Human-induced climate change is accelerating the deterioration of cave art in Indonesia, which includes some of the earliest works of art and the oldest known hand stencil on the planet, according to new research published Thursday that shows urgent action is required to preserve ancient rock art.

Researchers used X-ray fluorescence — a nondestructive X-ray technique that can determine the elemental composition of materials — to examine Pleistocene-age rock-art panels at 11 sites in the Maros-Pangkep limestone caves of Sulawesi, Indonesia. The team discovered that the deterioration of this cave art, which has slowly occurred over the past four centuries, has rapidly accelerated over the past 40 years because of anthropogenic, or human-caused, climate change.

The Maros-Pangkep caves house paintings made by humans between 20,000 and 45,000 years ago using red and mulberry-colored pigments. These paintings include figurative depictions of animals, human and animal composites, and narrative scenes, in addition to the oldest-known hand stencil — collages of handprints painted on cave surfaces — estimated to be at least 39,900 years old, according to the research, published May 13 in Scientific Reports.

This cave art, like those from ice age Europe in southwestern France and northern Spain, is a "globally significant assemblage," said Jillian Huntley, the lead author of the paper and a research fellow at the Griffith Centre for Social and Cultural Research.

"People all over the world at this time, these guys in Europe and Indonesia, were separated by a long way; they aren't the same population," Huntley told The Academic Times. "These populations and modern humans are doing very human things by making art at opposite ends of the world and doing them quite early."

Art in the Maros-Pangkep caves has continuously been discovered since the 1950s, but the scale and frequency of field surveys in the region are now rising to unprecedented heights, with new discoveries being made every year, according to Huntley. 

Local site custodians and Indonesian archaeological specialists had already started observing a slow deterioration of the cave art since the first site recordings in the 1950s, but over the last couple of decades, they've noticed that deterioration seems to be occurring more rapidly. 

Exfoliation of the limestone cave wall and ceiling destroys the rock surfaces that make up the artists' "canvas," and this is a driver of deterioration that seems to have worsened in recent decades. When Huntley and her colleagues entered the Maros-Pangkep caves to conduct this research, they suspected they would find evidence that climate change had increased the rate of deterioration, but the scale of what they found was something unexpected, Huntley said. 

"All the way across [the art panels], I was finding these chemical signatures for salts," she said. "I've worked in rock art and rock conservation for 10-15 years, and I've never seen anything chemically on that scale — it seemed to be quite a lot of salt."

The Maros-Pangkep region is in the world's most atmospherically dynamic region: the Australasian monsoon domain. Pair this with the rising frequency and severity of droughts from anthropogenic climate change and you've got a "perfect storm" for rock deterioration, Huntley said. 

Salt efflorescence weathering seems to be responsible for much of the increasing exfoliation of the Maros-Pangkep limestone cave surfaces. Salt efflorescence, also known as salt crystallization, is the growth of salt crystals deposited by saline solution. 

Because of climate change, the region is experiencing higher temperatures, longer-lasting droughts, more consecutive dry days and more extreme weather events. All of these events dry out the rock face, evaporating water that is holding salt on the cave surfaces but leaving the salt behind.

"What we have in the tropics, and particularly in the Maros-Pangkep region, is a recharge from a monsoon," Huntley said. "So, not only were there climate change impacts from the more hot and more consecutive dry days and droughts, but amongst that, we're also getting a regular recharge of water from monsoonal rains and far more extreme rain events and flooding that goes with climate change as well."

This increase in water leads to an increase in humidity, and when the salt on the rock surface is reexposed to humidity, again, it shrinks and swells. The salt then forms crystals on the rock surface, which causes the rocks to break apart. High levels of sulfur at all 11 sites suggest that this salt-induced deterioration is widespread throughout the Maros-Pangkep region. 

This research is just the "tip of the iceberg," Huntley said, as she and her colleagues look to fundraise more money in order to conduct broader-scale work in these caves, including the production of detailed 3-D maps of the area to track the weathering and deterioration of the rock surface in the Maros-Pangkep caves. 

Climate change is a global problem, and so reducing CO2 emissions worldwide is essential in preventing the deterioration of humans' earliest works of art, according to the study. Huntley suspects that if humans can't get global emissions under control, the more temperatures will rise and the worse the impacts on Maros-Pangkep will be. 

"I guess it's pretty easy for people to feel quite helpless about climate change," Huntley said. "I think there's been a bit of apathy about that because the problem seems so big — but it is small steps. Anything we can do to reduce those emissions … will make such a huge difference to how impacted we are and how severe those impacts are."

The study "The effects of climate change on the Pleistocene rock art of Sulawesi," published May 13 in Scientific Reports, was co-authored by J. Huntley, Griffith Centre for Social and Cultural Research; M. Aubert, Griffith Centre for Social and Cultural Research and Griffith University; A. A. Oktaviana, Griffith Centre for Social and Cultural Research and Pusat Penelitian Arkeologi Nasional; R. Lebe, L. Muhammad Aksa and M. Ramli, Balai Pelestarian Cagar Budaya; B. Hakim, Balai Arkeologi Sulawesi; B. Burhan and A. Brumm, Griffith University; I. Made Geria, Pusat Penelitian Arkeologi Nasional; L. Siagian, Museum Kepresidenan Republik Indonesia and Universitas Gadjah Mada; and H. E. A. Brand, Australian Synchrotron.