SINGAPORE (Reuters) - Google will buy carbon credits from an Indian initiative that turns large amounts of agricultural waste into biochar - a form of charcoal that removes carbon dioxide from the atmosphere and returns it to the soil, it said on Thursday.

The deal - signed by Google and Indian supplier Varaha - is one of the biggest ever involving biochar, and is the tech giant's first foray into India's carbon dioxide removal (CDR) sector.

Google is one of a number of big tech companies looking to offset emissions through CDR, which refers to a range of interventions designed to remove CO2 already in the atmosphere and oceans.

While some developers are looking at expensive new technologies that extract CO2 directly from the air, solutions like biochar could prove a cheaper near-term option.

"Biochar is a promising approach to carbon removal because it has the ability to scale worldwide, using existing technology, with positive side effects for soil health," said Randy Spock, Google's carbon removal lead.

Varaha will buy waste from hundreds of smallholder farms in India and build reactors to convert it into biochar, which can sequester CO2 for hundreds of years. It will also be supplied to farmers as an alternative to fertilisers.

Google will buy 100,000 tons of carbon credits from now until 2030. Varaha's chief executive Madhur Jain said there was scope for rapid growth, with waste from India's farms capable of generating enough biochar to store more than 100 million tons of CO2 every year.

CDR accounts for only a fraction of global carbon trading but is expected to grow rapidly as countries and corporations seek new ways to offset emissions.

However, critics say CDR is no substitute for emission cuts. They also warn that solutions like biochar offer no guarantee the CO2 will be removed permanently.

"We are going to face peak warming," said Jain. "Even if something just reduces (CO2) or removes it for only 20 to 40 or 50 years, I feel that we need to do everything that we can."