Lecture 19

Cycles of the Earth

Radar and Doppler Radar

Radar, introduced during WW II for military applications, has become a very important tool for weather observation and forecast. The basic principle of Radar is to broadcast a beam of electro-magnetic radiation (in the microwave range) and to detect its reflection from a distant object. As well as a plane or a ship, clouds and rain are also capable of reflecting the radar signal, allowing therefore to exploit Radars in meteorology. The time difference between broadcast and arrival of the reflection determines the distance of the reflecting object.

In an even more sophisticated application, Radars can also be exploited to estimate the velocity of the reflecting object by measuring changes in frequency of the microwaves, caused by the Doppler effect.

The Doppler effect is most easily observed as a variation of pitch in the sound emitted by a moving object : if a sound-emitting object moves towards you, the sound will be perceived at a higher pitch, if it's moving away at a lower pitch. The same effect applies to any type of wave, including electro-magnetic waves. A Radar beam reflected by a moving object (e.g. a cloud in a situation of high winds) will reach the detector with a frequency slightly different from the one it was emitted at, and this allows to assess the air speed.

Climate and some of the factors affecting it.

At variance with weather, that describes short term variations of the atmospheric conditions, climate describes typical weather patterns over extended periods of time. We don't need to spend many words to convince you of the differences in climate between, e.g., Alaska, Texas, Virginia or Hawaii.

Main factors determining the climate of a given region are

• the location of the region with respect to poles and equator. This is rather obvious, but are you sure you know the reason behind it? Related question : do you know what determines the pattern of seasons?
  
  A the variation in distance between earth and sun
  
  B the rotation of the earth around its axis
  
  C the inclination of the plane of revolution around the sun
  
  D the inclination of the axis of rotation with respect to the plane of revolution
  
  E none of the above
  
  
• the position with respect to the major land and ocean expanses (think of Hawaii..)
• the general circulation of the atmosphere and of ocean currents. For example, thanks to the Gulf Stream, Great Britain, has a mild climate, even though it is as far North as Northern Canada.
• the presence or absence of mountainous ranges and other land features. We did mention the difference in climate bewteen western and eastern Oregon or Washington State.
• etc.

Apart from the familiar regularity of the overall climate patterns (which nevertheless seems to have been somewhat disrupted by long lasting El Niños), over time scales of many tens of thousands of years there has been periodic major variations in climate. You have all heard of the ice ages: the latest one is not so remote, it occurred about 20,000 years ago.

Thanks to Mr. Milankovitch and to Yougoslavian wine, we now have a satisfactory theory for the recurrence of ice ages. The explanation relies on periodic variations of the direction of the earth rotation axis (think of the wobbling of a spinning top), together with variations in the axis direction and in the shape of the earth's orbit. A particularly marked ice age will occur when these factors combine to reduce in a substantial way the amount of solar illumination received by the northern or southern half of the hemisphere.

By extrapolation, one might predict a new ice age some 10,000 years from now, but on a much shorter time scale we should be concerned instead with the consequences of global warming.

Our endangered environment

In the course of the last two centuries, mankind has become "clever" enough to be in the position of seriously harming the ecology of the planet it is living on. The first episodes go back to the onset of the Industrial Revolution in the past century, and the consequent major increase in air pollution due to uncontrolled coal burning (see page 593, for an interesting, and in this case relatively harmless, response of Nature to human activities). While the main effects encountered during the worse days of air pollution have somehow been brought under control, more subtle "side-effects" of human activity have eneterd the scene. Following the book, we will discuss them in order of overall global impact and difficulty to control.

The Ozone Hole

The radiation coming from the sun includes a fair amount of wavelengths in the UltraViolet (UV) range. UV radiation is rather harmful, it not only burns you when sun-bathing but it can cause skin cancer. Appropriate doses of UV radiation will kill bacteria and other micro-organisms. Luckily, Nature has provided a shield against this radiation : at altitudes around 30,000 m (100,000 feet), the atmosphere contains large amount of Ozone, a three-atomic molecule of Oxygen, capable of interacting with UV radiation according to

$O_{3} + UV \rightarrow O_{2} + O$

The end effect of this reaction is to absorb the UV radiation, but also to destroy the Ozone. As a counterbalance, solar radiation in the visible range can cause the reaction

$light + O_{2} \rightarrow O + O$

which is then readily followed by $O_{2} + O \rightarrow O_{3}$ .

Under normal conditions, these sequences produce a dynamical balance, resulting in approximate constancy of the amount of upper atmosphere Ozone. Recently (1985), a severe depletion of Ozone was observed over the South Pole (where there were observation stations) and consequent observations also confirmed an overall decrease in Ozone concentration around the globe. After a period of uncertianty as to the cause, an ulikely culprit was identified in a chemical widely used in refrigerators, Air Conditioners and sprays, ChloroFluoroCarbons or CFCs. Freed because of leaks in your AC unit or through the use of sprays, CFC molecules reach the upper atmospheric levels, where sunlight breaks them into their constituents. Among them, free Chlorine is known to accelerate ("catalyze") the break-up of Ozone by sunlight, causing an unbalanced Ozone depletion.

It is remarkable how even relatively small amounts (when compared to the total volume of the atmosphere) of a given substance can create such major effects: this goes to show how little we still know about our global eco-system! We can nevertheless appreciate the harmfulness of upper atmosphere Chlorine by considering that a single Chlorine atom, once it has reached the Ozone layer, can continue doing its Ozone disruption work over and over, with cumulative effects.

After the problem was identified, international agreements put an ever increasing limit on the CFC usage, and positive results seem to be appearing already.

Acid Rain

Even though the emission of the worst pollutants (soot, etc.) have been to a large extent removed from the burning of fossil fuels (oil and coal), burning of coal releases in the atmosphere non negligible amounts of Sulphur, originally contained within the coal. High temperature oil burning on the other side releases Nitrogen Oxydes (NO, NO2, etc.). These substances react with rain water to form respectively sulphuric and nitric acid, producing the phenomenon of acid rain. Note that normal rain is always slightly acidic, but episodes of acid rain correspond to acid concentration much larger than normal. Acid rain has shown its negative effects not only on historical buildings and human lungs, but also on the health of forests and lakes.

Given the characteristics of modern energy production, acid rain is not easily controllable. Sulphur emissions can be reduced equipping coal burning plants with, rather expensive, "scrubbers", while Nitrous compounds are less easy to control, since they are inherent to the high temperature burning process.

Catalytic converters, electric cars, etc. can reduce the problem, but part of the difficulties are due to the fact that "producers" and "consumers" of acid rain are not in the same location (Eastern Canadians have complained for many years about the acid rain generated in the US Mid-West).

One meager consolation is that, within a few or several decades, we will run out of fossil fuels to burn and we will need some alternate energy source, but by then it might be too late to repair the damage.

Greenhouse Effect and Global Warming.

The last issue is even more controversial then the first two : even though the premises are universally agreed upon, the consequences are far from being universally acepted. Again, this is partly due to our very limited understanding of the dynamics of our eco-system.

Let us start with the uncontroversial facts :

• Carbon Dioxide, unavoidably an end product of any burning process, has the property of being transparent to visible light, but it can block and absorb longer wavelengths, typical of thermal radiation (Infrared).

  This is similar to what happens in a greenhouse : glass is patently transparent to light, but it blocks Infrared radiation. This is how a greenhouse gets warm: sun light in visible wavelengths enters the greenhouse and, once inside, its energy produces heat. The corresponding thermal radiation is trapped inside the greenhouse with relatively good efficiency. In this context, atmospheric CO2 acts as the equivalent of the greenhouse glass panels, but what gets warm is the whole earth.

• we are definitely experiencing an increase in the relative concentration of CO2 in the atmosphere. And (almost) nobody argues that this is due to our process of energy production.
• we also seem to be experiencing an increase in the average global temperatures (could this be at the root of the more frequent El Niños?).

Here the agreement ends : can we really say that the combinations of these effects is causing an irreversible global warming ? Or is this just an accidental statistical fluctuation? Note that an average warming of even a few degrees would have tremendous implications, that would take a long time and long suffering to assimilate:

Melting of the polar caps would raise the level of the oceans, submerging thousands of square miles of coastal plains : eventually we might not need to drive all the way to Virginia Beach to be at the sea-side....

Regions that currently have pleasant climate might become unlivable; on the other side, currently frigid regions might become habitable and productive (Canadians and Russians might not mind this too much...)

Still, no universally valid prediction can be made. As counter-arguments, it has been suggested that a temporary surface temperature increase would cause more evaporation, therefore more clouds, and clouds are obviously a good screening against solar heat. It has also been argued that an increase in CO2 concentration would cause trees to grow faster (provided there is any ample extent of forests left...), and in doing so they would absorb the excess CO2.

In summary, even though the subject is generating enough concern, we still do not have enough knowledge to assess the relative interplay of all these factors, and it will probably be several years before we have a satisfactory answer.