evidence: Levels of the three most important human-caused greenhouse gases – carbon dioxide (CO2), methane and nitrous oxide – continued their steady climb during 2023, according to NOAA scientists. [...] By far the most important contributor to climate change is CO2 , which is primarily emitted by burning of fossil fuels. Human-caused CO2 pollution increased from 10.9 billion tons per year in the 1960s – which is when the measurements at the Mauna Loa Observatory in Hawaii began – to about 36.8 billion tons per year in 2023. This sets a new record, according to the Global Carbon Project, which uses NOAA’s Global Greenhouse Gas Reference Network measurements to define the net impact of global carbon emissions and sinks. [...] About half of the CO2 emissions from fossil fuels to date have been absorbed at the Earth’s surface, divided roughly equally between oceans and land ecosystems, including grasslands and forests. The CO2 absorbed by the world’s oceans contributes to ocean acidification, which is causing a fundamental change in the chemistry of the ocean, with impacts to marine life and the people who depend on them. The oceans have also absorbed an estimated 90% of the excess heat trapped in the atmosphere by greenhouse gases.