The temperatures of sea surfaces today resemble those of the period during the last interglacial period, when sea levels were up to nine metres above the height they are at now.
The concerning finding is the conclusion of new research by a scientist from UCD along with colleagues in the US, and may help researchers to better understand how oceans will respond to global warming.
Interglacial periods are those warmer periods of the Earth's history, in between periods when the planet is experiencing glacial or cool climate trends.
During these interglacial periods, glaciers and ice shelves melt and retreat.
The last period like this happened between 129,000 and 116,000 years ago.
While previous research has indicated that it was a little hotter than today, the estimates of how much vary from no major difference to nearly 2 degrees Celsius.
This makes it an interesting period for scientists to study, as it enables them to predict what might happen to sea temperatures and levels during the current period of warming.
This latest research, published in the journal Science, saw 104 sea surface temperature data sets of the last interglacial period brought together.
The data was gathered at 83 sites where marine sediment cores were extracted.
The scientists, including Professor Andrew Parnell from UCD's School of Maths and Statistics and its Earth Institute, compared the core data to data sets from the periods 1870-1889 and 1995-2014.
They found that when the last interglacial period started 129,000 years ago, global ocean sea surface temperatures were broadly comparable to the average recorded over the 1870-1889 period.
But when they compared the average temperatures of the period 4,000 years later to the 1995-2014 data, they found they were almost identical.
They also discovered that the Atlantic Ocean was less warm than seas and oceans in the southern hemisphere at the start of the last interglacial period.
According to the research, the findings indicate that the historical global mean sea surface temperatures that have been produced by climate models are under estimates.
