More on measuring past climate conditions The article briefly mentions how scientists are able to determine climate conditions in the ancient past, long before any actual measurements were recorded. The study of past climates is known as paleoclimatology.
Climatologists divide climate study into three time periods that are based on the kind of data used to study climate change. The instrumental era includes the last 150 years when accurate weather records are available. The historical era includes the records from all of human history. Much of the information available from this time period is not very precise nor even quantitative, but it is actual data. The pre-historic era (or the period of paleoclimatology) relies on “proxy data” to infer changes in climate.
The time scale used when talking about climate change is also important. There are long-term trends in climate as well as shorter periods of change. Climatologists usually use four time periods:
The article mentions the study of ice core samples as one method of determining the composition of the atmosphere over time. The air samples trapped in the ice can be analyzed and from that analysis scientists can determine the composition of the atmosphere at a particular point in history. You might mention to students as part of this discussion that the gas analysis is done primarily by mass spectrometry. A mass spectrograph works this way: molecules in question are inserted into the instrument, ionized, accelerated to the same velocities and then acted upon by a magnetic or electric field. The magnetic field deflects the moving particles, the lighter ones being deflected to a greater degree than the heavier ones. So molecules and parts of molecules are separated according to their masses.
To see how the mass spectrometry works consider the analysis of water vapor tapped in an ice-encased air bubble. The oxygen component of water exists in two isotopic forms—O-16 and O-18. Remember that each oxygen atom has eight protons. Most (by far) oxygen atoms also have eight neutrons. That’s O-16. But a small percent of oxygen atoms have ten protons. That’s O-18, the heavier isotope. When the water vapor is analyzed in the mass spectrometer, the O-16 water molecules are deflected to a great degree than the O-18 molecules.
The O-18 water is heavier (because the O-18 isotope is heavier than the O-16 isotope). We know that the O-16 water has a higher vapor pressure and a slightly lower boiling point than water composed of O-18, and, therefore, it evaporates more easily than O-18 water. So, as past climate temperatures increased, the O-16 molecules evaporated to a greater degree, leaving a slightly increased proportion of O-18 molecules in the water of oceans and lakes. Thus, when chemists examine ice cores and determine the concentration of oxygen isotopes, a higher O-18 to O-16 ratio means that the climate was warmer and a lower ratio means the climate was cooler.
You can read a brief description of mass spectrometry here: http://masspec.scripps.edu/mshistory/whatisms_toc.php. Another method of analysis is gas chromatography, which is summarized here: http://chemwiki.ucdavis.edu/Analytical_Chemistry/Instrumental_Analysis/Chromatography/Gas_Chromatography.
There are other proxy methods used by paleoclimatologists. Some of them were listed in the Teacher’s Guide for the article on Mt. Kilimanjaro (see References, below).
For assessing the history of climate changes we must rely upon "proxy" climate indicators--natural archives that record seasonal or annual climate conditions such as ice cores, tree-ring measurements, laminated sediments, microbial life and corals--combined with the relatively small amount of available historical documentary or instrumental evidence available in prior centuries. Paleoclimatologists gather data from these natural recorders of climate variability, and by analyzing records taken from these and other sources, scientists can extend our understanding of climate far beyond the 100+ year instrumental record.
Listed below are some widely used proxy climate data types: