Close-up of an ice core yet to be extracted from a Hans Tausen drill during the Aurora Basin expedition, 2013. (Photo: Anthony (Tony) Fleming)

The climate has clearly changed in the past, affecting the planet and living things generally. For over two million years, the climate has been dominated by extended periods of globally colder temperatures than present (several degrees or more depending on location). These ice ages (or “glacial periods”) are interrupted by briefer warm (or “interglacial”) periods like the present one, which has lasted for approximately 10 000 years.

While the climate is complex it is clear from past history that it has at least these two semi-stable states. Exactly what triggers a change between these states is not completely understood. We know, for example, that some of the glacial/interglacial cycle is explained by changes in the earth's orbit, which influence the amount and distribution of heat input from the sun.

The precise timing of changes in orbital variations, temperature and other global indicators (e.g. greenhouse gas concentrations) is an important clue to advancing understanding.

These large climate changes have been accompanied by dramatic variations in vegetation and sea level, which have driven and controlled human migration in pre-history.

Indeed the warm, stable climate of the present interglacial, which has lasted some 10 000 years, has been the period in which modern agriculture and civilisations have developed. Such stability is very likely a necessary requirement for this development.

Besides the large climate changes, however, there are smaller changes in climate that may affect regions by up to a few degrees for decades to centuries.

One such change was the “Little Ice Age” which had a dramatic impact on European agriculture in approximately the 15th to 19th centuries. Other changes in say the severity and regularity of phenomena like El Nino, may also be linked to climate change and could have widespread impact on society.

The dramatic changes in greenhouse gas concentrations as a result of human activity exceed anything seen in the natural record. The complexity and even chaotic behaviour of the climate system make it difficult to predict its response to human and other forcing factors such as volcanic activity and solar variations. In an increasingly populated planet, such predictions are ever more important.

To meet this prediction challenge, complex computer models of the climate have been developed. These must be able to reproduce this past climate behaviour if they are to be useful in predicting future climate change.

Analysis of the air bubbles trapped in the ice cores allows us to see increases of greenhouse gases in the atmosphere from fossil fuel use. We can also detect the climate changes that occurred through this period.

We work mostly on ice cores from near the coast of Antarctica where snowfall is highest. This provides thick layers of snow which give very detailed climate records. Our longest ice core comes from near the summit of Law Dome, near Casey station. The ‘Dome Summit South’ (DSS) core is around 1200 m long, from surface to bedrock, and covers around 90 000 years of climate history. We are now working towards extracting a one million year old ice core, which will add to our knowledge of observed climate change in Australia, Antarctica and the Southern Hemisphere.

The ice core climate history work is also part of the Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC). This centre brings together researchers studying other aspects of climate change, including climate history studies using sediments laid down on the ocean floor and in lakes. We work with these and other international groups studying past climate from ice, tree rings and sediments.

How old is the core?

Read at http://www.antarcticglaciers.org/glaciers-and-climate/ice-cores/ice-core-basics/ 

If we want to reconstruct past air temperatures, one of the most critical parameters is the age of the ice being analysed. Fortunately, ice cores preserve annual layers, making it simple to date the ice. Seasonal differences in the snow properties create layers – just like rings in trees. Unfortunately, annual layers become harder to see deeper in the ice core. Other ways of dating ice cores include geochemisty, layers of ash, electrical conductivity, and using numerical flow models to understand age-depth relationships. Although radiometric dating of ice cores has been difficult, Uranium has been used to date the Dome C ice core from Antarctica. Dust is present in ice cores, and it contains Uranium. The decay of 238U to 234U from dust in the ice matrix can be used to provide an additional core dating.

What can ice cores tell us?

Accumulation rate

The thickness of the annual layers in ice cores can be used to find a snowfall rate. Past rates are often correlated to climate change, and it’s an essential parameter for many past climate studies.

Melt layers

Melt layers are related to summer temperatures. More melt layers indicate warmer summer air temperatures. Melt layers are formed when the surface snow melts, releasing water to percolate down through the snow pack. They form bubble-free ice layers, visible in the ice core. The distribution of melt layers through time is a function of the past climate, and has been used, for example, to show increased melting in the Twentieth Century around the Antarctic.

Past air temperatures

Past air temperatures from ice cores can be related directly to concentrations of carbon dioxide, methane and other greenhouse gases preserved in the ice. 

Snow precipitation over Antarctica is made mostly of water molecules containing the common oxygen and hydrogen isotopes  O-16 and H-1 (99.7%). There are also rarer stable isotopes: O-18 (0.2%) and H-2 (0.03%). Past snowfall  can be used to reconstruct past climatic temperatures. 

Snow that falls over Antarctica is slowly converted to ice. The oxygen and hydrogen isotopes are measured in ice through a mass spectrometer. Measuring changing concentrations H-2 and O-18 through time in layers through an ice core provides a detailed record of temperature change, going back hundreds of thousands of years.

Past greenhouse gases

The most important property of ice cores is that they are a direct archive of past atmospheric gases.  When the compacted snow turns to ice, the air is trapped in bubbles. This transition normally occurs 50-100 m below the surface.  The air bubbles are extracted by melting, crushing or grating the ice in a vacuum.

This method provides detailed records of carbon dioxide, methane and nitrous oxide going back over 650,000 years. Ice core records globally agree on these levels, and they match instrumented measurements from the 1950s onwards, confirming their reliability. 

Read and view video at https://untamedscience.com/biology/biomes/coral-reefs-biome/ 

Coral Reefs have been called the rainforests of the ocean because of their rich biodiversity. Unfortunately they are also in becoming increasingly threatened.  Not only is global warming going to affect the survival of coral reefs, but other human activities threaten the entire ecosystem. 

Corals are small animals that belong to the phylum Cnidaria together with anemones and jellyfish. Coral Reefs are almost exclusively found in tropical and sub-tropical waters across the globe. These reefs form the framework for an incredible diversity of other organisms: there are thousands of animals that make the coral reefs their home.  The longest coral reef system on Earth is the Great Barrier Reef off the east coast of Australia. This massive reef system stretches more than 2,000 km and can be seen from space!

For most reef building corals to survive they need to have a few special requirements met. For example, at any time the average water temperature cannot be less than about 18-20 degrees Celsius.  Since there is no shortage of sunlight, nutrients soon become the limiting factor for primary producers. So the waters around tropical coral reefs are in fact relatively nutrient poor. Yet still, they support this incredible diversity of life. The answer to the energy equation is working together. Most corals have developed a symbiotic relationship with a small microalgae called a zooxanthellae. This small organism is incorporated in the coral tissue and actually is what gives corals their beautiful colours. As other algae, the zooxanthellae use sunlight to photosynthesise and produce so much energy that it can also provide the coral with almost all of its energy needs (scientists have found that about 98 percent of the energy may be from the zooxanthellae). In return the algae can take up nutrient-rich waste products from the coral. Because of this symbiotic relationship corals can only grow relatively close to the surface where the water is clear enough for the zooxanthellae to perform photosynthesis.

The hard coral reef structure is made of calcium carbonate that the coral secretes as it grows and expands the colony.

Threats to Coral Reefs

Coral reefs are being threatened around the world because of many different factors. Their special requirements to survive also make them relatively sensitive to change. We know that most ecosystems are able to adjust to changes fairly well, given enough time. The problem today is that changes are happening so fast that most animals don’t have time to keep up. And coral reefs especially.

Many coral reefs are relatively close to land. This makes them easy to access and also easily affected by everything that goes on on land. Road construction, coastal clearing, agriculture, and more result in a lot of sediment and pollutants that get washed out to sea with the monsoon rains that fall in tropical areas. Simple sedimentation in the water may be enough to kill the reefs both by directly covering the corals but also by decreasing light penetration of the water so much that the symbiotic zooxanthellae algae are unable to photosynthesise.

Another factor is over-fishing. When too many fish are taken out of a system the balance is disturbed. Many of these fish eat algae and control the abundance of algae on the reef. When the fish is removed, the fast-growing algae can take over. Destructive fishing methods such as cyanide and dynamite used in some parts of the world also directly affect the structure of the coral reefs. A coral reef that has been blown away by dynamite needs many, many years to recover.

Global warming is also causing the sea temperatures to rise. 

Over the past century, the average global temperature warmed by more than 0.85 degrees Celsius, with most of the warming occurring since the 1970s. All of the warmest 20 years on record have occurred since 1990.

In Australia, mean surface air temperature warmed by 0.9°C since 1910. Our sea surface temperatures are increasing too, as 90 per cent of the excess heat in our atmosphere is stored in oceans. Sea surface temperatures in north-eastern Australia warmed, on average, by 0.12 degrees per decade since 1950. In the Coral Sea over the past century, 15 of the 20 warmest years occurred in the past 20 years. The sea surface temperature on the Great Barrier Reef, when averaged across the last 30 years, has increased by about 0.4 degrees, compared to records averaged across 30 years in the late 1800s. In 2016, sea surface temperatures on the Great Barrier Reef were the hottest ever recorded for the months of February, March and April.

Analysis of coral cores in centuries-old corals suggests current temperatures are warmer now than over the past three centuries.

The Intergovernmental Panel on Climate Change predicts that by 2035 the average sea surface temperature will be warmer than any previously recorded, and by 2100 sea temperatures off north-eastern Australia could be about 2.5 degrees Celsius warmer than the present average.

Effects

Rising sea surface temperatures are affecting every aspect of the Great Barrier Reef, as sea temperature is a key factor in controlling the diversity of marine life, and how far north or south an animal can live. Even though corals need relatively warm water to survive, there is a limit. Like all marine species, corals have adapted over many thousands of years within limited temperature ranges. This makes corals highly vulnerable to the potential effects of higher sea surface temperatures.

Water temperature helps determine the north-south limits of reefs, as well as their diversity. Temperature also helps control the rate of coral reef growth, making it critical in reef building.

When temperature limits are exceeded, corals are put under thermal stress, causing them to expel the algae that live within their tissues. The algae disappears from the coral and the corals become “bleached”. This basically means that the coral structure is left naked but still alive without the symbiotic algae. Most corals can only survive a short period without the symbiotic algae. If water conditions don’t change back to normal during this time also, the coral will starve and die. Coral bleaching is not always fatal, but has been one of the main causes of coral death around the world in the past two decades.

Severe bleaching is linked to climate phenomena such as El Niño events. These typically warm sea surface temperatures around the Great Barrier Reef, resulting in sustained elevated regional temperatures. Extreme El Niño occurrences are projected to increase due to climate change.

At least 10 mass bleaching events have affected the world’s reefs since 1979. The Great Barrier Reef was most severely affected by the 2016 event, and a second consecutive year of severe mass bleaching in 2017.

Coral bleaching is expected to occur more often and with greater severity in the future, making it difficult for corals to recover between bleaching events. This is likely to reduce the abundance of living corals, with flow-on effects for other species dependent on reefs.

Large, fleshy seaweeds (called macroalgae), which compete with corals for space, will likely also benefit from rising temperatures and coral bleaching. Degrading reefs can be rapidly overgrown by macroalgae, which in turn impede coral recovery.

What can you do to help?

The only way to protect these amazing environments is to provide an incentive for people to protect them. Ecotourism is one reason to save these reefs. Visit a coral reef, and learn as much as you can about it. Talk to the locals, and tell them how important a healthy reef is to your visit. If you want to do more, visit noaa.org.

One more thing… directly breaking of a branch of a coral could mean removing more than 10 years of construction. Don’t encourage breaking the corals for souvenirs. Leave them in the water to look at. Don’t encourage people who sell the corals to continue.