Causes of Climate Change*
J. Ernest "Sunny" Breeding, Jr., PhD Geophysics
Natural Causes of Climate Change
It is important to consider the natural causes of climate change to see if any of them can explain the current warming of our planet.
Fig. 5.1. Continental Drift. (Wikipedia)
Throughout geological time the land masses have drifted about the Earth. The positions of the land masses and the relationship of land masses to each other and the oceans that separate them all affect our climate. For example, it is known that when land masses are at the poles there is a greater degree of glaciation. In Fig. 5.1 on the left the continents are shown as they were about 135 million years ago. The Atlantic Ocean, which is so important to our climate today, was just beginning to be formed. The view on the right of Fig. 5.1 shows the Earth as it is today. It is obvious that the time scale of drifting land masses is on the order of millions of years. Since the global warming that we now experience is taking place on a much shorter time scale we do not need to consider continental drift any further.
Motions of the Earth
Fig. 5.2. Orbital Movements of the Earth. (ESA Science & Technology)
As the Earth moves in orbit about the sun it also rotates on its axis, which is at an angle to the ecliptic. The orbit about the sun, which depends upon the mutual gravitational attraction of the sun, moon, Earth, and planets is not circular but elliptical. But the ellipse changes in time, and this variation is referred to as the eccentricity. See the top illustration in Fig. 5.2. The changes in the elliptical orbit are periodic, and repeat about every 100,000 years. This is now known to have been the driving force of the recent ice ages, which have occurred about every 100,000 years.
The tilt axis of the Earth, shown in the center illustration of Fig. 5.2, is referred to as the obliquity. The tilt axis varies from about 21.5 to 25.5 degrees. This variation occurs at a periodicity of about 41,000 years. The tilt axis is very important to the amount of radiation from the sun that reaches the poles. When the axis tilts more towards the sun the amount of radiation reaching the high latitudes is increased for the hemisphere leaning towards the sun. This has an important effect on how much ice exists at the pole tilted towards the sun.
There is a third motion that also has an effect on our climate. The Earth precesses as it moves due to a "wobble." One way to visualize this effect is to think of what is directly above the north pole in the sky. Currently the star Polaris is above the north pole, and we call it our North Star. In Biblical times that was not the case. As the Earth precesses a different part of the sky is above either pole. So in time we have to choose different stars to be the "North Star." The precession has a periodicity of about 20,000 years. This effect has been found to be important to changes in our climate. A good example is the Sahara Desert. About 10,000 yeas ago there was rain and a forest in what is now a desert. From an analysis of climate data it has been found that the severe drought occurs about every 20,000 years with the lush vegetation occurring half way between the droughts.
Because of the different periodicities the eccentricity, obliquity, and precession can sometimes work to reinforce each other and sometimes work against each other. Their complicated effect on our climate was explained in equations by the Serbian mathematician Milutin Milankovitch in the early 1900s. It took him many years to derive the equations and make the calculations by hand. But his theory went in and out of favor by geologists because they had no way to properly date the glacial debris and other effects of the ice ages. It was not until radiocarbon dating techniques were developed and the time scale of the reversals of the Earth's magnetic field was discovered that it was possible to test Milankovitch's theory and prove that he was right.
Fig. 5.3. Incoming solar radiation by latitude. (IPCC)
The present day amount of solar radiation is shown in the left portion of Fig. 5.3. The middle figure displays the annual mean. It should not be a surprise that on an annual basis there is more incoming radiation in the tropical regions, and that it falls off towards the poles. The top figure shows the December to February latitudinal distribution. This is winter in the Northern Hemisphere, and since there is no sunlight at the North Pole the radiation goes to zero there. The bottom figure shows the latitudinal distribution from June to August. This is winter in the Southern Hemisphere.
Fig. 5.4. Amount of sunlight reaching the top of the Earth's Atmosphere in time. (NOAA)
The amount of sunlight reaching the Earth has been fairly consistent in recent years. The trend since 1880 has been a slight increase in radiation, which is seen in Fig. 5.4. There is a periodic variation of 11 years due to the sunspot cycle. Radiation from the sun has not changed enough to be the cause of the present global warming.
Fig. 5.5. The greenhouse effect. (IPCC)
Fig. 5.5 diagrams the effect of radiation from the sun and the greenhouse effect. Some of the incoming radiation is reflected and part of it is absorbed at the surface of the Earth. The incoming radiation is in the visible spectrum of light. Some of the absorbed radiation is emitted from the Earth's surface in the longer-wavelength infrared spectrum. A portion of this is lost to space, but most of the Earth's infrared radiation is absorbed by the greenhouse gases. This causes a warming of the Earth's surface and lower atmosphere, and is known to have an effect on our climate.
Ocean Conveyor Belt
Fig. 5.6. Great Ocean Conveyor Belt. (World Ocean Thermohaline Circulation - Maps)
The World oceans have a major impact on climate. You only have to consider the warming effect of the Gulf Stream on Western Europe to know this is true. The oceans are in continuous motion at all depths, and redistribute heat around our planet. In the upper layers of the ocean the currents are driven by the wind. In the deep ocean the currents are driven by changes in density. The heavier water sinks. This is known as thermohaline circulation, as the density is determined by both temperature (thermo) and salinity (haline). A Great Ocean Conveyor Belt was discovered to be important to climate variations and given that name by Wallace Broecker of the Lamont-Doherty Earth Observatory of Columbia University. It is diagramed in Fig. 5.6. In the North Atlantic the cold salty water sinks to great depths and travels as a deep ocean current south where it continues around South America, Australia, and into the Pacific Ocean. It rises to the surface and continues on passing through the Indian Ocean and finally back into the Atlantic returning to where it started. Broecker discovered that the conveyor belt can be shut down causing an abrupt change in climate. This can happen, for example, during a warming period when sea ice, ice sheets, and glaciers are melting and adding a lot of fresh water to the North Atlantic. The fresh water is not heavy compared to the ocean water, and does not sink. This stops or slows down the circulation in the conveyor belt and can affect currents like the Gulf Stream (Broecker, 2010.)
During the Heinrich events, discussed in Page 4, the melting icebergs dropped so much fresh water into the North Atlantic Ocean that the Great Ocean Conveyor Belt was significantly slowed or shut down causing the abrupt decrease in temperature. Once the supply of fresh water stopped the current system resumed with heat once again transported to the north, and leading to the abrupt increase in temperature. During the Younger Dryas event the large amount of melt water from the Laurentide Ice Sheet caused the temporary shut down of the Great Ocean Conveyor Belt. It is thought that the same or a similar explanation applies to the Dansgaard-Oeschger (D-O) events (Broecker, 2010.)
Fig. 5.7. Erupting volcano. (USGCRP)
Erupting volcanoes can have an effect on our climate. An eruption as seen in Fig. 5.7 can place a lot of debris in the atmosphere. This causes sunlight to be reflected with less of it reaching the Earth's surface. As a result, a volcanic eruption can cause a cooling effect that can last for a few years. Volcanoes also contribute greenhouse gases to the atmosphere. This may have caused warming of the Earth in the past when volcanic eruptions occurred with much greater frequency than today.
Causes of Climate Change Due to Human Activities
Fossil fuels include coal, oil, and natural gas. When any of these are burned carbon dioxide is produced, which is a greenhouse gas. Since the Industrial Revolution in 1750 human activity has added an enormous amount of carbon dioxide to the atmosphere which contributes to the greenhouse effect.
Fig. 5.8. Forest on fire. (USGCRP)
Fig. 5.9. Area denuded of trees. (USGCRP)
Forest fires, like the one pictured in Fig. 5.8, add greenhouse gasses to the atmosphere, which adds to the heating of the Earth. These fires are sometimes caused by lightning. Today man is also responsible for the burning of large wooded areas. This is occurring in the Amazon Jungle where large sections of the jungle are being cleared to make room for farming. This is adding lots of greenhouse gases to the atmosphere.
When the trees are gone, as shown in Fig. 5.9, it creates a cooling effect. By comparison the trees are much more effective at absorbing radiation from the sun. Without the trees the remaining surface is a much better reflector of light.
Observations and Predictions
Carbon Isotopes Determine the Source
We have seen that for most of the last two thousand years the concentration of carbon dioxide in the atmosphere remained fairly constant. It was after the Industrial Revolution in 1750 that things started to change. Since about 1850, and especially since the mid-nineteen hundreds, we have seen a dramatic increase in the concentration of carbon dioxide in the atmosphere. It is very important that we determine the source of this additional carbon dioxide. Is it from natural sources, man-made sources, or both? The atmosphere receives greenhouse gases from a limited number of reservoirs and sources: oceans, biosphere, soil (land), fossil fuels, and from time-to-time volcanoes. The biosphere, soil, oceans, and atmosphere exchange and store carbon dioxide in what is called the carbon cycle. The biosphere includes plants, trees, and animals. The burning of trees and other vegetation creates carbon dioxide. Fossil fuels include coal, oil, and natural gas; when they are burned carbon dioxide is created. Some of the carbon dioxide from fossil fuels is absorbed by the soil, biosphere, and oceans, but some also goes into the atmosphere. Carbon dioxide molecules remain in the atmosphere for one hundred years or more.
Climate scientists are able to determine the sources of carbon dioxide in the atmosphere, because each source has its own signature. There are different kinds of carbon atoms, and they are called isotopes. They differ in mass but react chemically the same way. There is a carbon 12 atom which has six protons and six neutrons in its nucleus. Additionally, there are carbon 13 and carbon 14 atoms which have one extra and two extra neutrons in their nuclei, respectively. The difference in mass is important and can be used to distinguish between the isotopes. Of the isotopes carbon 12 is the most abundant occurring 99% of the time. Carbon 13 is found about 1% of the time and carbon 14 occurs at the rate of about 1 in 1 trillion carbon atoms. As a convenience we define the signature ratios (carbon 13)/(carbon 12) and (carbon 14)/(carbon 12). We will consider these ratios as a function of the sources and reservoirs and how they vary with time.
Carbon 14 has the important property of being radioactive and is used to determine the age of many things. It decays with a half-life of 5,730 years, meaning that the original amount has been reduced by one half in that period of time. It does not take many half-lifes to reduce the carbon 14 atoms to a negligible amount, maybe ten half-lifes. So beyond about 50,000 years carbon 14 is nearly gone. The decayed carbon atoms turn into nitrogen atoms.
The soil (land), oceans, and the atmosphere absorb and outgas the different kinds of carbon isotopes. Their signature ratios (carbon 13)/(carbon 12) are similar. Fossil fuels are derived from very old plants and are millions of years old. Thus fossil fuels and the plants in the biosphere have a similarity in their signatures. Plants have a preference for the lighter carbon 12 atoms compared to the heavier carbon 13 and carbon 14 atoms. As a result, the signature ratios (carbon 13)/(carbon 12) for fossil fuels and plants are less than the same ratio found in the present day atmosphere. This simply means that there are a greater percentage of carbon 13 atoms in the atmosphere, oceans, and soil compared to fossil fuels or plants.
If the source of carbon dioxide is from the burning of trees and other vegetation the value of the signature ratio (carbon 14)/(carbon 12) will depend upon how many of the carbon 14 atoms have decayed. However, if the source is fossil fuels there will be negligible amounts of carbon 14 atoms remaining due to age. As a result, and since carbon dioxide molecules remain in the atmosphere for a time period on the order of a century, it is is easy to identify additions to the atmosphere from fossil fuels.
To determine the relative amounts of the different carbon isotopes a geochemist uses a mass spectrometer. This instrument can distinguish between and count the number of each of the isotopes found in samples of air (Shoemaker, 2010) or from samples of tree rings or other proxies. Today bottles of air are routinely collected at different remote locations around the Earth. From the measurements we obtain the ratios of the numbers of carbon 13 and carbon 14 atoms compared to the number of carbon 12 atoms.
The ratios (carbon 13)/(carbon 12) and (carbon 14)/(carbon 12) and how they change in time have been determined from air samples. For earlier times the ratios are determined from proxies. Tree rings track changes in the atmosphere with time. Studies of the air bubbles in ice cores, which are from the atmosphere, yield consistent results. Corals and sponges can be used to track changes in the carbon isotope ratios in the oceans, and these results are consistent with changes in the atmosphere. The change in both carbon isotope ratios with time have been determined from near the end of the last ice age to the present. It is found that the ratios remain nearly constant until the Industrial Revolution, and that after about 1850 both ratios show a dramatic and continuing decrease in their values. The only source of carbon dioxide that has the signature, that is, fingerprint required to cause an atmospheric reduction in both carbon 13 and carbon 14 is fossil fuels. The reduction occurs as the carbon dioxide from fossil fuels is added to the atmosphere and mixes with what is already there due to natural sources. Carbon dioxide in the atmosphere is well mixed. Approximately one-fourth of the carbon dioxide molecules in the atmosphere today are from fossil fuels. (For further discussions of this subject see Steig, 2004; Shoemaker, 2010; and Cullen, 2011.) Since the Industrial Revolution almost all of the additional carbon dioxide added to the atmosphere is due to human activities that burn fossil fuels.
Climate Model Predictions of Climate Change
Fig. 5.10. Comparing observations and predictions. (IPCC)
In Fig. 5.10 computer model predictions are compared with observations of the global mean temperature beginning in 1900. All data are shown as global mean temperature anomalies relative to the period 1901 to 1950. Consider first the bottom plots. The black curve is the result of measurements. The light blue curves are the results of different model simulations of the climate considering only natural causes of climate change. The thick blue curve is the plot of the multi-model ensemble mean. The model prediction of the climate is in fairly good agreement with observations (black curve) until about 1960. But in the later years the observations deviate from the prediction. The conclusion is that known causes of climate change do not predict the observed temperature values starting in the mid 1900s.
The plots in the top of Fig. 5.10 also repeat the observed temperature values in the black curve. The yellow curves display the results of multiple model predictions considering both natural causes and the man-made cause due to the burning of fossil fuels. The red curve shows the multi-model ensemble mean. It is possible to estimate the amount of carbon dioxide created from fossil fuels by referring to records of the amounts of coal, oil, and gas collected from wells and mines. The burning of large tracts of wooded areas are also considered. The model predictions are in good agreement with the observations for the entire period of study, more than 100 years. The conclusion is that greenhouse gasses added to the atmosphere by the burning of fossil fuels are the cause of global warming.
Look again at Fig. 5.10. Notice there are four vertical lines. These indicate the occurrence of volcanic eruptions. The first one is labeled Santa Maria, and occurred shortly after 1900. It can be seen that the observed (black curve) and predicted temperatures dropped for a few years after each eruption, as explained above.
We need to examine what might happen in the future due to global warming. We do that next.
Page 1: Climate Change and Definition
Page 2: Evidence of Global Warming
Page 3: Measurements
Page 4: Ice Ages
Page 5: Causes of Climate Change
Page 6: Predicting the Future
Page 7: How Can We Fix Our Climate?
Page 8: References
*A slide show version of these pages on climate change is available for presentations to groups. See References for more details.
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