Have you ever wondered why there are “caps” at the North and the South poles of Earth? Why is there so much ice there? The reasons for such things lie in the history of Earth. Over the years, geologists have divided the past of Earth into various divisions, and we call it the Geological Time Scale.
One of the ways to divide the history of Earth is by using the geological features present on Earth, such as the sedimentary rock layers. Every rock layer corresponds to a change in the Earth's environment and gives an idea of what has happened in the past. These layers are also known as boundaries that sometimes mark the end or beginning of an era.
For example, we have K – Pg boundary (the boundary that marks the end of dinosaurs) that marks the end of the Cretaceous Period, the last period of the Mesozoic Era, and marks the beginning of the Paleogene Period, the first period of the Cenozoic Era. By exploring the landforms, sediments, and fossils of the most recent period in the geologic time scale, spanning from about 2.58 million years ago to the present, the Quaternary Period, we can identify glacial periods of severe cold climate when great ice sheets formed in the high middle latitudes of the northern hemisphere and glaciers and ice caps advanced in mountain regions around the world. This is one of the clues to finding the answer to our question, and these clues lead us to an important age. So, be ready with a blanket and a heater to experience the recent chapter in Earth’s history: The Ice Age.
HOW WAS OUR EARTH DURING THE ICE AGE
Over the past, the Earth has undergone various Ice Ages or glacial periods - Great Ice Ages, Little Ice Ages, some of which also involve partial freezing of the Earth (one more clue to the answer to our question!).
The most recent major ice age, known as the Great Ice Age or Pleistocene Epoch, spanned approximately 2.6 million to 11,700 years ago. Extensive ice sheets and glaciers formed and retreated in a series of glacial and interglacial (a period of milder climate between Ice Ages) cycles during this epoch, shaping much of the Earth’s surface as we know it today.
The Great Ice Age, a recent chapter in the Earth's history, was a period of recurring widespread glaciations. During the Pleistocene Epoch of the geologic time scale, which began about a million or more years ago, mountain glaciers formed on all continents, the icecaps of Antarctica and Greenland were more extensive and thicker than today, and vast glaciers, in places as much as several thousand feet thick, spread across northern North America and Eurasia. So extensive were these glaciers that almost a third of the present land surface of the Earth was intermittently covered by ice. Much has been learned about the Ice Age glaciers because evidence of their presence is so widespread and because similar conditions can be studied today in Greenland, in Antarctica, and in many mountain ranges where glaciers still exist. It is possible, therefore, to reconstruct in large part the extent and general nature of the glaciers of the past and to interpret their impact on the physical and biological environments.
CLIMATE CHANGE AND ICE AGE
It is also important to recognise that the ice age isn’t just about advancing and retreating ice sheets. Major environmental changes also took place in the Mediterranean region and in the tropics. The Sahara, for example, became drier, cooler, and dustier during glacial periods, yet early in the present interglacial it was a mosaic of lakes and oases with tracts of lush vegetation. A defining feature of the Quaternary Period is the repeated fluctuation in climate as conditions shifted from glacial to interglacial, and back again, during the course of the last 2.5 million years or so.
Willi Dansgaard (1922–2011) was the first scientist to demonstrate that the ice sheets themselves provided an extended record of Earth’s climate history. Dansgaard was interested in oxygen isotope ratios in rainfall, snow, and ice. He made the landmark discovery that the oxygen isotope profile in ice cores provided a long-term record of changing air temperature in the Polar regions. He was able to show that as air temperature falls, more molecules of H2O containing the heavy (oxygen-18) isotope condense and are lost from clouds as rain and snowfall. Thus, atmospheric water vapor becomes more and more depleted of 18 - O in a poleward direction. In 1966, the Americans obtained a 1,390 m ice core from Camp Century—the first ice core to penetrate the Greenland ice sheet down to bedrock. Recent work on Greenland ice cores has allowed the end of the Pleistocene epoch and the onset of the Holocene interglacial to be dated very precisely to 11,700 years before AD 2000.
As layers of snow become compacted into ice, air bubbles recording the composition of the atmosphere are sealed in discrete layers. This fossil air can be recovered to establish the changing concentration of greenhouse gases such as carbon dioxide and methane. A Swiss physicist, Hans Oeschger (1927–98), made fundamental contributions to our understanding of ice age climate change. He pioneered the measurement of greenhouse gases in the bubbles trapped in ancient ice. In his laboratory at the University of Bern, Oeschger analysed many thousands of samples from Greenland and Antarctica. In 1979, his team was the first to show that CO2 concentrations during glacial stages were almost half those of the present. Note how the changes in temperature closely track the changes in methane and CO2. Methane is a potent greenhouse gas—it is stored in large volumes in the frozen biomass of the permafrost and as methane hydrate within sediments beneath the ocean floor. Ice core data have been fundamental in demonstrating that changes in the composition of the atmosphere played a key role in the shifting climates of the Quaternary, but there is still much debate about the processes involved and the leads and lags. The glacial and interglacial shifts during the Quaternary period can be explained by the Milankovitch Cycles using the link of CO2 exchange between the oceans and atmosphere. So, what is the Milankovitch Cycle really?
THE MILANKOVITCH CYCLES AND ICE AGE:
A brief discussion on the Ice Age should involve a discussion on Milankovitch Cycles, and this concept can solve the question that we are trying to solve, just from the start of this article. Milutin Milankovitch hypothesised that the long-term, collective effects of changes in Earth’s position relative to the Sun are a strong driver of Earth’s long-term climate and are responsible for triggering the beginning and end of Ice Ages. Specifically, he examined how variations in three types of Earth orbital movements affect how much solar radiation (known as insolation) reaches the top of Earth’s atmosphere, as well as where the insolation reaches. These cyclical orbital movements, which became known as the Milankovitch cycles, cause variations of up to 25 per cent in the amount of incoming insolation at Earth’s mid-latitudes (the areas of our planet located between about 30 and 60 degrees north and south of the equator). The Milankovitch cycles include the shape of Earth’s orbit, known as eccentricity. The angle Earth’s axis is tilted with respect to Earth’s orbital plane, known as obliquity; and the direction Earth’s axis of rotation is pointed, known as precession. Milankovitch combined the changes in each of these over the years due to various factors in our solar system to create a comprehensive mathematical model for calculating differences in solar radiation at various Earth latitudes along with corresponding surface temperatures. The model is sort of like a climate time machine: it can be run backwards and forward to examine past and future climate conditions. He calculated that Ice Ages occur approximately every 41,000 years. Subsequent research confirms that they did occur at 41,000-year intervals between one and three million years ago. But about 800,000 years ago, the cycle of Ice Ages lengthened to 100,000 years, matching Earth’s eccentricity cycle. While various theories have been proposed to explain this transition, scientists do not yet have a clear answer.
MILANKOVITCH CYCLES CAN NOT EXPLAIN EARTH’S CURRENT WARMING:
Milankovitch cycles can’t explain all the climate change that has occurred over the past 2.5 million years or so. More importantly, they cannot account for the current period of rapid warming Earth has experienced since the pre-Industrial period (the period between 1850 and 1900), particularly since the mid-20th century. Scientists are confident Earth’s recent warming is primarily due to human activities — specifically, the direct input of carbon dioxide into Earth’s atmosphere from burning fossil fuels. Milankovitch cycles operate on long time scales, ranging from tens of thousands to hundreds of thousands of years. In contrast, Earth’s current warming has taken place over time scales of decades to centuries. Over the last 150 years, Milankovitch cycles have not changed the amount of solar energy absorbed by Earth very much. In fact, NASA satellite observations show that over the last 40 years, solar radiation has actually decreased somewhat. Finally, Earth is currently in an interglacial period. If there were no human influences on climate, scientists say Earth’s current orbital positions within the Milankovitch cycles predict our planet should be cooling, not warming, continuing a long-term cooling trend that began 6,000 years ago. And there we are! The reason why there are ice caps at the North and South poles of Earth is that they are a part of the cooling cycle that was started 6,000 years ago!
In the above paragraph it is clear that if there was no human intervention then our planet should be cooling because of a cooling cycle, which takes us to a serious issue that our activities on Earth can even alter such big climatic conditions - our interventions on this planet can change the cycle from cooling to warming and this is a global concern because warming the planet is melting the polar ice caps which in turn leads to increase in water level of oceans that brings tremendous disaster in form of floods, tsunamis etc. It’s time to do something about this growing concern of melting icecaps. There are multiple research and startup-led efforts experimenting with different techniques to refreeze or preserve Arctic ice. Sustainable development is the key to making the required balance. All of us should practice sustainable development and inform those who are not practicing it. It’s time to come together as a whole, like we did during the concern of the depleting ozone layer.
