Earth and Space: Cycles on Earth

The Earth's climate has always been in a state of flux, naturally cycling through warmer and colder periods over millions of years. It's crucial to distinguish between 'weather' and 'climate'. Weather refers to the atmospheric conditions over a short period (like a rainy day), whereas climate is the average weather pattern for a region over a long period, typically 30 years or more. While the Earth has experienced dramatic climate shifts in the past, such as the age of the dinosaurs when it was much warmer with no polar ice, the current period of global warming is unique because of its rapid pace, driven primarily by human activities since the Industrial Revolution.

An 'ice age' is a long period in Earth's history (millions of years) when the overall temperature is lower, leading to the presence of large, permanent ice sheets on the continents and at the poles. It's a common misconception that an ice age is one continuous deep freeze. Instead, ice ages are punctuated by cycles of colder and warmer phases. The colder phases are called 'glacial periods', where glaciers and ice sheets advance and cover large parts of the continents. The warmer phases are 'interglacial periods', where ice sheets retreat towards the poles. We are currently in an ice age called the Quaternary Glaciation, but we are living through a warmer interglacial period that began around 11,700 years ago.

Scientists act like detectives, using clues from the natural world to reconstruct past climates. Two of the most important sources of evidence are ice cores and preserved pollen.

1. Ice Cores: By drilling deep into the ice sheets of Antarctica and Greenland, scientists extract ice cores that can be kilometres long. These cores contain layers of ice that have built up over hundreds of thousands of years. Trapped within the ice are tiny bubbles of ancient air. By analysing the concentration of greenhouse gases like carbon dioxide (CO₂) and methane (CH₄) in these bubbles, scientists can determine the composition of the atmosphere in the past. The isotopic composition of the water molecules in the ice itself also acts as a direct thermometer, allowing scientists to estimate past temperatures.
2. Pollen Analysis: Pollen grains from plants have tough outer walls that allow them to be preserved for thousands of years in oxygen-poor environments like the sediment at the bottom of lakes and in peat bogs. Since different plants thrive in different climates, the types of pollen found in a sediment layer reveal what was growing at the time. A layer rich in pine and birch pollen suggests a cold climate, while a layer with oak and lime pollen indicates a warmer period.

The primary driver of the long-term cycles of glacial and interglacial periods are slow, predictable changes in Earth's orbit around the Sun. These are known as the Milankovitch Cycles. They don't change the total amount of solar energy the Earth receives annually by much, but they do change the geographical and seasonal distribution of that energy. This is enough to trigger the advance or retreat of massive ice sheets. There are three main cycles:

1. Eccentricity (Shape of Orbit): The path of Earth's orbit around the Sun changes from being nearly circular to more elliptical and back again. This cycle takes about 100,000 years. A more elliptical orbit means more variation in the Earth-Sun distance throughout the year.
2. Axial Tilt / Obliquity (Angle of Tilt): The angle of Earth's axis of rotation tilts back and forth between 22.1° and 24.5°. This cycle takes about 41,000 years. A greater tilt leads to more extreme seasons (hotter summers and colder winters).
3. Precession (Wobble): The Earth wobbles on its axis like a spinning top. This cycle takes about 26,000 years and changes which hemisphere is pointed towards the Sun at different points in the orbit, affecting the timing of the seasons.