Weather and Climate – Cyclical Changes

Cyclical Changes

The long-term pattern of weather in a certain area is called climate. Weather changes can be seen from hour to hour, day to day, month to month, or even from year to year. For 30 years or more, different patterns of weather can be seen.

Generally, climates are consistent. Hence living things can adjust to them. For example, polar bears have adjusted to remain warm in polar climates, whereas cacti have adapted to store water that helps them to survive in dry climates.

Climates change very slowly over hundreds or even thousands of years. As temperatures change, living organisms in that particular area must adapt, relocate, or sometimes risk becoming extinct.

Earth’s Spin, Tilt, and Orbit

Depending on latitude, time of day, and time of year, Earth’s spin, tilt, and orbit change the amount of solar energy received by any particular region of the globe. Small changes in the angle of Earth’s inclination and the shape of its orbit around the Sun bring about changes in climate throughout 10,000 to 100,000 years and are not responsible for climate change today.

Once per year, the Earth orbits the Sun, but the Earth doesn’t do so in a simple and perfect way. For example, we know that the Earth’s orbit is not circular but elliptical, and its axis is not tilted straight. All these factors change over time, and that can make the Earth cooler or warmer because these factors affect the amount of radiation we receive from the Sun.

The rotation of the Earth causes daily changes in light and temperature, and the tilt of the Earth causes seasonal changes. When the Earth orbits the Sun, the Earth is pulled by the gravitational forces of the Sun, Moon, and giant planets present in the solar system, mainly Jupiter and Saturn.

Over long periods, the gravitational pull of other solar system objects slowly change Earth’s spin, tilt, and orbit. Approximately over 100,000 – 400,000 years, gravitational forces slowly shift Earth’s orbit among more circular and elliptical shapes, as shown by the blue and yellow dotted ovals in the image given below—the direction of Earth’s tilt shifts over 19,000 – 24,000 years.

Furthermore, over 41,000-year cycles, there is a change in the tilting of Earth’s axis towards or away from the Sun. Small changes in Earth’s spin, tilt, and orbit over these long periods can change the amount of sunlight absorbed and re-radiated by different parts of the Earth.

In the past, on various scales, changes in Earth’s spin, tilt, and orbit have affected the Earth system. Some of these changes involve:

More or less amount of sunlight absorbed by different areas of the Earth’s surface can affect Earth’s temperature.

Increasing or decreasing temperatures can change snow and ice cover distribution patterns. At high altitudes, increasing snow and ice cover increases the reflection of sunlight, reducing the amount of sunlight absorbed by the Earth’s surface.

Snow and ice cover affect the Earth’s system changes, including the carbon cycle and the amount of carbon (including the greenhouse gas carbon dioxide) transmitted between the atmosphere, biosphere, and ocean.

Milankovitch Cycles – Measurement of Earth’s Orbit

Our lives are based on cycles, which are a sequence of events that occur in the same order on a regular basis. Hundreds of distinct cycles exist in our planet and the universe.

Some are natural, such as seasonal changes, annual animal migrations, or our sleep cycles, which are governed by circadian rhythms. Others, such as producing and harvesting crops, musical rhythms, and economic cycles, are human-made.

Cycles also play important roles in both the short-term weather and the long-term climate of the Earth. For example, Milutin Milankovitch, a Serbian scientist, hypothesized a century ago that 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 glaciation periods (Ice Ages).

He specifically looked at how variations in three types of Earth orbital movements affect how much solar radiation (known as insolation) reaches the top of the Earth’s atmosphere and where it goes.

These cyclical orbital movements, known as the Milankovitch cycles, cause variations in the amount of incoming insolation at Earth’s mid-latitudes of up to 25%. (Our planet’s areas are located between around 30 and 60 degrees north and south of the equator).

Three main things cause the Earth’s natural climate cycles (Earth’s orbit around the Sun) are eccentricity, obliquity, and precession. The collective name of these three cycles is’ the Miankovitch cycle.’

The Milankovitch cycle comprises:

• The shape of Earth’s orbit called eccentricity;
• The angle at Earth’s axis is tilted concerning Earth’s orbital plane, called obliquity; and
• The path of Earth’s axis of rotation is pointed, called precession.

As per Milankovitch’s theory, the three cycles, namely eccentricity, obliquity, and precession, combine to affect the amount of solar heat that reaches the Earth’s surface and later affect climatic patterns, including glaciation periods (ice ages).

The period between these changes can be tens or thousands of years (precession and axial tilt) or more than hundreds or thousands of years (eccentricity).

The three Milankovitch Cycles affect the seasonality and position of solar energy around the Earth, thus affecting contrasts between the seasons.

Eccentricity:

When the Earth is closer to the Sun, the climate is warmer; this cycle also impacts the length of the seasons. The amount of a shape’s (Earth’s orbit) deviation from being a circle is called ‘eccentricity’.

Earth’s annual journey around the Sun is not entirely in a circular path, but it is very close to circular. Over a time, the gravitational force from our solar system’s two largest gas giant planets, i.e., Jupiter and Saturn, affects the shape of Earth’s orbit to change from just about circular to slightly elliptical.

Eccentricity calculates the shape change in Earth’s orbit from a perfect circle to somewhat elliptical. These changes cause an increase in the distance between Earth and the Sun.

Due to eccentricity, our seasons are slightly different lengths; that is, in the Northern Hemisphere, currently, summer is about 4.5 days longer than winter, and the spring season is almost three days longer than autumn. When eccentricity decreases, the length of the seasons slowly comes to normal.

When Earth’s orbit is at its most elliptic path, around 23 percent more solar energy reaches Earth at our planet’s (Earth) closest approach to the Sun every year than when it is farthest from the Sun. Presently, Earth’s eccentricity is most circular ( least elliptic) and is decreasing very slowly, in a cycle that covers about 100,000 years.

Due to the eccentricity cycle, the total change in global annual insolation is minimal. Likewise, because changes in Earth’s eccentricity are relatively small, there are relatively minor changes in seasonal, yearly climate.

Obliquity

Obliquity is an astronomical word explaining the angle of tilt of the Earth’s axis of rotation. In other terms, obliquity is defined as the angle of Earth’s axis of rotation tilts as it moves around the Sun. Obliquity causes Earth’s seasons. Since the last million years, obliquity has changed between 22.1 and 24.5ﹾ perpendicular to Earth’s orbital plane.

Suppose the Earth’s axial tilt angle is more significant. In that case, the seasons will be more extreme because each hemisphere gets more solar radiation in summer when the hemisphere is tilted toward the Sun and less in winter when it is tilted away.

In addition, larger tilt angles prefer days of deglaciation, i.e., the melting and retreat of glaciers and ice sheets. All these impacts are not consistent universally. For example, higher latitudes get a more significant change in total solar radiation than areas nearer to the equator.

Currently, Earth’s axis is tilted 23.4ﹾ or nearly halfway between its extremities, and very slowly, this angle is decreasing in a cycle that covers about 41,000 years. The last time it was at its maximum tilt was around 10,700 years ago, and it will go to its minimum tip around 9,800 years from now.

So while obliquity decreases, Slowly it helps to make our seasons milder, resulting in warmer winters and cooler summers that slowly allow snow and ice at high latitudes to develop into large ice sheets. So as ice covering increases, it reflects more solar energy back into space, encouraging even further cooling.

Precession

When Earth rotates, it slightly vibrates upon its axis, like a spinning toy top. This vibration is due to tidal forces initiated by the gravitational effects of the Sun and Moon that cause Earth to swell at the equator, which affects its rotation.

The tendency in the direction of this vibration compared to the fixed positions of stars is known as axial precession. The cycle of axial precession spans about 25,771.5 years.

Axial precession causes more severe seasons in one hemisphere and less severe in the other; presently, during winter, perihelion occurs in the Northern Hemisphere and in summer in the Southern Hemisphere. This causes the summer season in the Southern Hemisphere scorching and moderates Northern Hemisphere seasonal changes.

But in around 13,000 years, axial precession will make these conditions to reverse, with the Northern Hemisphere will experience more extremes solar radiation and the Southern Hemisphere will experience more moderate seasonal changes.

Climate Cycles over a Millenium

Changes in the Earth’s orbit around the Sun, known as Milankovitch cycles, initiate major glacial (cold) and interglacial (warm) periods. These cycles have occurred at varying intensities on multi-millennial time scales (10,000 – 100,000 year periods). The orbital changes occur gradually over time, influencing where solar radiation is received on Earth’s surface during various seasons.

These changes in the circulation of solar radiation are not strong enough to cause significant temperature changes on their own. They can, however, trigger powerful feedback mechanisms that amplify the Milankovitch cycle’s slight warming or cooling effect.

Changes in global surface reflectivity cause one of these feedbacks (also termed as albedo). Even a minimal increase in solar radiation at northern latitudes can contribute to increased ice melt. As ice melts, less sunlight is reflected from the ice’s bright white surface, and more is absorbed by the Earth, increasing overall warming.

A second feedback mechanism involves the concentrations of greenhouse gases in the atmosphere, such as carbon dioxide. The slight warming caused by changes in Earth’s orbit warms the oceans, allowing them to emit carbon dioxide.

As we’ve seen, more CO2 in the atmosphere causes more warming, amplifying the effect. Differential feedbacks in atmospheric CO2 concentrations can lag warming or cooling caused by orbital changes by up to 1000 years.

In this way, relatively minor changes in orbit can result in the last 800,000 years’ glacial and interglacial cycles. A most important concern with existing climate change is that similar feedback mechanisms will lead to a “runaway” warming effect in modern times that will be extremely difficult to stop or reverse.