What was done
The authors utilized 60 specimens of freshwater pearl mussels that had been collected alive between AD 1853 and 1930 (stored in museums) and between AD 1986 and 1997 from six rivers in northern, central and southern Sweden to reconstruct a 217-year history of summer (June-August) air temperature for the country, which they compared with the Scots pine tree-ring-derived summer air temperature reconstruction of Briffa et al. (1990).

What was learned
Schone et al. report that "both series do not reveal any significant long-term temperature trends," but they say that "the number of cold summers was higher prior to about AD 1900 as compared to the last 90 years."  Also, their data reveal that the peak temperatures of the late 1980s and early 1990s were no higher than those of the 1930s and 40s, while the coldest summer of the entire record (by far!) occurred on the last full year of the series.

What it means
Clearly, these temperature reconstructions reveal absolutely nothing that is unusual about either the entire 20th century or its last two decades, which are routinely described by climate alarmists as having experienced unprecedented warming over the last thousand years and perhaps even the past two millennia.  If such occurred anywhere, it wasn't in Sweden.

Reference
Briffa, K.R., Bartholin, T.S., Eckstein, D., Jones, P.D., Karlen, W., Schweingruber, F.H. and Zetterberg, P.  1990.  A 1400-year tree-ring record of summer temperatures in Fennoscandia.  Nature 346: 434-439.