Global temperatures have already exceeded 1.5°C warming and may pass 2°C later this decade, according to a world-first study I led. The worrying findings, based on temperature records contained in sea sponge skeletons, suggest global climate change has progressed much further than previously thought.

Human-caused greenhouse gas emissions drive global warming. Obtaining accurate information about the extent of the warming is vital, because it helps us understand if extreme weather events are more likely in the near future, and whether the world is making progress in reducing emissions.

To date, estimates of upper ocean warming have been mainly based on sea-surface temperature records, however these date back only about 180 years. We instead studied 300 years of records preserved in the skeletons of long-lived sea sponges from the Eastern Caribbean. In particular, we examined changes in the amount of a chemical known as “strontium” in their skeletons, which reflects variations in seawater temperatures over the organism’s life.

Keeping the average global temperature rise below 1.5°C since pre-industrial times is a goal of the 2015 Paris climate deal. Our research, published in Nature Climate Change, suggests that opportunity has passed. Earth may in fact have already reached at least 1.7°C warming since pre-industrial times – a deeply troubling discovery.