(Reuters) - (CORRECTION: This story has been corrected to identify Alyssa Auberger as Baker McKenzie's chief sustainability officer. A previous version of this story identified Victoria Halliday as the firm's CSO. Halliday is the firm's global communications manager.)

As pressure on corporations to cut carbon emissions grows, some companies are expecting their outside law firms, as vendors contributing to their footprint, to pull their weight.

This week BlackRock's chief executive warned companies it invests in they must show plans to reach net-zero emissions by 2050, and U.S. President Joe Biden signed a raft of executive actions to combat climate change. Delta Air Lines in 2020 pledged to become carbon neutral and Microsoft Corp pledged to become carbon negative, taking into account emissions tied to its law firms.

"Many (firms) are feeling that push from their clients to become more environmentally sustainable and to reduce their environmental impact," said Gayatri Joshi, vice president of client management at ecoAnalyze, a consultancy that helps law firms develop sustainability solutions.

Pressure to act on sustainability has also come from law firm recruits targeting firms' choice of clients. Last year students at top U.S. law schools boycotted Paul Weiss Rifkind Wharton & Garrison for representing ExxonMobil in high-profile climate-change lawsuits. But clients can exert a different kind of leverage when it comes to firms' own practices.

Joshi said she's noticed clients asking increasingly substantive questions about their law firms' sustainability efforts in recent years.

Law firms' biggest sources of carbon emissions include their paper use, the energy used in sprawling office spaces and the fossil fuels burned by extensive business travel, said Joshi and Daniel Krainin, a principal at Beveridge & Diamond, who are both Law Firm Sustainability Network board members.

Firms are taking steps. Around 2008 dozens of firms signed up for an American Bar Association and U.S. Environmental Protection Agency joint program aimed at helping law offices reduce paper and energy use.

At least one major law firm, Baker McKenzie, has a chief sustainability officer, Alyssa Auberger. In 2019 the firm pledged to reduce its emissions from energy consumption by 92% by 2030, and to curb its business travel emissions by using video conferencing.

Auberger in an email said Baker McKenzie made that pledge in part because its stakeholders, including clients, want to know it is "part of the solution" on climate change. The firm is seeing an increase in client requests for information about its carbon management program, she said, in part because clients are reassessing their own impact on the environment, including their supply chain.

Still, Krainin said, not all law firms are focused on sustainability, yet.

Joshi said they should be - in solidarity with their clients, if nothing else.

"As (lawyers) start to do all of these things themselves, they're able to advise their clients, they can talk about it," Joshi said. "With the knowledge of having done it, they can really make systemic change."