Constructed Climates: A Primer on Urban Environmentsby William G. Wilson
As our world becomes increasingly urbanized, an understanding of the context, mechanisms, and consequences of city and suburban environments becomes more critical. Without a sense of what open spaces such as parks and gardens contribute, it’s difficult to argue for their creation and maintenance: in the face of schools needing resources, roads and sewers
As our world becomes increasingly urbanized, an understanding of the context, mechanisms, and consequences of city and suburban environments becomes more critical. Without a sense of what open spaces such as parks and gardens contribute, it’s difficult to argue for their creation and maintenance: in the face of schools needing resources, roads and sewers needing maintenance, and people suffering at the hands of others, why should cities and counties spend scarce dollars planting trees and preserving parks?
In Constructed Climates, ecologist William G. Wilson demonstrates the value of urban green. Focusing specifically on the role of vegetation and trees, Wilson shows the costs and benefits reaped from urban open spaces, from cooler temperatures to better quality ground water—and why it all matters. While Constructed Climates is a work of science, it does not ignore the social component. Wilson looks at low-income areas that have poor vegetation, and shows how enhancing these areas through the planting of community gardens and trees can alleviate social ills. This book will be essential reading for environmentalists and anyone making decisions for the nature and well-being of our cities and citizens.
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Constructed ClimatesA PRIMER ON URBAN ENVIRONMENTS
By William G. Wilson
THE UNIVERSITY OF CHICAGO PRESSCopyright © 2011 William G. Wilson
All right reserved.
Chapter OneCities and Nature
Humans altered the biosphere, greatly expanded their population, and now feel the effects of those alterations. A hunter–gatherer lifestyle gave way to an agrarian one, and an agrarian lifestyle gave way to an urban lifestyle. Urban dwellers exist more detached from the biosphere and from the environment that shapes and interacts with it. In this book I review the ecological, environmental, and sociological features of urban life and a world increasingly changed by its human occupants.
The human population exploded in recent centuries, reaching numbers 60 times greater than the population 2,000 years ago and sixfold more than just 150 years ago, bringing with it serious environmental challenges. In this chapter I put human population densities into the context of other creatures and explain several important ecological concepts that show the stark immensity of our present population. I look at people's use of land and water, and how human densities and resources connect to important general ecological principles. Reasonable estimates show that our global population exceeds "natural" levels by up to 400-fold. If we were just another species, then, given our body size there ought to be about one person per square kilometer. However, suburban and urban areas of the United States have densities of 1,000 to 10,000 people per square kilometer.
All these people, mostly concentrated into cities, put tremendous stresses on our natural resources. Water and fertilizers represent two fundamentally important aspects of sustaining a large human population because both factors relate to important ecological features, including evapotranspiration and net primary productivity. These two features drive many aspects of nature, an important one being biodiversity, meaning the number or richness of species within broad groupings of organisms. For humans, evapotranspiration is important to demands of growing food, but I also demonstrate our immense dependence on producing nitrogen fertilizers using fossil fuel energy. While fertilizer greatly amplified food production and supported more people, along with increasing technologies, it meant an increased agricultural efficiency. That change in efficiency affected the economic viability of small family farms, and as these businesses collapsed, it pushed a tremendous land-use change toward urbanization.
An important feature of increased urbanization is that when precipitation falls on impervious surfaces, the water makes its way through constructed stormwater systems to urban streams. As a result, urban streamwater quality suffers, killing the sensitive organisms living in urban streams. These consequences correlate directly to the amount of impervious surface contained in a watershed. I refer to many examples from my city, Durham, North Carolina, and like most cities, Durham's stormwater system uses urban streams as above-ground stormwater pipes, and the stormwater flows into drinking water reservoirs. As a result, decreased water quality in urban streams correlates with various measures of increasing urbanization.
A book on cities and nature needs the context of just how many people exist in reference to the expectation of "natural" levels because the recent human population increase underlies many environmental features and concerns we experience today. More people means more cities, less "nature," and a greater role for urban environmental conditions that affect Earth in myriad ways, local and global, with consequences lasting well into the future.
I find current population numbers even more sobering when compared to how many people existed long ago. In the table at the top, I put our population explosion into a personal context: My oldest grandparent was born in 1888 when there were roughly 1 to 2 billion people in the world. My dad was born in 1930 when there were about 2 billion people. I was born in 1960 when there were about 3 billion people. My oldest child was born in the mid-1990s when there were about 5 to 6 billion people. Now there are about 6.7 billion people. Human population increased sixfold in 120 years, just four generations! (See Figure 1.1.)
Going back further in time, we find that human population increased by about a factor of 60 over the last 2,000 years. If you accept the idea that humans were once just another species, comparisons with other organisms yield around 15 to 150 million people for our "natural" population (see Figure 1.4), which we now exceed by as much as 200-fold.
Let me put population growth another way: Up until two millennia ago, during nearly all of humanity's existence, our population tripled. During the last 200 years, our population tripled and then tripled again. That's a lot of people added to Earth in just the last 100 years or so.
How densely are people packed? Earth's land surface measures 150 million square kilometers, and we have 6.7 billion people: simple division gives 40 people per square kilometer (km), or 104 people per square mile. Across the United States and as seen in Figure 1.1, populations range from very low densities, like the 0.1 people/mile (0.039 people/km2) in Loving County, Texas, and several counties in Alaska, to a very high density in New York County, New York, with 70,700 people/mile (27,300 people/km2) over its 22.8 square miles.
All these people need resources, and so we'll next examine food production and land-use change.
Humans need to eat food, food ultimately requires plant growth, and plant growth requires water, warmth, light, and nutrients. It really comes down to evaporating water. Ecologists sweep all the biologically relevant ways of turning liquid water into vapor under the term evapotranspiration, but only two really important ways catch my interest for the questions at hand: simple evaporation of liquid water, which accelerates when water is heated, and transpiration, which takes place when plants use and lose water while growing.
Two main ingredients limit evapotranspiration: heat and water. I took the desert photograph shown in Figure 1.2 out of a plane flying over the western United States, and, in a wonderful way, it shows the difference between potential evapotranspiration and actual evapotranspiration. A location's potential evapotranspiration measures how much liquid could change into vapor if only an unlimited water supply existed. Assuming infinite water means that heat ultimately limits potential evapotranspiration: How much water can a location's heat evaporate and transpire? In the Arctic, for example, there's plenty of (frozen) water, and if only there were more heat there'd be more evapotranspiration. Actual evapotranspiration, in contrast, tells how much evapotranspiration actually takes place in a spot given both its heat and water. A large difference between potential evapotranspiration and actual evapotranspiration of the desert motivates the farmer irrigating the fields.
The upper left plot turns my photograph into numbers, showing how the production of plant material increases as evapotranspiration increases across 23 vastly different ecological areas ranging from deserts to rainforests. My photo shows this graph's most extreme points in just one spot: the dry extreme by nature, another extreme made wet by irrigation.
For ecologists, plant growth means the uptake of carbon from the atmosphere, while appreciating that plants both take up carbon through photosynthesis and release carbon through respiration. We call the balance—uptake minus release— net primary productivity, or NPP for short. This plot shows that the net amount of carbon absorbed by plants depends on how much warm water they have available. Annual NPP totaled across the continental United States amounts to 3.4 × 1012 grams of carbon distributed over the United States' 7.9 million km2, giving an average plant growth of 430 gC/m2/year.
Biomass production excites people, historically for food and more recently for energy. However, packaging biomass into species excites botanists and ecologists even more. Along with increased plant biomass production, the upper right plot of Figure 1.2 shows that increasing evapotranspiration also correlates with an increasing number of tree species. Through some unclear mechanism, the process of natural selection yielded more species where greater plant growth occurs.
The number of species in a given ecological or evolutionary grouping is called its species richness, and we just saw that tree species richness increases with evapotranspiration. So too does the species richness of birds, mammals, amphibians, and reptiles (Figure 1.3). In other words, add heat, water, and sunlight, and out comes lots of plant growth with lots of species in each general grouping of organisms. As with body size scaling (Figure 1.4), the precise reasons behind these observations aren't well understood—yet there's the empirical observation. The data come from compendia of species range maps, perhaps one of the field guides that might be sitting on your bookshelf. An ecologist digitized and overlaid each map, creating a spatial map of species richness for each taxon of organism. Combining that information with independent evapotranspiration data generated these plots. Details differ between the taxa, such as the amount of scatter and saturation or downward trends at very high evaporation, but those details seem unimportant overall.
Evapotranspiration clearly drives not only how much plants grow, but also the conditions it promotes, like abundant plant growth, which influences plant and animal biodiversity. According to the study behind these figures, evapotranspiration accounts for more than 90% of the variation in species richness among these organisms! This high value means that in a natural area with minimal human disturbance, all other ecological factors besides evapotranspiration—interesting ecological interactions such as competition, predation, and disease — independently account for less than 10% of the variation behind how many species live there.
Species need evapotranspiration, and evapotranspiration needs water. Water comes from rain, and rain comes from evapotranspiration. Rain enters the ecological loop by soaking into the soil, then being pulled back out by growing plants or evaporation. This key feature has implications for cities. In various places throughout this book, I discuss impervious surfaces such as roofs, roads, buildings, and parking lots. Rainwater can't percolate into impervious surfaces, interrupting this crucial dependence of plant growth and species richness on evapotranspiration. A hypothetical city, half covered by impervious surfaces, might experience half the evapotranspiration it should because stormwater systems divert rainfall directly to streams, ponds, rivers, lakes, and ultimately oceans (see Figure 1.10). This reduction reduces plant growth, sustains fewer species, and results in fewer benefits from vegetation.
Imagining that humans were just another species, we'll now compare our population numbers with those of other organisms.
Let's put today's human population into perspective. Long before humans became really clever, our population fell in line with the populations of other organisms. That assumption seems reasonable to me, and it provides an estimate for prehistoric population sizes. The plots shown in Figure 1.4 demonstrate the connection between a carnivorous (meat-eating) species' population density and individual body mass. Mammalian species from different continents fall onto a general trend line, showing that the relation holds across different ecological communities. Species with small individuals have lots of them: many, many mice, but very few elephants. Quite some time ago biologists saw this clear and strong connection between population density and body size, not to mention many other quantities, but still there's no consensus as to why it should be so. Ecologists call this fascinating research area body size scaling, and many scaling relations exist for other ecological and physiological quantities. Humans, being mammals, were subjected to whatever biological forces impose this body size scaling, and these plots provide rough limits on a prehistoric population size estimate, as well as a stark indication of how far off the line our tremendous recent population increase has pushed us.
To find our place in the top plot, let's assume a human body mass of a nice, svelte 50 kilograms (kg), which corresponds to a weight of 110 pounds (lb), falls within empirical reason for prehistoric people. Putting this number on these plots provides a rough expectation for our "natural" population density: One person per 10 square kilometers if we were strict carnivores, and one person per square kilometer if we were strict herbivores (from results of a similar curve). We expect, then, that natural human densities ranged between 0.1 to 1.0 people/km2 (0.3–3 people/mi2), wherever conditions were suitable for primitive human habitation. Compare these densities to the modern ones across the United States (Figure 1.1), ranging from 0.04 to 27,000 people/km2 and shown in the bottom plot of Figure 1.4 for 1990 U.S. populations. Nowadays, rural might be considered anything less than 100 people/km2, urban more than 10,000 people/km2, with suburban in between. These numbers exceed humanity's present world-averaged density of 40 people/km2, which assumes we spread people out over every patch of Earth.
Multiplying the above natural density estimates by Earth's land surface, 150 million km2, yields the globe's prehistoric human population, somewhere between 15 and 150 million people. If humans were strict carnivores, the global human population would be about 15 million; if strict herbivores, there would be about 150 million humans. Regardless of diet, these natural estimates fall much lower than Earth's 6.7 billion people, a number roughly 40 times greater than for herbivores and 400 times greater than for carnivores. In sum, the human population greatly exceeds any sense of a natural carrying capacity.
More people should mean more food eaten, meaning more farmland needed. Instead, farmers grow crops much more efficiently today than just a few decades ago, with six times higher crop yields. Despite an increased population demanding more food, Figure 1.7 shows that U.S. farmland has held mostly steady over the last 60 years. We can feed all these people precisely because we've modified and engineered a few other species' productivities. Mechanical, environmental, and biological engineering supports our huge population.
I grew up on a farm and can put Figure 1.5 into a personal context. One of my uncles recalled how his Minnesota family traded in their team of workhorses for their first tractor in 1941, right when crop yields take off. In fact, my brothers and I never had a horse while growing up because my late father—a fantastic mechanic —disliked horse chores as a child. The rejection of horse power in favor of fossil fuel power keeps people fed.
The U.S. Department of Agriculture (USDA) estimated that a farm needs $250,000 in annual crop or livestock production to break even (see Figure 1.7), that is, just to have income meet expenses. What does that number mean to city people? Let's assume corn was priced at a historically high $3 a bushel, ignoring very recent, and very transient, off-the-axis prices from the biofuels push, and a great yield of 150 bushels an acre. What sized farm earns a farmer a quartermillion dollar gross income? The answer is more than 550 acres of land producing corn, nearly 2,000 acres producing wheat, or about 400 acres producing cotton. In contrast, Durham County farms average just 100 acres.
Are farmers economically sensible? Consider just the land assets involved in a corn-producing farm. If the farmland is worth $5,000 an acre, then its total value is $2.75 million, and that huge asset, not counting equipment, fuel, and labor, provides only a break-even return. On paper, the United States has millionaire farmers who can't make a living. A farmer could (an investment adviser might say should) sell all his or her land, invest it in mutual funds with a 10% annual return, and make $275,000 per year without backbreaking 16-hour workdays. The advice doesn't strongly depend on these figures: Divide the land price by two and the numbers remain astonishing. I'm surprised we have any farms at all in the United States. Essentially, farmers provide volunteer labor, not to mention a public service. Local, organic, and small-scale farming methods gained popularity recently, but it seems questionable that we can afford to scale back the increased food production efficiency from the last century. Certainly these interests can reenergize small farms and, perhaps, even prompt gardening by city dwellers.
Excerpted from Constructed Climates by William G. Wilson Copyright © 2011 by William G. Wilson. Excerpted by permission of THE UNIVERSITY OF CHICAGO PRESS. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Meet the Author
William G. Wilson is associate professor of biology at Duke University.
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