The first tweet has a picture (graph) that shows the North Atlantic Sea Surface Temperature Anomaly data over the past few decades, and the data for so far in 2023 is significantly higher than any previous year. That's the context.

The linked tweet was also the first in a thread which went on to establish the rest of what people are discussing. Twitter only shows the first tweet if you're not logged in though, so you may have reasonably thought it didn't continue beyond that.

There is also this video from Hank Green discussing this topic a little more:

https://youtu.be/dk8pwE3IByg