Question

Discuss the nature of an exponentially growing quantity and explain what the term doubling time means. Explain why the problem of exponential growth is easily overlooked. Discuss some of the differences in the factors affecting population growth in the U.S. 25 or 50 years ago compared to now. Discuss demand or resources in a growing population in one area (e.g. town, or county, or country) and globally (world economy, resources, food production supply and demand, living space, etc).

Answer 1

Explaination 1

Exponential growth is growth that occurs at a constant rate, such as an investment that grows at an annual 7 percent rate. The Rule of 70 provides a quick and easy way to determine how long it will take for an amount to double at a given growth rate. Simply divide the percentage number into 70 to obtain the approximate number of years required to double. A more detailed description is given below.

Populations can grow at a constant rate, thus the Rule of 70 can be used to approximate the doubling time of a population that is growing at a fixed rate of growth.

Human population has generally grown in an exponential manner throughout human history, and projections of exponential doubling times have been applicable.

Current dynamics of population growth are more complex and population growth can no longer be considered to be precisely exponential. Population issues of the 21^st Century include migration and immigration, aging, Baby Boomers and the youth bulge, as well as continued population growth in developing countries - all of which result in pressure on sustaining resources and the environment.

Many countries will continue to double their population. The United States is projected to double its population this century, practically within the lifetimes of children born today. Therefore it is appropriate to reference the time in which these countries will double their population. Doubling time is perhaps the most illuminating manner of presenting population growth to the lay person. However, it should be noted that the explanations below refer to a constant rate of growth, which as we note, is not as applicable to countries now as it was in the past.

Exponential Growth

A quantity grows exponentially when its increase is proportional to what is already there. A common example is compound interest, where $100 invested at 7% per year annual compound interest will double in 10 years. Similarly, if a population grows at 7% per year, it, too, will double in 10 years.

Exponential growth has surprising consequences. $100 invested at a 7% annual return will double in 10 years to approximately $200, double in another 10 years to approximately $400, and double again in the next 10 years to approximately $800. Significant gains can be made by simply relying on exponential growth over time. One way of saying this is that the longer you wait on your investment, the faster your returns come in. In the following graph, you can see that over time, returns increase dramatically.

There are also clear disadvantages to exponential growth. When populations continue to grow, the impact of growth becomes increasingly significant over time. Because of the nature of exponential growth, "when things get bad, they get bad in a hurry".

Consider a city with 100,000 people, growing at 7% per year. In 10 years, the population will double to 200,000 people, in another 10 years it will double again to 400,000 people, and ten years after that it will double again to 800,000 people. The following graph shows this exponential population growth. The shape of the curve is identical to the interest rate graph above!

Doubling Time.

The doubling time is the period of time required for a quantity to double in size or value. ... When the relative growth rate (not the absolute growth rate) is constant, the quantity undergoes exponential growth and has a constant doubling time or period, which can be calculated directly from the growth rate.

The Rule of 70 states that to find the doubling time of a quantity growing at a given annual percentage rate, divide the percentage number into 70 to obtain the approximate number of years required to double. For example, at a 10% annual growth rate, the doubling time is 70 / 10 = 7 years.

Similarly, to get the annual growth rate, divide 70 by the doubling time. For example, 70 / 14 years doubling time = 5, or a 5% annual growth rate.

Explaination 2.

Between 2010 and 2011, the U.S. population increased by 0.7 percent, after averaging 0.9 percent growth each year from 2000 through 2010.¹ The United States added just 2.3 million people from 2010 to 2011, compared with 2.9 million from 2005 to 2006, just five years earlier. The decline in U.S. population growth is likely due to a confluence of factors: lower levels of immigration, population aging, and declining fertility rates.

A drop in net immigration to the United States is a key factor in the country’s declining population growth rate. Around the turn of the millennium, the Census Bureau was estimating net international migration at about 1.4 million per year. By late in the decade, that annual number had been adjusted downward to less than 900,000. Between 2010 and 2011, net migration was estimated at around 700,000. The drop in immigration levels means that a smaller share of U.S. population growth is directly attributable to immigration, as opposed to natural increase (the excess of births over deaths). In the early 2000s, immigration accounted for roughly 40 percent of U.S. population growth, leaving 60 percent from natural increase.²During the past few years, however, net international migration is estimated to have accounted for only 30 percent of the annual population growth.

The decline in immigration has been linked to job losses in construction, manufacturing, and other occupations that are often filled by recent immigrants, as well as stricter enforcement of immigration laws. Net immigration levels from Mexico are estimated to have reached zero in recent years as an equal number of people entering the United States from Mexico are now returning to their home country.

SLOWER LATINO AND ASIAN POPULATION GROWTH

Because most immigrants to the United States arrive from Latin America or Asia, the decline in net international immigration has contributed to slower growth in the Latino and Asian American populations. Latinos and Asians remain the country’s two fastest-growing minority groups, but their growth rates have dropped below the peaks reported just a few years ago.

Population growth for Latinos averaged 3.6 percent per year during the previous decade, but fell to just 2.5 percent between 2010 and 2011 (see Figure 1). There was a similar drop for Asian Americans during the same period, from 3.6 percent to 2.2 percent.⁴ Although the rate of change fell for both groups, the number of Asians and Latinos is still increasing. And both groups are still growing faster than African Americans (1 percent) and non-Hispanic whites (0.1 percent).

POPULATION AGING

The recent decline in immigration has also accelerated population aging in the United States. Most immigrants are working-age adults, and many start families after they arrive in the U.S., creating an immigrant youth bulge.⁵ Less immigration therefore reduces the potential number of young people in the population. But even if immigrant levels had held steady during the recession, the U.S. population would still be growing older as the large cohort of baby boomers starts to reach retirement age.

A decade ago, children under age 18 made up a significant component of annual population growth and exceeded the growth of those ages 65 and older (see Figure 2). But by 2011, these patterns had reversed: The number of people under age 18 declined by 190,000 between 2010 and 2011, while the number of elderly increased by 917,000. Growth in the number of working-age adults, including those in prime childbearing ages, is also down sharply.

Because of its relatively young age structure, the United States still has a great deal of population momentum compared to many other developed countries.⁶ But as more baby boomers enter retirement and there are fewer people of reproductive age, we could see further declines in the number of births, and the age structure of the United States could start to resemble that of Europe.

DECLINING FERTILITY RATES

U.S. fertility rates have also declined in recent years, reducing levels of natural increase in the population. There were an estimated 4 million births between 2010 and 2011, down from 4.2 million at the recent peak of U.S. population growth between 2005 and 2006. The total fertility rate (TFR) stood at 2.0 births per woman in 2009—down from 2.1 a few years ago—but preliminary data from the National Center for Health Statistics (NCHS) suggest that the TFR could drop below 2 births per woman in 2010.

Latinos currently account for more than one-fourth of all births in the United States. Yet the Latina general fertility rate (births per 1,000 Latinas ages 15 to 44) dropped from 102 in 2007 to 80 in 2010, according to preliminary NCHS estimates. This represents a significant decline for a group with historically high fertility rates—approaching 3 births per woman just a few years ago. If current trends continue, the Latina TFR could drop below 2.5 in the near future.

The U.S. population is currently projected to reach “majority minority” status (the point at which less than 50 percent of the population is non-Hispanic white) in 2042. However, a sustained drop in immigration levels and fertility rates would slow the pace of minority population growth. It’s too soon to tell whether these are short-term trends resulting from the U.S. economic downturn, or whether they will continue in the future. But either way, these current trends represent a significant departure from developments over the past decade.

Explaination 3

The world is faced with challenges in all three dimensions of sustainable development—economic, social and environmental. More than 1 billion people are still living in extreme poverty, and income inequality within and among many countries has been rising; at the same time, unsustainable consumption and production patterns have resulted in huge economic and social costs and may endanger life on the planet.

Achieving sustainable development will require global actions to deliver on the legitimate aspiration towards further economic and social progress, requiring growth and employment, and at the same time strengthening environmental protection. y Sustainable development will need to be inclusive and take special care of the needs of the poorest and most vulnerable.

Strategies need to be ambitious, action-oriented and collaborative, and to adapt to different levels of development. They will need to systemically change consumption and production patterns, and might entail, inter alia, significant price corrections; encourage the preservation of natural endowments; reduce inequality; and strengthen economic governance.

The sustainable development with a focus on three important cross sectoral issues: sustainable cities, food security and energy transformation is a main thing.