Stephen covered several important points about the study of global climate change. These are summarized in this document.

Several "mitigation" approaches to global climate change were discussed, including:

• "Do nothing"
• Reducing greenhouse gas emissions
• Capturing and sequestering CO₂ emissions.

The "do nothing" approach rests on several aspects, including:

• Wait for more clouds – these will reflect more sunlight back to space – a negative feedback.
• The oceans will take up the additional CO₂.
• CO₂ fertilization will lead to grow of additional biomass.
• Sulfate aerosol.

"Not all clouds are created equal." Clouds high in the atmosphere appear best suited for scattering sunlight back to space. This would be a negative feedback. However, clouds at lower altitudes appear to be net absorbers of infrared radiation. This would be a positive feedback – ie, global warming causes increased evaporation of water, low altitude cloud amount increases, more thermal radiation is trapped in the atmosphere by the infrared absorbing clouds, the planet and its atmosphere heat further.

There is considerable uncertainty on the cloud issue. Should we wait for it in the hope it will mitigate the problem?

Table 14.1 in Bodansky shows the oceans absorb 92 GteC per year of CO₂, while releasing 90 GteC per year. Stephen also showed these values. Thus, the oceans are out of equilibrium with the atmosphere by 2 GteC per year. (Since 2 is a "small difference between big numbers", we need to ask how accurately the 2 is known.) Because of the lack of equilibrium, the oceans are a net CO₂ sink for the anthropogenic CO₂. The total storage of C in the oceans is estimated as about 40,000 GteC (compared to 5000-10000 GteC in fossil fuels and about 2000 GteC in the biosphere.) Can we anticipate more uptake of the atmospheric CO₂ by the oceans? Nature may not help too much here, since mixing in the intermediate and deep parts of the oceans is slow. Thus, the CO₂ absorbed by the upper layers of the oceans is very slowly transported to the deeper zones. This is not a favorable situation for "asking" the oceans to "naturally" take up significantly more CO₂.

It is argued that increased CO₂ in the atmosphere will lead to more growth of biomass, a negative feedback. However, CO₂ does not appear to be the "limiting" resource in plant growth. The limiting resource in plant growth is water, followed by various nutrients.

Sulfate aerosol is known to reflect sunlight, providing a cooling effect. Industrial regions of the planet and regions downwind of them have experienced little or no increase in mean temperature. Sulfate aerosol is caused by SO₂ emissions from fossil fuel burning and from other industrial activities. In the atmosphere, some of SO₂ is converted to sulfate (SO₄^-2) particles. The average "residence time" of the sulfate aerosol in the atmosphere is about 1 day – until it rains. This residence time (and wind speed) determines the size of the downwind region affected. Of course, acid rain is another consequence of the SO₂ emissions. Laws and regulations to limit SO₂ emissions from power-plants and other air pollution sources will decrease sulfate aerosol concentration in the atmosphere, reducing this cooling effect.

Volcanoes are another source of sulfate aerosol and sunlight reflecting dust. These can have a global cooling effect. However, "not all volcanoes are created equal" with respect to global cooling. Volcanoes near the equator have an advantage. In the equatorial regions, air rises, creating a large global circulation pattern. Some of this air "falls" to ground near 30 degrees N and S latitude, creating the permanent high pressure zones of the planet’s desert regions. Some of the air continues on the higher latitudes. Thus, aerosol from volcanoes in the 0-20 degree latitude range has the opportunity to cover much of the planet. This happened from the eruption of volcanoes in Mexico (ca. 1982) and the Philippines (ca. 1992). Stephen showed a plot of atmospheric transmissivity measured at the top of Mauna Loa on the Big Island of Hawaii. Normally, this is about 94%. However, in the 1982-83 and 1992-93 periods it significantly dropped, to as low as about 80%.

Stephen presented graphs of photosynthesis activity for the land and oceans. For the oceans, activity is greatest in coastal tropical waters, reaching as much as about 400 gC/m²-yr (where gC = grams of carbon of the biomass created). For the land, the maximum is about 3000 gC/m²-yr. This occurs in the wetter and warmer regions of the planet.

Mentioned was fertilization of the oceans with iron, leading to enhanced growth of algae – thereby converting atmospheric CO₂ into biological carbon, which will be deposited in the oceans. The downside is the consumption of ocean O₂ by the algae, leading to the killing of fish.

Some have proposed injecting sunlight-reflecting aerosol into the earth’s atmosphere, thereby promoting a cooling effect. Does this trade one environment impact for another?

Others have proposed capturing the CO₂ from power plants and placing it in the oceans. What will be the "unintended consequences" of placing large amounts of CO₂ in the oceans?

Perhaps a sounder idea would be to burn fuel with oxygen rather than air. This would make it much easier to capture the CO₂. If one burned fuel with oxygen, the main products of combustion would be CO₂ and H₂O. These could be relatively easily separated by cooling the exhaust gases and condensing the H₂O. The remaining CO₂ would be collected and pumped underground, perhaps "sequestered" in a used oil or natural gas formation. The two major costs would probably be: 1) the air separation unit for making the oxygen, and 2) the pumping of the CO₂ underground. The technical parts are available to do this, though the systems engineering and experience are incomplete. [Note air separation technology is used everyday to create medical oxygen, for example. About 5 large coal-fired power plants in USA use large air separation units. These are synthetic gas combined cycles. Coal is gasified in oxygen-blown gasifiers, creating a synthetic gas mainly composed of CO, CO₂, H₂, H₂O, and CH₄. This syn gas is cleaned up (it contains tar, H₂S, NH₃, and particulate matter) and is burned in a combined cycle. Overall efficiency is about 40%, and this is expected to improve in the coming years.]

Stephen discussed forest sequestration of CO₂ mainly from the standpoint of the Pacific Northwest – thus Douglas Fir was the focus. Stephen noted present agriculture is a net emitter of CO₂.

The rate of growth of Doug Fir peaks in about the 75^th year of the tree. It is estimated the harvesting and replanting of Doug Fir on a 100 year rotation would probably lead to the maximum sequestering of CO₂ by the Doug Fir forest. Also, selective thinning rather than "clear-cutting" would need to be practiced for harvesting the timber.

A 100 year rotation might not be acceptable to the forest products industry, since return on investment could be low. A shorter rotation tends to enhance return on investment.

The 100 year rotation finding implies that old growth forests, as preserved on federal lands, are not effective in the sequestration of carbon. However, these forests have other values.

Clearly, many "values" are involved in forest sequestration of carbon. The issue is complex.