Principal indicators of past climates - Climate Change: Biological and Human Aspects - page 58

2.3.2 Boreholes
As the climate warms and cools, so varying amounts of heat are absorbed into the
underlying geology. Consequently, it is theoretically possible to drill a deep borehole
to measure the temperature at varying depths as an indication of past climate. Depths
of borehole are typically of the order of a kilometre or more. Assumptions based on
geological knowledge of the area, especially those related to geothermal heat, need
to be made.
Problems with using boreholes as indicators of palaeoclimate include that the
resolution of the thermal record is low and decreases with depth. However, in 2000
Shaopeng Huang and colleagues from Michigan University, USA, and the Western
Ontario University, Canada, analysed temperatures from 616 boreholes from all
continents, except Antarctica, to reconstruct century-long trends in the past 500 years
(Huang et al., 2000). The results confirmed the unusual warming of the 20th century,
which was greater than for the previous four centuries. Their results also corroborated
the consensus as to past climatic change over this period from other palaeoindicators.
2.3.3 Carbondioxideandmethanerecordsaspalaeoclimaticforcingagents
In addition to indicators of past climates there are also records of palaeoclimatic
forcing agents. Here the principal examples are the ice-core records of the past
atmosphere. As snow falls on ice caps so bubbles of air become trapped and encased
in ice. Bubbles in ice cores from Vostok in Antarctica, and Greenland provide a record
of atmospheric composition going back hundreds of millennia. A history of both the
key greenhouse gases, carbon dioxide (CO 2 ) and methane (CH 4 ; see Chapter 1), can
be ascertained. These strongly reflect hemispheric climatic change as elucidated by
the deuterium isotope record and so demonstrate the strong link these forcing agents
have with climate. The methane record largely reflects the area of wetlands globally
that produce the gas. Its concentration is higher in the warmer, hence wetter (due to
increased ocean evaporation), interglacial times. Consequently, being a greenhouse
gas, its atmospheric concentration change is another example of one of the many
feedback systems operating in the biosphere. The warmer the global climate the more
wetlands there are generating methane, which being a greenhouse gas further warms
the climate, and so on. Conversely, the cooler the climate the less the wetland area,
hence less methane is produced, and less climate warming from atmospheric methane.
Of course wetlands are not the only source of methane. Oceanic methane hydrates
(clathrates) have been known to release pulses of methane into the atmosphere,
providing a warming burst.
The problem with carbon dioxide and methane gases as a record of climate forcing
agents is that other forcing factors operate as well and so the record cannot be said
to properly reflect climate. Indeed, one brief climatic return to glacial conditions
(the Younger Dryas) prior to our current Holocene interglacial took place in the
northern hemisphere but does not seem to have taken place to anything like the
same extent in the southern hemisphere. It is also less prominently reflected in
the Antarctic atmospheric carbon dioxide record. Furthermore, as we shall see in
Chapters 4 and 5, the so-called Little Ice Age, in the 15th to 17th centuries, took