Ice ages weren’t as dry as we thought, surprising new study of Australian caves finds

During the Ice Ages, dry, frozen land extended across much of northern Europe, Asia, and North America. Many plants and animals retreated from these desolate and harsh landscapes and sought refuge in pockets of more hospitable territory.

But what was happening in the rest of the world? For a long time, scientists thought that dry conditions prevailed everywhere on the planet during ice ages and that warm periods between ice ages were much wetter.

This interpretation has shaped our understanding of where plants, animals, and even humans lived during Earth’s past. However, this may not be correct.

Our new research published in Nature shows that ice ages were actually much wetter than previously thought, at least in the subtropical regions of the southern hemisphere (20° to 40° south).

Ice ages and hemispheres

Over the last million years, Earth’s climate has oscillated between cold glacial periods (or “glacial” periods) and warmer “interglacial” periods. We are currently experiencing an interglacial period known as the Holocene Epoch. It began around 11,700 years ago, following the last ice age which lasted around 110,000 years.

During ice ages, temperatures were lower, there was less carbon dioxide in the atmosphere, and ice sheets covered more of the globe. During interglacial periods, temperatures were higher, there was more carbon dioxide in the air, and large ice sheets remained only in Greenland and Antarctica.

Evidence from the Northern Hemisphere shows huge ice sheets spread across northern Europe, northern Asia and North America during glacial periods, and large areas south of the ice were covered in tundra. The idea that glacial environments were extreme and harsh later spread beyond these regions due to evidence that glacial periods were mostly treeless and with dusty atmospheres almost everywhere, including in Australia.

However, our new research reveals that some ice ages were actually wetter than today across much of the Southern Hemisphere.

Establish a climate record over 350,000 years

One way to understand how wet it was in the past is to look at mineral deposits called speleothems, found in underground caves. These deposits, which include stalagmites and stalactites, accumulate over time as rainwater seeps through the soil and limestone into the cave.

We can use the extent of speleothem growth over time to understand changes in water availability. Greater growth of speleothems reflects wetter conditions overall, while less growth suggests a drier environment.

Our understanding of past changes in the climate and environment of the Southern Hemisphere has been limited by the lack of well-dated, long-term data.

To address this issue, we collected speleothem samples from two cave regions in southern Australia, the Naracoorte Caves in the southeast and the Leeuwin-Naturaliste Caves in the southwest.

Using a dating technique based on the decay of natural uranium, we determined the ages of more than 300 individual speleothem fragments from the caves. As a result, we have produced a precipitation record spanning the last 350,000 years.

Wetter and colder, hotter and drier

Our study revealed some surprising but extremely consistent trends. Over the past 350,000 years, the wettest periods always occurred during the coldest glacial periods, while interglacial periods were consistently dry.

We also studied fossil pollen trapped in the same speleothems. It is harder to be a tree under the low atmospheric carbon dioxide of ice ages, but moisture-demanding grasses and shrubs thrived during ice ages but were suppressed during interglacial periods, confirming evidence of dating.

Next, we used our new records from southern Australia as references for subtropical regions around the Southern Hemisphere, and compared them with other published records from southern Africa and South America. We found that wet glacials and dry interglacials were not confined to southern Australia, but in fact formed a hemispheric-wide pattern.

Climate model simulations also showed a similar trend during the last glacial cycle.

Stable environments with abundant water

This new understanding of the conditions that prevailed in the Southern Hemisphere during ice ages will change the way we interpret the movement and expansion of plants, animals and even humans in the past.

It was previously assumed that during ice ages, reduced precipitation forced many plants and animals that needed higher humidity levels to move into small habitable areas called “refugia.”

However, our research suggests that, at least in the subtropical Southern Hemisphere, ice ages were often periods of relatively stable environments with abundant water, even though low levels of carbon dioxide meant plants had slow growth. and relatively unproductive.

Our research calls for a radical paradigm shift in how we perceive past Ice Age environments across Earth.

This article is republished from The Conversation under a Creative Commons license. Read the original article.