Part 2. Exploring Scientific Evidence and Evaluating Choices

What is global warming? The term "global warming" suggests that temperatures are increasing and that it is happening over the whole globe (earth). But is the whole atmosphere warming? What about the upper atmosphere? What about the oceans, the land, the ice masses on Greenland and Antarctica - are they all warming? How much are they warming? At what rate are they warming? And for what reason are they warming?

The questions below are designed to guide you in improving your understanding of the global warming issue. You are not required to specifically answer the questions. However, try to answer the questions mentally. This will greatly aid you when you formulate your recommendation to the panel. Begin by selecting a question below.

Section 1: What evidence suggests global warming?

Measurements of temperature near the earth's surface have been taken on a daily basis over most of the planet for about the last 100 years. Estimates of temperatures before about 1860 must come from indirect evidence, such as tree rings or lake sediments.

U.S. Global Change Research Program

Changes in temperature since 1880 are shown in the above graph. The reference point (the zero point) is the average temperature for the period 1951-1980, so the graph indicates changes that have occurred from this 30-year average value. The data show that the planet has warmed by about 0.7 degrees Celsius (about 1 degree Fahrenheit) in the last 100 years, with the most rapid rise occurring since the mid 1970s.

It should be noted, however, that some isolated locations have cooled over the last hundred years or longer, but when all stations are added together, the resulting global average temperature is increasing.

What about natural variations?
Residents of the Northern Hemisphere eagerly await the warm days of May and June after the cool winter and spring months. But we don't become alarmed about such warming for two reasons: (1) it's not global (the Southern Hemisphere is cooling by about the same amount, giving a near balance to global temperatures), and (2) it happens every year, so it is part of what we know to be the natural cycle of temperature.

If this year is hotter than last year, we may grumble and complain, but we rarely get nervous about global warming from one hot summer, because it has happened before and next year may be cooler again. So even changes from year to year are rarely cause for alarm.

We might become concerned if, over a long term (maybe several decades) the temperature continued a gradual increase. But even this might be misleading for those of us less than a few decades old, because the sun goes through slow cycles of more intense and less intense radiation and the earth has slight variations in its distance from the sun. These can lead to warming for a hundred years or so followed by cooling. Such changes are natural and have been happening for millions of years. (For additional information, read "Natural climate variability vs. man-made climate change" from the United Nations Environment Programme.)

However, a large international group of scientists known as the Intergovernmental Panel on Climate Change (IPCC) recently issued a consensus report stating that the evidence is mounting, based on several indicators, that the increase in temperature over the last several years is unlikely to have been caused by natural variations. They point to the build-up of greenhouse gases in the earth's atmosphere as a significant contributing cause for this temperature increase.

Click here for advanced information on global warming evidence.

Section 2: What effect will rapidly rising temperatures have on ecosystems?

The largest class of ecosystem is called a biome. Examples of biomes include the high latitude tundra and boreal forests of northern Canada and Russia, the temperate grasslands of the central US, and the tropical rainforests. Regions of transition between these biomes are very vulnerable to small changes in climate, particularly changes that occur on a short time scale compared to natural changes. For example, at the end of the last ice age, some 12,000 years ago, the temperature rose about 4 degrees C over a period of 1500 years, which is the equivalent of 30 generations of trees having lifetimes of 50 years. By comparison, the current increase in greenhouse gases is estimated to warm the climate by 4 degrees in about 130 years, which is equivalent to less than 3 generations of the same trees. This rather abrupt change of temperature would alter the ability of these transition ecosystems to adapt to rapidly changing conditions. Rare species of both flora and fauna with small ranges may be vulnerable to local or global extinction.

Ecosystem response to past climate change has been to shift gradually north or south depending on whether the temperature rises or falls. Barrier, such as highways, cities, and managed agricultural areas, now impede the gradual migration of ecosystems. For instance, the northward movement due to global warming of the temperate forests and their associated ecosystems in the southern US would be impeded by the agriculture, roads, and cities of the central US and Ohio River valley.

Adapted from IPCC, 1990

A further concern for the future of ecosystems is that the projected temperature rise due to increases in greenhouse gases will make the planet warmer than it has been in the last 160,000 years, so we can't compare future climate with climates that have occurred over this period.

High-latitude regions such as the frozen tundra of northern Canada and Russia will experience higher than global-average temperature increases (if the global average temperature rise is 2.5 degrees C, high latitude regions could experience a 5 degree C rise). This warming will increase the rate of the decay process and lead to yet more carbon dioxide and methane released to the atmosphere and further warming (called a positive feedback process).

Increases in carbon dioxide will cause some types of plants, known as C3 plants, to grow faster than others (called C4 plants). This will lead to some plants being more competitive and therefore more dominant in ecosystems. Some agricultural crops, such as soybeans, are C3 plants which will be more productive under increased carbon dioxide, while others such as corn, being C4 plants, will not benefit from this change. Increased growth rates for some crops and warmer climates for high-latitude agricultural nations, like Russia and Canada, could promote increased food production.

Section 3: What is the carbon cycle and how does it relate to global warming?

Carbon cycle is the name used to describe the collection of natural processes that move carbon through the atmosphere, soil, plants, animals, and the ocean.

Plants use carbon dioxide from the atmosphere to make tissue that may be eaten by animals or humans. Most plant carbon eventually ends up in the soil from dead plants or animal wastes. While some remains in the soil, most plant and animal carbon eventually goes back to the atmosphere as carbon dioxide, although some escapes as methane.

The ocean exchanges carbon dioxide with the atmosphere at a rate that depends on surface temperature. Ocean phytoplankton and larger plants use carbon dioxide from the atmosphere and serve as food for fish and other marine animals. When these marine fish and animals die, their skeletons go to the bottom of the ocean, thereby removing carbon from the carbon cycle.

Section 4: What are greenhouse gases and how do they relate to the carbon cycle?

We recognize the figure below as a description of the carbon cycle. To see how the carbon cycle relates to global warming, click on "Greenhouse Gases" in the figure below.

Section 5: Why have greenhouse gas concentrations risen?

The rise in greenhouse gases since the Industrial Revolution and the much more rapid rise of many of these gases in the last 50 years suggest that humans may be contributing to this rise.

Click on the red box in the image below to continue.

Section 6: If humans have caused and increase in greenhouse gases that lead to global warming, what are the projections of future atmospheric concentrations of these gases and future global warming?

We have seen that atmospheric concentrations of the major greenhouse gases have increased rapidly in recent years, primarily due to anthropogenic causes. Carbon dioxide is the most abundent greenhouse gas of anthropogenic origin, so we use it to demonstrate future scenarios of greenhouse gases and global warming.

We've looked at atmospheric carbon dioxide from the past up to the present, and now it might be informative for us to look to the future. Anthropogenic emissions of carbon dioxide to the atmosphere in the future, which occur mainly due to burning of fossil fuels, are very closely tied to economic development. Strong economic activity in developed countries and modernization of developing countries both relate closely to the production of electrical energy, use of fossil-fuel burning machines, and the use of cement. These agents of growth all produce carbon dioxide as byproducts. Presently the anthropogenic production of carbon dioxide increases about 2% per year. We can use different economic growth rates to project future anthropogenic production of carbon dioxide, as is shown in the next plot.

EPA

Here it can be seen that by continuing on our present rate of growth, atmospheric carbon dioxide levels will reach 600 ppm by about 2050. Reducing our growth in production to zero (keeping emissions constant at current levels) reduces the level to 440 ppm by 2050. If our goal were to limit atmospheric levels to less than 400 ppm, we would have to reduce emissions by 2% per year. If we turned off all fossil-fuel burning power plants, stopped using automobiles, and eliminated all other anthropogenic emissions, the atmospheric carbon dioxide concentration would return to about the 1980 level by 2050.

This figure reveals the dilemma that we face if we seek to limit the growth of atmospheric carbon dioxide. We seem destined to have a very high level of carbon dioxide in the earth's atmosphere, compared with levels of the last 160,000 years, by the middle of the next century.

It is not realistic to stop using power plants, automobiles, refrigerators, and other devices that contribute to greenhouse gases. So what other choices do we have, and what are the consequences of each choice?

Section 7: What choices do we have concerning global warming, and what are the consequences of each choice?

To explore the answers to the above questions, click on any red box in the image below.

Section 8: What do you think should be the solution to the problem of global warming?

Based on the web information and your own experiences, carefully formulate a recommendation to be presented to the panel. When you are ready to recommend a course of action, click on the Part 3 icon.

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