 2040 net-zero target for greenhouse gas emissions.

The e-commerce giant counts in its Scope 3 the greenhouse gases from the manufacturing, use and disposal of its own-brand products, such as Kindle e-readers and Amazon Basics. These account for about 1% of sales, according to Amazon.

The company doesn’t count the emissions from making, using or disposing of anything else it sells, such as products it buys from manufacturers or from products sold by third parties.

That is a different approach from some rival retailers.

Target Corp.

for example includes emissions from non-Target branded products it sells in stores and online, according to its climate disclosures.

An Amazon spokesman said in a statement that Amazon is an “online marketplace where many products sold are from third-party sellers, who control their own carbon emissions accounting.” Amazon’s emissions disclosures are verified by an independent third party and follow the guidance of a widely used standard known as the GHG protocol, the statement added.

Scope 3 emissions can be hard for companies to measure accurately or to control, according to some business groups and academics.

“The quality, provenance, methodologies and content of Scope 3 reporting leave a lot to be desired among even sophisticated companies, even in advanced economies such as in the U.S.,” said Anant Sundaram, a finance professor at Dartmouth College’s Tuck School of Business.

The SEC proposals give businesses protection against being taken to court for inaccurate Scope 3 disclosures, provided the information is reasonable and given in good faith.

The agency has also decided not to require Scope 3 numbers to be audited. That contrasts with the planned requirement for Scope 1 and 2 disclosures by larger companies to be verified independently.

Hester Peirce,

a Republican SEC commissioner who voted against the proposals, said this difference in approach reflected the flaws in Scope 3 data. “Nobody can credibly provide assurance for numbers that are inherently unreliable,” she said.

Kristina Wyatt, a former SEC official, said the proposals would make it much easier for investors to compare companies’ carbon footprints, even if different methodologies are used. “Maybe it’s not entirely perfect, but it’s certainly a huge leap forward from where we are right now,” said Ms. Wyatt, deputy general counsel at Persefoni Inc., a carbon-accounting startup.

The SEC said Monday the current system of voluntary disclosure makes it hard for investors to compare carbon-cutting pledges. Two-thirds of S&P 500 companies by the end of last year had set a carbon target, up from less than half at the end of 2017, according to data provider Refinitiv. But many companies don’t give investors enough information to judge if their commitments have substance or are likely to be met, the SEC said.

Despite mushrooming net-zero targets, carbon emissions at many big companies have gone up in recent years. Between 2015 and 2020, the combined Scope 1 and 2 emissions increased at 71 of 252, or more than a quarter, of the S&P 500 companies reporting these numbers, the latest Refinitv data show. Scope 3 emissions increased over the same period at 90 of the 154 companies reporting this data, according to Refinitiv.

Gary Gensler,

the SEC chair, said Monday the agency isn’t trying to push more companies into setting climate goals. “This is a disclosure rule,” he told reporters. “Whether or not a company [has] a target … that’s up to them.”

Write to Jean Eaglesham at jean.eaglesham@wsj.com