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Unearthing the Past The Geological Evidence of Climate Change
In the light of rising temperatures and shifting weather patterns it is difficult to ignore the significant transitions taking place on the planet due to climate change. Some call it the crisis of our time, and it is a topic which often sparks controversy. Well, while there might be truth in some perspectives, geological evidence shows that temperature and weather patterns have fluctuated naturally across time scales, ranging from decades to millions of years.
On geological timescales, climate change is caused by changes in the earth’s orbit, quantity of greenhouse gases in the atmosphere, axial tilt and precession, ocean currents and carbon dioxide content, plate tectonics and volcanic eruptions as well as changes in land cover and meteorite impacts. Each of these processes is a unique contribution to changes in the earth's climate, but matters are complicated by how they interact with one another. They can cause additional, or reduced alterations in others.
Earth has experienced several major ice ages. During these periods, Earth’s temperature decreased, causing an expansion of ice sheets and glaciers. The oldest glaciation is the Huronian about 2.5 billion years ago. The exact causes of this glaciation are not known, but it is believed to have been triggered by a combination of factors, including reduced greenhouse gas concentrations, changes in solar radiation, and continental plate tectonics.
The end of the Proterozoic era saw another glaciation, known as the Snowball Earth hypothesis. It occurred about 700 million years ago. During these events, the planet experienced near-global ice cover, with ice extending from the poles to the equator. Just as with the Huronian ice age, this event is believed to have been the result of a combination of factors, including reduced greenhouse gas levels, possible changes in Earth's orbit, and a concentration of continental mass near the equator. The release of volcanic gases and weathering processes probably played a crucial role in ending these extreme glaciations.
As the Paleozoic era unfolded, the earth transitioned from a predominantly glaciated state to a warmer climate. This was caused by an increase of oxygen and a subsequent drop in carbon dioxide, most likely produced by the rise of land plants. The Late Paleozoic experienced a significant cooling phase, culminating in the Carboniferous-Permian glaciation (approximately 323-300 million years ago). This glaciation was characterised by extensive ice sheets that covered vast regions of the southern hemisphere, including parts of Gondwana. The glaciation eventually gave way to a warmer climate, heralding the Permian-Triassic transition. During the Cenozoic era, the climate started out warm and moist but gradually cooled, spanning from about 66 million years ago until the present day.
How do we know about past climates? Well, geological records have preserved a wide range of settings, including marine and lake sediment layers, ice cores, as well as fossil evidence. By using these proxy indicators, geoscientists have been able to reconstruct the long-term climate patterns. For example, cores taken from ice sheets have provided a record of polar temperatures, precipitation patterns and air composition dating back 120,000 to 800,000 years ago. Layers of sediment contain remnants of ancient plants and animals which lived during a given time and are now preserved as fossils. The presence of fossilised marine organisms in rocks found in areas that are currently arid indicate that these regions were once submerged under water. Similarly, the presence of coal deposits in regions that are now tropical indicates a significant shift in climate over geological time.
Dendrochronology, the study of tree rings, is another indicator, because it provides insight into past growth patterns of trees. Each ring in a tree trunk corresponds to one year of growth. The width of the rings is influenced by environmental factors such as temperature, precipitation and sunlight. By analysing tree rings from ancient trees or preserved wood found in archaeological sites, scientists can reconstruct past climate conditions. Variations in ring width and density can indicate periods of drought, forest fires or other climate-related events.
Coral reefs, often referred to as the "rainforests of the sea”, are sensitive indicators of climate change. The growth of coral reefs is influenced by water temperature, salinity and other factors. When water temperatures exceed certain thresholds, corals undergo a process known as "bleaching”: they expel the symbiotic algae living within them, which leads to their eventual death. Studying the growth patterns and chemical composition of coral skeletons allows geoscientists to reconstruct past sea surface temperatures and identify periods of climate stress or stability.
From a geological perspective the current rate of climate change is alarming. Geological records show that similar changes have occurred in the past – but over much longer timescales, which allowed ecosystems and species to adapt and evolve. The rapid pace of today's climate change may exceed the ability of many species to adapt, which would result in biodiversity loss and ecosystem disruptions. Understanding these geological correlations can help us develop strategies to mitigate the effects of climate change and preserve the planet's ecosystems.
