LONDON—The central bank to the world’s central banks, the Bank for International Settlements, has warned of the growing risk of a price bubble in environmental-focused asset markets. Some estimates indicate Environmental, Social, and Governance (ESG)-focused assets have soared to a value of $35 trillion and now account for more than a third of all assets professionally managed by banks and investment funds. A narrower definition including only exchange-traded funds (ETFs) and mutual funds with ESG or socially responsible investment (SRI) mandates points to even faster, tenfold growth, to approximately $2 trillion. This is evidenced in assets such as clean energy and electric car stocks and green bonds, which have soared in recent years. “There are signs that ESG assets’ valuations may be stretched,” the BIS, which holds regular meetings for the world’s central banks, said as part of its latest quarterly report. Claudio Borio, head of its monetary and economic …