The modern rate at which sea levels are rising is comparatively higher than previously believed, Geophysics professor Jerry X. Mitrovica said on Monday during a lecture at the Harvard Geological Lecture Hall. The evidence, he said, comes from Roman fish tanks and ancient eclipses.

The data Mitrovica collected show that the rate of rising sea levels in ancient times was lower than previously thought. He pointed out that this made today’s rate seem even more concerning.

Mitrovica explained that wealthy Romans built cement pens for fish along the coast of the Tyrrhenian Sea during the time of Emperor Augustine in order to keep seafood fresh until consumption.Today, they serve as indicators of sea levels between 100 B.C. and 100 A.D. The tanks were standardized so that they were neither too low—so as not to let the fish escape—nor too high—so as not to waste wall material. These structures can inform modern scientists about ancient sea levels.

Record on ancient eclipses yielded similar data. Mitrovica said that inconsistencies between water clocks, which were engineered to operate at a specific water level, and the time that eclipses occurred indicated fluctuations in the sea level.

According to Mitrovica, these data show that ancient sea levels rose more slowly than believed.

“Conventionally, the subset of these [data points] say that sea levels have been rising between 1.5 and 1.8 millimeters per year,” he said. “The sea levels actually rose at a rate of 1.2 millimeters per year. Some think that that’s a good thing, but it’s actually a bad thing.”

The troubling aspects of these data, Mitrovica concludes, are two-fold. In addition to showing that sea levels are rising at an even more accelerated pace when compared to times past, it also further implicates humans as the main cause of climate change, since the dramatic rise of sea levels in the past century corresponded with industrialization.