Study Aid/Quiz, CO₂ and Climate Change

1.  What causes the annual variations in the plot of CO₂ concentrations measured at Mauna Loa, Hawaii, during the last 50 years?

The fluctuations of CO₂ levels are due to seasonal changes in solar radiation, vegetation cover and photosynthesis rates in the Northern Hemisphere (where Hawaii is located). In the N.H summer, the CO₂ absorbed during photosynthesis is greater than the CO₂ produced during respiration and decomposition of plant remains.  In the N.H. winter, temperate forests lose their leaves, and respiration and decomposition proceed faster than photosynthesis.  Also, FYI, the seasonal fluctuations are greater in the N.H. than the S.H. because the N.H. contains more land mass and more temperate forests.

2.  Match the reservoir with the amount of carbon it contains:

39,040 Gt
610 Gt
4200 Gt
40,000,000 Gt
760 Gt

760 Gt Atmospheric CO₂
39,040 Gt Oceanic dissolved CO₂, carbonate ion CO₃^2-, and bicarbonate ion HCO₃^-
610 Gt Living biomass
4200 Gt Fossil fuel
40,000,000 Gt Limestone in sedimentary rocks

3.  How do oceanographers measure primary productivity in the oceans?  What affects the distribution of the abundance of phytoplankton?

Oceanographers use satellites with a color scanner that quantifies the intensity of the photosynthetic pigment, chlorophyll, in the ocean waters. Chlorophyll is a green pigment that is the dominant pigment in algae.  Nutrient concentrations, light intensity, and water temperature affects the abundance and distribution of phytoplankton. (see page 138 for more detail).

4.  If the biomass increases due to increased CO₂ levels, will the greenhouse effect be reduced and global warming impeded?

At first glance, this answer “yes” seems logical. However, the statement is not supported by what we know about the dynamics of the carbon cycle.  On land, terrestrial plants may incorporate more CO₂ into biomass, but much of this is simply recycled back into the atmosphere through decomposition and respiration within a few years.  In addition, terrestrial plants may not be CO₂ limited but are limited by the availability of other nutrients (nitrogen or phosphorous) or environmental conditions (sunlight, temperature, moisture, soil properties etc). So even if more CO₂ is available, this increase would not affect growth rates of plants.  Phytoplankton in the oceans are known to be nutrient limited rather than CO₂ limited, so a significant increase in primary production in oceanic waters would not be expected to occur.

5. Does carbonate or silicate weathering (or both) remove CO₂ from the atmosphere?

Silicate weathering removes CO₂ from the atmosphere, carbonate weathering does not, at least on the long term (when carbonate precipitation in the ocean, the next step, is considered). See page 146 for a comparison of reactions.
So if CO₂ levels were to increase, silicate weathering would have a negative feedback effect on CO₂ concentrations in the atmosphere.