Our Changing Climate

Reports to the Nation on Our Changing Planet

Number 4

Draft: December 17, 1996

Dennis L. Hartmann

Table of Contents:

Climate and American People

Earth's Climate: A Dynamic System

Why Does Our Climate Change

Can We Change the Climate?

The Greenhouse Effect

Why are Greenhouse Gas Amounts Increasing?

Aerosols: Sunscreen for the Planet?

How has Climate Changed in the Last Century?

How Do We Predict Climate Change?

What do Climate Models Tell Us about Our Future?

Where do We Go from Here?

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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 suggest that the first humans in America came from northeast Asia, and it is likely that the ice age climate made it possible for these people to walk across the land bridge between Siberia and Alaska. After crossing this plain, these hardy people made their way south between the great ice sheets and spread across America.

Caption: Ice sheets and shifted coastlines at the time of the last glacial maximum opened an access route from Asia to America. The current continental outline is given for reference. [Graphic by D.L. Hartmann and Kay Dewar.]

We know that some of these early Americans were big game hunters. Their camps are marked by distinctive fluted spear points which they used to hunt mastodons--extinct relatives of the modern elephant. They also shared the land with saber-toothed cats, woolly rhinoceroses and giant ground sloths. These and a variety of other species all became extinct about 10,000 years ago. Some researchers argue that efficient human hunters caused these extinctions, but others believe that environmental change was the key factor.

The extinctions coincide with a time of enormous change in the global climate. Some 14,000 years ago the great North American ice sheets began to melt rapidly, and by 7,000 years ago they were gone. This end to the ice age caused dramatic changes in North America. As the ice melted and the climate warmed, the once-wet region between the Cascade Range and Rocky Mountains became the relatively dry landscape that we know today as the Great Basin. Features like Utah's Great Salt Lake turned into shadows of their former selves. Fifteen thousand years ago this body of water was 1200 feet deeper and covered an area the size of Lake Michigan.

Such changes had a marked impact on ice age plants and animals. Cold-loving spruce trees, for example, withdrew their range northward by about a thousand miles, giving way to grassland and broadleaf trees. Mastodons and other large mammals that preferred cold climates may not have been able to adapt to the warmer, drier conditions. As their favorite game animal disappeared, the earliest Americans would also have been forced to adapt.

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. Further to the south, climate changes also affected the civilizations set up by the earlier Asian immigrants to America. The Anasazi people of the Four Corners region of the American Southwest provide an interesting example. 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. Beginning about 1150AD the Four Corners region experienced a series of profound droughts, and by 1300AD the Anasazi had abandoned this area.

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 people were forced to move away to find new sources of livelihood.

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. Yet 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 in weather are random; like the lucky and unlucky days of a gambler they occur without any apparent cause. The atmosphere, in isolation, has only short-term memory, and so acting alone it cannot produce random variations that last longer than about a month. 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. Similarly, long-term effects can result from changes in the biology of the climate system as well as its chemistry. Once perturbed, for example, the carbon dioxide content of the atmosphere takes more than a century to return to normal.

If weather varies over long intervals and climate does too, how do we distinguish one from the other? One simple way to think of it is that climate is what we expect; weather is what we get. To describe climate, researchers look at the average weather over a number of years in a particular region during a particular season. Variations in the weather from year to year usually cancel each other out and therefore the region's climate stays relatively constant.

But sometimes changes in temperature or precipitation continue for a few years or even a decade. We can think of these shifts in weather as climate fluctuations. One example of an important climate fluctuation is that associated with the El Nino-Southern Oscillation of the tropical Pacific. The ocean and atmosphere are closely linked in this region and together produce important climate fluctuations on intervals of two to five years that have a significant impact on the seasonal weather in regions far removed from the tropical Pacific. Weather phenomena ranging from droughts in Australia to flooding in the U.S. 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 1930s.

In addition to its fluctuations from year to year or decade to decade the climate also varies on time scales of centuries or longer. Great continental ice sheets have appeared and disappeared again and again over the last several million years. What caused these long-term variations? Scientists believe they stem from something other than the internal workings of the climate system. 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 as a result of shifts in the planet's external conditions.

Why Does Our Climate Change?

In 1930 the Serbian mathematician Milutin Milankovitch offered a theory for what caused the advances and retreats of ice sheets. He hypothesized that the critical factor in determining ice sheet growth was the amount of sunshine reaching high latitudes of the Northern Hemisphere in the summer. We call the energy provided by sunshine the insolation. Milankovitch predicted that ice sheets would grow when the insolation reaching the high latitude continents was less than normal during summer, since this would allow snow cover to last through the melting season and gradually accumulate over the centuries.

Caption: The oxygen isotope record in ocean sediments allows us to estimate the mass of water contained in continental ice sheets 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 500,000 years. Figure prepared by D.L. Hartmann from data supplied 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.

He showed that changes in insolation result from subtle variations in Earth's orbit. The planet's tilt as it revolves around the sun, for example, varies with a period of about 41,000 years. Today the angle is about 23.5 degrees, but it ranges between 22 and 24.5 degrees. The amount by which Earth's orbit deviates from a perfect circle also varies, with periods around 100,000 and 400,000 years. And the month of Earth's closest approach to the sun--currently January--varies on a 23,000-year cycle. The effects of all these orbital variations are largest in middle and high latitudes, where ice is more likely to form.

Over the last several decades Milankovitch's theory has received a large boost. Modern techniques have allowed scientists to calculate how much land ice once existed, based on information contained in layered ocean sediments. For the last several million years, the ice sheets have varied with the same regularity as Earth's orbit. Summertime insolation at high latitudes drops at roughly the same times that global ice volume peaks, in accordance with the Milankovitch theory. In particular, the period of rapid ice sheet melting about 10,000 years ago occurred at a time when greater summertime insolation came to the high latitude continents of the Northern Hemisphere.

Yet while external shifts of insolation appear to be the pacemaker of ice ages, the nature and magnitude of the resulting climate changes are still 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 Earth's orbital changes. One way that this amplification takes place is via a process known as ice-albedo feedback. "Albedo" is, in short, a measure of Earth's reflectivity. Snow and ice bounce the sun's rays back into space far more effectively than unfrozen ocean or ice-free land. When temperatures are cold enough for snow cover to last through a summer season, the planet absorbs much less of the energy available in sunshine than it would 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 like shifting insolation.

An important clue to understanding how the climate can get cold enough to sustain summertime snow comes from measurements of carbon dioxide (CO2). CO2 is a greenhouse gas that tends to warm the climate. Scientists can determine how much CO2 existed in ancient air because some of that air is trapped in bubbles inside cores of ice from the Greenland and Antarctic ice sheets. These cores show that the atmosphere contained 40% less CO2 when the ice reached its maximum extent 20,000 years ago than it did just prior to the Industrial Revolution. Calculations show that the reduced CO2 may account for nearly half of the approximately 10_F global cooling during this glacial maximum.

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. Loetscher, H. Oeschger, U. Siegenthaler, and B. Stauffer. 1994. Historical CO2 recor 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. ]

Some things cause climate to change over periods shorter than glacial cycles. Climate change could, for example, be produced by variations in the energy output of the sun. Observations taken over the last few decades indicate that output is about 0.1% greater when the number of dark spots on the sun is at its maximum--roughly every 11 years--than when it is at a minimum. This change in energy output is too small to cause important climate variations, but the sun's output may vary more on longer time scales. Some evidence suggests that weakened solar energy output may have helped produce the Little Ice Age of 1350-1800AD. During this period temperatures were a few degrees colder than now in middle latitudes. But while mountain glaciers expanded, major ice sheets did not form.

Volcanic eruptions can affect the climate over the short term by sending large amounts of sulfur dioxide (SO2) gas into the stratosphere, about ten miles above Earth's surface. 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 of the planet. The eruption of Mt. Pinatubo in June of 1991 cooled the climate by a few tenths of a degree for about a year. But such effects fade as the volcanic particles slowly fall out of the stratosphere. Only 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 ice age forebears. These technologies also allow us to see the invisible impact of our population on the broader terrain of the planet's skies.

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. And in fact, measurements show that the levels of this gas have increased by about 30% since the late 1700's. That time coincides with the beginning of the Industrial Revolution when the use of coal as an energy source began to increase rapidly. Burning coal releases CO2 to the atmosphere. Other fossil carbon fuels, like petroleum and natural gas, also release CO2 when they are burned. Such fuels are used in electrical generation plants, automobiles, home heating, and in a variety of other ways. Carbon dioxide also escapes 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 levels 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). 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 just as Arrhenius predicted?

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. Loetscher, H. Oeschger, U. Siegenthaer, 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. Yet in spite of the fact that so little of it is around, CO2 can have a big effect on the climate. To understand why we need to understand the greenhouse effect of the atmosphere. Earth's atmosphere lets in the rays of the sun which warm the surface. The planet, in turn, keeps cool by emitting heat back into space in the form of infrared radiation--the same radiation that warms us when we sit near a campfire or stove. But while the atmosphere is fairly transparent to sunshine, it is almost opaque to infrared radiation. Much like a garden greenhouse, it traps the heat inside.

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. In addition, greenhouse gases like CO2 as well as clouds can re-emit this radiation, sending it back toward the ground. The fact is, Earth's surface receives almost twice as much energy from infrared radiation coming down from the atmosphere as it receives from sunshine. If all greenhouse gases were removed from the atmosphere, the average surface temperature of Earth would drop from its current value of 59F (15C) to about 0F (-18C). 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.

It is the distinctive molecular structures of the greenhouse gases that allow them to absorb and re-emit infrared radiation in this way. Although the atmosphere is about 78% nitrogen and 21% oxygen, these gases have a simple structure consisting of two identical atoms. As a result, they have a relatively minor effect on the transmission of solar and infrared radiation through the atmosphere. But the three-atom molecules of water vapor, carbon dioxide, ozone and a host of other gases can efficiently absorb and emit infrared energy by storing and releasing it in molecular vibration and rotation. Though some of these gases constitute only a tiny fraction of the atmosphere, they can nevertheless make significant contributions to the greenhouse effect.

The molecule that makes the largest contribution is water vapor, which 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. Because atmospheric water vapor tends to increase with increasing temperature, however, it can amplify climate changes produced by other means--a process called water vapor feedback.

Why are Greenhouse Gas Amounts Increasing?

Carbon dioxide has a much longer lifetime in the atmosphere than water vapor. 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. That's because CO2 is cycled between the atmosphere and the ocean or land surface by chemical and biological processes. Plants, for example, use it to produce energy in a process known as photosynthesis. 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 man-made greenhouse gases such as halocarbons have been introduced into the atmosphere. Many of these gases 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 mainly from rice paddies, domestic animals, and leakage from coal, petroleum and natural gas mining. Halocarbons are a family of industrial gases that are manufactured for use in refrigeration units, as cleaning solvents, and in the production of 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.

Climate Forcing Pie Chart

Caption: Changes in the atmospheric concentration of CO2, methane, nitrous oxide and halocarbons that have occured since the Industrial Revolution have altered the energy budget of Earth. The difference is about 2.4 Watts per square meter, or roughly 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 levels of greenhouse gases in the atmosphere is the most important direct influence that we can have on 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. 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. Most of the human-induced aerosols come from sulfur released in fossil fuel burning and from burning vegetation to clear agricultural land. Human production of sulfur gases accelerated rapidly in the 1950's.

It appears that the cooling effect of atmospheric aerosols has canceled out part of the warming that might have been associated with greenhouse gas increases. Aerosols can reflect solar radiation or absorb and emit infrared radiation, and are often visible as haze or smog. By reflecting sunlight, they cool the climate. The human-induced increase in atmospheric aerosols since preindustrial times is believed to have reduced the energy absorbed by the planet by about half a Watt per square meter, which would offset about 20% of the greenhouse gas warming effect.

The aerosols produced by humans could also have a significant effect on the numbers or properties of 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. The increased number of aerosols produced by humans could cause the water in clouds to be distributed into more, but smaller, cloud droplets. With their water spread more diffusely, the clouds would reflect more solar radiation. The existence of such 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 also keep in mind 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 entirely. 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 effects 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 so 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. 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.

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. However the period of this warming also coincides with the period when human activities have increased CO2 and other greenhouse gases in the atmosphere. 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 greenhouse 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 is occurring 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 the 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 decade-long time scales is fairly large, and it is not yet easy to separate this variability from changes that have resulted from human activities.

Temperature Anomaly Plot

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 computer models, and the simulations can then be tested against observations of the current and past climates. 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. But while such models capture many of the key features of the present climate, they do have shortcomings.

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 ranging over thousands of miles, are just as important as small-scale processes like 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 standard approach is to represent the globe with a grid 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 much 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 decline 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 extend the past century's increase in greenhouse gases into the next century and see how the climate model responds to this change. Because of the approximations in the models, however, the predicted warming over the next century is quite uncertain, ranging from a modest warming of 2_F(1_C) to a very substantial warming of 8_F(4.5_C). 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 larger increases in evaporation than in precipitation over midlatitude land areas, which 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 many aspects of human life, including public health, agriculture and the way we manage our water and energy resources. 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 indicates 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 the availability of water might change in a particular state or county, where measures to adapt to climate change would need to be taken.

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 processes that determine how much climate change to expect, and continuing 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. An accurate assessment of what those shifts will bring will help us to either mitigate climate change or adapt to its effects.

Given the current level of uncertainty and the complexity of the climate system the future will certainly bring surprises, both of the pleasant and the unpleasant variety. Information about how the climate is changing, the assignment of causes to these changes, and accurate prediction of future climates will be very important for the public and policy makers. Efficient communication of this information to all concerned will be an important part of the process of deciding how to respond to the challenge of our changing climate.


Meltzer, D. J., 1993: Search for the First Americans. Smithsonian Books, Washington, D.C., 176.

Hartmann, D. L., 1994: Global Physical Climatology. Academic Press, San Diego, 411.

Graedel, T. E. and P. J. Crutzen, 1995: Atmosphere, climate, and change. W.H. Freeman, 196pp.

Imbrie, J. and K. P. Imbrie, 1979: Ice Ages: Solving the Mystery. Enslow Publishers, Short Hills, N. J., 224.

Boxed text and wavy text for top channel:

page 1, boxed text:

"We have now entered an era when actions by humanity may have as much

influence on Earth's climate as the natural processes that have driven climate

change in the past. Our future climate will be partly of our own making."

page 2, boxed text:

"Favorable temperatures and abundant water near the surface of Earth

support a rich diversity of life. Patterns of temperature and rainfall have

shifted significantly over time in response to natural forces, and these

changes in climate have had important effects on people and the natural

world we live in. "

page 3, wavy text in top channel:

"Life on Earth responds to the climate and also helps to shape it."

page 14, wavy text in top channel:

"Without the greenhouse effect, Earth would be a frozen planet."

pages 24-25, wavy text in top channel:

"Humanity, long affected by Earth's changing climate, now plays a role in shaping it."