Constructed Climates

Figure 1.1: The table shows  global population numbers exceeding estimated “natural” levels by as much as 200-fold,  increasing more than sixfold since just  the mid-1800s. The year -10,000 simply means pre-agrarian. Across the United States, the U.S. Census map shows county population densities varying from 0.1 people per square mile (Loving County, Texas) to more than 70,000 people/mi² (New York County, New York).

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[1]. Human population increased six-fold 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[2]. Across the United States and as seen in Figure 1.1, populations range  from very low densities, like the 0.1 people/mile² (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/mile² (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.
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[1] The U.S. Census Bureau summarizes historical population numbers from several sources at www.census.gov/ipc/www/worldhis.html.

[2] I make many conversions from one unit to another in this book, for example from people per square kilometer to people per square mile, and here’s how to do it. We all know that 1 kilometer equals 0.62 miles. (Actually, few people have all these numbers stored in their head, including me, and I find them somewhere online, for example, onlineconversion.com.) That equivalence means the fraction (1 kilometer)/(0.62 miles) equals one. We can multiply any number by one without changing its value, so write our global population density as (40 people/km²)(1 km/0.62 mi)². The km² in the numerator cancels with that in the denominator, and 40/(0.62)²=104. Thus, our global population density equals 104 people per square mile.