Using sponges collected off the coast of Puerto Rico in the eastern Caribbean, scientists have calculated 300 years of ocean temperatures and concluded the world has already overshot one crucial global warming limit and is speeding toward another.

These findings, published Monday in the journal Nature Climate Change, are alarming but also controversial. Other scientists say the study contains too many uncertainties and limitations to draw such firm conclusions and could end up confusing public understanding of climate change.

Sponges — which grow slowly, layer by layer — can act like data time capsules, allowing a glimpse into what the ocean was like hundreds of years ago, long before the existence of modern data.

Using samples from sclerosponges, which live for centuries, the team of international scientists was able to calculate ocean surface temperatures going back 300 years.

They found human-caused warming may have started earlier than currently assumed and, as a result, global average temperature may have already warmed more than 1.5 degrees Celsius above pre-industrial levels. Researchers say the results also suggest global temperature could overshoot 2 degrees of warming by the end of the decade.

Under the 2015 Paris Agreement, countries pledged to restrict global warming to less than 2 degrees above pre-industrial levels, with an ambition to limit it to 1.5 degrees. The pre-industrial era — or the state of the climate before humans started burning large amounts of fossil fuels and warming the planet — is commonly defined as 1850-1900.

The study authors argue their findings suggest the pre-industrial era should be pushed further back to between the 1700s and 1860. Changing that baseline would mean the world has already warmed at least 1.7 degrees (scientists say long-term global warming currently stands at between 1.2 to 1.3 degrees).

“The big picture is that global warming, and the urgent need for emission reductions to minimize the risk of dangerous climate change, has been brought forward by at least a decade,” Malcolm McCulloch, lead author of the study and marine geochemist at the University of Western Australia, said at a news briefing. “So, this is a major change to thinking about global warming.”

However, several climate scientists have questioned the study’s findings, especially using one type of sponge from one location in the Caribbean to represent global temperatures. Gavin Schmidt, a NASA climate scientist, said estimating global average temperature requires data from as many locations as possible, as climate varies across the planet.

“Claims that records from a single record can confidently define the global mean warming since the pre-industrial are probably overreaching,” he said in a statement.

Gabi Hegerl, a professor of climate system science at the University of Edinburgh, said the study was “a nice new record that illustrates how temperatures in the Caribbean started to rise over the industrial period.” But, she added in a statement, “the interpretation in terms of global warming goals overstretches it.”

Some went further. Yadvinder Malhi, professor of ecosystem science at the Environmental Change Institute, University of Oxford, said the way the findings have been communicated were “flawed” and have “the potential to add unnecessary confusion to public debate on climate change.”

A co-author of the study defended its robustness and argued that temperature changes in the part of the Caribbean the sponges came from have always mimicked changes around the globe.

“It’s probably one of the best areas if you’re trying to figure out global averaging on the Earth,” said Amos Winter, a professor of geology at Indiana State University. Ocean temperatures in the region are predominantly affected by planet-heating pollution, rather than natural climate variability like El Niño, he said.

Whatever the baseline for measuring global warming, what remains clear, experts say, is that the impacts will worsen with every fraction of a degree of warming.

“It’s exciting to see new research that allows us to peek centuries in the past,” said Joeri Rogelj, director of research at the Grantham Institute at Imperial College London, in a statement. But, he added, “relabeling the warming that has occurred until today by using a different starting point does not change the impacts we are seeing today, or the impacts we are aiming to avoid.”

Winter hopes the study will function as a call to action. “Hopefully this will help change our viewpoints of what is happening in the globe, make us act now, and not wait for some disaster to happen for us to change our habits.”