Baltic Sea past climate
A few years ago, the American scientist Michael Mann launches his thesis on global temperature variations during the past millennium. Using indirect observations from earlier times and measurement series from 1850 and onwards, he showed that the variations were small until a hundred years ago, when there was a steep rise in the curve. Many have used Mann's curve to support the theory that the higher quantity of greenhouse gases impacted the climate in the 20th century. Others have questioned both Mann's method and his results. Mann's conclusions are based on a skewed distribution of data. While the mass of information certainly becomes denser when we gain access to instrumental data, its reliability is inadequate. Data on temperature in the sea are few. The measurements were taken on land and frequently in areas of strong population growth. Sites and instruments were often changed.
From the majority of the period, we have only sparse, indirect observations of tree rings, sediments, and drill cores; observations that have been translated to temperature. These are reinterpretations that entail a large measure of uncertainty. Nor is ground temperature alone sufficient to characterize something as complex as the climate. Our studies of the Baltic Sea show that the total heat balance must be taken into account (Omstedt and Nohr , 2004). Such an analysis will perhaps not become reliable for the entire earth's heat content until we can combine instrumental and global satellite data. Here, there is a great deal for future research to do.
Mann's thousand-year curve should not be taken as gospel. The conclusion that the 20th century was unique may be a hasty one. What if the deviation is only an expression of longer and better measurement series, or constitutes a normal variation that becomes apparent only when longer periods of time are studied in detail? What was the situation in the 1930s.the early 1700s.the Middle Ages?
If anything, Mann's work emphasizes the need for more in-depth studies. Existing climate archives must be supplemented with new ones. Let us give one example. One of the most intriguing long climate series shows how the Baltic Sea has frozen year by year from the early 1700s to the present day. The bar chart shows (Figure 1) the size of the ice cover every year. The greatest climate change in the period takes place in the late 1800s (Omstedt and Chen, 2001, Omstedt et al., 2004). The breakpoint marks the end of the "Little Ice Age" and the beginning of the milder period we are in today. What caused the climate change is uncertain, but our studies show that many low-pressure fronts moved in over the Baltic Sea during this phase.
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Figure 1. Annual size of the Baltic Sea ice cover since the 1700s based on various types of observations. The annual variation is large, but the variations are considerably smaller when illustrated as 30-year mean values (red curve). The green line shows the average size of the ice for the entire period and the circle marks a breakpoint where the climate changed and the Little Ice Age ended.
Our understanding of the distribution of the ice is based on various types of information. In modern times, data are provided by weather stations, ice-breakers, the coast guard, and satellites. Earlier observations are based on information found in newspapers, lightship logs, other log-books, travelogues, and similar sources. But that kind of information is becoming increasingly thin. To find out about the distribution of the ice before 1720, we must employ other methods.Our research team analyzed the correlation between winds and ice. Using reconstructed atmospheric pressure, we could connect to how the winds blew and the ice distribution. As usual, it involved a combination of measuring data and indirect methods. There are only a few indirect measurement series available for Northern Europe before 1720. Unevenly distributed information of varying quality is a problem. Despite that, we believe our model can tell us something about how the climate has varied in the Baltic Sea.
We also got help from an unexpected source: the old Swedish monarch Carl X Gustaf, who walked with his army across the ice in the Little Belt and the Great Belt in 1658. Our model calculations, shown in the diagram, also indicate that it was cold that year. A documented historical event thus supports our calculations and shows how historical archives can be meaningful to climate research.
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Figure 2. The climate model's calculations of the size of the ice cover in the 1650s. The model indicates that there was a great deal of ice in 1658, which is consistent with the historical fact that King Carl X Gustaf and his army walked across the Little Belt and the Great Belt that year.
Swedish researchers have formed a network aimed at mapping the climate of Sweden for the past two thousand years (MUSCAD). We are meeting annual and one of the subjects of discussion during last meeting 2004 in Göteborg was the value of historical documents as sources of knowledge about the climate. Sweden has unique documents that have never been examined in this context. Working together, humanist researchers and scientists can piece together a picture of past seas and ancient climates.