Climate and the rise and fall of civilizations: a lesson from the past


Andrew Glikson, Australian National University

2015 will likely be the hottest year on record, beating the previous record set only in 2014. It is also likely to be the first year the global average temperature reaches 1℃ above pre-industrial temperatures (measured from 1880-1899). Global warming is raising temperatures, and this year’s El Niño has pushed temperatures higher still.

Although 2015 is unusually hot, 1℃ symbolically marks the halfway point to 2℃, widely considered to be the threshold of “dangerous” climate change. In fact an additional 0.5-1℃ is actually masked by sulphur aerosols which we have added to the atmosphere alongside greenhouse gases.

A temperature level of 1℃ (above pre-industrial levels) is similar to or warmer than the peak temperatures of the early Holocene epoch approximately 8,000-7,200 years ago. Studies of the early Holocene provide clues to what was such a world like.

The climate roller-coaster

The last ice age (or Last Glacial Maximum) peaked around 26,000 years ago. The earth warmed over the coming millennia, driven by an increase in radiation from the sun due to changes in the earth’s orbit (the Milankovic cycles) amplified by CO₂ released from warming water, which further warmed the atmosphere.

But even as the earth warmed it was interrupted by cooler periods known as “stadials”. These were caused by melt water from melting ice sheets which cool large regions of the ocean.

Marked climate variability and extreme weather events during the early Holocene retarded development of sustainable agriculture.

Sparse human settlements existed about 12,000 – 11,000 years ago. The flourishing of human civilisation from about 10,000 years ago, and in particular from 7,000 years ago, critically depended on stabilisation of climate conditions which allowed planting and harvesting of seed and growing of crops, facilitating growth of villages and towns and thereby of civilisation.

Peak warming periods early in the Holocene were associated with prevalence of heavy monsoons and heavy floods, likely reflected by Noah’s ark story.

We can’t measure historical temperatures directly, so scientists use oxygen measurements instead. Human civilisation arose in a period of mostly settled climate.
Bruce Railback’s Geoscience Resources

Early civilisations

The climate stabilised about 7,000 – 5,000 years ago. This allowed the flourishing of civilisations along the Nile, Tigris, Euphrates, Indus and the Yellow River.

The ancient river valley civilisations cultivation depended on flow and ebb cycles, in turn dependent on seasonal rains and melting snows in the mountain sources of the rivers. These formed the conditions for production of excess food.

When such conditions declined due to droughts or floods, civilisations collapsed. Examples include the decline of the Egyptian, Mesopotamian and Indus civilisations about 4,200 years ago due to severe drought.

Throughout the Holocene relatively warm periods, such as the Medieval Warm Period (900-1200 AD), and cold periods, such as the Little Ice Age (around 1600 – 1700 AD), led to agricultural crises with consequent hunger, epidemics and wars. A classic account of the consequences of these events is presented in the book Collapse by Jared Diamond.

It’s not just Middle Eastern civilisations. Across the globe and throughout history the rise and fall of civilisations such as the Maya in Central America, the Tiwanaku in Peru, and the Khmer Empire in Cambodia, have been determined by the ebb and flow of droughts and floods.

Changing the game

Greenhouse gas levels were stable or declined between 8,000-6,000 years ago, but then began to rise slowly after 6,000 years ago. According to William Ruddiman at the University of Virginia, this rise in greenhouse gases was due to deforestation, burning and land clearing by people. This stopped the decline in greenhouse gases and ultimately prevented the next ice age. If so, human-caused climate change began much earlier than we usually think.

Rise and fall in solar radiation continued to shift the climate. The Medieval Warm Period was driven by an increase in solar radiation, while the Little Ice Age was caused at least in part by a decrease.

Now we’ve changed the game again by releasing over 600 billion tonnes of carbon into the atmosphere since the Industrial Revolution, raising CO₂ concentrations from around 270 parts per million to about 400 parts per million.

One of the consequences of this rise is an extraordinary decline in the North Atlantic Ocean Circulation as cold water from melting of Greenland ice enters the sea. This could potentially lead to a collapse of the Atlantic Meridional Ocean Circulation and a short, regional human-caused cold period, or “stadial”, mostly affecting Europe and North America, similar to those that occurred in the early Holocene.

CO2 concentrations in November 2015. The blue circle shows an area of reduced CO2 corresponding to cooler sea temperatures in the North Atlantic Ocean.
NASA, Author provided

While this may sound like “global cooling”, a cold period could have deleterious effects on agriculture and is bound to be succeeded by further warming due to the high atmospheric CO₂ concentrations.

The current shift in state of the atmosphere-ocean-ice system signifies a return to conditions such as existed at the early Holocene, which were less favourable for farming. But it doesn’t stop there.

A further rise in CO₂ and temperature would lead to conditions which existed in the Pliocene before 2.6 million years ago, including many metres of sea level rise (around 10-40 metres), posing an existential threat to the future of civilisation.


Andrew will be on hand for an Author Q&A between 2 and 3pm AEST on Friday, December 11, 2015. Post your questions in the comments section below.

The Conversation

Andrew Glikson, Earth and paleo-climate scientist, Australian National University

This article was originally published on The Conversation. Read the original article.

Delving deep into caves can teach us about climate past and present


Gabriel C Rau, UNSW Australia; Andy Baker, UNSW Australia; Mark O Cuthbert, University of Birmingham, and Martin Sogaard Andersen, UNSW Australia

Have you ever enjoyed the cool refuge that an underground cave offers from a hot summer’s day? Or perhaps you have experienced the soothing warmth when entering a cave during winter?

When descending into a cave, you may not only enjoy the calm climate, you may also admire the beauty of cave deposits such as stalagmites, stalactites and flowstones, known by cave researchers as speleothems.

Perhaps you already know that they grow very slowly from minerals in the water that drips off or over them. This water originates from rain at the surface that has travelled through soil and limestone above, and seeped into the ground and ended up in the cave.

As speleothems grow, they lock into their minerals the chemical signatures of the environmental and climatic conditions of the time the rainwater fell at the surface. So, as a stalagmite grows, the surface climate signature is continuously trapped in the newly created layers.

Some very old stalagmites hold climatic signatures of the very distant past, in some cases up to millions of years. They contain an archive of the past climate as long as their age, often predating global weather station records.

Above and below

But if a cave remains cool during summer and warm during winter, how is its climate related to that of the surface? And how does this affect the chemical signature recorded by speleothems?

To understand the relationship between surface and cave climate, our research group, Connected Waters Initiative Research Centre at UNSW Australia, conducted multiple field experiments at the Wellington Caves Reserve in New South Wales.

During the experiments, the surface and the cave climates were measured in detail. For example, highly accurate temperature sensors were used to measure the water temperature at the surface, and at the point where water droplets hit the cave floor forming stalagmites.

Installation of high-resolution temperature sensors inside the cave
Martin S Andersen

The research team initiated controlled dripping in the cave by irrigating the surface above the cave with water that was cooled to freezing point to simulate rainfall.

The cold water allowed us to determine whether the drip water in the cave is affected by the conditions at the surface or those along its pathways through the ground.

We also added a natural chemical to the irrigation water, which allowed us to distinguish whether the water in the cave originated from the irrigation or whether it was water already present in the subsurface.

Our results revealed a complex but systematic relationship between the surface and the cave climate. For example, surface temperature changes are significantly reduced and delayed with depth.

Our research illustrates how to decipher the surface temperature from that in the cave. Understanding this is necessary to correctly decoding past surface temperature records from their signatures preserved in stalagmites.

Keeping it cool

We also discovered that air moving in and out of the cave can cool cave deposits by evaporating water flowing on the cave deposits. This cooling can significantly influence the chemical signature trapped in the cave deposit and create “false” signals that are not representative of the surface climate.

In other words, it will make the surface climate “look” cooler than it actually was, if not accounted for. While this is more likely to occur in caves that are located in dry environments, it may also have to be considered for stalagmites in caves that were exposed to drier climates in the distant past.

Temperature loggers installed on stalactites to measure the drip water temperature
Martin S Andersen

Our new knowledge can also help scientists select the best location and type of stalagmite for the reconstruction of past climatic or environmental conditions.

This new discovery is significant because it can improve the accuracy of past climate signals from cave deposits. It may also help us understand previously unexplained artefacts in existing past climate records. By improving our understanding of the past climate we can better understand future climate variations.

The Conversation

Gabriel C Rau, Associate Lecturer in Groundwater Hydrology, UNSW Australia; Andy Baker, Director of the Connected Waters Initiative Research Centre, UNSW Australia; Mark O Cuthbert, Research Fellow in Hydrogeology, University of Birmingham, and Martin Sogaard Andersen, Senior lecturer, UNSW Australia

This article was originally published on The Conversation. Read the original article.

The Wilderness Society Being Torn Apart


Infighting is threatening to destroy the environmental group, ‘The Wilderness Society.’ A court battle now looms in order to sort out the mess that has become The Wilderness Society.

This is a group that I have supported in the past and depending on the outcome of the court case and what then happens with The Wilderness Society will determine whether I support the group again.

The Wilderness Society is a well known environmental group in Australia. It was formed to fight the Franklin Dam project in Tasmania in the 1970s.

The following link is to an ABC news article reporting on the story:

http://www.abc.net.au/news/stories/2010/05/03/2889021.htm

The Wilderness Society website:

http://www.wilderness.org.au/

The video below is a reflection on the Franklin Dam project protest.

 

Next Trip: Back to Trekking


With the road trip now done and the car returned to Budget (rental), my thoughts have turned to my next holiday. I have a few road trip style adventures in the works, but my next holiday is likely to be a trekking adventure.

In the past I have twice walked across the Barrington Tops, completing the ‘Tops to Myall Heritage walk’ section as far as the township of Craven. I now plan to do the second section of the walk, from Craven to Hawks Nest – not far from where I now live. This has been something I have wanted to do for some time and now I intend to actually complete it.

There is no intended date at this stage, though I would hope to do it well before the end of this year. I would think it likely to be towards the early part of the second half of the year – if that makes sense.

Prior to setting out on this trek, I will be looking to update some of my gear (most of which has vanished in recent moves). I will be looking at a new tent and a sleeping bag for starters, as well as a new back pack. I’m sure there will be a few other things I will need to acquire before I set off as well.

Planning for this trip can all be followed on this Blog… stay tuned for further developments.