the assumption of responsibility for the welfare of the world















Climate change, in a modern environmental context, is the broad changes in climate resulting from global warming, itself the result of an increase in the greenhouse effect. The increased energy input into the Earth’s climate systems, in which water and air transfer energy from warmer to cooler areas, has complicated effects; global-scale warming can lead to local cooling, among other things, as well as changes in storm frequency and intensity. The expression ‘climate change’ is meant to capture these other effects, since global warming is not experienced as a universal increase in temperature.


There are three main terms that apply to climate change as an environmental issue: ‘climate change’ itself, ‘global warming’, and ‘greenhouse effect’. All three are accurate in this context, but they have different meanings, of course. Climate change is simply a change to local or global climate, which is medium- to long-term, year-on-year conditions of sunlight, temperature, pressure, humidity, and precipitation in a given location. Global warming is a rise in global average temperature, as an indicator of increased retention of energy in the Earth’s system. The greenhouse effect is the trapping of energy in the Earth’s atmosphere through the presence of certain atmospheric gasses. Any of these three can be anthropogenic — caused by humans — and typically when discussing environmental issues, it is the anthropogenic versions that we have in mind, even if that is not specified.

In short, the anthropogenic greenhouse effect causes global warming, which causes global and local climate change. The shift towards ‘climate change’ as the main term (rather than ‘global warming’) has been to emphasize the various changes we can observe, and to avoid the retort that any local cooling effect somehow disproves global warming. In fact, local cooling may be caused by global warming, as can numerous other local climate changes.

Climate change in history

Climate change is normal. It has been a part of Earth’s longest-scale history. It happens, for example, through tectonic-plate activity, but very, very, very slowly. We would expect land on a plate moving north to south to experience a change in climate, since latitude is a major determiner of climate. The sun disproportionately heats the Tropical Zone and heats the polar zones very little. But the Earth’s surface is covered with two fluids, water and air, that serve to transport energy around the Earth, so that this disproportionate energy received in the tropics will to some extent be distributed north and south towards the other zones. But the Earth is not a smooth surface, and its irregular surface affects how water and air move, so that tectonic plate movement, which changes the shape of the land, would also change the patterns of fluid transport of energy around the Earth.

There are also regular cycles, or recurring patterns, of climate change, based on the way the Earth functions in space relative to the sun. The patterns were discovered by Milutin Milanković (Милутин Миланковић « Milutin Milanković ») and are named after him. There are three main Milanković cycles:

Axial tilt: The Earth’s axis of rotation is tilted 23.4° with respect to its plane of orbit around the sun, and this determines the size of the tropical, temperate, and polar zones, and thus the areas of the Earth that receive direct sunlight or no sunlight at times throughout the year (for more detail, see latitude). The tilt actually changes; every 41,000 years, it cycles from 21.4° to 24.5° and back. We are currently in a period of decreasing axial tilt.

Eccentricity: The Earth’s orbit is not a perfect circle; Johannes Kepler (1609) discovered that the orbit is in fact an ellipse, with the sun at one focus. An ellipse has two foci, on opposite sides of, and equidistant from, its center. The eccentricity of an ellipse is a measure (roughly) of how oblong the ellipse is. Below is an exaggerated diagram of the Earth’s orbit as an ellipse; the orbit is much closer to a circle.

Every 100,000 years, approximately, the Earth’s orbit goes through a cycle, becoming more eccentric and then less. While the angle of the sun, and thus latitude, plays the largest role in relative sun exposure, it is also true that the Earth will receive the most energy overall when the Earth is closest to the sun (perihelion) and the least energy overall when it is furthest away (aphelion). Eccentricity is lowest when the orbit is closest to a circle; this evens out the energy received throughout the year.

Axial precession: The Earth’s axial tilt maintains a fixed orientation to the orbit, not to the sun. That is only on a year-by-year basis, though. Every 21,000 years, this orientation goes through a complete cycle. We currently arrive at perihelion (closest to the sun) 14 days after the December solstice, and over time are reaching the perihelion later and later in the year. Being close to the sun means a warmer season, thus a milder winter for the northern hemisphere and a hotter summer for the southern hemisphere. Being further away from the sun means a cooler season, meaning we currently have a milder summer in the northern hemisphere and a harsher winter in the southern hemisphere. In about 10,000 years, that effect will be reversed.

Each of these changes has a predictable effect. When we overlay these cycles, we get a complicated but still predictable pattern of Milanković climate changes.

While not regular or predictable, there are catastrophes (essentially, major disturbances) that affect climate as well. These include meteor impact, such as the most famous climate-change catastrophe, when a meteor struck the Yucatán peninsula 66 million years ago. This was identified by geologists in rock layers as the Cretaceous-Tertiary (K-T) boundary; the strike threw dust and debris into the atmosphere in massive amounts, leading eventually to the extinction of most dinosaurs (with the sole exception of the bird line), and the subsequent rise of mammals as the world’s dominant animals.

The greenhouse effect

The greenhouse effect is the trapping of energy by gasses in the Earth’s atmosphere, which warms the Earth. The effect is natural; indeed, it is a requirement for life on Earth. The problem lies in the aggravation of this effect by human action.

There are four primary gasses responsible for the anthropogenic greenhouse effect (and all also naturally occurring); they include water (H2O) and nitrous oxide (N2O), but the most important are carbon dioxide (CO2) and methane (CH4). Carbon dioxide is produced by the burning of fossil fuels, and so has increased dramatically in the atmosphere with the advent of industrialization. Methane is twenty times more potent than carbon dioxide as a greenhouse gas, and has increased due to agriculture.

Electromagnetic (EM) radiation is a spectrum that includes many familiar forms of energy, which can be classified by wavelength. The shorter wavelengths include gamma rays and x-rays. The longer wavelengths include microwaves and radio waves. In the middle is visible light, ranging from violet light (shorter) to red light (longer). Ultraviolet radiation is just shorter than visible light; infrared radiation is just longer than visible light. The EM radiation that we receive from the sun is concentrated in the visible portion of the spectrum. (This is presumably not a coincidence; rather, humans evolved to see radiation in the portion of the spectrum where it is most abundant.)


When the sun’s radiation hits the greenhouse gasses in the atmosphere, it is either reflected (bounced off), absorbed, or transmitted (allowed to pass through). When the transmitted radiation hits the Earth, it can also be reflected, but the rest is absorbed and warms the Earth. A warm body radiates heat; this heat is also electromagnetic radiation, in the infrared range. When that infrared radiation gets back to the greenhouse gasses in the atmosphere, it can, again, be reflected, absorbed, or transmitted. But the rate of reflection is higher for these longer wavelengths of infrared than for the shorter wavelengths of the incoming energy. This is the key to the greenhouse effect: because of the change in wavelengths, energy is more likely to be reflected on its way out of the atmosphere than on its way in. This creates the warm environment on which life depends.

The introduction of anthropogenic greenhouse gasses into the atmosphere increases the warming effect. Not only does that increase the average global temperature (global warming), but it disturbs the heat-transfer system to which all present ecosystems are adapted, leading to many indirect effects that may not be obviously the result of increased heat. Extreme weather events are an example of this climate change; the storm records broken in recent years are a consequence of changing the energy equilibrium on the planet.

There is also a global-warming feedback loop caused by melting ice and snow, especially in the Arctic. Ice and snow have a high albedo (reflectivity; from Latin, meaning “whiteness”); they cause the Earth to reflect solar energy rather than absorbing it and being warmed. As the Earth warms from increased greenhouse gasses, the ice and snow are melted, and the Earth is covered with something less reflective than ice and snow. Water, in the thawed Arctic, has a very low albedo (except for energy striking from a low angle), meaning the energy is mostly absorbed. This leads to more radiating infrared from the Earth, and even more global warming.

Ozone depletion

Finally, it is important to note what climate change is not: it is not the environmental issue involving the ozone layer. Ozone, a molecular form of oxygen (O3, as opposed to the normal O2), exists in a normal layer in the atmosphere. (This atmospheric ozone is distinct from ground-level ozone, an air pollutant that is a major component of smog. That is yet another environmental issue.)

The primary value to humans of the atmospheric ozone layer is the filtering of ultraviolet radiation. Scientists discovered a thinning of the ozone layer, which was at its worst over Antarctica. They also eventually discovered that this thinning was caused by chlorofluorocarbons, or CFCs. CFCs were used in consumer products (aerosols like hairspray) and as refrigerants in air conditioners. When released into the atmosphere, the CFC molecule interacts with ozone and breaks it down, and a single CFC molecule can break down many ozone molecules. The solution to that problem was to stop using CFCs, and, thanks to legal and voluntary action, that has largely happened, and the ozone layer has begun to recover: a rare environmentalist success.

Climate change as an issue

The greenhouse effect, again, is natural and normal. Global warming, if understood as a rise in global average temperature, is a simple fact. Climate change is a part of normal historical processes. The question that arises in politics is whether any of these things is happening anthropogenically. Note that this is no longer really a question in science.

To dismiss the climate change in the industrial era as something normal or natural is to ignore what we already know about climate change in the past. Past climate change is not mysterious or random. The longest, slowest process of climate change involves tectonic plate movement. We can explain much of climate change beyond tectonics using Milanković cycles. That leaves catastrophes to account for most of the rest of historical climate change. After considering tectonic movement and Milanković cycles, industrial-era climate change is something new, something apparently catastrophic, without an obvious source like a meteor impact. And we know the greenhouse effect is real, and we know that we are releasing an increasing amount of greenhouse gasses into the atmosphere.



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