Controversy about climate change has changed dramatically in the past decades. In Chapter 4 we took a careful look at questions about global warming, examining Dr. James Hansen’s statement that the “globe is heating up.” We saw, through International Panel on Climate Change (IPCC) Reports and more recently the Berkeley Earth Surface Temperature (BEST) project, that there is consensus in the scientific community that indeed the average global temperature has gone up.
One of the disagreements scientists have about the theory of global warming and the greenhouse effect is whether the warming of Earth’s atmosphere is just a part of the natural climate cycle or the result of human activities. If the warming is part of the natural cycle, then Earth may become cooler on its own. If, however, human activities are responsible, Earth will continue to warm, unless people decide to do something about it—primarily reduce the burning of fossil fuels. But change is not easy.
Nearly all climatologists agree on each of these points:
Controversies, like those reported in the newspaper articles in Investigation 4.1, have been researched and debated by scientists over the past three decades. While many of the questions are substantially resolved, many of the questions are still unanswered. However, government officials and industry leaders still need to decide what to do, even before all the evidence is in. So it’s especially important for them to keep informed about the latest research. To do that, they need to understand what the issues are.
As you read the latest reports on these controversies, imagine you are a government policy maker—a U.S. representative, senator, cabinet member, or even the president of the United States. Based on the latest information, what actions do you think are warranted: strong, moderate, or no action to curb increase in greenhouse gases?
III. Are natural cycles such as
Proxy DataA very simple example of proxy data would be estimating the number of mice living in an area. If for some reason you are unable to count the mice directly, you can count mice droppings. The amount of mice droppings will increase with the number of mice in the area and a relationship can be determined between the amount of mice droppings and the number of mice. If we determine how many droppings an average mouse produces in a specific period of time, the number of droppings is the proxy data for mice.
Can you think of other examples in which proxy data could be useful?
Proxy data about past climate changes can be found in tiny bubbles of atmospheric gases trapped layers of ice near the North and South Poles and in the composition of the water, itself. Since the ice does not melt in the Arctic and Antarctic regions, older layers are buried by newer layers. Over the past decades, many ice cores have been extracted from the Greenland ice sheet and from the deep ice of Antarctica. Ice cores are cylinders of ice cut by a drill shaped like a long tube. Yearly layers can be seen and can be counted like annual rings on a tree.
annual rings on a tree.
Photo by Lonnie Thompson,
Byrd Polar Research Center, Ohio State University.
Ice cores are important in two ways. First, by melting or crushing the ice, the samples of past atmospheric air, especially carbon dioxide, methane, nitrogen, and oxygen, can be released and analyzed. Second, the actual composition of the water can be analyzed in terms of the relative abundance of isotopes of oxygen or hydrogen in the water molecules.
Isotopes are atoms that are chemically the same, but have different numbers of neutrons, so have different atomic weights. A “heavy” oxygen atom (18O) has 18 neutrons in its nucleus , two more neutrons than a normal oxygen atom (16O) has, though either can be part of a water molecule. Similarly water contains hydrogen atoms which can be in any of 3 isotopes: 1H (normal hydrogen), 2H (deuterium), or 3H (tritium).
Studies have found that the amount of water with the 18O isotope of oxygen changes with temperature because it differs from the water with 16O in how easily it condenses to precipitate as snow. That's why the abundance of 18O in a sample of water can serve as a proxy for temperature. A comparison of some of the results of past global temperature determinations by researchers using proxy data gathered different methods is shown in the graph below.
Temperatures for the Past 1000 Years
In the graph of temperatures for the past 1000 years, each line on the graph is from a different study, using a different set of temperature-proxy data with different methods of analysis. They show striking similarities:
- The Earth experiences oscillations in global temperature over time.
- There has been a steep increase in temperature since the early 19th century.
- The Earth’s temperature is higher than it has been in the last 1000 years.
- Proxy data measurements closely match
recorded temperatures for the past 100 years.
This is especially important because it makes us
confident that proxy data methods are accurate.
Although this graph makes clear that something unusual is happening, we're still left with the question, “Might the Sun be responsible for the observed temperature increase.” Another part of the answer to that question involves study of the sunspot cycle. A sunspot is an area on the Sun that is cooler than the surrounding area, making it appear dark by comparison.
Records of sunspots started in 1610, shortly after Galileo Galilei made his first observations of the Sun with his new telescope. Continuous counts of sunspots since 1849 reveal a regular cycle of sunspot numbers that oscillates from high to low every 11 years. This cycle is seen clearly on the "Sunspot Numbers” graph with one notable exception, the Maunder Minimum, 1645–1715, when very few sunspots were recorded, in a climatic period called the “Little Ice Age.” Temperatures in northern Europe during this time were 0.5 C lower than average.
We now know that increased sunspot activity indicates increased solar activity and greater amounts of solar radiation sent to Earth, heating the planet. The amount of radiation coming from the Sun varies by only 0.1% but the effect is still noticeable on a large scale. In recent years, ice cores in both northern and southern hemispheres have confirmed the 11 year cycle. When this cycle is matched up with data about global temperatures, we see a strong relationship between the two: average global temperature varies directly with sunspot activity.
With the understanding of the relationship between sunspots and global temperature in hand, can we answer the question of whether or not something unusual is happening with our Sun that might be contributing to increased global temperatures in the present. The results of one such study were published in the journal Nature and are summarized in the graph below. The results of this study show that although the 11 year sunspot cycle continues, we have been experiencing a period of unusually high solar activity.
This graph of sunspot numbers over time was taken from research by Solanki et al. (2004).
The red line is observed sunspot number over the last 400 years,
and the blue line the estimated sunspot number from
their C-14 tree ring data (proxy data) over the last 11,400 years.
Other groups are working on improving estimates of historic sunspot numbers, but it appears that the solar cycle may be contributing to global warming of the last century. But how much? Does it account for all of the temperature increase? This question is still being investigated and is a point of controversy.
In the late 1990s, Dr. Sallie Baliunas from Harvard University’s Center for Astrophysics, argued that that most of the climate change observed in the 20th century century, over 90% of it, was due to changes in the Sun’s activity.
|Sallie Baliunas at
Mt. Wilson Observatory.
of Sallie Baliunas.
Baliunas argued that changes in solar activity affect Earth’s climate, causing changes in the paths of storms, and the amount of cloud cover, rain, and snow, which in turn affect the planet’s overall global temperature. Support for that theory came from deep in an ice core taken from the Greenland ice sheet, from time back to more than 160,000 years ago. A greater concentration of dust of dust in some layers suggested there must have been storms that year that carried the dust from areas where there was no snow to the Greenland ice sheet, and the concentration of dust varied with an 11-year cycle.
More recent estimates for the contribution of increased solar activity on current global warming have ranged from less than 5% to as high as 25%. More study is needed, but indications are that global warming cannot be explained without a large impact from greenhouse gases. Current levels of carbon dioxide gas in the atmosphere are higher than at any time in the past 420,000 years (Petit; Bibliography).
Baliunas’ argument about contribution of solar variations to climate change cultivates our desire to understand what other factors besides CO2 concentrations may be factors in climate change and why the global climate cooled slightly from 1940 to 1970. This decrease cannot be explained by an increased greenhouse effect, which would predict a continuous increase in temperature since the industrial revolution. Is solar variability the dominant cause or is something else to blame?
As important as the greenhouse effect is, it is not the only cause of climate change. Both natural and human factors can cause global cooling. Industrial and volcanic activities emit large amounts of sulphate aerosols (very tiny particles suspended in a gas) into the atmosphere that reflect energy back into space, cooling the atmosphere. Evidence of sulfur concentrations in Greenland ice cores points to the industrial boom after World War II causing rising global concentration human-produced aerosols in the atmosphere. The rise that is reflected in ice core records drops off in the early 70’s when the US and Europe enacted strong clean air regulations. CO2 emissions continued to rise throughout that time period.
Sulfur trapped in the Greenland Ice Sheet, 1900–2000.
Graph by Robert Simmon, based on data from McConnell et al.,
NOAA/NCDC Paleoclimatology Program.
Computer models are not able to replicate the observed cooling between 1940 and 1970 without factoring in aerosol concentrations along with solar variability and CO2 concentrations. Once again we see how vigorous scientific debate reveals the complex interactions that contribute to global climates and the difficulties associated with explaining short term changes in global temperature.
7.1 Keeping the facts straight
Use evidence to support or qualify a claim.
Let's revisit the controversial question that follows from the 1988 statements made by Dr. James Hansen. Are humans responsible for global warming?
Though we do not know precisely how much of the atmospheric warming observed in the past 100 years is due to the increase in greenhouse gas concentrations, scientific consensus has human generated greenhouse gases at center stage. One indicator of the growing consensus can be seen in the conclusions from the International Panel on Climate Change (IPCC): 1990 First Assessment Report
- “. . . .emissions from human activities are substantially increasing the atmospheric concentration of greenhouse gases and that this will enhance the greenhouse effect and result in additional warming of the Earth’s surface. “ .
- 1995 Second Assessment Report.
- “. . greenhouse gas concentrations have continued to increase.”
- “. . . the balance of evidence suggests a discernible human influence on global climate.”
- “There is new and stronger evidence that most of the warming over the last 50 years is attributable to human activities.”
- “Global average temperature and sea level are projected to rise under all IPCC scenarios.”
- Warming of the climate is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and the rising average sea level.
- Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.
7.2 How Have IPCC Reports Changed?
Although the IPCC reports cover much more than whether or not humans are responsible for global warming, it is helpful here to take a close look at how statements in this area have changed over the years:
The reports make increasingly strong statements about human or anthropogenic contributions to climate change but the reports acknowledge areas where consensus has not been reached. Most of these areas are connected with predicting the future of climate change which we will examine next.
The concentration of carbon dioxide in the atmosphere is increasing at the rate of half a percent per year. That’s barely noticeable. But if that rate continues, about 100 years from now there will be twice as much carbon dioxide in the atmosphere than there was before the industrial revolution. Other greenhouse gases (methane, nitrous oxides, and CFCs) contribute another half a percent per year in equivalent heat-trapping capacity.
The IPCC report envisions various scenarios, or possible futures. According to some scenarios, increasing world population will cause the amount of greenhouse gases in the atmosphere will grow even faster than it is growing now. As world population increases, so does the demand for food, energy, and manufactured goods. Much of this population growth is taking place in developing nations. As the people in these nations strive to have the same standard of living enjoyed by most people who live in industrialized nations, the amount of greenhouse gases in the atmosphere will increase even further. According to these projections, it seems likely that in the decades to come there will be even more greenhouse gas emissions than now. Moreover, the destruction of rain forests, which is occurring around the world at an increasing rate, means that fewer trees will be around to absorb large quantities of carbon dioxide.
Other scenarios are more optimistic. If energy use can be cut back through conservation, less fossil fuels will be burned, and greenhouse gas emissions will be reduced. For example, new conservation technologies are being developed to light and heat homes in ways that use less energy; and cars are being developed that use gasoline more efficiently. Then there is the growth of non-fossil fuel energy industries, including wind, solar, nuclear and geothermal power plants that do not emit greenhouse gases. It is also possible that Earth’s natural systems will adjust and the climate will not change significantly, even if the concentration of carbon dioxide continues to increase.
According to the IPCC report, “most emission scenarios indicate that, in the absence of mitigation policies [steps to reduce emissions], greenhouse gas emissions will continue to rise during the next century and lead to greenhouse gas concentrations that by the year 2100 are projected to change climate more than that projected for twice the pre-industrial concentrations of carbon dioxide.”
It’s difficult to imagine what life will be like in five years—let alone in 30, 40, or 50 years. Perhaps it's appropriate to take a moment to reflect on the interconnectedness of science with our everyday lives. If we take the predictions of climatologists seriously, we realize more than ever that our actions or inaction today will affect the lives of future generations, children and grandchildren. The ability to predict change is at the heart of current scientific uncertainties and those uncertainties can lead to policy controversies.
"We are looking ahead, as is one of the first mandates given us as chiefs, to make sure and to make every decision that we make relate to the welfare and well-being of the seventh generation to come…
Where are you taking them? What will they have?"
---Oren Lyons, Chief of the Onondaga Nation
It is helpful to keep in mind that, although climate change prediction has inherent scientific uncertainty, there is another related issue where there is more certainty: the amount of fossil fuels powering a large part of our society and world economies is in limited supply. How soon sources of fossil fuels run out and are completely replaced by sustainable alternative energy sources is a subject in the GSS Energy use book, but play an integral role in solutions to any problems associated with climate change. In other words, solutions to society's energy problems are also solutions for climate change problems.
Predictions of future climates are currently in the form of computer models that predict warming as greenhouse gases continue to increase, and possible effects that result from the warming. Scientists construct computer models to understand things they are not able to analyze in a laboratory. Models have become more accurate as computers have become more powerful over the years. In addition to needing computers that can handle large, complex programs, climate models need accurate descriptions of as many factors that control climate in order to make accurate predictions. Just like a video game programmer needs detailed information about how people walk in order to create realistic graphics, climate models need detailed information about how the Earth’s climate system works.
Climatologists try to make their models more realistic by including more factors. Early on, the effect of aerosols—particles in the air emitted by cars and factories were included in models. Unlike the invisible greenhouse gases, aerosols are visible. Their effect on climate is much like a cloud of ash from a volcano. Aerosols cool the earth by reflecting some of the Sun’s light back into space. The result is that instead of a 5°C increase expected by Arrhenius, only a 1° to 3.5°C increase in temperature is expected with a doubling of greenhouse gases in the atmosphere.
Predicting how warm it will get is very difficult. Imagine a time—perhaps when our children are grown—when new sources of power are developed which are “clean.” They do not produce either greenhouse gases or aerosols. Although aerosols have short lifetimes, since the particles settle within a few years, greenhouse gases hang around for a very long time—as much as 500 years in the case of carbon dioxide. With the cooling effect of the aerosols gone, the atmosphere could heat very rapidly in less than a decade, causing even greater climate disruptions. Over the years, research in new areas such as the exchange of heat between the oceans and the atmosphere and the role of ocean and clouds add more detailed information into computer models, so presumably more accurate information comes out.
the mid-70’s, mid-80’s, the First IPCC Assessment Report (FAR; 1990),
Second Assessment Report (SAR; 1995), Third Assessment Report (TAR; 2001)
and the Fourth Assessment Report (AR4; 2007).
We can test the precision of computer models by having them “predict” conditions in the past. Today’s computer models are getting better and better at describing past conditions.
When carbon dioxide levels double, many climate models predict that the consequences will be devastating and irreversible in the near term. Knowing when doubling is likely to happen and how high the global temperatures will go affects the urgency of policy changes and the amount of financial sacrifice individuals and nations are willing to tolerate.
To address this, the IPCC has developed several different scenarios which it refines with each new report as computer modeling is perfected. Using the best physical modeling available, the modelers enter different factors that depend upon human dependent changes, primarily population and use alternative sources of energy. Each scenario gives a different answer for the question of carbon dioxide doubling time and temperature increase.
The graph below summarizes the results of each of the scenarios reported in the Fourth Assessment Report in 2007.
Summary for Policymakers IPCC Working Group 1 2007
It is somewhat discouraging that no scenarios lead to stable global temperatures. If carbon dioxide emissions are halted at the year 2000 levels, temperatures will continue to increase. But it's also important to remember that our actions can determine the rate of average global temperature rise.
See also Climate Modeling 101 website at National Academy of Sciences.
If Earth becomes warmer because of the greenhouse effect, more water
will evaporate from the oceans. Increased water vapor means more
clouds, and clouds are very effective at reflecting sunlight. (Just
think of the cooling effect of a cloud passing overhead on a hot day.)
On the other hand, clouds can also warm Earth. Anyone who lives in the
desert knows that you must dress warmly at night because the land cools
off very rapidly. But if there are clouds overhead, they will absorb
some of the heat radiated by Earth and send it back toward the ground.
The only way to determine for sure how to enter clouds into the models
is to study their effects in different parts of the world and in
The study of clouds is a very active area for researchers today. Ali Omar is involved in a research project designed to answer some of these questions. He is one of the researchers studying the effects of clouds from a satellite launched in 2006. Named CALIPSO, the satellite is a joint project of the United States and France. It is equipped with a laser pointed towards Earth’s surface. The satellite measures laser light that is reflected from cloud layers to determine where the cloud layers are located and how thick they are. It also measures infrared (heat) energy radiated into space. Scientists expect that data from the satellite and from ground-based measurements will enable them to determine whether the overall effect of the clouds is to cool or warm Earth’s surface.
Ali Omar is assistant professor of Physics at Hampton University in Hampton, Virginia. When he was a young boy in Kenya he wanted to be a pilot, but his parents would not let him because they thought it was too dangerous. So, he decided he would learn how to build jet engines instead. He came to the United States to study aeronautical engineering. One of his required courses concerned how jet exhausts affect the atmosphere. That sparked his interest in environmental science. “Now,” he says, “I’m kind of glad my parents did not allow me to become a pilot.”
Lynne Talley is a researcher at Scripps Institution of Oceanography in La Jolla, California, where she studies the role of ocean circulation in climate. She explained the importance of her research by posing an interesting question: “In February, you would not want to go swimming off the coast of Labrador (north of New England). The ocean surface temperature is only about 1°C. But Ireland is the same distance north of the equator, and water there is a much warmer 10°C. Why are they so different?” The difference, she said, is due to ocean currents. Labrador is bathed in frigid waters that come from the Arctic, while the coast of Europe is warmed by currents that come from the equator.
Talley said the temperature of the ocean changes far less than the temperature of the land. That affects the interaction between ocean circulation and world climate on different time scales, from a single day to hundreds of years. Let’s start with daily changes. In the daytime the land heats quicker than the ocean, so the air above the land is warmer than the air above the ocean. Warmer air is less dense than cool air. Differences in air density can give rise to air movements. In some locations the difference is great enough for cool sea breezes to move landward, pushing the warmer air upward. At night the land cools off quickly, cooling the air just above it. When the air over the land becomes colder than the air over the sea, the winds reverse.
This description of how winds are created by the relative warming of land and sea also applies to the far stronger seasonal changes in Asia referred to as monsoons. In the summertime the land mass in Asia stays warmer than the sea. So, moist air moves from the ocean toward the land, bringing life-giving rains. In the winter, the ocean is warmer than the land, so the winds reverse, and people who live in Asia experience hot, dry weather.
Changes also occur every few years. The most intensively studied of
those is El Niño—a weather condition in the Pacific Ocean that occurs
every four to seven years. (El Niño means “The Child” in Spanish. It
refers to the Christ Child since these weather conditions always occur
around Christmas time.) During El Niño, normal winds and ocean currents
reverse for several months, and extreme weather conditions can occur at
other places in the world, ranging from droughts, forest fires, and
floods to changes in the monsoons and trade winds. It is estimated that
the 1982–83 El Niño event caused $8 billion dollars in damage and the
death of 2,000 people worldwide. Estimates of the impact of the 1997–98
El Niño season are even greater, with property damage estimated in the
$30 billion to $60 billion range, and as many as 21,000 lives lost (see
Mackenzie in the Bibliography). While the cause of El Niño events is
still not understood, scientists have determined that it involves the
interaction of the ocean and atmosphere systems.
Over a period of hundreds of years, the ocean transports both heat energy and dissolved carbon dioxide in a current so huge it weaves through all of the world’s oceans. It is sometimes called the “global ocean conveyor belt.” Part of the driving force for this current is in the North Atlantic where salty water (which is already denser than fresh water) cools and becomes even denser. The cold, salty water sinks and begins a very long, deep journey around the world (as shown in the diagram on page 24) until it finally wells up somewhere in the Indian and Pacific, and maybe even around Antarctica. As it becomes part of a warm surface current, the water absorbs heat from the Sun and carbon dioxide from the air before it again sinks to the bottom of the Atlantic, several hundred years later. As pointed out by Talley, the precise course of the current is not thoroughly understood. However, the overall idea of long-term ocean circulation is well accepted by nearly all oceanographers
Some scientists are concerned global warming might cause enough melting of the Arctic and Greenland ice sheets for fresh water to flood the Atlantic, slowing or stopping the global conveyer belt. Some researchers reported evidence that the global conveyer belt had, in fact, stopped or weakened at least twice since the last ice age, suggesting that this may occur again if melting of the Arctic and Greenland ice sheets increases (see Ruhlemann in the Bibliography).
Until we have a better understanding of how the clouds and oceans interact with the atmosphere, we will not be able to predict accurately how much our planet will warm if the concentration of greenhouse gases doubles.
Analysis of layers of ice in Greenland provides an answer to this
question. The record in the ice shows Earth started to emerge from the
last ice age about 15,000 years ago. Then about 12,900 years ago, it
suddenly cooled again. In the relatively short period of about 10
years, the temperature plunged 15°C, returning almost to ice-age
temperatures. The cold period lasted for 13 centuries, until about
11,600 years ago, when, in just about a decade, Earth finally started to
warm again. It has been relatively warm ever since. This cold period
was also observed in the analysis of layers of ice from Antarctica, but
was not quite so intense.
Why did the climate change over just one decade, then remain cold for 13 centuries, only to warm rapidly again? Scientists do not have certain answers to this question, but one popular theory is that the deep ocean current was “switched” off, and then “switched” back on again 13 centuries later. Here’s how it may have happened.
The Global Conveyor Belt - A theory of how water circulates through the world’s oceans over a period of several hundred years.
We know the conveyor belt current makes a huge difference in the
climate, and it is driven by changes in the temperature and salinity
(saltiness) of the seawater. If the climate warmed too quickly, the
warmer temperature could have caused a glacier to melt, flooding the
North Atlantic with fresh water. The fresh water would float on top of
the denser salty water, and stop the conveyor belt current from flowing.
For the global ocean current to be “switched” back on, the ocean would
have to become salty again. This could happen as the world cools in an
ice age and less fresh water is added to the ocean. Eventually the
ocean would become salty enough for the current to start flowing again
and warm temperatures to return.
While we don’t know for certain what will happen in the future, we do know abrupt changes in climate have occurred in the past, suggesting that they may occur in the future. There are some indications of changes in ocean salinity and temperature already being reported, which we'll see more in the next chapter (8).
7.3 State of the Climate
Visit the National Oceanic and Atmospheric Administration (NOAA) website on "State of the Climate" at http://www.ncdc.noaa.gov/sotc/
Look at latest information about particular climate factor(s) such as temperatures, precipitation, ice and snow, drought, and/or storms. Compare with the same factor(s) in prior time period(s). Write about your findings and conclusions and share with your teacher and fellow students.
Now, how about the last two questions in our list of active scientific questions:
- Has the warming caused any noticeable effects so far, and if so, what are those effects?
- How will life on Earth be affected by global warming?
| For new material relating to this chapter,
please see the GSS website
“Staying Up To Date” page:
Global Warming & Climate Change Myths
(sorted by recent popularity vs what science says).
Back to the top