CLIMATE CHANGE AND GLOBAL WARMING,

 

             The Earth’s climate is changing and the scientific consensus concludes that by altering the composition of the atmosphere, humans are increasing the average temperature of the Earth's surface. This process has already begun; the planet is measurably warmer than it was at the start of the last century. However, scientists predict the change that will occur over the 21st century will be even greater. This increase will have unpredictable impacts on weather patterns around the globe. We are all experiencing climate change. Our descendants will likely experience far more.

We recognize that climate change can be a controversial subject and that prescriptions for solutions quickly take on a political character, which can raise suspicions of bias. Some argue that the climate is too complicated to predict, and others suggest that natural variations can explain the observed changes in the climate.

These objections have some merit. It should be no surprise that the Earth's climate is a complicated subject. First, the atmosphere is vast: it extends over 600 km (370 miles) above the ground, and it weighs over five quadrillion tons (that's a five followed by 15 zeros). Second, the atmosphere is a dynamic system, creating blizzards, hurricanes, thunderstorms, and all the other weather we experience. And it is true that this dynamic system is largely controlled by natural processes; the Earth's climate has been changing continually since the atmosphere was produced.

·         The Celsius scale is the standard international unit for temperature that scientists use when discussing the climate. In the Celsius scale, water freezes at 0 oC and boils at 100 oC. A comfortable room might be heated to 20 oC (which is equivalent to 68 oF). Temperatures can be converted from the Celsius scale to the Fahrenheit scale with the following equation:

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o    Weather describes the short term state of the atmosphere. This includes such conditions as wind, air pressure, precipitation, humidity and temperature. Climate describes the typical, or average, atmospheric conditions. Weather and climate are different as the short term state are always changing but the long-term average is not. 

On the 1st of January, 2011, Chicago recorded a high temperature of 6 oC; this is a measure of the weather. Measurements of climate include the averages of the daily, monthly, and yearly weather patterns, the seasons, and even a description of how often extraordinary events, such as hurricanes, occur.

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Figure 3.1: A Thermometer (7)

So, if we consider the average Chicago high temperature for the 1st of January (a colder 0.5 oC) or the average high temperature for the entire year (a warmer 14.5 oC) we are comparing the city's weather with its climate. The climate is the average of the weather.

§  Insolation, Albedo and Greenhouse Gases

§   What controls the climate? The average temperature of the Earth is about 15 oC, so the Earth’s water is mostly in the liquid state. The average temperature of Mars is about -55 oC, so all of the water that can be found on the surface of Mars is frozen. This is a big difference!

One reason Earth is so much hotter than Mars is that Earth is closer to the Sun. Mars receives less than half as much energy from the Sun per unit area as Earth does. This difference in insolation, which is the measure of the amount of solar radiation falling on a surface, is a very important factor in determining the climate of the Earth. 

On Earth, we notice the effects of varying insolation on our climate. Sunlight falls most directly on the equator, and only obliquely (at an angle) on the poles. This means that the sunlight is more concentrated at the equator.

The insolation angle shows that the same amount of sunlight covers twice as much area when it strikes a surface at an angle of 30^o compared to when it strikes a surface directly: the same energy is spread more thinly, weakening its ability to warm the Earth.

Insolation is the effect of incidence angle on sunlight intensity. Note that the same amount of sunlight is spread out over twice the area when it strikes the surface at a 30-degree angle.

Insolation Comparison

Due to this occurrence, the tropics receive about twice the insolation as the area in the Arctic Circle. The difference in energy between these two regions will explain why the equator has a hot climate and the poles have a cold climate. Differences in insolation can also explain why we have various seasons.The Earth has its axis tilted at 23^o compared to its orbit, and over the course of the year, each hemisphere alternates between directly facing the Sun and obliquely facing the Sun.

When the Northern hemisphere is almost directly facing the Sun (the months of May, June and July), insolation is higher and the climate is warmer. The variation in insolation does explain why the summer and winter occur (there is less energy from the sun in winter than in summer), and why the timing of the seasons is opposite in the Southern and Northern hemispheres.

 

The image shows an insolation comparison of how latitude is important in determining the amount of insolation. The same amount of sunlight (yellow bars) is spread out over twice the planet's surface area when the rays strike the Earth at an angle 

·         Equatorial and Seasonal Impacts of Insolation

The image on insolation shows both the equatorial and seasonal impacts of insolation. High levels of insolation are shown in warm colors (red and pink) and low levels of insolation are shown in cold colors (blue). Note that in January (image 1), the maximum levels of insolation are in the Southern Hemisphere, as this is when the Southern Hemisphere is most directly facing the sun. The Arctic receives very little insolation at this time of year, as it experiences its long polar night. The reverse is true in April 

·         Annual Mean Temperature

·         The equator always receives plenty of sunlight, however, and has a much higher average temperature as a consequence; compare the average temperature of the equator with that of the poles.

Reflectivity of Earth's Surface

The level of insolation that affects the Earth depends on the amount of light (or solar radiation) that is being emitted by the Sun. Over the current geologic period, this is very slowly changing - solar radiation is increasing at a rate of around 10% every billion years, however, this change is much too slow to be noticed by humans. The sun also goes through an 11-year solar cycle, in which the amount of solar radiation increases and decreases. At the solar cycle peak, the total solar radiation is about 0.1% higher than it is at the trough.

The Earth's orbit is not perfectly circular, so sometimes the Earth is closer to or further from the Sun than it is on average. This also changes the amount of insolation, as the closer the Earth is to the Sun the more concentrated the solar radiation. It is important to note that these orbital variations have made a big difference in conditions on the Earth during the period in which humans have inhabited it.

In addition to considering how much energy enters the Earth system through insolation, considering how much energy leaves is also important. The climate of the Earth is controlled by the Earth's energy balance, which is the movement of energy into and out of the Earth system. Energy flows into the Earth from the Sun and flows out when it is radiated into space. The Earth's energy balance is determined by the amount of sunlight that shines on the Earth (the insolation) and the characteristics of the Earth's surface and atmosphere that act to reflect, circulate and re-radiate this energy. The more energy in the system the higher the temperature, so either increasing the amount of energy arriving or decreasing the rate at which it leaves would make the climate hotter.

·         Reflectivity of Earth's Surface

One way to change how quickly energy exits the Earth system is to change the reflectivity of the surface. Compare the difference in dark surface of tilled soil (Figure a) with the blinding brightness of snow-covered ice (Figure b).

·         The dark soil absorbs the Sun’s rays and therefore, heats the Earth surface, while the brilliant snow reflects the sunlight back into space. Albedo is a measure of how reflective a surface is. The higher the albedo, the more reflective the material: a perfectly black surface has zero albedos, while a perfectly white surface has an albedo of 1 - it reflects 100% of the incident light. If a planet has a high albedo, much of the radiation from the Sun is reflected back into space, lowering the average temperature.
Today, Earth has an average albedo of just over 30%, but this value depends on how much cloud cover there is and what covers the surface. Covering the soil with grass increases the amount of light reflected from 17% to 25% while adding a layer of fresh snow can increase the amount reflected over 80%.

The dark soil absorbs the Sun’s rays and therefore, heats the Earth surface, while the brilliant snow reflects the sunlight back into space. Albedo is a measure of how reflective a surface is. The higher the albedo, the more reflective the material: a perfectly black surface has zero albedos, while a perfectly white surface has an albedo of 1 - it reflects 100% of the incident light. If a planet has a high albedo, much of the radiation from the Sun is reflected back into space, lowering the average temperature.

Today, Earth has an average albedo of just over 30%, but this value depends on how much cloud cover there is and what covers the surface. Covering the soil with grass increases the amount of light reflected from 17% to 25% while adding a layer of fresh snow can increase the amount reflected over 80%.

·         Changes in Albedo

Changes in albedo can create a positive feedback that reinforces a change in the climate. A positive feedback is a process which amplifies the effect of an initial change. If the climate cools, (the initial change), snow covers more of the surface of the land, and sea-ice covers more of the oceans. Because snow has a higher albedo than bare ground, and ice has a higher albedo than water, this initial cooling increases the amount of sunlight that is reflected back into space, cooling the Earth further (the amplification, or positive feedback).

Imagine what would happen if the Earth produced even more snow and ice as a result of this further cooling. The Earth would then reflect more sunlight into space, cooling the planet further and producing yet more snow. If such a loop continued for long enough, this process could result in the entire Earth is covered in ice! Such a feedback loop is known as the Snowball Earth hypothesis, and scientists have found much supporting geological evidence

The most recent period in Earth's history when this could have occurred was around 650 Million years ago. Positive feedback are often described as "runaway" processes; once they are begun they continue without stopping.

Albedo does not explain everything. However, the Earth and the Moon both receive the same amount of insolation. Although the Moon is only slightly more reflective than the Earth, it is much colder. The average temperature on Earth is 15 ^oC, while the Moon's average temperature is -23 ^oC. Why the difference? A planet's energy balance is also regulated by its atmosphere. A thick atmosphere can act to trap the energy from sunlight, preventing it from escaping directly into space. Earth has an atmosphere while the Moon does not. If the Earth did not have an atmosphere, it would have an average temperature of -18 ^oC; slightly warmer than the Moon since it has a lower albedo.

·         How does the atmosphere trap the energy from the Sun? Shouldn't the Earth's atmosphere reflect as much incoming radiation as it traps? It is true the atmosphere reflects incoming solar radiation - in fact, only about half of the insolation hitting the atmosphere actually reaches the Earth’s surface. The reason the atmosphere is able to warm the Earth is because the nature of light radiation changes as it reaches the planet’s surface. Atmospheres trap more light than they reflect.

A human will see the Earth’s atmosphere as mostly transparent. This is because we see light in the visible spectrum, which is the light radiation in the range of wavelengths the human eye is able to perceive, and visible light is able to travel a long way through the Earth's atmosphere before it is absorbed. Light is also transmitted in wavelengths we can't see, such as in the infrared spectrum, which is sometimes referred to as infrared light, heat, or thermal radiation. Compared to visible light, infrared light cannot travel very far in the Earth's atmosphere before it is absorbed.

Solar radiation striking the Earth is largely in the visible part of the spectrum. The surface of the Earth absorbs this energy and re-radiates it largely in the infrared part of the spectrum. This means that solar radiation enters the Earth in the form of visible light, unhindered, but tries to leave in the form of infrared light, which is trapped. Thicker atmospheres keep this infrared radiation trapped for longer, and so warm the Earth - just like an extra blanket makes you warmer in bed.

·         Greenhouse Effect

·         Here, the visible light radiation enters the atmosphere and quickly exits as infrared radiation if there is no atmosphere (top Earth). When there is an atmosphere (the middle Earth), visible light enters unhindered but the infrared light is partially reflected back to the surface, increasing the amount of energy and thus the temperature at the Earth's surface. If the atmosphere is made thicker (bottom Earth) the infrared radiation is trapped for longer, further warming the planet's surface.

The way the atmosphere acts to trap light radiation is referred to as the greenhouse effect, and the gases that prevent the thermal radiation from exiting the Earth system are described as greenhouse gases. 

The four most important greenhouse gases in the Earth's atmosphere are water vapour, carbon dioxide, methane, and ozone. All four are found naturally in the Earth's atmosphere. 

However, human activities are adding to the natural amount of carbon dioxide and methane, and even adding new greenhouse gases, such as chlorofluorocarbon (CFC).