The Swiss central bank could be required to pull its $800 billion balance sheet out of investments in fossil fuel companies in a move by one of the world's biggest reserve banks to tackle climate change.

Swiss lawmakers are preparing a campaign that would make targeting climate change one of the policy objectives of the Swiss National Bank, alongside the traditional monetary targets of ensuring price stability and fostering economic growth.

The drive will begin this month with a motion in parliament's lower house, two lawmakers behind the campaign told Reuters. The push is coming from the centre-left Social Democrats (SP) and the Greens, which achieved record results in October's elections.

The SNB built up a $800 billion balance sheet during efforts to rein in the safe-haven Swiss franc, giving it an unusually large investment portfolio for a mid-sized country's reserve bank, around a fifth the size of the balance sheets of the European ECB or the US Fed.

Its investments include stakes in fossil fuel companies such as Arch Coal, Exxon Mobil Corp and Chevron.

"This is a climate emergency and we need to take action as soon as possible," said Adele Goumaz Thorens, a Green politician who proposes this to the upper house early next year. "The SNB has invested so much money it is incredible and could really be an actor for change."

Sandro Leuenberger from pressure group Climate Alliance Switzerland said: "The SNB is the eighth biggest institutional investor worldwide, and it gives Switzerland a huge weight internationally... It's the biggest lever Switzerland has and we have to use it."

The SNB declined to comment on the politicians' proposals. It shuns shares in companies which cause severe environmental damage but has resisted using its investment clout to promote green causes because of the risk of conflicts of interest or politicizing monetary policy.

Central banks are under pressure to do their bit in the fight against climate change.

The new chief of the European Central Bank has described tackling climate change as "mission critical," although its narrow mandate to fight inflation limits what it can do. Federal Reserve chief Jerome Powell has said fighting climate change was not the U.S. central bank's job.

The French and Dutch central banks are factoring climate into investment decisions, but as they are part of the ECB's euro system the impact of those decisions is limited. Norway's central bank can exclude companies with "unacceptable greenhouse gas emissions," but its reserves are dwarfed by the Swiss.

The Swiss campaign must pass both houses of parliament. Passing the upper house which has a centre-right majority will also be tough, the campaigners said. The SP is part of the four-party coalition government, which the Greens hope to join.

The authors are banking on concern in Switzerland, where temperatures are rising twice as fast as the global average, triggering record hot summers and melting Alpine glaciers.

Environmental group Artisans de la Transition says companies in the SNB's US equity portfolio generated 48.5 million tonnes of CO2 in 2017, slightly more than the carbon emissions attributable to Switzerland itself.

"At present the SNB doesn't have a mandate, so they don't tackle these risks," said SP lawmaker Beat Jans, who wants the SNB to dump its holdings in fossil fuel companies. "It's our job as politicians to give them the tools to do this."