More than 340 institutional investors representing $24 trillion in assets on Thursday called on government leaders attending next week's United Nations climate summit to set carbon pricing policies that encourage the private sector to invest in cleaner technologies.

Firms signing a joint letter include BlackRock, Calvert Investments, BNP Paribas Investment Partners and Standard Bank.

They want countries to set a price tag on pollution by taxing carbon emission or implementing cap and trade emissions policies to create incentives for investing in cleaner technologies.

"Stronger political leadership and more ambitious policies are needed in order for us to scale up our investments," the investors' statement said.

The UN climate change summit on Sep. 23, devised and led by Secretary General Ban-ki Moon, will bring heads of state and senior officials from more than 125 countries, including U.S. President Barack Obama, to New York to mobilize global action to address climate change.

The institutional investors added that gaps and delays in implementing climate change policies increase risks to their investments since the physical impacts of climate change "will increase the likelihood that more radical policy measures will be required to reduce greenhouse gas emissions."

UN summit to focus on business, financial communities

In addition to political declarations, the UN summit will place a heavy focus on the role the business and financial communities can play to drive low-carbon economic growth.

"Investors are owners of large segments of the global economy as well as custodians of citizens' savings around the world. Having such a critical mass of them demand a transition to the low-carbon and green economy is exactly the signal governments need in order to move to ambitious action quickly," said Achim Steiner, director of the UN Environment Programme.

UNEP coordinated the joint investor statement with four investor groups focused on climate change, including Ceres' Investor Network on Climate Risk and the Asia Investor Group on Climate Change.

Both the UN and the World Bank have been calling on CEOs to embrace internal carbon pricing policies ahead of the summit.

On the sidelines of the summit, Ban will also host a private sector forum that will help build up support for carbon pricing in businesses and governments.

Among participants in that event are the CEOs of Air France , China's Sinopec, the McDonald's Corporation and Royal Dutch Shell.