Global Warming and the Changing Land


My subject today is global warming. Although these two words are used almost daily in the media, most people don’t really know what it is. So I’d like to spend a few minutes defining what we mean by “global warming.”

For the last several years, I've been part of an international organization called the Intergovernmental Panel on Climate Change (IPCC), which was established in 1988 by the United Nations Environmental Programme (UNEP) and the World Meteorological Organization (WMO). The panel was formed at a time when scientists first began to worry that there was some connection between human activities and changes in our climate.

Let me describe the panel a bit more. The IPCC consists of three working groups. Working Group I assesses the scientific aspects of the climate system and climate change, so this group consists of about 200 climate scientists who meet on their own. Working Group II addresses the impact of climate change—that was my group, and there are about 300 of us. Another 200 social scientists make up Working Group III, which assesses options for mitigating climate change. The recent report actually took three years of work, and during that period we were meeting every two or three months, either as small committees or the full working group.

To date, the IPCC has issued three full assessment reports. The most recent of these, the Third Assessment Report, was released in the summer of 2001. The main conclusion of this report—which involved the work of hundreds of experts in meteorology, the cryosphere (ice and snow), glaciology, biology, agriculture, and economics—was twofold: (1) human activities have caused a large increase in what are called greenhouse gases, which I’ll explain in a moment, and (2) this increase in greenhouse gases is most likely responsible for the observed rises in global temperature over the past 50 years.

Now we come to global warming. In general, it means that the average temperature of the globe is warming from one year to the next. But it also means much more than this.

In concrete terms, global warming means that the weather itself is changing and weather patterns are changing. We’re seeing not only a general increase in temperature, but also changes in precipitation patterns over the last 50 to 100 years. For example, rainfall is occurring in fewer but more extreme events. In other words, floods have increased over the past 100 years.

So what is the greenhouse effect that is supposedly causing these changes? Greenhouse gases have actually been around for a long, long time. These gases, such as carbon dioxide, methane, and various nitrogen compounds, occur naturally as by-products of certain geological and biological processes. When they rise into the atmosphere, they build up and form a kind of a blanket.

When solar radiation passes through the atmosphere and strikes the earth’s surface, some of this radiation is absorbed, but much of it is converted into heat. As this radiated energy rises and escapes, it is trapped by a “blanket” of greenhouse gases that absorbs and reflects the heat back toward earth. The effect of this trapped energy is to warm the earth. Without greenhouse gases, the earth would be about 60 degrees Fahrenheit colder than it currently is, and life as we know it would not exist.

The greenhouse effect is a perfectly natural phenomenon that has been taking place for millions of years. The problem has to do with what’s been happening in the last 140 years or so.

These same molecules that make up naturally occurring greenhouse gases—carbon dioxide, methane, nitrous oxide—are likewise produced by the burning of fossil fuels and other human activities. So human beings have been increasing the levels of greenhouse gases in the atmosphere. More of the sun’s heat is retained, warming the surface of the earth and the atmosphere nearest to the earth. So while the greenhouse effect is natural, human beings have strengthened this process in the period between 1860 and the present.

Let’s take a look at carbon dioxide in particular, which is thought to be responsible for most of the warming observed over the past century. Atmospheric concentrations of carbon dioxide have steadily increased from about 290 parts per million in 1860, the beginning of the Industrial Revolution, to a little above 360 parts per million at the current time (Figure 1). That’s about a 30 percent increase in carbon dioxide concentrations.

{insert Figure 1 here}

What is this increase due to? Essentially the Industrial Revolution. Around that time, we began to burn enormous amounts of fossil fuels that had been locked below the surface since the Cretaceous period, when the dinosaurs roamed the earth. Burning these fuels released carbon dioxide into the atmosphere, and with the spread of industrialization around the globe, concentrations of carbon dioxide have steadily increased.

Carbon dioxide lasts about a hundred years—it’s a very stable molecule. Therefore, once it reaches the atmosphere, it hangs around for quite a long time before it breaks down. The burning of fossil fuels—to supply power for our homes, automobiles, and industrial processes—is responsible for about 75 percent of carbon dioxide emissions caused by human activity. The remaining 25 percent is caused by other sorts of human activities, such as deforestation.

When a forest is cut down, trees are removed from the ecosystem and replaced by bare dirt or agricultural crops, which require much less carbon dioxide for photosynthesis than trees do. Thus, deforestation results in less total biomass for storage, so the extra carbon dioxide that is being produced remains in the atmosphere. Deforestation is often accompanied by clearing, especially in many Third World countries. Once the large logs are taken off, the residue is cleared by burning, which releases yet more carbon dioxide.

And the rate of deforestation is increasing. In Brazil, for example, deforestation in was largely confined to the coastal and southern regions in the 1970s; just ten years later, large areas of forest were being cut in the Amazon Basin and this trend has been continuing.

Now, what about temperature? Over the last 140 years, global average temperatures have fluctuated between warmer periods and cooler periods (Figure 2). But if we consider the general trend, it’s clear that the temperature has been steadily climbing. Global average surface temperature has risen by about one degree Fahrenheit over the last century. In most regions of the United States, temperatures have risen by between 1.8 degrees and 5.4 degrees Fahrenheit. In the past 30 or 40 years, this temperature increase matches the increase in carbon dioxide concentration very closely.

{insert Figure 2 here; scale on right is in degrees C; don’t know if label is on chart}

To study temperature trends, scientists analyze data from climate stations around the world. Records are rather sporadic for the years before 1910, although various kinds of historical records can shed light on weather and climate. But since 1920 good meteorological data has become increasingly available—at least from Europe, the United States, and Asia. If we look at the details of this temperature increase, what you see is that 1998 was the hottest year on record. The data show that the 13 warmest years on record have all occurred in the period since 1980, with 1998 the warmest. Indeed, temperatures have increased phenomenally over the last century.

But a lot of people are saying, well, temperature goes up and down; this is just normal variation in climate. So let’s see what we can learn about natural variability by looking at temperature trends that go back 1,000 years.

Of course, there were no meteorological stations then, but scientists are able to estimate temperatures further back in time by using proxies such as tree rings, coral reef growth, which is much like a tree ring, and ice and ocean sediment cores. The use of ice cores involves drilling deep into glaciers where the ice has trapped little bubbles of gas and oxygen isotopes, which can indicate not only exact atmospheric concentrations of carbon dioxide at that time but temperatures as well.

It’s certainly true that the combined data show huge fluctuations in temperature from year to year, warming periods followed by cooling periods, with the pattern repeated again and again until the last 140 years (Figure 3). But these ups and downs are fairly regular and in the same plane until the start of the Industrial Revolution, when temperatures increase sharply and climb to levels much hotter than we’ve seen before.

{insert Figure 3 about here}

If we go back even further, say 160,000 years, the ice core samples again point to major fluctuations in temperature as the earth experienced glacial and interglacial cycles (Figure 4). As the figure shows, large increases is carbon dioxide concentrations have been followed by gradual declines and then subsequent large increases. For thousands of years, this cycle—what we call natural variability—was due to biological and geological processes. However, current carbon dioxide levels are well outside the bounds of natural variability. They are far higher than those of any peak period in the last 160,000 years, and the rate of change in carbon dioxide concentration is also unprecedented. But even more striking is the tight correlation between carbon dioxide concentrations and global temperatures. In the same time period, temperature has risen and fallen but always closely tracking those carbon dioxide levels.

{insert Figure 4 about here}

This is what first gave scientists the idea that temperature was indeed very tightly linked to carbon dioxide concentrations. And since then, the numerous experiments of atmosphere scientists have shown that there is indeed a mechanistic link between carbon dioxide and temperature and that carbon dioxide does effectively act as a blanket.

Now, ice core samples can actually go back 420,000 years. While they show the same kind of cycling, with both carbon dioxide and temperature rising and falling in tandem, at no point in that long period are carbon dioxide levels as high as they are now—about 30 percent higher than they have been in the last half million years or so.

If we go back even further on the timeline, we find small peaks of warming that are hotter than modern temperatures, but we must go back millions of years—to the Cretaceous period and the time of the dinosaurs—before we see evidence of temperatures that were quite a bit hotter than what we are experiencing today. The Cretaceous period was associated with what we would now consider very tropical conditions—very high temperatures, extremely high humidity, lots of rain, and a totally different set of plants and animals. In other words, the world’s climate was fundamentally different from the climate of today.

The last 10,000 years is the period in which human civilization arose and in which we have really flourished as a species. Prior to this, human beings weren’t really very numerous.

In the Cretaceous, when the dinosaurs were king, life was very different. The only mammals that existed were rodent-like creatures. They were our closest relatives. Large mammals like horses, elephants, and lions did not evolve until much later. And the land, of course, was very different. The area that is now Texas was mostly under water, and those ancient oceans deposited all the lovely fossils that are used in our beautiful limestone homes today.

Climate change was actually one of the reasons for the disappearance of the dinosaurs. We hear a lot about a meteor striking the earth as a cause, and that very likely did happen. A large meteor would have spewed out a huge cloud of dust, blocking the sunlight, cooling the earth down, and possibly making it drier.

At the same time, the continents were drifting away from the equator and towards the poles, a move into cooler latitudes. This very gradual climate change, a natural process that was possibly helped along by a meteor crash, is what caused quite a few species of dinosaurs to gradually disappear. They simply were not adapted to a cooler, drier climate.

All modern species of plants and animals are equally adapted to their local climates. This is why you don’t expect to see a Texas armadillo living with an Arctic penguin. The very idea seems pretty silly to us because species are restricted to fairly small regions of the globe, and biogeographers and ecologists have shown that these restrictions are largely due to climate. We call it the climate envelope—the climate that will sustain a particular species.

Now, humans are one of the very few species that occur throughout the globe. In fact, we may be the only one except for certain microorganisms. But we are also adapted to the climate in which we evolved in. In the far north, the tribal peoples have evolved with thick bodies and short limbs to retain the heat. In much hotter climates, the people have darker skin to protect against solar radiation, and they are also generally taller, with longer limbs, so that they can lose heat. In Europe the people are in between those two extremes.

So let’s look at what’s been happening to species over the past century with these increases in temperature. Since species are restricted to a fairly small area by their adaptation to climate, then one result that you would expect to find with climate change is the movement of species from one geographical area to another—in other words, dying out in some parts of their range and expanding in other parts. And that is indeed what we are finding.

Let me start with an example from my own work with butterflies—Edith’s Checkerspot (Euphydryas editha), which occurs in the western United States. This species is a very good candidate for climate-related studies because it has a fairly large range. It extends all the way from Mexico to Canada, encompassing many climate zones, and it is also a species that biologists have been studying for 30 years.

Furthermore, we know from hundreds of small-scale studies that climate really does drive its populations. As part of its natural biology, populations become extinct all the time because of various extreme weather events.

Historically Edith’s Checkerspot has lived in many different habitats, from the coastal meadows of California to the highest mountains in California, Oregon, Washington, and Canada, where its host plants grow. Over the last century, a large number of population extinctions have been observed. Since populations become extinct as part of the natural biology of that species, this alone doesn’t necessarily mean much until we look at the pattern of those extinctions (Figure 5).

{insert Figure 5 about here}

In the southern or lower range, in Mexico and southern California, about 75 percent of the populations have become extinct. In the middle of the range, about 40 percent have become extinct, and in Canada, only 20 percent. It should be emphasized that all the extinctions occurred in natural habitat, so human destruction of habitat was not a factor. This pattern in rate of extinctions has effectively shifted the range of the species northward by about 55 miles on average.

We see the same kind of shift with altitude. In the last hundred years, the populations at the low elevations have become extinct at a fairly high rate—about 40 percent—and in areas where the habitat is intact. In contrast, only 15 percent have become extinct in the high elevations. This means that the butterflies have moved nearly 400 feet up the mountain as well. Interestingly, this shift in range northward and upward matches the shift in temperature that has occurred over the same area. By tracking the temperature isocline—that is, a line on the map connecting areas where mean yearly temperature is the same—scientists have discovered that the mean temperature for, say, June has moved northward by about 63 miles.

Higher temperatures are a problem for the butterflies in the southern area and at lower elevations because the host plants dry up before the caterpillars fully develop. The overall effect of increasing temperatures, then, is to favor the populations at the northern and upper range limits, to the detriment of populations at the southern and lower range limits.

Another species that is undergoing a range shift is the anopheles mosquito, which carries malaria. The anopheles mosquito occurs in Texas, but we’re at its very northern limit, so we really don’t have a problem with malaria in Texas right now. But scientists predict that this mosquito is not only going to spread farther north but also become more abundant in areas where it currently occurs. Warmer conditions encourage the growth of the malarial parasite and the transmission of the disease, so there is likely to be a much higher incidence of malaria in the wild than it is today in Texas. Now, this isn't necessarily a problem for Texans because we have very good mosquito control programs and very good health care. The IPCC working group concerned with health issues—and many of these people came from the Center for Disease Control in Atlanta—concluded that, although malaria in the wild may become more prevalent, the risk of infection will depend on how we manage health care, sanitation, and education initiatives. And the same is true for most developed industrial countries.

Unfortunately, Third World countries currently lack good health care and effective mosquito control programs. So the spread of the mosquito into parts of northern Africa and the likelihood of an increase in malarial transmission is of great concern.

As I mentioned earlier, long-term changes in physical and biological systems as a result of regional increases in temperature have been widely documented (Figure 6). These studies show changes in cryospheric systems, in water flow, in glacial extent, in sea ice, and in the range shifts of plants and animals all over the world—much like that of Edith’s Checkerspot butterfly. These published studies have covered 400 plant and animal species or systems and span the globe reasonably well.

{insert Figure 6 about here}

Of those 400 species and systems, 91 percent of the changes that have been documented are what you would expect from climate change.

For example, the red fox has shifted its range northward by about 150 miles, threatening the Arctic fox. Historically the red fox has occupied much of North America, including Canada, the northern extent of its range. It is not as well adapted to cold conditions as the Arctic fox, which is generally confined to Arctic regions. Apparently the two species cannot coexist. The Arctic fox is much smaller and quite passive whereas the red fox is much more aggressive. As the red fox has expanded farther north over the past century, the Arctic fox has been forced to retreat to a narrow region along the Arctic Ocean.

The changes are not confined to the northern latitudes. A study of bird communities in Costa Rica’s Monte Verde National Preserve revealed similar range shifts. The preserve extends from lowland valleys up to cloud forest in the mountains. Over the past 30 years, the keel-billed Toucan and other tropical birds that live down in the valleys have started moving up the mountain slopes and coming into contact with the birds of the cloud forests. One such bird is the tiny, shy, brilliant green quetzal, which is highly revered in Mayan legend. The males have a tail that can be a long as three feet, which prevents the birds from defending themselves very well. As the toucan invades the mountain habitat, it competes with the quetzal for nesting sites. And since it’s much bigger and much more aggressive, it’s going to win every time. Many of the cloud-forest species have declined or already become extinct with the destruction of habitat, and range shifts increases the pressure on populations even more.

Increases in average temperatures produce changes in physical systems as well. Take glaciers, for example. Glaciers have been photographed and measured for 50 to 100 years, and the historical record demonstrates that snow and ice cover in glacial areas has dramatically decreased around the world. In general, the glaciers of the European Alps have lost 30 to 40 percent of their surface area and about 50 percent of their volume. In Africa, the glaciers on Mount Kenya and Mount Kilimanjaro have lost 60 percent of their area in the last century. In Glacier National Park in Montana, more than 70 percent of the glaciers have already melted, and they will probably disappear by 2030 if warming continues.

The shrinking of glaciers has a significant socioeconomic impact. Many city water systems, for example, are supplied by melting winter snow. In addition, the melting of mountain glaciers contributes to rising sea levels. It is estimated that about 30 percent of the projected change in sea level by 2100 will come from melting glaciers.

Another observed change that is believed to be a response to warming has to do with disruptions in the timing of events. Everyone knows that spring is associated with warmer temperatures, and that’s when the flowers come out. The flowers germinate and bloom, the trees start leafing out, and the butterflies appear.

For several hundred years, people have informally documented when these events occur. To take one example, the Marsham family in England has been recording this information in diaries since the 1700s. For the last 50 years, scientists have kept much more rigorous records of these kinds of observations. We’ve discovered that, over the past century, the events associated with fall and spring are changing by as much as one week to three weeks.

Spring is coming earlier in a very real sense. Migratory birds and butterflies are arriving to their nesting or breeding grounds earlier. Frogs, insects, and birds are breeding up to three weeks earlier. Many plants in Europe and North America are flowering between two and four weeks earlier in the spring. And if you look at the timing of fall events, such as the turning of leaves and the dropping of leaves from the trees, they are occurring later by about one or two weeks. With spring coming earlier and fall later, we’re experiencing an absolute increase in the growing season.

This isn’t necessarily a bad thing, of course, especially in the northern latitudes. An increase in the growing season may allow agriculture in regions where it has always been limited before.

Changes in plant and animal communities are also taking place. Laura was talking about grasslands and native prairies, and in the last 30 to 40 years these areas have undergone a noticeable increase in trees and shrubs. Woody plants have become more abundant, with woodlands starting to encroach into some of the prairies. Again, humans have not caused the change by planting trees. It’s simply been a fairly natural change.

There are some indications that the loss of prairieland is due not only to heavy grazing but also to climate change—that is, changes in precipitation, rainfall, and temperature—and possibly to high levels of carbon dioxide. This last possibility certainly deserves more study. Experiments in environments of artificially increased carbon dioxide indicate that this gas favors woody plants to the detriment of the native remnant prairies.

I’ve been talking about changes that have occurred in the past century, such as higher temperatures or changes in rainfall patterns that are resulting in drought in some areas and more flooding in others.

But what about the future?

For glimpses into the future, we have to rely on computer models. People have heard a lot about these models and the claim that, depending on the model, you can get whatever result you want. But that isn’t quite true, especially for models that work with global averages. The bigger scale, the better the models work.

Let’s look at how several different computer models predict average temperature increases for the United States over the next century (Figure 7). Each line on the chart represents a different model: the UK model, the Canadian model, and so on. We have our own modeling group in Colorado, the National Center for Atmospheric Research. These models are known as the “Hadley model” and “Canadian model.”

{insert Figure 7 about here}

The chart shows annual average changes in surface air temperature that have been observed for the United States between 1860 and 2000, along with projections for the next 100 years. Modeling groups run their analyses by starting with a common point in the historical record--say the year 1910. They then run their model through time, using particular assumptions about the mechanisms underlying the climate system. That is, each model will input a specific effect of each of several known climate-driving factors: such as carbon dioxide, methane, solar orbital changes, or volcanic activity. Each model uses slightly different mixes of these factors as well as slightly different numbers as to how concentrations of these gases will change in the next 100 years. This is one reason they give different results. Then the output of the model from 1910 to present day is compared with the actual climate record. If the model predictions fit the observed climate, then the model is considered to be a good one. Notice that all of the models are pretty good fits to the actual climate records. The differences are mainly in predicted climate from the present day forward 100 years. These differences are partly due to different assumptions as to how much carbon-dioxide emissions will increase (or not) over the next 100 years.

The negative numbers indicate temperatures that are cooler than the average for the period from 1960 to 1990; the positive numbers indicate warmer temperatures. Today, temperatures are a bit warmer than the average, and the past ten years have been warmer than that.

As you see, there is a lot of agreement among the models, and the predictions are fairly consistent for the next 50 years. Then they start to diverge quite a lot. This means that the models are not as good in predicting temperature increases into the second half of the century as they are in the first half.

Now, you often hear in the media that scientists are uncertain about the models, and I want to explain what that means.

The uncertainty refers to the divergence in these different models. The models all predict warming. The uncertainty is whether this warming is going to be as little as two degrees Fahrenheit or as much as eight degrees Fahrenheit. The bottom line—and there is strong consensus on this—is that the United States will see warmer temperatures over the next 100 years.

Let’s talk about Texas and which areas in the state are the most vulnerable to climate change.

People who live in Texas are accustomed to variable weather. Texas experiences fairly extreme temperatures—extreme freezes, extreme heat. Native plants and animals are adapted to this and go dormant during mid-summer and mid-winter, more or less avoiding these extreme temperature conditions.

The effects of droughts and floods are more difficult to adapt to. Changes in precipitation may have a much greater effect on our ecosystems than temperature.

Unfortunately, the models for precipitation change in Texas vary widely. Some say it’s going to be drier, some say wetter. It’s very difficult to predict the future climate of Texas.

But we know for certain that the sea level is going to continue to rise, and predictions range from 4 inches to as much as 35 inches. Because Texas has such a long coastline and so much development along the Gulf, many regions are vulnerable.

Galveston is a case in point. The sea level at Galveston has risen about 25 inches over the past century, due to a combination of the rising level of the ocean and the sinking of the land as groundwater is pumped out to provide drinking water. Furthermore, it’s a very low island, even relative to other barrier islands. Galveston has actually become a hotspot for research scientists because of all these converging factors, and it’s likely to experience some serious problems in the next 20 to 50 years. Similarly, other barrier islands—such as Padre Island, with its wonderful National Seashore recreation area—is going to be highly vulnerable to sea level rises.

Other areas of concern are the tidal flats and salt marshes, like the Aransas Pass National Wildlife Refuge, which is breeding habitat for the whooping crane. The salt marshes also provide a home for oysters and clams as well as the nursery grounds for young shrimp, crab, and fish. So these marshlands not only play an essential role in natural systems but also have economic benefit for commercial fisheries.

Now, you may very well say, if animals and plants change their range and plants change their timing, why won’t the salt marshes just recede? This is a perfectly reasonable expectation, but there are two reasons why the marsh ecosystem can’t migrate inland.

The first is the rate of change. Things are happening very fast, and a single heavy storm can eliminate a lot of habitat for good. The second reason is the extensive development of the surrounding area. There are towns, there are buildings, there are people, not just vacant land where animals and plants can move around. The landscape is very much dominated by humans, and that restricts many of these movements.

Another thing that we know for certain about the future climate of Texas relates to something called the heat index, which is a combination of temperature and humidity that measures the effects on human comfort. For example, in drier air we can cope with the same temperature that would make us very uncomfortable in high humidity.

There is fairly high agreement among the computer models that Texas will experience an increase in the heat index over the next 100 years. This means we will see not only higher temperatures but also higher levels of humidity.

So what can we do about it? Well, I’d argue that the Hill Country will still be the best place in Texas to live—with our cold spring-fed rivers and creeks.

In closing, I’d very much like to thank the Environmental Sciences Institute at the University of Texas for helping me put this fancy presentation together. The extremely helpful graphs used in the slides came from the Union of Concerned Scientists, the U.N. Intergovernmental Panel on Climate Change, the U.S. Environmental Protection Agency, the U.S. Global Change Research Program, the Centers for Disease Control, and several special people. We thank you very much.


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Figure 1. Increases in CO2 concentrations, 1860\-2000. Source: Office of Science and Technology Policy.

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Figure 2. Increases in CO2 concentrations and global average temperatures, 1860\-2000. Source: Office of Science and Technology Policy. 

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Figure 3. Temperature data for the Northern Hemisphere, 1000\-2000. Source: Intergovernmental Panel on Climate Change, Third Assessment Report (2001).

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Figure 4. Correlation between CO2 concentrations and temperature over the past 160,000 years. Source: Office of Science and Technology Policy; Union of Concerned Scientists (talking points).

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Figure 5. Patterns of population extinctions of Edith’s Checkerspot butterfly, 1860\-1996.

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Figure 6. Locations of observed changes in physical and ecological systems. Source: Intergovernmental Panel on Climate Change, Third Assessment Report (2001).

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Figure 7. Projected temperature increases for the United States based on various models, 2000\-2100. Source: U.S. Global Change Research Program.