San Francisco: Eight US lawmakers have come forward to push Bitcoin mining companies into revealing how much electricity they use for crypto mining, as cryptocurrency mining's impact on energy is being felt across the globe. The US Senators have sent letters to six companies that mine Bitcoin in the US, asking them about how much electricity they use, where it comes from, and how they plan to grow. "Given the extraordinarily high energy usage and carbon emissions associated with Bitcoin mining, mining operations raise concerns about their impacts on the global environment, local ecosystems, and consumer electricity costs," according to the letters. The letters were signed by Senators Elizabeth Warren (D-MA), Sheldon Whitehouse (D-RI), Jeff Merkley (D-OR), Margaret Hassan (D-NH), and Ed Markey (D-MA), Katie Porter (D-CA), Rashida Tlaib (D-MI), and Jared Huffman (D-CA). The letters were sent amid an oversight hearing on crypto mining's impact on energy by the House Energy & Commerce Committee. Also ReadIMF warns El Salvador from using Bitcoin as official currency Cryptocurrency mining threatens targets to limit global warming to 1.5 degree Celsius under the 2015 Paris Agreement, according to the Swedish Financial Supervisory Authority and the Swedish Environmental Protection Agency. The most common method for producing crypto-assets requires enormous amounts of electricity and generates large CO2 emissions. "Crypto assets have a significant negative impact on the climate as mining leads to both large emissions of greenhouse gases and threatens the climate transition that needs to happen urgently. This is alarming, and crypto-assets therefore need to be regulated," according to Erik Thedeen, director of the Swedish Financial Supervisory Authority. Bitcoin mining relies on an energy-intensive process called "proof of work" to keep ledgers secure. The University of Cambridge and Digiconomist estimate that the two largest crypto-assets, Bitcoin and Ethereum, together use around twice as much electricity in one year as the whole of Sweden. Digiconomist estimates that crypto-assets at their current market value lead to release of up to 120 million tonnes of CO2 in the atmosphere per year.