Don't tell Greta. But in the Scandinavian part of the Arctic around Svalbard Island, about ten thousand years ago, summers were 6°C warmer than they are now, for a millennium. Then they cooled for a while, warmed up again, and about 6,000 years ago cooled again and have never warmed up since. This natural variability is encoded in the shells of molluscs partial to warm water. Which is why they don't live around Svalbard any more. See, there was a climate crisis that caused it to become way warmer than now and species flourished.

According to the authors of the study in Sage Journals, "Shallow marine molluscs that are today extinct close to Svalbard, because of the cold climate, are found in deposits there dating to the early Holocene. The most warmth-demanding species found, Zirfaea crispata, currently has a northern limit 1000 km farther south, indicating that August temperatures on Svalbard were 6°C warmer at around 10.2–9.2 [thousand years ago], when this species lived there." Moreover, they found, the Blue Mussel, Mytilus edulis, recently returned to Svalbard after being away for 4,000 years due to cold temperatures. Aha, warming!

Not so fast, argues another recent study, while there might be some immigration via water currents, the situation is complicated by modern shipping traffic: "The occurrence of blue mussels on Svalbard is most likely the result of at least two distinct dispersal vectors: natural larval advection by ocean currents and human introduction by ship traffic." But shipping traffic doesn't explain why these warm-water molluscs thrived in Svalbard for many centuries during the earlier stage of the Holocene. The only explanation is that the waters were warmer and so were the seasons on land.

Why was it warmer? Because climate is complicated. Which is why we prefer to learn about it from scientists who study it, not 16 year-old high school drop-outs with no idea how little they know.