Accounting firms and financial powerhouses such as Ernst & Young, PricewaterhouseCoopers, Deloitte, and KPMG are investing billions in assembling their own in-house sustainability teams.

Paying attention to environmental, social, and governance (ESG) issues is becoming increasingly critical for all companies across all industries.

Sustainability consultants are required to help companies measure their greenhouse-gas emissions, achieve climate and diversity pledges, overhaul supply chains, and comply with forthcoming regulations.

For these reasons, Deloitte announced a $1 billion investment in its sustainability and climate practice and offered training to all 340,000 of its employees, while KPMG said in October it would spend more than $1.5 billion to expand its environmental, social, and governance.

Following the same trend, PwC unveiled its investment plan of $12 billion over five years and 100,000 new hires in ESG and artificial intelligence.

With the Big Four's now acting as regulators and preparing to force firms to report climate information and disclose their carbon footprints, companies need to rely on sustainability consultants to crunch data on emissions, water use, waste, and climate risks.

It is not just the big accounting firms that are staffing up. Corporations large and small are hiring and even training sustainability experts.

The hiring and training come as firms look to track emissions and comply with proposed SEC rules.

"It's not unlike the digital disruption when companies had to figure out how to adapt to the internet," Bruno Sarda, a principal in the climate change and sustainability practice at Ernst & Young, said.

"Now sustainability is everybody's business. It's no longer confined to a handful of people who have it in their job title," he added.