When you go to the beach, it might seem like there are more jellyfish every summer. Is this true? We study the barrel jellyfish, which lives in the Mediterranean and Black Seas. Since the end of the XIX century, many scientists and interested members of the public have been recording the sightings of this jellyfish along the coasts. We collected this information to see whether jellyfish numbers are increasing with the warming of the ocean waters. We found that, due to the warming of the sea caused by climate change, the barrel jellyfish are appearing earlier in the year, and there are greater numbers of barrel jellyfish in some regions now compared with years past. Although other factors, such as food, predators, or habitat changes may also affect marine species, our results indicate that ocean warming could have direct effects on living organisms, changing the functioning of marine ecosystems.