They give off a lot of greenhouse gases (especially CO2) into the air. This warms the earth.

它们向空气中释放大量温室气体（尤其是二氧化碳）。这使得地球变暖。

youdao

These are already cheaper and cleaner than burning coal, oil and gas with all their pollutants, especially CO2.

WSJ: The New Nuclear Revolution

"Given the pressing need to reduce our dependence on fossil fuels, especially oil, and to cut CO2 emissions, the future for hydrogen as an alternative means of storing and utilising energy cost-effectively has never been brighter, " said Jim Heathcote, chief executive of ITM.

BBC: Hydrogen refuel station unveiled

Absorption is particularly fast in cold water so the Arctic is especially susceptible, and the recent decreases in summer sea ice have exposed more sea surface to atmospheric CO2.

BBC: Arctic Ocean 'acidifying rapidly'