Background
The authors write that the IPCC (2007) contends that "the anthropogenic release of greenhouse gases is very likely the cause of most of the present global warming," and they add that "ongoing global warming and the conclusion that the Baltic Sea region has experienced a temperature increase greater than the global mean [our italics] over the past century together suggest that at least part of the recent climate change in the Baltic Sea region may be attributed to the increased release of greenhouse gases." However, they rightly suggest that to be confident of this conclusion, the natural variability of the region's climate "must be better understood."

What was done
To obtain this "better understanding," Hansson and Omstedt analyzed a multi-proxy-derived gridded dataset spanning the period 1500 to 2001 with the goal of quantifying the natural variability of water and air temperatures over the Baltic Sea (which occupies the region from roughly 10°E to 30°E and 53°N to 66°N).

What was learned
The Swedish researchers report that over the past 500 years, "three warm periods stand out, two occurring in the twentieth century (1930s and 1990s) and one in the first half of the eighteenth century (mid 1730s to mid 1740s)," and they write that "the eighteenth century warming [was] comparable in magnitude to the late twentieth century warming." What is more, they found that "the 1730s and 1740s [had] even less ice than the 1930s."

Another of their key observations was that "multi-decadal variability in water temperature is on the order of 0.9-1.4°C, while even greater variability is present in air temperature," and they report that "the larger end of the variability is not isolated in the twentieth century but occurs throughout the past 500 years, indicating that shifts like these are within the natural range of variability."

What it means
In light of their several findings, Hansson and Omstedt conclude -- in the final sentence of their paper -- that "even though different climate forcing may operate on the climate system today compared to over the past 500 years, it is not possible to clearly state that the region is experiencing a climate change outside the natural limits of the past half millennium," even after previously acknowledging that the Baltic Sea experienced a temperature increase "greater than the global mean" over the past century, which further suggests that the same may hold true for other parts of the world as well.

Reference
IPCC. 2007. Climate change 2007: the physical science basis. In: Solomon, S., Qin, D., Manniing, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M. and Miller, H.L. (Eds.), Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge.