GLASGOW, Nov 3 (Reuters) - The United States on Wednesday announced its support for a new capital market mechanism that will issue investment-grade bonds and raise significant new finance for scaling clean energy and sustainable infrastructure in emerging economies. Underscoring the urgency of acting to stop global warming, Treasury Secretary Janet Yellen told the COP26 climate conference in Glasgow, Scotland, that the United States would join Britain in backing the Climate Investment Funds" (CIF) new Capital Market Mechanism.She said the initiative would help attract significant new private climate funds and provide $500 million per year for the CIF"s Clean Technology Fund, as well as its new Accelerating Coal Transition investment program. CIF was established in 2008 to mobilize resources and investments in low- and middle-income countries. It has drawn some $10.5 billion in pledges from 14 contributor countries and leveraged $61 billion in funding from other sources for projects that have benefited 72 countries to date. "The climate crisis is already here. This is not a challenge for future generations, but one we must confront today," Yellen told hundreds of conference participants. "Rising to this challenge will require the wholesale transformation of our carbon-intensive economies." She told a separate event later Wednesday that she was the first U.S. Treasury secretary to take part in the annual U.N. climate conference, a testament to its importance. "The reason I am here is because climate change is not just an environmental issue. It is not just an energy issue. It is an economic, development and market-destabilizing issue, and I would not be doing my job if I did not treat it with the seriousness warranted," said Yellen, who previously headed the U.S. Federal Reserve. Getting to net-zero emissions is expected to cost between $100 trillion and $150 trillion over the next three decades, and also offers enormous opportunities for growth and investment, she said. CIF said the new mechanism could generate an estimated $50 billion for developing countries over the next decade. It said it was the first time a multilateral climate fund was focusing on bonds to "help narrow the clean infrastructure gap" in developing countries. Under the program, CIF said it would monetize existing assets from its Clean Technology Fund to issue bonds, and then disburse them through multilateral development banks as equity, debt, mezzanine finance, guarantees and other forms of finance. "Emerging economies need significantly more resources to realize their climate ambitions and we are heeding their call," CIF Chief Executive Mafalda Duarte said, adding that the capital mobilization model was "scalable, replicable and game-changing." Developing countries are home to two-thirds of the world"s population and on track to consume 70% of energy supply, but investments in clean energy lag far behind in these countries, threatening efforts to reach a net-zero economy by 2050. U.S. President Joe Biden has announced plans to quadruple American support for climate finance for developing countries by 2024 to more than $11 billion, but the private sector needed to participate as well, Yellen said. "As big as the public sector effort is across all our countries, the $100-trillion-plus price tag to address climate change globally is far bigger," she said. Yellen, who met with financial executives on Tuesday, said financial institutions with collective assets under management of nearly $100 trillion had united under the Glasgow Financial Alliance for Net Zero, or GFANZ, pledging to make their portfolios carbon-neutral by 2050. In the United States, Yellen said the Financial Stability Oversight Council was working to enhance climate-related data and disclosures available to investors, market participants, and regulators, and that Washington would work with its partners to support similar efforts on a global scale. It was also critical to increase the transparency of data about infrastructure projects, Yellen said, underscoring her support for a new Group of 20 initiative to implement specific principles for infrastructure investments. Reporting by Andrea Shalal; Editing by David Gregorio and Jonathan Oatis