The Global Temperature Record

Figure 23.1 also shows the global temperature record of the last 1000 years, as estimated from a variety of sources. The so-called Medieval Warm Period (MWP), extending from 1000 to about 1400, was followed by a span of considerably colder climate, the Little Ice Age, (LIA) from 1450 to 1890, when the global mean temperature may have been 0.5-1,0°C lower than today. During the LIA glaciers moved into lower elevations, and rivers that rarely freeze today were often ice-covered in winter. 

A rise in average temperature of 1 °C may not seem enough to cause dramatic shifts in the weather, but the historical record shows otherwise. The period from 1500 to 1850 is called the Little Ice Age, because there were extensive increases in the sizes of the glaciers in all alpine regions. During that period, the average global temperature was just 0.5 °C lower than in 1900. 

Fig. 4.3. (A) Composite multispecies benthic foraminiferal Mg/Ca records from three deep-sea sites DSDP Site 573, ODP Site 926, and ODP Site 689. (B) Species-adjusted Mg/Ca data. Error bars represent standard deviations of the means where more than one species was present in a sample. The smoothed curve through the data represents a 15% weighted average. (C) Mg temperature record obtained by applying a Mg calibration to the record in (B). Broken line indicates temperatures calculated from the record assuming an ice-free world. Blue areas indicate periods of substantial ice-sheet growth determined from the S 0 record in conjunction with the Mg temperature. (D) Cenozoic composite benthic foraminiferal S 0 record based on Atlantic cores and normalized to Cibicidoides spp. Vertical dashed line indicates probable existence of ice sheets as estimated by (2). 3w, seawater S 0. (E) Estimated variation in 8 0 composition of seawater, a measure of global ice volume, calculated by substituting Mg temperatures and benthic 8 0 data into the 8 0 paleotemperature equation (Lear et al., 2000).

Current concentrations of GHG have already caused the mean global temperature to increase by 0.76 °C in the period from 1850 to 2005 owing to the inertia of the climate system this will lead to at least a further half-degree warming over the next few decades. Eleven of the twelve years from 1995 to 2006 rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850). 

Evidence in support of the Iron Hypothesis has been obtained from ice cores that contain records of past dust deposition, atmospheric CO2 levels and global temperatures. As shown in Figure 5.13, during the past 160,000 years, periods during which dustborne iron levels have been high coincide with lower atmospheric CO2 levels and global temperatures, i.e., the most recent Ice Ages. 

Variations in the benthic foraminifera record after 33 Ma indicate fluctuations in global ice volume in addition to temperature changes. Since then the majority of the 5 0 variations can be attributed to fluctuations in the global ice volume. Thus, Tiedemann et al. (1994) demonstrated the presence of at least 45 glacial-interglacial cycles over the last 2.5 Ma. 

Figure 11.2 shows the excitation polarization spectrum of protoporphyrin IX in propylene glycol at —55°C (full line) and bound to the heme pocket of apohemoglobin recorded at 20°C (dotted line). One can see that polarization at a low temperature is higher than that observed when porphyrin is embedded in the heme pocket of apohemoglobin. This is the result of fluorophore local motions within the pocket, independently of the global rotation of the protein. 

Figure 12 Alkenone data acquired with planktonic foraminiferal can be used to remove the temperature effect from oxygen isotopic signals in order to assess global (ice volume) and regional (evaporation/precipitation balance) contributions to isotopic change. Data come from an equatorial Atlantic core studied by Schneider et al. (1996) the isotopic deconvolution was made by the present author. Note that the structure and amplitude of the temperature-corrected planktonic record are consistent with current thinking on the global ice volume signal for the late...

---