Climate Change May Have Already Passed Key Limit, Controversial Study Says

Global warming may have already exceeded a critical threshold, a new study, published in the journal Nature Climate Change, says.

By analyzing slow-growing sponges, researchers from the University of Western Australia, Indiana State University and the University of Puerto Rico-Mayagüez have estimated ocean surface temperatures from before the Industrial Revolution, calculating that the Earth has warmed even more since the preindustrial period than we once thought.

According to the International Panel on Climate Change, global temperatures have risen by over 1.0 degrees Celsius over the last 150 years. The IPCC uses this baseline because it is the earliest period from which we can draw reliable, near-global measurements for global warming data.

In 2015, officials from around the world pledged in the Paris Agreement to limit global warming to no more than 2 degrees Celsius—but preferably 1.5 degrees Celsius—above preindustrial levels by the end of this century.

But according to the new study, we may have already crossed this threshold. By its estimates, global warming may already be 1.7 degrees Celsius above preindustrial levels. “Our result is 0.5 degrees Celsius higher than IPCC estimates, with 2 degrees Celsius global warming projected by the late 2020s, nearly two decades earlier than expected,” the study says.

To reach these results, the team, led by Malcolm McCulloch, studied the preserved skeletons of sclerosponges—a slow-growing sponge species whose hard limestone skeletons act as a sort of natural archive for past climate records, going back centuries.

Historical observations and data for ocean temperatures before 1850 are very limited. But these simple skeletons can act as a proxy record for temperatures before this date through climate-driven chemical changes in their skeletons. Based on this data, the team concluded that human-driven global warming began decades earlier than the preindustrial baseline used for IPCC measurements.

“This paper presents a detailed sclerosponge record from the Caribbean that shows that ocean surface warming (something they term industrial-era warming) in this region began in the 1860s, a time earlier than the existence of good ocean temperature records,” Yadvinder Malhi, a professor of ecosystem science at the University of Oxford’s Environmental Change Institute, said in a statement.

He continued: “This finding is used to argue that the pre-industrial reference baseline should be pushed back to the period from the 1700s to 1860, rather than the 1850-1900 period usually used by the IPCC and others. With this new baseline, an extra 0.5C is added to our estimates of industrial-era warming, and hence the studies [imply] we are already well past 1.5C of industrial era warming and approaching 2.0C.

Malhi said the way these findings have been communicated is “flawed and has the potential to add unnecessary confusion to public debate on climate change.”

“While the Paris climate targets set reference warming targets against pre-industrial baselines, there is an implicit assumption this is dominated by human-caused warming caused by the industrial era. It is unlikely that warming of 0.5C in the 1800s is human-caused,” he said.

Then there is another issue: McCulloch and his team used data gathered from the Caribbean to infer changes in global temperatures.

“The paper shows a new record that tracks known anomalies such as the cool period following two severe eruptions,” Gabi Hegerl, a professor of climate system science at the University of Edinburgh, said in a statement. “But a single location cannot substitute global data, as climate varies across the globe, which is why the only way to measure global temperature is to get data from across the globe.”

Hegerl went on: “It’s a nice new record that illustrates how temperatures in the Caribbean started to rise over the industrial period, punctuated by volcanic episodes that caused temporary cooling and significant variability. However, the interpretation in terms of global warming goals overstretches it.”

Kate Hendry, a professor on the Polar Oceans Team of the British Antarctic Survey, said in a statement that our understanding of how these sponges grow is still somewhat limited.

“We must keep in mind some considerable complexities surrounding the use of sponge skeleton chemistry as archives of past ocean change,” she said. “We need to know far more about how these animals make their skeletons, and exactly how their chemistry relates to ambient temperatures—something we don’t have a good handle on now. We need a better understanding of these proxies before we can use them to make important statements about the state of the climate with confidence.

Hendry said we clearly need to know more about these sponge archives.

“We need more high-resolution climate records from more locations to build a robust, global picture. However, the importance of this paper is that it makes us ask the question: what if the planet has already warmed more than we thought?” she said.

In a response article, also published in Nature Climate Change, Wenfeng Den, a professor at China’s Guangzhou Institute of Geochemistry, said that while more research is needed to replicate these results in other locations across the globe, McCulloch and his team have highlighted the need for more precise and comprehensive climate models to enable us to make more accurate climate predictions in the future.

Richard Allan, a professor of climate science at the U.K.’s University of Reading, said that regardless of this study’s results, global temperatures are rising and action must be taken now to avoid catastrophic climate change.

“This indirect measure of ocean warming adds to the pile of evidence of warming since the pre-industrial era before substantial emissions of heat-trapping greenhouse gases were pumped into the atmosphere by human industrial activities,” he said in a statement.

He continued: “Using this regional estimate to recalibrate global warming estimates above the dangerous 1.5 degrees Celsius threshold is something of a red herring since rapid climate change is already fast approaching or already at this point regardless of the mix of evidence used.

“Rapid cuts in greenhouse gas emissions to net zero remain absolutely paramount in avoiding an even more dire future for societies and the ecosystems upon which we depend,” Allan said.