Summary CO and Climate

Both theoretical modeling and proxy estimates point to similar trends for CO2 over Phanerozoic time. High levels of CO2 in the early Paleozoic are followed by a large drop to low levels during the Permo-Carboniferous. A primary reason for this drop, based on GEOCARB modeling, and the models of Mackenzie et al. (2003) and Bergman et al. (2003), was the rise of large vascular land plants, which accelerated the weathering of Ca and Mg silicates and later led to the burial of increased quantities of organic matter in sediments. At the end of the Permian, possibly at the very end, the level of atmospheric CO2 rose to high values during the early Triassic. After this, CO2 remained high in the Mesozoic and began a gradual decline, punctuated by short- and medium-term excursions, which extended into and through the Cenozoic, reaching the rather low

Royer 2004 Co2

Figure 5.21. CO2 and climate (Modified from Royer et al., 2004). A: Comparison of model predictions (GEOCARB III) and proxy estimates of CO2. Shaded area represents range of error for model calculations. All proxy results are averaged within successive ten million years time slots. B: Intervals of glacial (black) or cool climates (light gray). C: Latitudinal distribution of direct glacial evidence (tillites, striated bedrock, etc.). (After Royer et al., 2004.)

Figure 5.21. CO2 and climate (Modified from Royer et al., 2004). A: Comparison of model predictions (GEOCARB III) and proxy estimates of CO2. Shaded area represents range of error for model calculations. All proxy results are averaged within successive ten million years time slots. B: Intervals of glacial (black) or cool climates (light gray). C: Latitudinal distribution of direct glacial evidence (tillites, striated bedrock, etc.). (After Royer et al., 2004.)

level, compared to most of the Phanerozoic, found at present. Contributing to this drop was increased mountain uplift in the Cenozoic and a continual increase in solar radiation throughout the Phanerozoic. The solar increase exerted a major influence on weathering rate, helping to bring about an overall long-term decline in CO2.

Changes in CO2 over the Phanerozoic correlate rather well with changes in paleoclimate. Times of minimal CO2 coincide with the two most widespread and long-lasting glaciations of the Phanerozoic, that during the Permo-Carboniferous (330-270 Ma) and that of the past 30 million years (Crowley and Berner, 2001; Royer et al., 2004). This is illustrated in figure 5.21. Also, periods of unusual warmth at high latitudes, such as occurred during much of the Mesozoic (250-65 Ma) correlate with periods of elevated CO2. Together these observations give support to the greenhouse theory of climate change on a Phanerozoic time scale (Royer et al., 2004). Short-lived glaciations and cooling episodes, such as occurred during the late Ordovician at 440 Ma, can be explained by other causes but may also involve short-term changes in atmospheric CO2.

There are problems with all of the methods of CO2 estimation, and they have been pointed out in this and (for modeling) in previous chapters. The GEOCARB standard curve is not intended to be used as an accurate CO2 measure (as is sometimes mistakenly done), but rather as a suggestion of how CO2 has changed over the Phanerozoic. New advances in the various fields that contribute to the modeling (e.g., volca-nology, biogeochemistry of weathering, tectonics, and erosion) will undoubtedly cause modifications to the GEOCARB curve, and to those of Mackenzie et al. (2003) and Bergman et al. (2003). However, the overall qualitative trend described above, I believe, will withstand the test of time. It has now stood for more than 14 years.

Organic Gardening

Organic Gardening

Gardening is also a great way to provide healthy food for you and your loved ones. When you buy produce from the store, it just isnt the same as presenting a salad to your family that came exclusively from your garden worked by your own two hands.

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