With growing investor interest in sustainable investing, especially among younger investors, 2019's flows were at a record pace, a year-end report by Morningstar.

Estimated net flows into open-end and exchange-traded sustainable funds that are available to U.S. investors totaled $20.6 billion for the year, an increase of nearly four times the previous annual record for net flows set in 2018, the data showed.

Morningstar based its data on 300 mutual funds that integrate ESG factors into their investment processes, pursue sustainability-related investment themes, and seek to measure sustainable impact alongside financial returns.

The data also showed that during the fourth quarter, estimated net flows jumped to $7.1 billion, easily surpassing the record high for a quarter of $4.8 billion set in 2019's second quarter.