This is the fifth instalment from the annual series 10 New Insights in Climate Science, which aims to synthesise and communicate the latest and most essential scientific findings on climate change. It is the result of a collaboration between Future Earth, Earth League and the World Climate Research Program (WCRP). This report constitutes a climate science year-in-review for journalists, policy makers, and the general public. A peer-reviewed academic article published in parallel to this policy report, provides an in-depth explanation of the ten insights, as well as a complete account of the reviewing process. These are the 2021 ten New Insights in Climate Science: Stabilizing at 1.5°C warming is still possible, but immediate and drastic global action is required. Rapid growth in methane and nitrous oxide emissions put us on track for 2.7°C warming Megafires – climate change forces fire extremes to reach new dimensions with extreme impacts Climate tipping elements incur high-impact risks Global climate action must be just Supporting household behaviour changes is a crucial but often overlooked opportunity for climate action Political challenges impede effectiveness of carbon pricing Nature-based solutions are critical for the pathway to Paris – but look at the fine print Building resilience of marine ecosystems is achievable by climate-adapted conservation and management, and global stewardship Costs of climate change mitigation can be justified by the multiple immediate benefits to the health of humans and nature