29 April 2005

Lecture 44

Reading, Chapter 29, 30, and 31

B. Ecosystem ecology

2. Biogeochemical cycles

b. Global climate change resulting from rising carbon dioxide

"Global warming" has been a contentious political issue because it is difficult to predict with confidence and because it will cost money to prevent it. For such issues, it is important to be aware of what is known with confidence and what is uncertain and still subject to debate.

Known facts about global warming:

1) It is known with confidence that the carbon dioxide levels in our atmosphere have increased by over 35% in the last 200 years. Unambiguous measurements of atmospheric composition in the last 40 years have shown this plainly. It is also known that most of this new carbon dioxide is the result of burning fossil fuels, since fossil fuels have a distinctive carbon isotope ratio.

2) It is known with confidence that carbon dioxide is a greenhouse gas. This means that it allows sunlight to pass through the atmosphere but traps heat near the Earth's surface. In theory, increased carbon dioxide in the atmosphere should cause the Earth to warm. This was predicted over 100 years ago by the chemist Arrhenius.

3) It is known that average global temperature has increased by over half a degree Centigrade in the last 100 years. Few scientists are still sceptical of this any longer.

 

Uncertainties about global climate change include the following

1) At present, the rate of CO₂ increase in the atmosphere is about half what would be predicted from the rate of fossil fuel burning and deforestation. The "missing sink" to which this carbon dioxide is going and whether its rate of uptake will remain stable is unknown. Current estimates are that carbon dioxide in the atmosphere will double by the year 2100. This rate of increase could accelerate or slow down, depending on such things as how warming affects the growth of forests and the respiration of bacteria in the soil.

2) It is not known how quickly or how much global warming will occur. Estimates of warming go from 2 to 6 ° C in the next 100 to 200 years. (6°C would be catastrophic). Factors that complicate these estimates include:

- As ice melts at the poles, soil is exposed and absorbs more solar radiation than the ice did. This is a positive feedback that would accelerate the rate of warming.

- Changes in ocean circulation caused by the melting of ice, which adds fresh water to the ocean, may cause cooling to occur in at least some parts of the globe. Cooling of the east coast of the United States is expected if ocean circulation is perturbed. This is a negative feedback that would slow warming.

- Vegetation loss caused by regional drought may increase the level of erosion and dust in the atmosphere and thereby decrease the absorption of solar radiation. This is a negative feedback that would slow warming.

- As ocean temperatures rise, dissloved CO₂ and methane, which is also a greenhouse gas, may show increased diffusion from the oceans. This is a positive feedback that would accelerte warming

 

3) Perhaps the greatest uncertainty about global warming is what it will cost if allowed to proceed unchecked. Some examples of impacts include:

Rising sea levels inundating coastal cities and farmland with seawater.

Northward spread of tropical diseases in the western United States.

Disruptions to agriculture by drought in some areas. Improvement of agriculture in other areas, such as northern Canada

Extinctions of plants and other organisms that cannot adapt or migrate toward the poles.

Cooling of the eastern United States owing to disruption of the ocean currents that warm that region now.

 

c. Cycles of solar radiation.

Some attribute the current period of warming to changes in solar radiation. The orbit of the Earth around the sun changes a little over time such that solar radiation goes up and down in a 23,000 year cycle. There is also a slower cycle of 100,000 years that occurs on top of the 23,000 year cycle. Solar radiation has a major effect on global temperature but the effect is complex. Over the last 400,000 years, periods of low global temperature (ice ages) have alternated with warmer (interglacial) periods. Past warming from ice ages has occured when solar radiation is in the high part of the 100,000 year cycle but the relationship is weak, indicating that some of the feedbacks described above are complicating the solar effect.

A point to remember about the solar hypothesis is that we are already in an interglacial period. Any additional warming will be unprecedented in human history.

There has been much study of the global carbon cycle and its interaction with global climate. A new field of "paleoclimatology" has arisen that attempts to decipher past climate fluctuations in order to make better predictions about current climate change. An excellent website devoted to this work can be found at this link.

There has also been a recent and very thorough study of warming in the Arctic, which has warmed more than elsewhere over the last 50 years. A very accessible and authoritative summary of this study can be found at this link.