Steward’s most important points are that CO2 is not the only climate driver and that many interacting factors control our climate. Factors Steward deals with include
- Variations in the sun’s activity, especially the magnetic activity which controls the incidence of cosmic rays in the lower atmosphere, which in turn form ionization trails for clouds to condense on. The clouds increase the earth’s albedo (tendency to reflect light) and have a cooling effect.
- The tendency of CO2 to become less effective as a greenhouse gas as it becomes more concentrated.
- Variations in the earth’s orbit, together with variations in the sun’s position caused by gravitational interactions with Jupiter and Saturn.
- Various geologic processes such as weathering of rocks and plate techtonics which release calcium to combine with and sequester CO2.
- Changes in earth’s albedo due to desertification, vegetation growth, and the exposure of more or less beach as sea levels advance and retreat.
In Chapter 7 Steward discusses two scenarios that show how the various factors can interact. The first is pretty typical of the conventional wisdom of the AGW advocates, although it includes some drivers such as water vapor, methane, changes in albedo due to increased vegetation and melting snow. In this scenario the earth warms.
In the second scenario Steward notes that
- The earth’s orbital alignment is moving in a cooling direction
- The greenhouse effect of CO2 saturates as the CO2 concentration increases
- Warming of the atmosphere allows it to absorb more water vapor, which in turn leads to more low-level clouds, increasing albedo and leading to a negative feedback, mitigating the heating. But water vapor is itself an important greenhouse gas, and whether increased albedo due to clouds or the water vapor greenhouse effect wins out is a matter of research.
- In the past, the Paleocene period for example, CO2 concentration was as high as 1200 PPM but temperatures were around 27 degrees C (80 deg F). Since then CO2 and temperature have both declined to the present levels (with some ups and downs) of 380 PPM and 59 deg F (15 deg C)
- The great civilizations of the past have been associated with warm periods, not cool periods.
Steward does not advocate expensive efforts to reduce CO2 emissions, neither does he propose doing nothing. He advocates continuing research, and some reduction in CO2 emissions (including sequestering CO2 and storing it for later release if needed)