By Libby George

VIENNA (Reuters) - Definitions of what a green bond is must be streamlined urgently to attract funds needed for the green transition, the World Bank Group's private investment arm said, as markets brace for a backlash on sustainable investments from the incoming Trump administration.

The number of 'taxonomies', a system of classification, for green bonds has jumped in recent years as countries try to move more money into activities and projects that will help them meet their climate goals.

The International Finance Corporation (IFC), aimed at creating markets, is one of the world's biggest issuers of green debt, and has sold nearly $14 billion across 207 green bonds in 21 currencies since 2010.

Alfonso Garcia Mora, IFC vice president for Europe, Latin America and the Caribbean said on Tuesday that to facilitate the estimated $2.4 trillion needed annually for the green transition worldwide, reforms were "absolutely essential".

This included creating globally accepted definitions regarding sustainable bonds, Mora said.  

"Today, [there are] more than 30 green taxonomies in the world," he said at the Invisso Central & Eastern European Forum in Vienna.

"How can we actually close the gap between investors and needs if what we have is 30 different ways of understanding what a green bond is? We are complicating the life of every single investor in the world ... how do they allocate their money if what we have is a very different way of understanding?"

The return of Donald Trump as U.S. president - expected to herald a turbo-charged U.S. political backlash over environmental, social and governance-related (ESG) policies - has cast a shadow over green bond markets seen as vital to help finance the green transition.

Mora said if financial markets are following different taxonomies, "we have a big problem".

"So we really need to coordinate much more on that."

(Editing by Kirsten Donovan)