Climate variations on orbital timescales

The glacial cycles are driven by variations in Earth's orbit. Climate reconstructions on these timescales come from ice cores and ocean sediment cores. We looked at an ice core data set in Fig. 8.3 showing concentrations of CO2 and CH4 in the atmosphere, recorded in bubbles in the ice.

Figure 8.3c, which we haven't talked about much yet, is a reconstruction of the temperature where the ice accumulated, in Antarctica (not a global average). The temperature estimate is based on the relative abundances of different isotopes of hydrogen and oxygen in the water. Two isotopes of an element have the same number of electrons and protons, and hence pretty much the same chemistry. That is what we mean when we say that they are both the same element. The isotopic sisters have different number of neutrons, however, so they have different masses. Some isotopes, like carbon-14, decay radioactively. Other isotopes, such as oxygen-16 and oxygen-18, are both perfectly stable and do not decay.

The different masses cause two isotopes of the same element to behave slightly differently from each other in chemical reactions. These are subtle effects; the raindrops might have a ratio of oxygen-18/oxygen-16 that is only a percent or so different from the isotopic ratio of the vapor. In order to make the numbers a little easier to remember, isotope geochemists describe ratios not as percentage differences but rather as per mille, written as %o. One percent is one part in 100, whereas one per mille is one part in 1000. Ten % = 1%.

Mean annual air temperature (°C)

Fig. 11.11 Correlation between the S18O of rain or snow with the mean annual temperature where the precipitation fell.

Mean annual air temperature (°C)

Fig. 11.11 Correlation between the S18O of rain or snow with the mean annual temperature where the precipitation fell.

One example of a process that affects isotopic ratios is rain and evaporation. A water molecule comprised of a heavier isotope of oxygen or hydrogen will tend to rain out earlier than would a water molecule made of the lighter isotope. As the temperature over an ice field decreases, the water vapor that remains in the atmosphere to snow down on the ice becomes increasingly depleted in the heavy isotopes. For this reason, there is a systematic relationship today between the temperature and the relative abundances of the heavy and light isotopes in water. Snowfall has less heavy isotopes left when the temperature of snowfall drops (Fig. 11.11). A small caveat: the data points in Fig. 11.11 are all present-day precipitation. If the oxygen isotopic composition of the evaporating source water were different in the past, the entire oxygen-isotope/temperature line might move up and down. Scientists use a combination of oxygen and hydrogen isotopes to correct for this effect.

Sediment cores also provide information about past temperatures. The oxygen isotopic composition of CaCO3 shells tells us something about past temperatures, as do measurements of the chemical composition of the shell. A family of protozoa called the foraminifera supply most of the shells that are analyzed from deep sea cores. Some, like the planktonic foraminifera, live in the sunlit surface waters and leave a record of SSTs. Others, the benthic foraminifera, live on the sea floor, and can tell us about past deep ocean temperatures.

The surface of the Earth was perhaps 5-7°C cooler during the Last Glacial Maximum, about 20,000 years ago (Fig. 8.3). The changes in greenhouse gas concentrations changed the radiative forcing of the Earth, by about 2-3 W/m2. There was an even

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Age (million years)

Fig. 11.12 The Paleocene Eocene Thermal Maximum event. Replotted from Zachos (2001).

Age (million years)

Fig. 11.12 The Paleocene Eocene Thermal Maximum event. Replotted from Zachos (2001).

larger change in the radiative forcing from albedo changes, perhaps 4-7 W/m2. This is the result of ice sheets, sea ice, and changes in the terrestrial biosphere. Here is another chance to test the climate models. There are uncertainties about the circulation of the ocean during glacial time, but in general, climate models are able to simulate the observed cooling of the Earth's surface. If the climate models were somehow too sensitive to greenhouse gases, then we would expect the climate models to predict a colder Last Glacial Maximum than we piece together from the proxy records. CO2 is only part of the reason for the colder glacial climate, but it is striking to look at how similar the CO2 and temperature records are. Every little wiggle lines up between the two records. It is difficult to argue that CO2 is somehow unimportant to climate, after looking at this record (Fig. 8.3).

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Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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