Our Changing Climate

Reports to the Nation on Our Changing Planet

Number 4

Draft: October 1996

Dennis L. Hartmann

Our Changing Climate (7,000 words)

Table of Contents

Climate and American People (800 words)

Earth's Climate: A Dynamic System (500 words)

Why Does Our Climate Change (600 words)

Can We Change the Climate? (300 words)

The Greenhouse Effect (800 words)

Why are Greenhouse Gas Amounts Increasing? (500 words)

Aerosols: Sunscreen for the Planet? (600 words)

How has Climate Changed in the Last Century? (500 words)

How Do We Predict Climate Change? (400 words)

What do Climate Models Tell Us about Our Future? (300 words)

Where do We Go from Here? (400 words)

Climate and American People.

Climate has always had a profound effect on life in America. The first people arrived in America between 15,000 and 30,000 years ago. During that time much of North America was covered by two great ice sheets that were nearly two miles thick in places. One ice sheet followed the coastal mountains from Alaska to Washington State, and another extended from the eastern slope of the Rocky Mountains to the Atlantic Ocean and from the Arctic Ocean to Ohio. Because so much water was piled up on land in ice sheets, the sea level was about 350 feet lower at the peak of the last ice age about 20,000 years ago. The lowered sea exposed a wide plain between Siberia and Alaska, creating a land bridge across the Bering Sea. Genetic, linguistic, and fossil evidence suggests that the first humans in America came from northeast Asia, and it is likely that they walked across the land bridge between Siberia and Alaska sometime during the last ice age. After crossing this plain, these hardy people made their way south between the great ice sheets and spread across America.

Caption: Distribution of ice sheets and sea level/coastlines at the time of the last glacial maximum and the access route from Asia to America, Current continental outline also given for reference. Graphic by D.L. Hartmann and Kay Dewar.]

[Artwork of ice sheets with mastodons and spear-wielding paleo-Americans in foreground: The above is copied from Search for the First Americans by David J. Melt! St. Remy Press, Montreal, Smithsonian Books. page 17. It is a suggestion for a starting point. I would want to have the ice sheet be closer and much more prominent. and the mastodons maybe a little closer also.

The oldest confirmed evidence of human habitation in America dates from about 11,500 years ago. These people were big game hunters, and their camps are marked by distinctive fluted spear points. They hunted mastodons, extinct relatives of the modern elephant, and shared the land with saber-toothed cats, giant ground sloths, and a variety of other now-extinct species. The mastodon, mammoth, woolly rhinoceros and saber-toothed cat all became extinct about 10,000 years ago. Some argue that efficient human hunters caused these extinctions, but others believe that environmental change was the key factor. Enormous changes in the global climate were occurring about 10,000 years ago. The great North American ice sheets began to melt rapidly about 14,000 years ago and by 7,000 years ago they were gone. As the glaciers melted the summers became much warmer, creating very different conditions for plants and animals. Spuce trees, which like colder climates, retreated northward by about a thousand miles, giving way to grassland and broadleaf trees. Many large mammals like the Mastodon that preferred cold climates were not able to adjust to these changes. The climate warming and melting of the ice sheets caused other dramatic changes in North America. The Great Basin between the Cascade Range and Rocky Mountains now has a dry climate, but during the ice age it was a wetter place. About 15,000 years ago the Great Salt Lake in Utah was about 1,200 feet deeper and covered an area about the size of Lake Michigan. As the ice melted and the climate warmed, the west became the relatively dry region we know today.

The effect of climate on human settlement of America continued into medieval times. The first Europeans to set foot on America were Vikings who settled Greenland under the leadership of Eric the Red in about 1000AD. His son, Leif Erikson, led an expedition to colonize America that probably settled in Newfoundland. The colony in Greenland was abandoned in about 1400AD when cooler temperatures associated with the Little Ice Age made farming there too difficult. Well before this time the earlier Asian immigrants to America had developed civilizations, but continued to be affected by changing climate. The Anasazi people of the Four Corners region of the American Southwest are an interesting example of how climate can affect a people. They had an economy centered around corn farming, and built large dwellings in river valleys and along the ridges between canyons. The most famous of these are the cliff dwellings and pueblos of the Mesa Verde region near the junction of Colorado, Utah, Arizona, and New Mexico. This region experienced a series of profound droughts beginning about 1150AD, and by 1300AD a large area in the Mesa Verde region was abandoned.

[Suggested Graphic: Mesa Verde cliff dwellings or Pueblos. Combine this with dried corn stalks.]

Although we have more advanced technology than the Anasazi, modern Americans are also affected by variations in our climate. Between 1934 and 1937 parts of Texas, Oklahoma, Colorado, New Mexico, and Kansas became known as the Dust Bowl when severe drought afflicted the area. Clouds of dust rolled across the vast area affected by the drought, and many residents of this area were forced to move away to find new sources of livelihood.

[Suggested Graphics: Some images from the Dust Bowl or other images of drought]

Earth's Climate: A Dynamic System

Weather changes both rapidly and slowly. The passage of a thunderstorm can change a bright sunny day into a dark, windy, rainy one in less than an hour. Weather also varies over longer time intervals. Just as baseball players have hitting streaks and slumps, and gamblers have lucky and unlucky days, weather can vary widely from one year to the next. Farmers know that in one year the amount and timing of rainfall can be nearly ideal for growing crops, while the next year might bring drought or floods. In some years no hurricanes reach the Atlantic Coast, while in other years coastal states are battered by one storm after another. In many cases, these variations are examples of random weather variations that are part of climate. The atmosphere, in isolation, has little memory of past events beyond a month or so, but the climate is determined by the workings of the climate system, which is composed of the atmosphere, oceans, ice sheets, land, and the plants and animals that inhabit them. Because the ocean has a large capacity to store and release heat, it gives the climate system a long memory which can result in variations lasting decades or longer. The number of hurricanes in the Atlantic, for example, is known to vary from decade to decade in synchrony with subtle shifts in the sea surface temperature. Chemical and biological processes can support climate variations over periods that are much longer than the periods usually associated with random fluctuations of the weather. Once perturbed, the carbon dioxide content of the atmosphere takes more than a century to return to normal.

What is the difference between weather and climate, and how do we distinguish normal variations in the weather from climate change? Climate is the expected weather of a particular region during a particular time of the year, averaged over a number of years. Climate is what we expect, weather is what we get. Most of the variations in the average weather for a particular month or season are caused by internal variations in the climate system and are not associated with long-term changes in the climate. We can think of variations in temperature or precipitation lasting a few years to a decade as climate fluctuations. Examples of important climate fluctuations are those associated with the El Niño-Southern Oscillation phenomenon of the tropical Pacific. The ocean and atmosphere are closely linked in the tropical Pacific and together produce important climate fluctuations on intervals of two to five years that have a significant impact on the weather averaged over whole seasons in regions far removed from the tropical Pacific. These weather variations are not caused by external forces, but result from the intimate slow dance of the atmosphere with the ocean. Another example of a climate fluctuation is the Dust Bowl of the 1930's in America. While it had a very serious influence on the lives of many people, it lasted only a few years and did not represent a long-term change in the climate. We can't give a simple explanation for the series of warm, dry years that produced the Dust Bowl event of the 1930's, but it is probably an example of a natural fluctuation of the climate system. The effects of this fluctuation were worsened by the agricultural practices in use in the region at that time, and improved conservation techniques were adopted after the Dust Bowl experience.

Caption: The time series of summertime temperature and rainfall at Topeka, Kansas gives a useful illustration of natural year-to-year variations in local climate, as well as the major climate fluctuation associated with the Dust Bowl period of the 1930's.

Why Does Our Climate Change?

Just as a baseball player's home run statistics might change when the fences are moved closer to home plate, the weather statistics can change when external conditions change. So what causes the climate to vary on time scales of decades and longer? Why did great continental ice sheets appear and disappear again and again over the last several million years? In 1930 the Serbian mathematician Milutin Milankovitch offered a theory that the timing for the advances and retreats of ice sheets is set by variations in Earth's orbit about the sun. Milankovitch hypothesized that the critical factor in determining ice sheet growth was the amount of sunshine reaching high latitudes of the Northern Hemisphere during the summer season. We call the energy provided by sunshine the "insolation". He predicted that ice sheets would grow when the insolation reaching the high latitude continents during summer was less than normal, since this would allow snow cover to last through the melting season and gradually accumulate over the centuries.

The oxygen isotope record in ocean sediments allows us to estimate the mass of water contained in continental ice sheets as a function of time in the past. The global ice volume has varied dramatically from ice age conditions to interglacial conditions more like today's many times over the past 3 million years. This plot shows the variation of global ice volume over the last 150,000 years. Figure prepared by D.L. Hartmann from data suplied by Raymo, M. E., W. F. Ruddiman, N. J. Shackleton and D. W. Oppo, 1990: Evolution of Atlantic-Pacific d 13C gradients over the last 2.5 m. y. Earth Planet Sci. Lett., 97, 353-368.

The angle of the tilt between Earth's axis of rotation and the plane of Earth's orbit about the sun is currently about 23.5 degrees, but it varies in the range between 22 and 24.5 degrees with a period of about 41,000 years. The amount by which Earth's orbit deviates from a perfect circle varies with periods around 100,000 and 400,000 years. Currently the Earth is closest to the sun in January, but the month of Earth's closest approach to the sun varies with a period of about 23,000 years. Variations in all these orbital parameters cause changes in the amount of insolation that reaches particular latitudes during each season. The variations are largest in middle and high latitudes, where ice is more likely to form.

Milankovitch's theory was given a large boost during the last several decades when modern techniques allowed the chronology of past variations in the global volume of land ice to be inferred from information contained in layered ocean sediments. For the last several million years, the ice sheets have varied with the same periods as Earth's orbit, and the association of reduced high latitude summertime insolation with increased global ice volume is basically in accord with the Milankovitch theory. In particular, the period of rapid ice sheet melting about 10,000 years ago occurred at a time when Earth's orbit was arranged such that greater insolation came to the high latitude continents of the Northern Hemisphere during summer.

[Provided Graphic: A schematic diagram showing the orbital parameters or their effect on insolation. Made by D.L. Hartmann]

Milankovitch calculated seasonal and latitudinal shifts of insolation that appear to be the pacemaker of ice ages, but the nature and magnitude of the resulting climate changes are determined by processes that take place within Earth's climate system. In order for the climate to swing from ice age to warmer conditions, the climate system must amplify the response to the influence of Earth's orbital changes. A "positive feedback" is a process within the climate system that amplifies the response of temperature to external influences such as changes in Earth's orbit. One such process is ice-albedo feedback. Snow and ice absorb a smaller fraction of the incoming solar radiation than unfrozen ocean or ice-free land, reflecting more of it back to space. The "albedo" measures the fraction of the incoming solar radiation that gets reflected back to space. When temperatures are cold enough for snow cover to last through a summer season, much less of the energy available in sunshine is absorbed than would be without a covering of snow. Thus, as the ice expands, less solar heat is absorbed, which tends to cool the climate further, and leads to further expansion of the ice cover. This ice-albedo feedback process can make the climate more sensitive, so that more temperature change results from influences such as changes in incoming solar radaition.

An important clue to understanding the change in climate between ice age and warmer conditions comes from measurements of carbon dioxide (CO2) trapped in bubbles in ice cores taken from the Greenland and Antarctic ice sheets. These data show that the atmosphere contained 40% less CO2 at the peak of the last ice age than it did just prior to the Industrial Revolution. Because CO2 is a greenhouse gas that tends to warm the climate, reduced atmospheric CO2 is part of the explanation for how the cold climates of the ice ages were produced. Calculations show that the reduced atmospheric CO2 during the ice age may account for about half of the approximately 10_F colder global temperature at the time of maximum glacial advance. The knowledge that variations in the chemical composition of the atmosphere are important for explaining the ice ages has caused scientists to broaden their view of the climate system to include not only the physical processes that constrain energy and moisture, but also the chemical and biological processes that control atmospheric composition and land surface characteristics. Over the longer periods of time that are required for major glacial cycles, the atmospheric CO2 content is closely tied to the amount of CO2 in the ocean. The amount of CO2 in the ocean is dependent on marine organisms that use CO2, sunlight, and nutrients in the process of photosynthesis. Lowered atmospheric CO2 may have resulted from increased productivity of these marine organisms during the ice age.

Caption: Estimates of past carbon dioxide concentrations derived from ice cores drilled at Vostok, Antarctica and Siple Station, Greenland are combined with the modern instrumental record from Mauna Loa Observatory to show the relationship between atmospheric CO2 changes associated with ice ages and the modern increase in CO2 associated with human activities. Natural control of atmospheric CO2 ended at the time of the Industrial revolution, when humans began burning fossil carbon fuels, manufacturing cement, and removing forests at an increasing rate. [Prepared by D.L Hartmann from public data sources. Data references are Barnola, J.M., D. Raynaud, C. Lorius, and Y.S. Korotkevich, 1994. Historical CO2 record from Vostok ice core. p.7-10 in Trends '93: A Compendium of Data on Global Change.; Neftel, A. H. Friedli E. Moor, H. Lötscher, H. Oeschger, U. Siegenthaler, and B. Stauffer. 1994. Historical CO2 record from the Siple Station ice core. pp. 11-14; and Keeling, C.D, and T.P. Whorf, 1994. Atmospheric CO2 records from sites in the SIO air sampling network. pp. 16-26. in Trends '93: A Compendium of Data on Global Change. .]

While the Earth's orbit seems to be an important pacemaker of climate change over glacial time periods, other factors can cause climate changes over shorter periods. Climate changes could be produced by variations in the energy output of the sun. The number of dark spots that appear on the sun varies with a period of about 11 years. Observations taken over the last few decades indicate that the energy output of the sun is about 0.1% greater when the sunspot number is at its maximum than when the number of sunspots is at a minimum. This change in energy output is too small to cause important climate variations, but the sun's energy output may have larger variations on longer time scales. Some evidence suggests that weakened solar energy output may have helped produce the Little Ice Age of 1350-1800AD. During the Little Ice Age temperatures were a few degrees colder than now in middle latitudes, and mountain glaciers expanded, but major ice sheets did not form. This cold period followed a medieval warming of Viking-Norman times that extended from about 900 to 1300AD.

Volcanic eruptions can affect the climate by sending large amounts of sulfur dioxide (SO2) gas into the stratosphere, about ten miles above the surface of Earth. In the stratosphere the SO2 gas is converted into tiny sulfuric acid droplets that remain there for a year or more. These droplets reflect sunlight and reduce the solar heating at the surface of Earth. The eruption of Mt. Pinatubo in June of 1991 cooled the climate by a few tenths of a degree for about a year, but the effect fades as the volcanic particles slowly fall out of the stratosphere. A succession of major volcanic eruptions could cause a longer-lasting change in climate.

Can We Change the Climate?

At the end of the last ice age, there were perhaps a million people in North America, or about one for every 7 square miles. Today, excluding Alaska and Hawaii, there are about 80 people for every square mile of land area in the United States. To sustain this population growth and raise our standard of living, we employ natural resources and technologies that were unknown to our forebears. Is it possible that, because of our numbers and our greater use of resources and technology, modern humans are directly influencing the global climate of Earth? In 1896 the Nobel-Prize-winning Swedish chemist Svante Arrhenius predicted that humans would warm the global climate by increasing the carbon dioxide content of the atmosphere. At the beginning of the Industrial Revolution in the late 1700's, the use of coal as an energy source began to increase rapidly. When coal is burned, energy is produced, and CO2 is released to the atmosphere. Other fossil carbon fuels such as petroleum and natural gas also release CO2 when they are burned. Measurements show that atmospheric CO2 has increased by about 30% since the late 1700's. This documented increase results primarily from the use of fossil carbon fuels in electrical generation plants, automobiles, home heating, and in a variety of other ways. Carbon dioxide is also added to the atmosphere during the process of cement manufacture and as a result of deforestation. The yearly rise in CO2 has increased in recent times, and continued growth of both population and per capita energy use will force atmospheric CO2 to even higher levels. In addition, the amounts of other greenhouse gases in the atmosphere have increased during the industrial age, in most cases as a direct result of human activities. These include halocarbons, methane(CH4), nitrous oxide(N2O), and tropospheric ozone(O3).

Caption: Atmospheric carbon dioxide has increased from a value of about 275 parts per million before the Industrial Revolution to about 360 parts per million in 1996, and the rate of increase has speeded up over this span of time. The amount of CO2 in the atmosphere has been measured with instruments since 1957. CO2 concentrations farther into the past can be estimated from CO2 amounts trapped in bubbles in ice cores from Greenland and Antarctica. Atmospheric CO2 began to rise rapidly in about 1700 at the beginning of the industrial revolution. It is certain that the predominant cause of this increase is burning of fossil carbon fuels such as coal, oil and natural gas. Prepared by D.L Hartmann from public data sources. Data references are Neftel, A. H. Friedli E. Moor, H. Lötscher, H. Oeschger, U. Siegenthaler, and B. Stauffer. 1994. Historical CO2 record from the Siple Station ice core. pp. 11-14; and Keeling, C.D, and T.P. Whorf, 1994. Atmospheric CO2 records from sites in the SIO air sampling network. pp. 16-26. in Trends '93: A Compendium of Data on Global Change.

The Greenhouse Effect:

Carbon dioxide gas constitutes a tiny fraction of the atmosphere. Only about one air molecule in four thousand is CO2. How can CO2 have a big effect on the climate, if there is so little of it around? To understand this we need to understand the greenhouse effect of the atmosphere. The Earth is warmed by absorbing sunshine and is cooled by emitting infrared radiation to space. Infrared radiation is the heat radiation we feel when we sit some distance from a campfire or warm stove. In equilibrium, the input of energy to the climate system by absorption of solar radiation is just equal to the output of energy by emission of infrared radiation to space. Energy moves through the climate system at a rate of 235 Watts per square meter of surface area (W m-2), when averaged over the whole globe. This is roughly equivalent to two 100-Watt light bulbs for every square yard of surface area. The critical characteristics of the atmosphere that enable it to raise the temperature of the surface of Earth are that the atmosphere is fairly transparent to sunshine, but is almost opaque to infrared radiation. So the atmosphere lets in the heat from the sun, but is reluctant to let it escape again. In this way it is a little like a garden greenhouse. About half of the solar energy that reaches Earth passes through the atmosphere and is absorbed at the surface. About 90% of the infrared radiation emitted by the surface is absorbed by the atmosphere before it can escape to space. Because the atmosphere is good at absorbing infrared radiation, it is also good at emitting it. The surface of the earth receives almost twice as much energy from infrared radiation coming down from the atmosphere as it receives from sunshine. The infrared radiation coming from the atmosphere is emitted from greenhouse gases like water vapor and CO2 and from clouds. If all greenhouse gases were removed from the atmosphere, the average surface temperature of Earth would drop from its current value of 59_F (15_C) to about 0_F (-18_C). Without the atmosphere's greenhouse effect, Earth would be a frozen and probably lifeless planet.

Caption: The atmosphere allows solar radiation to enter the climate system relatively easily, but absorbs the infrared radiation emitted by the Earth's surface. Although about half of the energy coming from the sun is absorbed at the surface of the Earth, almost twice as much heating of the surface is provided by downward infrared emission from the atmosphere. This "greenhouse effect" causes the surface of Earth to be much warmer than it would be without the atmosphere. This diagram shows the flow of solar(yellow) and infrared(red) radiative energy through the climate system in Watts per square meter of surface area. On average, 168 Watts of solar radiation energy reach each square meter of the surface area, but the heating of the surface from the downward infrared radiation emitted by the atmosphere is almost twice that, 324 Watts per square meter. Prepared by D.L. Hartmann and Kay M. Dewar from data supplied in Kiehl, J. T. and K. E. Trenberth, 1996: Earth's annual global mean energy budget. Bull. Amer. Meteor. Soc., 77, submitted..

The extent to which a gas enhances the greenhouse effect of the atmosphere by absorbing and emitting infrared radiation is rooted in its molecular structure. Although the atmosphere is about 78% nitrogen and 21% oxygen, because these gases have a simple two-atom structure, they have a relatively minor effect on the transmission of solar and infrared radiation through the atmosphere. The gases that are most important for the greenhouse effect are the three-atom molecules water vapor, carbon dioxide and ozone. A host of other gases with three or more atoms make significant contributions to the greenhouse effect, even though they constitute a tiny fraction of the atmosphere. The structure of these molecules is such that they can efficiently absorb and emit infrared energy by storing and releasing energy in molecular vibration and rotation.

The molecule that makes the largest contribution to the atmospheric greenhouse effect is water vapor, because its bent, three-atom structure absorbs and emits infrared radiation easily, and it is relatively abundant in the atmosphere. The amount of water vapor in the air is determined by the balance between evaporation from the surface and precipitation as rain or snow. An average water molecule stays in the atmosphere only a few days between evaporation from the surface and falling out of the atmosphere as precipitation, and the water vapor content of the atmosphere adjusts quickly to changes in surface temperature. There is little that humanity can do to directly control global atmospheric water vapor amounts, although water vapor will respond quickly to climate changes resulting from other natural or human causes. Because atmospheric water vapor tends to increase with increasing temperature, it can amplify climate changes produced by other means, a process we call water vapor feedback.

Why are Greenhouse Gas Amounts Increasing?

Carbon dioxide has a much longer lifetime in the atmosphere than water vapor. It is cycled between the atmosphere and the ocean or land surface by chemical and biological processes. If CO2 is suddenly added to the atmosphere, it takes between 50 and 200 years for the amount of atmospheric CO2 to establish a new balance, compared to several weeks required for water vapor. Through millions of years of Earth's history, trillions of tons of CO2 were taken out of the atmosphere by plants and buried in sediments that eventually became coal, oil or natural gas deposits. In the last two centuries these deposits have been employed at an increasing rate as an economical energy source, and today humanity releases about 5.5 billion tons of carbon to the atmosphere every year through fossil fuel burning and cement manufacture. Approximately another 1.5 billion tons per year are released through land use changes such as deforestation. These releases result in an increase of atmospheric CO2 of about one-half percent per year.

Other naturally occurring greenhouse gases such as methane and nitrous oxide have also been increasing, and entirely manmade greenhouse gases such as halocarbons have been introduced into the atmosphere. Many of these gase are increasing more rapidly than carbon dioxide. The amount of methane, or natural gas, in the atmosphere has doubled since the Industrial Revolution. Although its sources are many, the increase is believed to come from rice paddies, domestic animals, and leakage from coal, petroleum and natural gas mining, among other sources. Halocarbons are a family of industrial gases that are manufactured for use in refrigeration units, as cleaning solvents, and for making insulating foams. They were first manufactured in the 1940's, and because they do not easily react with other chemicals, they can have a lifetime in the atmosphere of more than 100 years. Halocarbons are also responsible for the Antarctic Ozone Hole and a more general decline in global stratospheric ozone, but this is a separate problem from the greenhouse warming contributed by the halocarbons. Production of some of the halocarbons that are most important for climate have been regulated by international agreements to preserve Earth's protective ozone layer, so their influence on climate will decline in the future.

Caption: It is estimated that the changes in atmospheric CO2, methane, nitrous oxide and halocarbons since preindustrial times would, if all else remained equal, have changed the radiation balance of Earth by about 2.4 W/m-2. This is about 1% of the energy flow through the climate system. Prepared by D.L. Hartmann from public data supplied in Houghton, J. T., L. G. Meira Filho, B. A. Callander, N. Harris, A. Kattenberg and K. Maskell, Eds., 1996: Climate Change 1995: The Science of Climate Change. Intergovernmental Panel on Climate Change, Cambridge, 572.

Aerosols: Sunscreen for the Planet?

Although our effect on the amount of greenhouse gases in the atmosphere is the most important way in which we can directly influence the global climate, humans also contribute to the aerosol content of the atmosphere. Aerosols are tiny particles of liquid or solid matter that are suspended in air. They are different from water cloud droplets or ice particles in that they appear even in relatively dry air. Atmospheric aerosols have many sources and are composed of many different materials including sea salt, soil, smoke, and sulfuric acid. They can reflect solar radiation or absorb and emit infrared radiation directly, and are often visible as haze or smog. By reflecting sunlight, they cool the climate. Another way that aerosols affect climate is through their effect on clouds. Every cloud droplet or ice particle has at its center an aerosol, called a cloud condensation nucleus, on which the water vapor collected to form the cloud droplet. Aerosols that attract water, such as those composed of salt or sulfuric acid, are particularly effective as cloud condensation nuclei.

Although there are many natural sources of aerosols, it is estimated that aerosols resulting from human activities are now almost as important for climate as naturally produced ones. Human production of sulfur gases accelerated rapidly in the 1950's and it appears that the cooling effect of the resulting aerosols has canceled out part of the warming that might have been associated with greenhouse gas increases. Most of the human-induced aerosols come primarily from sulfur released in fossil fuel burning and from burning vegetation to clear agricultural land. The human-induced increase in atmospheric aerosols since preindustrial times is believed to have reduced the energy absorbed by the planet by about 0.5 W m-2, which would offset about 20% of the greenhouse gas warming effect. The aerosols produced by humans could also have a significant effect on the amount or properties of clouds. It may be that this would make the clouds reflect more solar radiation. More reflection of sunlight by clouds would cause a cooling that might offset part of the greenhouse gas warming, but the size of this effect is very uncertain.

We must keep in mind, however, some very important differences between the greenhouse warming and the aerosol cooling. While greenhouse gases such as CO2 and halocarbons remain in the atmosphere for about a century after being released, only a few weeks transpire between when an aerosol is released into the lower atmosphere and when it is washed out of the atmosphere. Therefore human-produced aerosols are not distributed evenly over the globe, but tend to be concentrated near the points where they are released into the atmosphere. Aerosols that result from human actions originate predominantly in industrialized countries of the Northern Hemisphere where fossil fuels are burned, and in land areas where vegetation is burned. Because their affects are more localized, aerosols may cause regional shifts in climate. Also, because of their short lifetimes in the atmosphere, the effect of aerosols on today's climate is determined by the release of aerosols that occurred during the previous couple of weeks. In contrast, the CO2 that we release into the atmosphere today will affect the climate for 50 to 100 years into the future. For these reasons the greenhouse gas warming must eventually overwhelm the human-induced aerosol cooling. Nonetheless it is important to understand the effect of aerosols on the climate, in order that we may better predict how changing greenhouse gas amounts will affect the future climate, and assign causes to temperature changes when we observe them to occur. Efforts are underway to reduce the release of SO2 gas from coal-fired energy plants, because it causes acid rain and lung disease, and this may have the effect of reducing aerosol amounts in some regions.

Ship Track Image

[Suggested Graphic: A good picture of an aerosol haze. Or a good image of cloud tracks in stratus clouds. The above is a possible example. It needs a little work to get the red out and enhance the ship tracks a little.]

How has Climate Changed in the Last Century?

Measurements suggest that global mean surface temperature has increased by about 1 _F in the last century. The warming has been greatest over the continents between 40 and 70 degrees north latitude. Over this same period of time measurements indicate that global sea level has risen between 4 and 10 inches. Scientists do not yet know with certainty what part of these changes is caused by humanity and what part would have occurred without us. Part of this warming may be a rebound from the cooling of the Little Ice age during the 1350-1800 period, and the causes of the Little Ice Age were probably unrelated to human activities. The period of this warming also coincides with the period when human activities have increased CO2 and other greenhouse gases in the atmosphere, however. Many scientists are convinced that human activities have made a major contribution to the warming of the last century, and that warming caused by greenhouse gas increases will be a continuing part of our future.

A rapid warming of the climate would cause serious problems. Because such a warming, once initiated, would last for a long time, scientists and civic planners are very interested in knowing how much warming has occurred and whether that warming can be attributed to human actions. The record of global temperature obtained from thermometers around the world extends back in time only a little over a century. This record shows a steady increase up until about 1940's, followed by a period of slight cooling between the 1940's and 1970's. Since the 1970's the temperatures have gone up rapidly, and many of the warmest years in the global temperature record have occurred in the last 15 years. It is not known with certainty whether this recent warming trend will continue, nor whether it is caused by the increasing trend of greenhouse gas concentrations in the atmosphere. The natural random variability of the climate system on decadal time scales is fairly large, and it is not yet easy to separate this variability from changes that might have resulted from human alteration of the global environment.

Caption: The record of global mean surface air temperature from thermometer readings indicates a global warming over the past century, with many bumps and wiggles suggesting the natural year-to-year variability of climate. Prepared by D.L Hartmann from data supplied in Hansen, J., R. Ruedy, M. Sato and R. Reynolds, 1996: Global surface air temperature in 1995: Return to pre-Pinatubo levels. Geophys. Res. Lett., 23, 1665-1668. and in Wilson, H. and J. Hansen. 1994. Global and hemispheric temperature anomalies from instrumental surface air temperature records. pp. 609-614 in Trends '93: A Compendium of Data on Global Change. and including later additions to the online data set.

How do We Predict Climate Change?

The behavior of the climate system can be simulated with a computer model, and these simulations can be tested against observations of the current and past climates. Although these models have shortcomings, they do capture many of the key features of the present climate. They can be used to study the response of the climate to changing amounts of greenhouse gases and aerosols, to changes in land surface conditions, and to other natural or human-caused changes.

Modeling the climate on a computer is difficult because processes with very large spatial scales, such as the transport of energy from the tropics to the poles by atmospheric motions with scales of thousands of miles, are equally important with small-scale processes such as the collection of water molecules into raindrops. How do we represent this wide range of spatial scales and also produce a model that is efficient enough to run on available computers in a reasonable length of time? The approach taken is to represent the globe with a gridwork of boxes about 100 miles on a side and then predict the average properties in these boxes using known laws of physics. The effects of processes that occur on smaller scales are represented with approximate formulas that relate them to the averaged properties in the grid boxes. The problem with this approach is that some of the small-scale processes that must be treated in a more approximate fashion are also central to the feedback effects that determine how big a climate change will result from human actions. For example, clouds have a huge influence on the transmission of solar and infrared radiation through the atmosphere, yet the processes that determine the properties of clouds occur on scales that are much smaller than a climate model grid box. A large part of the uncertainty in forecasts of future climates derives from uncertainty about how to treat clouds in climate models. Important feedbacks such as those involving surface ice and atmospheric water vapor also involve processes occurring on small scales and must be treated with approximate formulas. As computer power and understanding both increase, some of the uncertainty associated with feedback processes will be reduced and more accurate climate forecasts will become available.

What do Climate Models Tell Us about the Future?

Once a climate model has been tested against current and past observations, it is reasonable to ask what it can tell us about future climates. A typical experiment of this nature is to extrapolate the increase in greenhouse gases over the past century into the next century and see how the climate model responds to this change in greenhouse gases. Because of the uncertainties in the models described previously, the predicted warming over the next century is quite uncertain, ranging from a modest warming of 1_C(2_F) to a very substantial warming of 4.5_C(8_F). Models consistently predict that the warming would be greater in high latitudes than in the tropics, and greater over land than ocean. Many models predict that a greater increase of evaporation than precipitation over midlatitude land areas would result in drier conditions in those regions, especially during summer in North America and Southern Europe. Warming may cause agricultural zones in North American to move northward, which would benefit some communities and harm others. Changes in the climate of specific small regions and changes in the activity of tropical storms cannot yet be predicted with much confidence. When natural climate fluctuations cause sea surface temperature in the tropical Atlantic to increase, hurricane activity also increases, but it is not certain that a global surface temperature rise caused by greenhouse gas increases would have a proportional effect on hurricane activity. The effect of the warming on humanity depends on the magnitude of the warming, the speed with which the warming occurs, and the way society is organized to adapt to climate change. If the warming is as fast and as large as some of the models predict, then the effects on people and our natural environment could be very serious. Agriculture and water supplies take decades to adapt, and natural ecosystems take centuries. Therefore, rapid change would pose more difficult problems.

Where do We Go from Here?

When planning for the future, most Americans assume that the climate we have experienced in the past will continue, but this may not be the case. Rain, snow, and temperature affect human life and commerce in a variety of ways, including water and energy resource management, human health, and agriculture. We know that the amounts of some greenhouse gases in the atmosphere are increasing as a result of human activities. The well-understood physics of the greenhouse effect indicate that the changing composition of the atmosphere should warm the surface climate of Earth. Current estimates of the expected climate change over the next 50 years range from a future climate modestly warmer than today to one warmer than any that has occurred on Earth for more than a million years. This range of uncertainty is uncomfortably large. Moreover, current models cannot make accurate predictions of how temperature and water resource availability might change in a particular state or county, where measures to adapt to climate change would need to be made.

Given the current level of uncertainty and the complexity of the climate system, there will certainly be surprizes in the future, both of the pleasant and the unpleasant variety. Information about how the climate is changing and the assignment of causes to these changes will be very important for the public and policy makers in deciding how to respond to the challenges that our role in shaping climate may present. Efficient communication of this information to all concerned will be important in this process.

Scientists are working hard to improve our understanding of the climate system and our ability to predict its future course. This work involves taking careful observations to monitor subtle changes in the climate system, conducting intensive observational programs to study the critical processes that determine the size of the expected climate change, and continuing efforts to improve climate models and test them against observations. We also need to better understand the complex relationship between humans and climate. Because of the long lifetime of greenhouse gases in the atmosphere, and the slow but steady response of the climate to them, it is very important to have accurate forecasts of how the future climate will evolve in response to both natural and human forces. The potential exists for very significant shifts in climate, and better predictions will enable us to act more effectively to mitigate climate change or adapt to its effects. Accurate predictions of future climates can provide a basis for decisions about how best to respond to the challenge of our changing climate.


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