Global Warming: Introduction

The intertwined problems of population growth, water shortages, and food shortages have been recognized for over one hundred years, and no one disputes that they are important global problems. However, until very recently much of the public was unaware of the related problems of peak oil and global warming. As a result, I will spend more time focusing on these less familiar problems.

At the time of this writing (2009), there is growing public acceptance of the reality of human-induced global warming. I would argue that the scientific community reached consensus in the late 1990’s on the reality of global warming, and in the early 2000’s on the idea that warming is primarily human-induced. As expected for an issue this complex, it is taking longer for the public to reach a consensus. This is not surprising, as the culprit is fossil fuel use, and there are extremely powerful and wealthy business concerns that have campaigned against this consensus to protect their profits. This situation closely parallels that of the tobacco companies in the 1970’s, who paid lobbyists and scientists large sums of money to spread falsehoods about the link between smoking and cancer (see the book “Thank You For Smoking” for an insightful and amusing illustration of how corporations conspire to hide the truth). Unfortunately, this has led to a politicization of the global warming issue. Although Al Gore did an admirable job of raising public awareness on this issue (for which he won the Nobel Prize in 2006?), his political associations led many to close their minds to the possibility that he was right. However, Al Gore did not invent the theory of global warming, nor did he participate in any of the scientific investigations; he was merely publicizing an issue that was hidden from view. Most scientists are not very good at public outreach, and they need well-known figures like Al Gore to carry their message to the masses. This effort has been only partly successful, so now it is up to scientists like me to help the public understand the reality and the importance of issues like global warming and peak oil.

The concept of global warming is really quite simple. Energy in the form of sunlight passes through earth’s atmosphere and heats the surface; the surface warms and gives off heat. Without greenhouse gases like CO2 in the earth’s atmosphere, that heat would radiate into space and be lost, and the average surface temperature of the earth would be only 0°F, meaning that all water on the earth’s surface would be frozen, and life would not be possible. Fortunately, the greenhouse gases in our atmosphere absorb and trap the heat, increasing the average observed surface temperature of the earth to a very hospitable 59°F. We are fortunate to have greenhouse gases in our atmosphere. However, like Goldilocks we need it not too cold and not too hot, but just right. If the concentration of greenhouse gases gets too high, it will be too hot for us.

Recognition of the greenhouse effect goes back to Joseph Fourier in the early 19^th century, and the role of carbon dioxide (CO2) was identified in 1859 by John Tyndall. It was Svante Arrhenius in 1896 who predicted that human activities could contribute to the greenhouse effect, but it wasn’t until the 1970’s that scientists like Roger Revelle and Wallace Broecker began to raise the alarm. Their concern was based on measurements by Charles Keeling, who showed that CO2 concentration in the atmosphere was increasing at an alarming rate:

Fig. 1: http://en.wikipedia.org/wiki/File:Mauna_Loa_Carbon_Dioxide-en.svg

Although there are seasonal fluctuations related to plant growing seasons (see inset of Fig. 1), the long-term trend shown in red is of steadily increasing CO2 concentration. The measurements in Fig. 1 were made at the famous Mauna Loa observatory in Hawaii, but similar measurements have been made at many other observatories and show the same trend. So how is this related to human activity? In the Peak Oil section, we described how oil contains the energy of sunlight that fell on earth millions of years ago, trapped in organic molecules that were manufactured in plants through photosynthesis. The simplified chemical reaction is:

Eq. 1: 6 CO₂ + 6 H₂O + energy from sunlight = C₆H₁₂O₆ + 6 O₂

The glucose molecule C₆H₁₂O₆ represents the organic matter that stores the energy in fossil fuels. When we combust fossil fuels, we undo the work of photosynthesis, promoting the reverse reaction by heating the organic matter in the presence of atmospheric oxygen so they react and liberate the stored energy. The troubling product of this combustion is CO₂, which accumulates in earth’s atmosphere, leading to the steadily increasing atmospheric CO₂ concentrations exemplified by Keeling’s curve (Fig. 1).

Another example of this delicate balance maintained by earth’s atmosphere is the oxygen concentration of the atmosphere. From Eq. 1 above we can see that combustion consumes O₂ while producing CO₂. Thus, we would predict that if combustion of fossil fuels are now the primary source of atmospheric CO₂, then with time increasing CO₂ should be balanced be decreasing O₂ concentration in the atmosphere:

*Insert link to figure

The current atmospheric O₂ concentration of 21% is just right for trees: If O₂ rose to 25%, forests would burn after every lightning strike, but if it fell to 13%, we wouldn’t be able to start a fire. In fact, it is life that regulates the composition of the atmosphere, as illustrated vividly by James Lovelock’s conception of Gaia, which posits that earth behaves like an organism because it’s components act in concert to maintain life-support systems at optimal levels. Just as our body maintains a constant temperature of 98.6°F, the earth can maintain global temperatures within a narrow range that is conducive to life. How? Eq. (1) gives us some insight. Because temperature and atmospheric CO2 concentration are positively correlated, when CO2 is increased, then temperature increases, and the combined effect is to induce plant growth through photosynthesis (Eq. 1). This causes plants to extract greater amounts of CO2 from the atmosphere, thereby decreasing atmospheric CO2 concentration and therefore temperature. In effect the earth system works to counteract environmental changes, a process called negative feedback (similar to LeChatlier’s principle in Chemistry). Thus, life helps to regulate the composition of atmosphere and therefore helps maintain an optimal temperature, and the earth system of which life is a part is self-regulating (homeostatic). Essentially, the solid earth and atmosphere (geochemistry) and life (paleontology) have co-evolved.

If life maintains the composition of the atmosphere at an optimal level, why worry about greenhouse gas (CO₂) emissions?

· They may exceed the capacity of the system to maintain constant temperature and composition

· They may kill coral reefs and other marine organisms

CaCO₃ + H₂O + CO₂ = 2 HCO₃^- + Ca²⁺

· They could shut down the “ocean conveyor belt” and cause drastic cooling of Europe

· Severe weather events such as El Nino and hurricanes may become more frequent and intense

· Specific regions may become uninhabitable due to desertification

How do we know that the new CO2 added to earth’s atmosphere derives from human use of fossil fuels, rather than some other natural source such as volcanic degassing? There are several lines of evidence that clinch the case, two of which involve the use of carbon isotopes. When plants grow through photosynthesis (Eq. 1), they preferentially extract the light isotope ¹²C from the atmosphere, so organic matter has a low ¹³C/¹²C ratio. Volcanic degassing would not change the ¹³C/¹²C ratio of the atmosphere, but returning organic carbon back to the atmosphere through burning of fossil fuels should lead to a decrease in the ¹³C/¹²C ratio of the atmosphere, which is what we observe. Another carbon isotope found in the atmosphere is ¹⁴C, which is radioactive and has a half-life (the amount of time it takes for half of the ¹⁴C to decay) of 5700 years (*check; it continuously formed by cosmic rays in the earth’s upper atmosphere). Modern plants incorporate ¹⁴C from the atmosphere and are therefore slightly radioactive. However, the organic matter from plants that grew millions of years ago that is now contained in fossil fuels has no remaining ¹⁴C, so burning fossil fuels (as opposed to modern biomass) should cause a decrease in atmospheric ¹⁴C concentration, which is again what is observed[1].

So we can agree that CO₂ is a greenhouse gas, and that human activity has increased the CO₂ concentration in the atmosphere. This should lead to warming of the atmosphere, which will thermally equilibrate with the land surface and oceans through heat transfer, causing them to also warm. Thus, the entire earth will warm, as is evident in the following plot:

We can fit a straight-line to all of the data to obtain the red curve, which shows a ~150-year trend of increasing global average temperature. If we fit a line to the data from the last 25 years, we obtain the yellow line that has a steeper slope than the red line, suggesting that the rate of heating was higher in the last 25 years than observed over the 150-year period. This acceleration of warming to rates higher than ever recorded in geologic history is what has scientists concerned.

Global warming is documented by many global changes. Instrumental records (corrected for the urban “heat island” effect) and natural evidence (shrinking and thinning of Arctic ice, loss of Antarctic ice shelves, receding of most Alpine glaciers globally, lengthening of growing season, migration of animals & plants to higher latitudes, borehole measurements) all show that the earth’s surface has warmed 0.4-0.8°C (~1°F) during the 20th century. The probability that warming is real is > 99% (IPCC, 2007).

Why is the greenhouse effect so hard for humans to detect (Pollack, 2005)?

• It is difficult for humans to focus on small incremental changes worldwide when big things are happening at home.
• Our senses are tuned to detect rapid change (e.g., lobster in boiling water).
• We can easily detect changes in weather (short-term), but not changes in climate, which is the long-term characterization of the average weather.
• It becomes harder to interpret human-induced changes in climate when they are superimposed on longer-term natural changes (Milankovitch cycles, continental drift, and oceanic circulation patterns).
• We are tempted to interpret short-term departures from the norm as long-term trends.
• Small changes such as a 1° increase in average global temperature can have a large impact because the earth, like our bodies, is a complex, finely-tuned machine that cannot tolerate small changes in temperature.

What is Causing the Warming?

Possible causes of climate change include variable sun, strengthening greenhouse, increased atmospheric aerosols, and volcanic eruptions. Computer simulations based on real-world measurements show that the natural drivers, solar variability and volcanic eruptions, have actually caused earth’s surface temperature to decrease during the 20^th century. Aerosols also cause cooling. Therefore, the only remaining cause of global warming is increased greenhouse gas concentrations from burning of fossil fuels.

*More to come...

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[1] Note that it is the changing ¹⁴C content of the atmosphere that makes accurate ¹⁴C dating of material less than 100 years old impossible.