Population - Environmental Science - page 67

The IPAT Model
As a way to look at the human impact on the environment, in 1974 Paul Ehrlich and John Holdren developed
the IPAT model, which examines how technology, affluence, and population all work together to impact the
environment. It's shown as I = P × A × T , where
• I stands for the impact on the environment.
• P stands for population. Population can affect the environment because more people mean more land and
resources are used and more waste is produced.
• A stands for affluence. Greater affluence means larger resource consumption per person due to increased
wealth.
• T stands for technology. Technology can act positively or negatively either by creating ways to exploit re-
sources faster and more easily, or by developing better ways to reduce our impact, such as better pollu-
tion controls or renewable energy.
Sensitivity can also be added to the IPAT model, taking into account the sensitivity of an environment when
being used by humans. For example, deserts and grasslands are more susceptible to degradation if they are not
managed properly.
The Rule of 70
Overall, a population's net change in size, per 1,000 individuals, is measured by its growth rate , which is cal-
culated as follows:
Growth Rate = (Birth Rate + Immigration) - (Death Rate + Emigration)
For example, if the birth rate is 10 individuals, immigration is 20 individuals, the death rate is 15 individuals,
and emigration is 5 individuals, it would be calculated as follows:
(10 + 20) - (15 + 5) = 30 - 20 = 10 per 1,000 individuals
This means that the population will grow by 10, so in one year the population will add 10 individuals per 1,000
individuals. Expressed as a percentage, the growth rate is
10 per 1,000 × 100 percent = 1 percent annually
As mentioned earlier, with exponential growth, a population grows by a fixed percentage each year. Therefore,
although the population of 1,000 individuals grew by 10 in one year, if that same 1 percent growth rate contin-
ues each year, the population grows by 1 percent each year. For example, a population of 1,000 mice that
grows at a rate of 1 percent each year will end up at 1,010 mice after one year. Although this doesn't sound like
a large increase in population, due to exponential growth, the population will grow to 1,100 after ten years if it
remains at a 1 percent growth rate. Look at this on a larger scale, such as with a population of 1 million indi-
viduals, and it is evident how quickly a population can multiply and grow.
The human growth rate was at its highest in the 1960s at 2.1 percent. Since then, it has dropped to 1.2 percent,
but this rate still reflects growth, so the human population continues to grow exponentially. In the early 1800s,
the human population was close to 1 billion people; in 1950 the human population was approximately 2.5 billi-
on; and in 2005 humans totaled about 6.5 billion. By 2050, the human population could reach over 9 billion.