From unprecedented wildfires across the US to the extraordinary heat of Siberia, the impacts of climate change were felt in every corner of the world in 2020. We have come to a \"moment of truth\", United Nations Secretary General Antonio Guterres said in his State of the Planet speech in December. BBC Future brings you our round-up of where we are on climate change at the start of 2021, according to five crucial measures of climate health. CO2 levelsThe amount of CO2 in the atmosphere reached record levels in 2020, hitting 417 parts per million in May. \"We saw record levels again this year despite Covid.\"