Introduction
Records of past climate change are necessary to understand the current changing climate and predictions for the future. These records can potentially provide insight into modern-day natural climate variability. Over recent decades, there has been increasing interest in the events known as the Medieval Warm Period (MWP), also known as the Little Climatic Optimum, and the Little Ice Age (LIA). There are differing views regarding when both the Little Ice Age and Medieval Warm Period occurred, if they actually did occur, the causes of these phenomena and if the two events occurred on a global scale. Until recently, the Medieval Warm Period and the Little Ice Age were assumed to have occurred at a global scale, yet their effects vary quite dramatically around different regions of the earth and at different times. These phenomena have been suggested as periods of significant warming and cooling respectively, extending hundreds of years and occurring in numerous areas across the earth. The so-called Medieval Warm Period is suggested to have occurred approximately between 900 and 1400 AD, while the Little Ice Age is proposed to have taken place roughly from 1550 to 1850 AD (Hughes & Diaz, 1994; Nesje & Dahl, 2003). Having supposedly happened in the more recent past, albeit while the climate was somewhat more variable (Whyte, 1995), these events could give a sneak preview of what is likely to occur or how the climate system is likely to respond to increasing anthropogenic effects.
Outline
This paper will examine the investigations and findings of literature on the Little Ice Age and Medieval Warm Period, revealing the contrasting opinions and conflicting evidence, problems with the various methods of climatic reconstruction as well as highlighting areas with gaps in the research and what direction future research should take. The questions of: if, when and where the LIA and MWP occurred, and the theorised causes will be reviewed. Subsequently, the various forms of methodology commonly utilized in the climatic reconstruction of these two events will be assessed and their benefits and drawbacks considered. Lastly, future possibilities will be brought forward and the main theories noted in this account summarised.
Medieval Warm Period And Little Ice Age: Occurrence, Causes And Effects
The LIA and MWP were climatic anomalies that were widespread, nearly synchronous phenomena with worldwide imprints (Soon & Baliunas, 2003), or were they? There is much dispute over whether these phenomenon actually occurred; if so, when and where and to what extent. These two major climatic episodes are agreed by many researchers to have occurred (Esper et al. 2005), however, the Intergovernmental Panel on Climate Change (IPCC) in their 2001 report produced a reconstruction of past climate in which neither the MWP or LIA were identified. The later IPCC report (2007) concluded that the MWP was of a heterogeneous nature, a statement which was refuted by Esper & Frank (2009). Nesje & Dahl (2003) offered another opinion, suggesting that rapid glacial advances in the 18th century were attributed to the North Atlantic Oscillation (NAO) and that the asynchronous events during the LIA could be attributed to multi-decadal trends in the north-south dipole NAO pattern. There is no solid agreement in the literature on when the LIA or MWP began or ended. Mann & Jones (2003) concluded that the MWP extended from 800 to 1400 AD; O'Hare et al. (2005) suggests the MWP lasted from 900 to 1200 AD; while Hughes and Diaz (1994) estimated from the 9th to 15th centuries. Rodrigo et al. (1994) claims the LIA began in the 1600s and ended in the 19th century; Pfister et al. (1998) suggested the LIA occurred from 1300 to 1900 AD; Bradley et al. (2001) puts forward 1550 to 1850 AD. Scientists and researchers are still developing and reworking theories on the causation of both the Little Ice Age and Medieval Warm Period. Hughes & Diaz (1994) proposed the notion that solar variability, volcanic activity and ocean-atmospheric interactions all represented plausible forcing mechanisms for the LIA. O'Hare et al. (2005) and Henson (2006) similarly indicate that from 1650 to 1730, sunspot numbers declined almost to zero and that the MWP was associated with a lull of volcanic activity and the LIA coincided with higher levels of volcanic activity. Crowley (2000) states that from the 14th to mid 18th century the volcanic contribution to decade-scale variance increased to 41-49%, suggesting that volcanic activity was a major contributory factor of the LIA. Additionally, it has been suggested that orbital scale forcing, variability linked to the millennial orbital-seesaw, and anthropogenic factors may have also been causal factors (Ruddiman, 2008).
Medieval Warm Period
Some studies have demonstrated that the MWP did not occur simultaneously across the globe (Hughes & Diaz, 1994). While the MWP was associated generally with increased temperatures in Europe, in other regions it is more strongly associated with drought (O'Hare et al., 2005; Henson, 2006). Esper et al. (2002) suggests that the MWP appears to be more temporally variable than the warming trend of the last century. However, Esper & Frank (2009) have argued that currently, there is not sufficient widespread, high resolution proxy data to decisively conclude on the spatial extent of warmth during the MWP. Additionally, Jones et al. (1998) concluded that little evidence could be found to support or reject Medieval warming. Conversely, Hughes & Diaz (1994) reasoned that evidence presented doesn't support a global MWP; however they agree that such a phenomenon could be drawn from high-elevation records. Temperatures in Scandinavia, China, Sierra Nevada, the Canadian Rockies and Tasmania appear to have been higher during some part of the MWP than those that occurred until the most recent decades of the twentieth century (Hughes & Diaz, 1994). It is suggested that temperatures in the higher latitudes of the Northern Hemisphere were approximately 1ºC warmer during the MWP than compared to the mid 20th Century (O'Hare et al., 2005).
Little Ice Age
Generally, the majority of the literature agrees that a LIA did occur, some suggesting it was global scale phenomenon (Henson, 2006). Ruddiman counters this, arguing that while the trend of a MWP to LIA is a suitable account for Greenland, Iceland and Northern Europe, it may not describe changes around the rest of the globe. The LIA had cooler phases that are recorded at various sites, specifically in the Northern Hemisphere, however most do not seem to be synchronous or if they were, then only short in length such as 30 years or less. (Ruddiman, 2008; Whyte, 1995). The LIA was possibly the coolest three century period, in the Northern Hemisphere, in the entire post-glacial (O'Hare et al., 2005). Marked regional variables and available evidence indicates that the LIA was principally a Northern Hemisphere phenomenon (O'Hare et al., 2005). Conversely, it is proposed that the LIA was the coldest period globally in thousands of years (Henson, 2006). Fagan (2000) noted that the LIA wasn't constant and was marked by "an irregular seesaw of rapid climate shifts, driven by complex and still little understood interactions between the atmosphere and the ocean". During the LIA, hot, dry summers and mild winters did occur, however these milder intervals happened at a significantly lower frequency than in more recent times (O'Hare et al., 2005). It is suggested that mean global temperatures dropped by 0.5ºC during the LIA with a decrease of around 1.3ºC in European winters, compared to the early 20th Century (O'Hare et al., 2005). Ruddiman (2008) concurs, claiming that reconstructions of Northern Hemisphere temperatures during the LIA were considerably cooler than during the last century.
Evidence
Various sources of evidence are utilized to support or nullify the occurrence of the MWP and LIA. Methods used to evaluate these events can include: historical records, ice cores, glacial-geological evidence, and bore holes. Documentary evidence has shown crops being planted and maintained father north and at higher elevations in the MWP than in some parts of the 20th century as well as the expansion of settlements (Hughes & Diaz, 1994; O'Hare et al., 2005). The construction of a wooden aqueduct across the valley below the Grosser Aletsch Glacier in approximately 1200 AD indicates warmer conditions across the Swiss Alps (Kininmonth, 2004). Ice core evidence has indicated above average isotope oxygen 18 levels at the end of the first millennium A.D. and the early centuries of the second millennium A.D., at sites close to 3000m elevation or higher, indicating a warm period (Hughes & Diaz, 1994). Glacial-geological evidence has demonstrated montane glacier advances in Europe before AD 900 and after 1250 AD, a lack of advances in between, as well as evidence of considerable glacier retreats between 900 and 1250 AD in the Canadian Rockies and European Alps (Hughes & Diaz, 1994). Advancing mountain glaciers around the globe (excluding Antarctica) reached their maximum extent around 1860 AD and further evidence suggests a globally synchronous cooling during the LIA (Kininmonth, 2004). Borehole temperature evidence has revealed a fluctuation in the Greenland profile which matches the timing of the change from the MWP to the LIA (Kininmonth, 2004). Additionally, continental boreholes from around the earth indicate that 500-1000 years ago, temperatures were significantly warmer and that approximately 200 years ago, temperatures were significantly cooler (Kininmonth, 2004).
Other methods used to evaluate the MWP and LIA can include tree rings and sediments as well as other proxy data. Tree ring evidence has demonstrated that there was an increased tendency toward cold conditions in the early and mid 17th century and a warm period sometime between the mid 12th to early 14th centuries (Hughes & Diaz, 1994; ) Furthermore, averages of tree ring chronologies support the large scale occurrence of the MWP over the Northern hemisphere extratropics (Esper et al., 2002). Tree ring data has also revealed that the MWP in the Northern Hemisphere extratropics may have begun in the early 900s, with the warmest period between 950-1045 (Esper et al., 2002; Briffa, 2003). Evidence from sediment stratigraphy, the species compositions of fossil diatom and midge assemblages has shown that in Lake Naivash (Kenya) lake levels and salinity fluctuations in 1000 to 1270 AD were significantly direr than today, while between 1270 to 1850 AD they were significantly wetter, corresponding to the MWP and LIA respectively (Pittock, 2005; Verschuren et al., 2000). Sediment cores from the bottom of a sinkhole located in Belize show that there was a long δ18O low during the Medieval Warm Period from 1000-1400 AD, and a δ18O high around 1500 AD, representing the Little Ice Age (Gischler et al., 2008). Henson (2006) remarks that the Mayans abandoned cities between 750 and 950 AD; sediments examined in the nearby Caribbean indicate strong multi-year droughts during this time, supporting the claim that the MWP was associated with droughts in the Americas. Lichen halos in the Canadian Arctic show an interval of expanded snow fields during the LIA, implying cooler temperatures (Ruddiman, 2008). In parts of the North Western US, Yellowstone Park and central Idaho, pollen and charcoal analyses from sediments reveal greater fire frequency during the MWP and less during the LIA (Pittock, 2005).
Osborn & Briffa (2006) provide evidence for intervals of significant warmth in the Northern Hemisphere within the MWP (890 to 1170 AD) and for significantly colder intervals during the LIA (1580 to 1850 AD), using multiple data sources. Beltrami (2002) concluded that his multiproxy reconstructions, from tree rings and oxygen isotopes in ice cores, did not reveal a strong signal for the LIA, however from his geothermal data, he suggested that there was a cool period between 1500 and 1800 AD. It is evident from the various proxy data reconstructions that a wide range of results can be produced; perhaps this can be attributed to issues with the methods used to evaluate these proxy data.
Methods Of Identification
The various proxy variables that are used to assess climates prior to instrumental recording have limitations, such as spatial, seasonal and timescale restrictions (Jones et al. (1998). Many of the proxy reconstructions are limited seasonally, as they are most representative of summer or growing season conditions (Jones et al., 1998). All proxy variables have possible timescale limitations (Jones et al., 1998). Tree-ring based reconstructions commonly have temporal replication changes, which are often ignored (Esper & Frank, 2009). These replication changes can affect the quality of the climate signal and introduce more random fluctuations (Esper & Frank, 2009; Wigley et al., 1984). Reconstructions produced from ice cores, stalagmites or lake sediments can be associated with lowered resolution the further back in time the data is used as well as reduced dating control (Blass et al., 2007; Esper & Frank, 2009; Fisher et al., 1996; Tan et al., 2006). Jones et al. (2009) states that the traditional method of spatial calibration of the isotopic thermometer may be unsuitable in analyses of time series of ice-core isotope data and may alter models. Changes in ice sheet elevation and in climatic conditions upstream of an ice-core drill site can introduce non-climatic biases in isotopic series (Jones et al., 2009). Documentary data is also under scrutiny as the evidence procured from them can be biased or over exaggerated by the person who recorded it, that is, the information can be subjective (Jones et al., 1998; Hughes & Diaz, 1994). The use of historical documents is limited to regions with long-written histories (Jones et al., 2009). As the above statements suggest, there are a number of problematic issues related to proxy data and climatic reconstruction.
Limitations And Areas For Further Research
Most research seems to be biased towards the northern hemisphere or current global warming. Ruddiman (2008) states that very large uncertainties related to sparse records make it difficult to support the occurrence of a MWP followed by the LIA. Reports produced by the IPCC seem to be biased towards the idea of Global Warming and the Greenhouse effect, their data, while being contradicted by numerous studies (Briffa & Osborn, 2002; Esper & Frank, 2009; Kininmonth, 2004; McIntyre & McKitrick, 2003), is supportive of a steep rise in temperature in the twentieth century. Data used in some studies has been rather outdated, for example, Nesje & Dahl (2003) used data from 1977 and 1974. Kininmonth (2004) and Pittock (2005) comment that the MWP and LIA have widespread documentation for the European North Atlantic region, but to a lesser extent elsewhere. The majority of the data seems to be geographically limited to these areas. Data sparseness and low replication is a problem before approximately 1200 AD (Esper & Frank, 2009). Mann et al. (2003) stated that it is still a challenge to reduce uncertainties and properly synthesize global means in relation to past climates. As noted by Briffa & Osborn (2002), more reconstructions need to be produced from improved proxy records to create a detailed assembly of temperatures and climates for the past 1000 years. It has been suggested that integrated analyses might provide insight into past climate changes and that there is a need to compare results from many climatic reconstructions to obtain an accurate assessment of climate (Beltrami, 2002). However, Soon and Baliunas (2003) suggested that due to the different nature of the proxy data the results of each cannot be combined into a global quantitative synthesis. Further research should attempt to develop the number of high-quality records for as much of the last 2000 years as possible (Hughes & Diaz, 1994).
Conclusion
In conclusion the occurrence, timing, extent, causes and effects of the MWP and the LIA are still being debated amongst researchers. Although plausible theories have been proposed, the conclusive evidence to support these theories is lacking, and more data collection on a global scale is required. Current proxy data methods need to be improved upon and their restrictions taken into account when interpreting results and making conclusions. Future research should focus on developing accurate constructions of past climate rather than focussing solely on the MWP and LIA.
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