1-1: Overview of Global Change

Introduction

Half of the Earth's atmosphere lies within 5 km of the surface of the earth, three-fourths is located within ten km and 90% is within 16 km (10 miles). Among the many functions of this atmosphere are two that are critical to life as we know it: the atmosphere shields the surface of the earth from lethal ultraviolet radiation and allows life to exist on the surface of the planet. It also serves to process energy that comes from the sun and energy that is reradiated from the surface of the earth. Absorption of this energy establishes an average surface temperature that allows water to exist in all three phases -- liquid, solid, and vapor. The favorable environment established through these two functions allows for the flourishing of life on the planet over the past million years.

Earth's Changing Atmosphere

Recent measurements have revealed that the characteristics of this atmosphere are changing. Figure 1 is a plot of atmospheric carbon dioxide, in parts per million, as a function of time since 1972. You can see that at all locations, the carbon dioxide concentration of the atmosphere is increasing. We know why it's increasing: the burning of fuels (coal, oil, natural gas), the burning of vegetation, and the production of cement. It is an uncontested fact that CO₂ in the atmosphere has risen from a pre-industrial (1780) level of 280 parts per million (ppm) to the current (2007) level of 382 ppm - a 34% increase.

There are other gases in the earth's atmosphere whose composition has changed in recent years. Methane is an example. Figure 2 shows the concentration of methane in parts per million over the last thousand years. You can see that for the first 700 years of this record, until the 1700's, the concentration of methane in the earth's atmosphere was fairly constant. But since the Industrial Revolution, methane concentration has increased quite dramatically to the levels more than double the pre-Industrial Revolution values. The sources of methane are reasonably well known: cattle, termites, rice paddies, boreal forests, and high latitude tundras are sources of methane gas.

Global-Average Surface Temperature

Measurements of the global-averaged surface temperature of the Planet Earth show that this temperature has increased by about 0.5^oC over the last 100 years. This increase with time has not been uniform. Figure 3 shows there was a reasonably steady rise from 1860 to a peak in the 1940's and then a drop off to the 1970's followed by a very dramatic rise since about 1970. Recent data show that 1998 has been the hottest year in the 138-year historical record and that six of the eight warmest years have occurred since 1998. The extraordinary warmth in 1998 was largely due to a very strong El Niño. The demise of this El Niño allowed global temperatures to drop back somewhat, but then resume an increase to the point where at the end of 2005, global mean temperatures equaled those of the 1998 El Niño year even though we had no El Niño in progress at this time.

There is now good evidence that at least part of this rise in temperature is caused by the increases in carbon dioxide, methane and other trace gases that are produced as a result of anthropogenic (human) activities. There is good physical evidence to support this because we know carbon dioxide and methane absorb infrared radiation and are contributing to this recent rise in temperature.

Temperature and CO₂ Correlation

From a plot of atmospheric CO₂ and Earth surface temperature over the last 400,000 years (Figure 4) we see a strong correlation between temperature and CO₂. It is noteworthy that the concentration of CO₂ during this period did not exceed 300 ppmv, in contrast to recent measurements that show concentrations exceeding 370 ppmv and rising steadily.

Reconstructed Earth Surface Temperature, 1000-2000

A 1999 report by Mann et al (1999) shows that the surface temperature change for Planet Earth over the last 1,000 years as reconstructed from ice cores, lake sediments and tree rings (Figure 5). The yellow bars on this graph show the range of uncertainty for each proxy measurement. Overall the graph shows that the temperature of the planet has decreased gradually over the period from year 1000 to about 1900. Since then, the temperature has risen abruptly at a rate uncharacteristic of the behavior of the preceding 900 years. This graph provides strong evidence that conditions of the planet have changed dramatically in the last 100 years.

If we plot current atmospheric CO₂ levels on the historical record of Figure 4 we see that current levels far exceed levels of the last 400,000 years (Figure 6). If human-induced emissions of the CO₂ continue at current rates of increase, atmospheric CO₂ will surpass 300% of pre-industrial levels by 2100 (Figure 7). The question then is what happens to global mean temperatures on the graph in Figure 7? Is the recent rise in temperature shown in Figure 3 the early sign of the trend to be expected in Figure 7?

Other Chemicals for Concern

We also have other chemicals in the earth's atmosphere that are increasing. Chlorinated fluorocarbons (CFCs) were invented in the late 1930's, so they did not exist in the earth's atmosphere before that time. They are very stable compounds and are removed from the atmosphere very slowly. This allows time for these compounds to diffuse into the stratosphere where they can be broken down by ultraviolet light creating free chlorine atoms that can combine with ozone to create diatomic oxygen and therefore serve as a means of depleting stratospheric ozone.

Nitrous oxide is another gas in the earth's atmosphere whose concentration is increasing. Natural sources of nitrous oxide in the soil are augmented by the use of nitrogen fertilizers leading to increased atmospheric concentrations. Nitrous oxide, like the CFCs, has a very long lifetime in the atmosphere and also can lead to destruction of stratospheric ozone. A loss of stratospheric ozone allows increased amounts of ultraviolet light to reach the surface of the earth and damage living tissue. Cases of skin cancer in New Zealand and Australia are experiencing a dramatic rise, in part due to this increase in ultraviolet light. Tiny ocean organisms, called phytoplankton, in regions where stratospheric ozone has been depleted are vulnerable to damaging levels of ultraviolet radiation.

Population, Consumption, and Sustainability

The root cause of these faster-than-normal changes in our global environment is a rapidly rising global human population with increasing amounts of per capita consumption and energy resource use. These anthropogenic (i.e., human) demands put strains on natural and managed (agricultural and forest) ecosystems that raise questions about irreversible change to the global environment that might jeopardize the ability of future generations to meet their basic human needs. Serious questions are raised about the "sustainability" (indefinitely continuing current practices that deplete resources or degrade the environment) of our use of energy, agricultural and forestry practices, use of fresh water, and others.

Global Change Issues

Issues commonly listed under global environmental change include

• Deforestation
• Reduced Biodiversity
• Stratospheric Ozone Depletion
• Global Warming
• Desertification
• Human Population Increase
• Acid Deposition
• Sea-level Rise
• Groundwater Depletion or Degradation
• Invasive Species
• Land Degradation
• Decreased Availability of Fresh Water

The US Global Change Research Program was established by Presidential order to coordinate research and related activities in the US to help provide a well-founded scientific understanding of the Earth system to ensure the availability of future resources essential for human well-being, including water, food, fiber, ecosystem, and human health. It provides the foundation for increasing the skill of predictions of climate change and climate variations and sponsors research to understand vulnerabilities to changes in climate, ultraviolet radiation at the Earth's surface, and land cover. Such scientific knowledge is an essential basis for informed decision making on environmental issues and to ensure the social and economic health of future generations. In this course we will examine the scientific basis for global change, explore the implications of these changes and discuss some of the political implications of global change.

Summary

In summary, we live on a planet that, from space, looks blue. It has water existing in all three phases. On closer inspection, we can see that we have a green planet, with a rich diversity of biological species. However, the habitable zone of this planet that supports biological activity is a very thin shell around the surface of the planet. The region of the earth in which humans can live without life support systems is a thin spherical shell about 3 kilometers thick on a planet of radius 6,370 km. If the earth was the size of a basketball, this zone of human habitation would be about the thickness of a sheet of paper. This is the only known zone in the universe where humans can exist without life support systems. And now humans are performing a global experiment -- a chemistry experiment -- on this habitable zone without knowing what the consequences might be. As concerned citizens and future leaders of the planet, we have two options: we can cover our eyes and pretend there is no problem, or we can use the tools at our disposal to study this problem -- to evaluate the evidence, to look at the scientific results, and answer for ourselves what level of certainty we have about these issues. And what are the consequences -- environmentally, socially, and economically -- of acting or of failing to act in determining the nature of our global chemistry experiment. What future do we leave to our grandchildren?

Course Objectives

This is a course in Global Change that addresses these and many other issues. A key objective of the course is to demonstrate the interconnectedness of the earth system. A second objective is to instill in students the value of authoritative literature on global-change issues. A third objective is to engage students in dialog among themselves and with outside experts on the economic, social, political, and ethical implications of these changes and finally, we will learn to use higher-level critical thinking skills to gain an understanding of these global problems and the difficultly of finding solutions. A central objective of this course is to recognize and use such higher-level skills.

Updated atmospheric CO₂ datasets and graphs can be obtained here