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U4735x Environmental Science for Decision Makers
Lecture 18: Climate Change on a Variety of Time and Space Scales.
Jim Hays
Temperature change over
the last century Projection
#2.1, Projection #2.2.
Temperature change over the last millennium Projection
#2.3.
Climate is the long term average of a variety of conditions including temperature, precipitation, wind velocity and direction etc.
The temperature record of the last century or thousand years is but one aspect, all be it an important one, of a very complex system known as the Earth's climate system. The term system is used in a variety of ways. You may be familiar with it as used to describe the educational net work of schools, teachers, students buildings and administrators, known as the educational system. Political systems are another example. In the most simple sense a system is, or consists of, two or more interacting components or parts. This definition emphasizes the interaction between components. So the school system or a political system would not exist if the various parts didn't interact. The term Climate System is used in exactly the same way.
The Earth is a complex dynamic system consisting of five closely interacting subsystems: the surface of the solid earth, three fluid subsystems - the atmosphere, the ocean and continental and ocean ice - which surround the solid earth, and the life system. The life subsystem resides primarily on the solid earth or within the fluids. We belong to this latter subsystem.
All subsystems are dynamic,
with processes operating on time scales ranging from seconds to billions of
years, and space scales (notes
on size) ranging from the atomic to the global. The state of any subsystem
is at any moment a consequence of past states of itself and the other subsystems.
All subsystems are powered by gravitational energy and energy generated at the atomic level, within the sun. From this latter source, radiant energy streams through 93 million miles of nearly empty interplanetary space to warm our planet's surface.
The flow of matter and energy is continuous between the subsystems. The climate subsystem's fluids originated in the solid earth and continue to recycle through it. Life most likely originated in these fluids, and draws chemicals from the atmosphere, ocean and rocks thereby modifying their composition. Today it is responsible for maintaining the non-equilibrium composition of the atmosphere.
The boundaries between subsystems are largely transitional. The top of the atmosphere, more than one hundred kilometers above the Earth's surface, merges imperceptibly with interplanetary space. Water, air and, in polar regions, ice extend kilometers below the solid Earth's surface, and recent data indicate that life does too. The traditional disciplines of biology, meteorology, oceanography, and geology look at various pieces of the system and so we have had a tendency to arbitrarily divide it into compartments that correspond to the domains of these disciplines. This compartmentalization of a single system into arbitrary subsystems tends to obscure the properties of the whole system and in this course we will make an effort to put the system back together, recognizing that we must of necessity focus on aspects of the system and processes within the system. We will draw from the traditional disciplines of physics, chemistry, biology, and geology for concepts to help us understand the planet's processes and history.
We will learn about our
planet by exploring it. In this the computer can help, by allowing us to view
images, explore measurements of planetary properties, and deduce from this information
the dynamic processes that make it and keep it a beautiful and livable planet.
Start an exploration of the planet Earth through looking at it and comparing it with the other planets. Images of the planets (except Pluto) and Earth's moon (Projections #2.4-#2.13):
As you might expect the amount of energy supplied to a planet's climate system is important to the nature of the resulting system. So much that is characteristic about our solar systems planets is a result of their particular climate. There is a tug of war going on between the energy supplied by the sun that tends to tear away a planets atmosphere and the gravitational tug of the planet that tends to hold the atmosphere near its surface. For planets close to the sun (out to Mars) the suns energy is strong and has stripped away all the light gasses (hydrogen and helium with molecular weights of 2 and 4) that make up the bulk of the sun. Beyond Mars the sun's energy is too weak to accomplish this and the planets have retained atmospheres dominated by hydrogen and helium much like the sun. So the outer planets are huge and much like small stars. Jupiter if it were a little bigger would have become a star. The inner planets have lost all these most abundant light gasses and have retained only relatively heavy gasses such as nitrogen (molecular weight 28) Carbon dioxide (molecular weight 28) or in the case of earth Oxygen (molecular weight 32). Only earth has abundant oxygen in its atmosphere, why do you think this is?
For more on the planets, visit the following web sites:
Why is Earth so different from its nearest neighbors? Look at the statistics for Venus and Mars above.
To answer this we need to go back a long way in time to a time when the earth's atmosphere was more similar to the atmosphere of Venus and Mars than it is today.
Our present atmosphere is more than three quarters nitrogen and about one fifth oxygen, with a number of lower abundance gases that play a critical role in the heat budget of the atmosphere and in other process (Projection #2.14). Water vapor, which accounts for less than one percent of the mass of the troposphere, is the most important single component of the "greenhouse gases" that are responsible for maintaining average temperature conditions at the Earth's surface (15°C or 59°F) that are well above the freezing temperature of water. Without their presence, the mean temperature of the planet would be -20°C (-4°F), well below the freezing point of water (0°C) (Projection #2.15) and nearly all water molecules would be in the form of ice as is now true for Antarctica and Greenland. The term "greenhouse gas" is often used as a shorthand description of the ability of some gases to permit visible radiation from the sun to pass through to reach the Earth's surface, but to absorb longer-wave infrared radiation emitted from the surface before it can escape to space, somewhat analogous to the effect of the panes of glass in a greenhouse. Many of the important greenhouse gases other than water vapor are present in extremely small concentrations (orders of magnitude less than nitrogen and oxygen) yet have major effects on the heat balance of the earth due to their very high efficiencies at absorbing outgoing long-wave radiation. Note that all the greenhouse gas molecules have three or more atoms per molecule, which makes them far better able to interact with the low energy radiation emitted from the Earth's surface in the infrared part of the spectrum. Current reconstructions of the history of the atmosphere indicate that appreciable free molecular oxygen (at about one percent of the present atmospheric level) was not present until about 2 billion years ago, and accumulated to approximately one tenth of the present level sometime between 2.0 and 0.6 billion years ago (Projection #2.16). Thus accumulation of an oxygenated atmosphere essential to all current animal life required several billions of years, a major fraction of earth history, and was the result of the evolution of single cell plant life capable of photosynthesis. No other planet in our solar system has an atmospheric composition even remotely like that of earth because our planet was apparently the only one to develop life on a significant scale. The evolution of living organisms and the composition of the atmosphere have been linked through almost the entire history of earth. Without the extremely long history of free oxygen accumulation in the atmosphere as the result of photosynthesis by primitive green single cell plants (prokaryotic cells), none of the higher life forms would have been able to evolve.
One of the effects of rising oxygen in the atmosphere was the formation of ozone in the stratosphere as high energy solar ultraviolet (UV) radiation impacted oxygen molecules at elevations of 15 to 30 kilometers above the earth's surface. The subsequent presence of ozone in the stratosphere provided a shield to further penetration of UV radiation that is destructive to the organic compounds that comprise living organisms. Without that shield green plants would have been unable to live on the land surface exposed to direct sunlight. Again one of the critical steps in the evolution of life was linked to the interplay between the global environment and living organisms.
There is considerably evidence that the first half of earth history had an atmosphere largely free of oxygen. One of the most dramatic clues involves large masses of unique iron mineral deposits, known as banded iron formations, that accumulated at a number of sites around the world between 2.5 and 2 billion years ago (Projection #2.17). The discrete period for formation of these deposits indicates that very special environmental conditions were required for them to develop. They required very low, but not zero free oxygen in the atmosphere to form. Iron is a peculiar element that is slightly soluble in water in the absence of oxygen, but extremely insoluble with trace amounts of oxygen (Projection #2.18) The assemblage of minerals, especially siderite (FeCO3) and other iron carbonate minerals, that make up banded iron formations suggest that the iron was initially in solution, and then precipitated out with almost no associated detrital minerals such as clays and other aluminosilicate minerals. Such deposits could not form in estuaries or coastal waters of the modern ocean because of the abundance of clays and silts found in such environments today. The oceans of this mid Precambrian world must have had appreciable areas where iron was a moderately abundant dissolved ion, in great contrast to today's oxygen-rich ocean which contains essentially no dissolved iron. No mineral formations even remotely similar to these Precambrian banded iron deposits are being accumulated anywhere in the world today. For our modern mineral-hungry human society, these banded iron formation represent the primary source of the element that is refined and transformed into steel, one of the key materials that underlies the modern industrial economy. In the USA, about 75% of our annual supply of iron ore is derived from the large banded iron Precambrian deposits of the Lake Superior District (Projection #2.19). Other famous deposits of this iron ore type include western Australia (Hammersley Range), South Africa (Transvaal), and southern Brazil (Minos Gerais). The locations of these deposits include many of the oldest fragments of continental plates from early in Earth history. The unusual chemistry of dissolved Fe, influenced by the presence or absence of dissolved O2, will be discussed later in the course during consideration of arsenic contamination in Bangladesh groundwaters. The dependence of dissolved Fe behavior on reducing/oxidizing conditions is sometimes described in terms of a "redox" sensitive environmental cycle. The chemistry of many ions dissolved in natural waters can be strongly influenced by the presence or absence of dissolved oxygen.
One of the effects of rising oxygen in the atmosphere was the formation of ozone in the stratosphere as high energy solar ultraviolet (UV) radiation impacted oxygen molecules at elevations of 15 to 30 kilometers above the Earth's surface. The subsequent presence of ozone in the stratosphere provided a shield to further penetration of UV radiation that is destructive to the organic compounds that comprise living organisms. Without that shield green plants would have been unable to live on the land surface exposed to direct sunlight. Again one of the critical steps in the evolution of life was linked to the interplay between the global environment and living organisms.
One issue of interest concerning any material that plays a central role in industrial society is the order of magnitude of the total global amount available, compared to the amount consumed each year. Qualitatively, you would not expect that the world supply of iron ores was likely to be depleted in the near future (i.e. during the 21st century). Iron is one of the most abundant elements in the earth's crust (5% by weight). World per capita consumption of steel, the predominant use of Fe in industrial human activity, is about 150 kg per year (Projection #2.20), equivalent to about 3 times the weight of an average human! Note that we will generally use metric units during this course, since they are most common in scientific reports and more convenient for calculations. Multiplying annual per capita steel consumption by current global population (about 6 billion), total steel consumption is about 1 billion metric tons per year. Dividing this rate of Fe use into recent estimates of Fe ore reserves (2.2 x 1011 T), indicates a supply lifetime of these reserves about 240 years. The term "reserves" for mineral deposits usually refers to those ores which could be exploited economically at their current market price and cost of production. If we were to assume that the entire global volume of banded iron deposits, multiplied by 0.2 because they are only about 20% by weight Fe, could be eventually mined (global "resource"), the total time to deplete all such deposits at current consumption rates would be about one hundred thousand (105) years! Although it is not plausible that the entire volume of banded iron deposits in the world would be eventually mined, this order of magnitude calculation provides strong indication that all accessible Fe ores in the world are not likely to be depleted in the next few centuries.
The Earth's mean temperature had to be above freezing or the oceans would have frozen and no life could have existed. In fact since life must have existed during this long interval of time it indicates that the earth had a climate more like our present climate than that of either Venus or Mars.
There is evidence of past ice ages when large parts of the continents were covered with ice (Projection #2.21)and there is evidence that there were times when there was no continental ice except perhaps on high mountains. We are currently in an ice age regime by that we mean we recently came out of an major glaciation (11,000 years ago) and will at sometime in the future probably go into another one. There is evidence from studying bubbles in ice cores that atmospheric carbon dioxide was much less during the time of maximum ice build up than it is now. This drop in carbon dioxide atmospheric carbon dioxide concentration in the atmosphere may have amplified the glacial age cooling. We will return to this topic later in the course.
