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You can also view a pdf of Chapter 6: Learning from Climates Past
1. CLIMATE IN CONTEXT
"Rain, heavy at times, will begin in late morning and continue into the evening hours as a cold front sweeps across the area...." Ah, the weather forecast—what would we do without it? There is no shortage of conversation about the weather, which, after all, touches our daily lives. For some, the weather is pretty important—especially if their harvest depends on it. For others, it is more tangential. I'm thinking of myself here—most days I just want to know about my trek in and out of New York City, where I work. Will rain or snow make it impossible or just more miserable than usual? And climate? What would a climate forecast be like? "The next decade will bring persistent showers and mild temperatures in January to March, and extensive periods of no rainfall at all throughout the summer months." Hmm... that seems a bit remote from my immediate worry of getting to work.
Weather and Climate
The musings here illustrate the essential difference between weather and climate, the topic of this book. It also illustrates one of the conundrums in reducing carbon dioxide (CO2) emissions from the use of fossil fuels, the main culprit in global warming. Weather refers to conditions in the atmosphere at any one time. The now familiar radar images on television show that local weather systems develop and dissipate rapidly over the course of hours to a day. On a continentwide scale, weather systems form and decay over days to perhaps a week. A persistent weather system, such as a warm spell, may last for a couple weeks, especially in midlatitudes where the tracks of the systems are commonly determined by the position of the polar jet stream, as chapter 2 explains.
Climate, in contrast, can be thought of as the "average weather" for a particular region over some time. Region here can mean the entire globe, as in global warming; it can mean a large land area, say, eastern North America; or it can even mean a small one, as in the "microclimate" of a wine-producing valley near Bordeaux, France. Over all these scales, however, climate implicitly refers to the long term. One conundrum, therefore, is that it is difficult to marshal either the individual or the collective will to make the changes necessary to avoid negative impacts of global warming because they generally do not affect our immediate lives.
Although we have become adept at forecasting weather over hours to days, predictions beyond that become progressively more uncertain with distance into the future. Weather is inherently chaotic. Strictly speaking, the term chaotic means that small differences in initial conditions result in large differences in how a system will eventually develop. In other words, to predict weather accurately, one would have to know the temperature, humidity, barometric pressure, wind velocity, precipitation, and other characteristics of a weather system everywhere across an affected region; the more information at hand, the farther out in time a reliable forecast becomes possible.
Being an average condition, climate is more stable and displays distinctive patterns of change on distinctive timescales. Examples include annual changes such as monsoons, which are shifts in winds that bring seasonal rains to a number of regions in the tropics and subtropics. They also include fluctuations that occur only every several years, the most notable of which is El Nino, referring to the periodic warming of the equatorial eastern Pacific Ocean that commonly infl ences climate across much of the globe.
The Climate System
Climate is a dynamic system resulting from the combined interactions of various parts of Earth with one another and with the Sun. The parts are the atmosphere; the ocean (the hydrosphere); glaciers, terrestrial ice sheets, and sea ice (collectively known as the cryosphere); the living biomass (the biosphere); and even the solid Earth (the lithosphere ) (figure 1.1). Think of it as your body, with all of its parts interacting in a pulsating whole. And like your body, the climate system is not just a set of physicals interactions. It is also a dynamic chemical system, with matter flowing through its various parts.
The atmosphere, being the medium that we live in, is the part of the climate system that affects us most directly. It plays a major role in transporting heat around the planet. Because Earth is a sphere, the Sun's heat is more intense near the equator than near the poles. This uneven distribution generates winds that carry heat from the equator to the poles and from the surface to the upper atmosphere. The atmosphere is not isolated from the ocean, however. The ocean circulates, in part driven by the winds and guided by the positions of continents, and thereby also transports heat toward the poles. Indeed, the ocean holds far more heat than the atmosphere, but it flows much more slowly.
As for the chemical interactions, the most important are the exchanges of carbon among the atmosphere, ocean, and biosphere (here we can also include the dead biomass held mainly in soils). In fact, we can think of each of these spheres as reservoirs where nearly all the carbon on or near Earth's surface is stored. This description leads to the concept of the carbon cycle, referring to the flow of carbon among the various reservoirs. In time periods of months to decades, photosynthesis by plants and decay of organic materials affect the amount of CO2in the atmosphere, but over longer times it is the ocean that exerts the dominant control on atmospheric CO2content because the amount of carbon in the ocean is more than 50 times that in the atmosphere (or in the entire living biomass). If we think of the climate system as something like our body, the atmosphere and ocean are its main organs, and the carbon cycle is the circulation system that connects them and other organs. Because these elements are so central, they serve as the focus of the first several chapters of this book.
Most of the carbon (more than 99.9 percent) on Earth exists not in the ocean, atmosphere, or biosphere (the "surface" reservoirs), but in a deep reservoir in the form of rocks—that is, the lithosphere. The lithosphere is part of the climate system mainly because carbon flows between it and the surface carbon reservoirs, but this flow is far slower than the flow of carbon among the surface reservoirs. Over millions of years, a close balance has apparently persisted between the amounts of carbon flowing from the surface to the rock reservoirs via removal of CO2from the atmosphere and ocean by the formation of carbonate- and other carbon-bearing rocks and the return of CO2to the atmosphere by the breakdown of those rocks at the high temperatures and pressures of the deep Earth. In fact, this long-term balance appears to have acted as a natural, planetary thermostat, maintaining conditions on Earth's surface that are conducive to the evolution and survival of life since nearly the beginning.
The different parts of the climate system also interact through feedbacks, or phenomena that amplify or diminish the forces that act to change climate. An example helps to envision them. As the Arctic warms due to buildup of greenhouse gases, sea ice melts. As sea ice melts, there is less bright ice to reflect solar energy back to space, so more energy is absorbed by the dark ocean. The greater absorption of energy in turn further warms the ocean and overlying atmosphere and causes even more ice to melt. In this way, greenhouse-gas warming is amplifi ed. This feedback in part accounts for why the Arctic is generally more sensitive to global warming than is the rest of the planet. Feedbacks can be complex and operate in unpredictable ways, and they are one reason that projecting future climate is fraught with uncertainty.
The climate system is complicated in other ways, one of which is that the various climate phenomena operate on different timescales (table 1.1). Some of these phenomena and their associated timescales are familiar—for example, the daily variations of warm days and cool nights, and the annual passage of the seasons. Other phenomena occur on longer or irregular intervals, and still others occur at timescales beyond the human experience and are consequently difficult to imagine. Our knowledge of the latter may also be incomplete because the evidence for them is buried (commonly and literally) in the geological record.
Climate Change: Separating Facts from Fears
What we do know from the available records, both geological and observational, is that the climate is changing. Hardly a day goes by without some mention of it in the news. Earth's climate is warming; CO2and other greenhouse gases have been building up in the atmosphere mainly as a consequence of the burning of fossil fuel; and the scientific evidence is now overwhelming that this buildup is causing the warming. These statements are the facts of climate change.
Less certain are how much the climate will warm in response to growing emissions and to what extent the warming will change the world around us. Should the warming be substantial, it may have huge, negative impacts on biodiversity, ecosystems, agriculture, the global economy, and the health of human societies everywhere. These possible results are the fears of climate change. It is important to separate the facts from the fears because although the facts give us insight, the fears reflect uncertainty. We will need knowledge and ingenuity to respond to global warming. To gain them we must start with the facts.
OBSERVATIONS OF CLIMATE CHANGE: THE FACTS
In addition to CO2, the greenhouse gases include methane (CH2), ozone (O3), and water vapor (H2O). These gases reside mostly in the troposphere, the lower 10 to 15 kilometers (30,000 to 50,000 feet) of the atmosphere where the weather occurs. Here the greenhouse gases absorb heat radiated from Earth's surface and thus act as a giant, insulating blanket.
Greenhouse gases have been building up since the beginning of the industrial age, but only since 1958 has the CO2content of the atmosphere been measured directly, beginning first on the top of Hawaii's Mauna Loa volcano (figure 1.2). The remarkable Mauna Loa record shows two interesting features. First, CO2concentrations exhibit regular annual fluctuations, reflecting the growth in the springtime and the death in the fall of Northern Hemisphere plants. Second, CO2has been climbing on a steady, unbroken path over the years. In 1958, the average CO2content of the atmosphere was 315 parts per million (ppm) by volume (that is, 315 ppm = 0.0315 percent); by 2008, it had reached about 385 ppm and is rising at a rate of about 2 ppm per year. Both that rate of increase and that amount of CO2now in the atmosphere are greater than at any time in the past 800,000 years, the time over which a continuous record of atmospheric CO2contents exists. Furthermore, a number of observations make quite clear that the CO2is originating mainly from the burning of fossil fuels.
At the same time, global mean surface air temperature has been rising, too (figure 1.3). The warming began around 1910 and has proceeded in two distinct intervals, the first from 1910 to 1940, and the second beginning in the late 1970s and continuing today. In the intervening interval, global temperature changed little, possibly because of an increase in the amount of pollutants being injected into the atmosphere then. Global mean land-surface air temperature has risen about 1°C (1.8°F) in 100 years, and over the past three decades the rate of increase has accelerated to 0.27°C (0.49°F) per decade.
What is causing the warming? The evidence is overwhelming that it is a result of the rising levels of greenhouse gases in the atmosphere. Several lines of evidence lead to this conclusion. First, there is the basic physics—greenhouse gases absorb the radiant heat, or infrared (IR) energy, being emitted from Earth's surface. As was recognized more than a century ago, the climate should warm because now more of that energy is being trapped in the atmosphere rather than escaping to space.
Second, there are no other known natural forces external to the climate system, such as changes in the amount of solar irradiance (that is, the amount of sunlight) reaching Earth, that might account for any but a small fraction of the warming. In the particular case of solar irradiance, there is indirect evidence that irradiance changes with time and that it may account for some of the cool and warm spells in the past, but except for the 11-year sunspot cycle, which represents only a minuscule fluctuation in irradiance, there have been no detectable changes in solar output since the advent of precise measurements by satellite in 1978.
Internal variations in the climate system—that is, fluctuations occurring on timescales of decades and less and resulting mainly from the system's dynamic nature—may conceivably account for warming, at least regionally. These variations include phenomena such as El Nino--Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO), both of which are ocean--atmosphere interactions resulting in large-scale redistributions of heat (discussed more fully in chapters 2 and 3). However, a variety of observations argue against internal variations as being responsible for the warming.
First, warming has been occurring more or less everywhere—it is a global not regional phenomenon, as would be expected if the warming were due to internal variability. Second, the lower atmosphere below about 10 kilometers (33,000 feet) has also been warming, while at the same time parts of the upper atmosphere have been cooling, as expected from the basic theory of greenhouse-gas warming. Third, both the annual average maximum (daytime) temperature and the annual average minimum (nighttime) temperature have increased, but the nighttime temperature has increased more than the daytime temperature. This observation is consistent with what one would expect from increased insulation by greenhouse gases, as explained chapter 5. Fourth, the oceans have been warming by far more than can be accounted for by natural, internal variations in the climate system.
Those are the essential facts in a nutshell: Earth's climate is warming; CO2and other greenhouse gases are increasing in the atmosphere; and the scientific evidence overwhelmingly points to the greenhouse-gas buildup in the atmosphere as the cause of the warming.
POTENTIAL CONSEQUENCES OF CLIMATE CHANGE: THE FEARS
The fears concern how much the planet will warm and what the consequences might be, but there is much uncertainty about this future. Three possible consequences that may severely impact society illustrate both the fears and their associated uncertainties.
One concern associated with global warming is the possibility of significant sea-level rise. Sea level is currently rising at a rate of 2.6 ± 0.04 millimeters (0.1 inch) per year, equivalent to 26 centimeters (10 inches) per century. The 2007 report by the United Nations Intergovernmental Panel on Climate Change (IPCC), a massive document assessing the current scientific understanding of climate change, estimated that by the year 2100, sea level may be between 20 and 60 centimeters (8 and 24 inches) higher than it is today. The main contributions to this rising level are the melting of glaciers and thermal expansion of the oceans (that is, warm water is less dense than cold water and therefore occupies more space). Not included in the IPCC estimate is the loss of ice from the Greenland and West Antarctic ice caps (the relevant studies had not yet been published by the time the IPCC report was written). The loss of ice from these ice caps should cause sea level to rise even more, but because we do not know by how much or how quickly the ice will disappear, we do not know by how much or how quickly sea level will rise.
The stakes, nonetheless, are high. Worldwide, two-thirds of cities with populations of more than 5 million people are vulnerable to the effects of rising sea level (the most serious of which are flooding during storms and coastal erosion). In China, for instance, a sea-level rise of just 0.5 meter (20 inches) might affect most regions that are within 10 meters (33 feet) of present sea level, currently home to 144 million people, or 11 percent of China's population.
A second concern is that warming will increase the incidence of harsh and extended droughts, which may significantly affect agriculture and the water supplies of millions of people. A number of regions are particularly vulnerable to drought, including southwestern North America, Southeast Asia, and the Sahel of Africa (the southern borderland of the Sahara Desert). About 1,000 years ago, southwestern North America, for example, experienced a number of "megadroughts," each lasting several decades over a 300-year interval of relative warmth. Such megadroughts have not been seen since. The megadroughts then and the multiyear droughts that have plagued these areas in the recent past appear to be related to conditions in the tropical oceans, but exactly how those conditions influence rainfall patterns is not completely understood. The theory is that warming increases the probability of the occurrence of megadroughts; the fear is that such droughts will occur and have severe economic consequences.
A third concern is that warming will also increase the incidence of extreme weather events. Indeed, these events have been appearing with more frequency over the past half-century. Two recent examples are the heat wave in Europe in 2003 and Hurricane Katrina, which destroyed much of New Orleans in September 2005. Eighteen hundred lives were lost to Katrina, and the storm cost more than $100 billion. In Europe, the summer of 2003 was the hottest in more than 600 years, rainfall was 50 percent below normal, and 22,000 to 45,000 people died in a two-week period as a consequence of the heat.
Extreme events are by definition unusual and may occur in the absence of greenhouse-gas warming, so whether global warming has anything to do with hurricanes such as Katrina is a matter of current debate. Nonetheless, global warming makes the improbable more probable; the European heat wave and Hurricane Katrina illustrate the devastation that such extreme events can bring.
THINKING ABOUT THE FUTURE IN THE FACE OF UNCERTAINTY
We do not know by how much or how rapidly sea level will rise, and we do not know if or when megadroughts and severe weather events will strike. We cannot even state the odds that such events will occur. Yes, we are for the most part, ignorant. But that is exactly the point. We are smart enough to know that we are putting ourselves at risk, but we cannot gauge the risks.
Most of us buy insurance to mitigate risk, such as the personal financial risk associated with one's house burning down. In the case of climate change, we can buy insurance, in a sense, by trying to minimize the change. But there is a big difference in the latter case: unlikely as it might be, if climate change brings human society to its knees, we are out of luck because we will not be able to buy a new planet. In other words, we can seek only to reduce the risk. This is an important point: efforts to limit climate change and to mitigate its impacts are exercises in risk management, and understanding the problem in that light helps to guide our response.
Two characteristics of the climate system also shape our response. First, the climate system possesses inertia: it takes time for the system to reach a new balance in response to the forces that have acted to change it. In other words, even if greenhouse emissions were to be immediately capped at today's levels, warming would continue for several decades. By one estimate, there is currently more than 0.6°C (1.1°F) worth of warming already locked in.
Second, climate can reach tipping points, or large, abrupt shifts in climate in response to factors that gradually cause climate to change, such as the buildup of greenhouse gases in the atmosphere. The geological record is replete with instances of abrupt and dramatic shifts in climate.
A prime example is the Younger Dryas (named after a tundra wildflower), an interval of extreme cold that abruptly descended on the northern Atlantic Ocean and neighboring regions beginning about 12,900 years ago and that ended just as abruptly 1,300 years later. In Greenland, the end of the Younger Dryas was marked by an increase in temperature of 7°C (13°F) and a doubling of the rate of snow accumulation in just a few decades and perhaps only a few years. The abrupt beginning and end of the Younger Dryas did not come about because of any sudden, external change. Rather, the ultimate cause was the gradual warming following the most recent glacial period. This is the notion of a tipping point—the climate system or any complex system crosses a threshold to a new state at a rate that is determined by the system's internal dynamics and that is faster than the cause. The existence of tipping points and our inability to predict them form together one element of the risk we are taking. We are forcing climate to change, and we do not know how it will react.
On a related note, having described the geological evidence for abrupt, large, and rapid swings in climate, I have been surprised by many students' question why, considering that climate has changed so dramatically in the past in response to natural forces, we should concern ourselves with the human-induced changes. The answers are simple. First, human-induced changes may very well be substantial. Second, complex societies were not around to experience the huge shifts of the past. The climate of the past 11,600 years, known to geologists as the Holocene, has been stable by the standards of the past million years, and complex societies have been around for only about the past 6,000 of those years. The climate system, in short, has within it the possibility of bringing about dramatic changes that modern societies have not experienced but that may seriously challenge their abilities to adapt.
So, what does this book add to the discussion? As noted, chapters 1 through 4 deal with the climate system. In chapter 5, I then look at the systematic way that the scientific community thinks about climate change, the greenhouse effect, the various factors causing climate to change, and feedbacks and other features of the climate system. Chapter 6 explores the fascinating story of the climate of the past, or paleoclimate, which is recorded in tree rings, cave deposits, corals, lake sediments, deep-sea sediments, and even ice. Paleoclimate not only is fascinating, but gives us essential insight into how the climate system works today and how it will change in response to greenhouse-gas emissions. The story focuses on the past 800,000 years, but we shall also visit more distant times to seek additional insight.
Chapter 7 describes the rapid increase in global temperature over the past century, some of the changes that we are beginning to see as a consequence of that warming, and some changes that we can expect based on our understanding of how the climate system works. The Arctic is especially sensitive to warming and at the same time has an outsized influence on global climate. Accordingly, chapter 8 investigates the melting of the Arctic ice cap and permafrost, or perennially frozen ground, and the resulting changes in the tundra. The sensitivity to warming raises another important concern discussed in chapter 8: the fate of the Greenland and West Antarctic ice caps and the extent to which they will contribute to rising sea level.
Chapter 9 seeks to explain climate models. The models offer a way to understand what the future may bring based on fundamental physical laws and an understanding of how the climate system works. The future fundamentally depends, however, on how greenhouse-gas emissions change. Because greenhouse-gas emissions are largely a consequence of energy production, we cannot talk about climate change without talking about energy. Chapter 10 deals with the central question of our energy future—how to satisfy the world's insatiable appetite for electricity while keeping emissions in check.
That is the story. It is complex: it suggests that we face a difficult future, but it also implies that we can avoid the dire consequences of climate change by intelligent action and innovation.
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