A small correction: Seawater temps today are not the “warmest they have ever been”. That is a misstatement. They are the highest
since scientists began recording them by using thermometers in 1850. 1850, coincidentally, marked the end of the northern hemisphere’s “Little Ice Age” (1350AD-1850AD). That was the period that followed the earlier “Medieval Warm Period” (900AD to 1300AD). Below is a chart of sea temps since 1850, that takes into account variations in sampling and recording methods:
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Methods other than the direct measurement with thermometers (such as the TEX-86 Index, “clumped” isotope paleothermometry, oxygen isotope comparison, and other techniques) show that the sea temperature has fluctuated wildly over time, but for most of the world's history, it has been much warmer than it is today. 10,000 years ago, it seems it was about the same as it is today. It has been hotter, and it has been colder (see Smithsonian chart, below).
I am not trying to argue pro/con on climate change. I'll leave that to the "sane" and "insane" folks. I am just giving some facts that often are not included in popular website and news articles that use a lot of overstatement and exaggeration to make their point, like the statement “warmer than it has ever been”.
Below is a temperature chart going back 500 million years, published by the Smithsonian:
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