The US Environmental Protection Agency defines climate change as “any significant change in measures of climate (such as temperature, precipitation, or wind) lasting for an extended period (decades or longer).” The EPA then defines global warming as “an average increase in the temperature of the atmosphere near the Earth’s surface and in the troposphere…”.

The important point to note is that we often think of climate change and global warming as both referring to “unsustainable increases in global temperature caused by the impact of humanity’s activities on the natural environment”. Recent trends in global warming may certainly have been caused by human economic activity (and we will cover this in a separate article). However climate change (both warming and cooling), generally, is both a natural and dramatic process that has taken place since the Earth was created, around 4.5 billion years ago (4,500,000,000 years ago).

These changes can be dramatic. For example the Earth has gone through at least four major ice ages, when permanent ice cover extended over large areas of sea and land. Two of these ice ages (the Huronian and Cryogenian) may even have been so extreme that the Earth became one big snowball! And within each of these ice ages, and in the interglacial periods that separated them, there were further cycles of global warming and cooling that caused fluctuations in ice cover, global temperatures, and environmental conditions.

As a footnote to present discussions about global temperatures, the most recent glacial period (a glacial period is smaller than an ice age) ran from 110,000 years ago to around 10,000 years ago. The Earth experienced the “little ice age” from roughly the 16th century until the middle of the 19th century. So let’s look at what causes these environmental changes in a little more detail.

The Greenhouse effect
Firstly there is the Greenhouse effect. Although always thought of as being a bad thing, by some measures, the average temperature of the earth would be -18 degrees Celsius without the Greenhouse effect. The main greenhouse gases, are, in descending order of importance, water vapour (ie clouds), carbon dioxide, methane, nitrous oxide, ozone and CFCs. These gases are all significant because they absorb infra red radiation.

The precise mechanism by which the Greenhouse effect works is complicated, however, basically, the Earth is warmed by the Sun’s radiation. Earth then radiates heat, as infra red radiation, back into the atmosphere. In turn the greenhouse gases absorb this infra red radiation, radiated by the Earth, warming up and heating the rest of the atmosphere. In turn, as the atmosphere is warmed by the greenhouse gases, it also radiates infra red radiation in all directions further heating the planet and its atmosphere.

A key factor of this mechanism is the reflectivity of the atmosphere and of the Earth, called the albedo. Loosely speaking, white things reflect radiated energy, while dark things absorb it. So a dark rainforest will absorb the Sun’s energy while light wheat fields will reflect more. Similarly clean white glaciers reflect more energy than ones sooty from industrial pollution, or than bare mountainsides. The more energy reflected, the less is absorbed and the cooler the system remains.

Therefore, at an atmospheric level, an increase in high level clouds will reduce the amount of solar energy reaching the planet keeping the earth cooler, while low level cloud cover tends to trap the heat in the lower atmosphere keeping the world warmer. Strangely, industrial pollution, and in particular small particles (called aerosols and generated as a result of industrial pollution or volcanic eruptions) can directly cool the earth by reflecting sunlight, and indirectly by seeding cloud formation.

Orbital effects
Earth takes one day to spin on its axis, and around 365.25 days to orbit the sun. However the earth is actually tilted on its axis by 23.5 degrees, so that at any time of the year one hemisphere of the earth is closer to the Sun than the other, and hence we get winter and summer. Although the seasons aren’t normally considered as an aspect of climate change, there are other imperfections in the Earth’s orbit that do act over much longer time frames, called the Milankovitch cycles, which are believed (but not proven) contribute to glacial cycles.

Firstly the eccentricity of the orbit of the earth increases and decreases over a 100,000 year period making the earth unusually close to, or far from, the Sun at different times. Also the Earth’s axis of tilt changes between about 22 and 24.5 degrees over a roughly 40,000 year cycle. Similarly the precise direction this tilt points also changes over an approximately 26,000 year cycle. Finally, although the earth travels around the Sun on a flat plane, this plane is slightly inclined increasing and decreasing in dip and inclination over a 70,000 year period.

Solar radiation
Then there is the variation of the intensity of solar radiation. The brightness of the Sun fluctuates unpredictably over time causing changes in the amount of solar radiation reaching the earth: a brighter Sun means a hotter Earth. There are also the solar flares which emit bursts of high energy particles. When these interact with the planet’s atmosphere they can contribute to cloud formation changing the albedo and helping to trap in, or keep out, the Sun’s warmth. Although, not strictly related to solar radiation, it has also been suggested that the movement of our solar system through the arms of the Milky Way galaxy has also contributed to environmental change on earth.

Plate tectonics
Over a geological timescale plate tectonics (ie the movements of the continents) will also contribute to environmental conditions on Earth for a variety of reasons. For example the location of land masses will dictate the size and location of desserts: for rainfall you need to be reasonably close to the sea and so larger continents will also tend to have a dessert at their centre. Land masses close to the equator, or the poles, will also tend to be dessertified by the heat or the cold. Another factor is that land based glaciers will help to reduce sea levels, while floating ice has no impact. Therefore the ice caps that form on large land masses located over or close to the poles, such as Antarctica and Greenland, will tend to contribute to lower sea levels.

The position of the continents also impacts on the flow of the Earth’s sea currents that circulate heat from the equator to the polar regions. Famously the UK, which is at the same latitude as New York, experiences chilly and rainy winters, while the US and the mid West experience arctic white-outs. The cause being that the British Isles are warmed by the Gulf Stream which brings warm water from the Gulf of Mexico. Changes in the location of land masses can interrupt these flows warming and cooling different areas of land unpredictably.

Another aspect of plate tectonics affecting global temperatures is volcanic activity. The critical factor is the smoky emissions from the volcano. These particles enter the atmosphere helping to blot out the Sun and cause global cooling.

Conclusion
In this article we have intentionally steered away from how humanity is driving environmental change (we will cover that in a later article) and have focused instead on what drives environmental change generally. It should be remembered that there are a range of complex feedback loops that amplify or moderate the impact of all of these factors. However, understanding the key underlying mechanisms does help us to better understand current environmental change discussions, and also to put them in better context.