The extent of summer sea ice during the  19th century, insofar as it is shown in patterns  of navigability inferred from ship tracks, the  direct observations of explorers, and a number  of native accounts, is remarkably similar to  present ice climatology. A chart of northern  Canada (Figure 2) shows the routes followed  by discovery expeditions and their wintering  locations between 1818 and 1859, and also  displays the frequency that sea ice has  occurred during the recent 30-year reference  period 1971–2000. It is perhaps surprising that  most of the Northwest Passage was navigated  during the 19th century, with expedition ships  coming within 150 km of completing the passage on a number of occasions. Most   significant is that even in years that were   recognized as unfavorable at the time, ships  were still able to reach locations that would  be consistent with the worst ice conditions  that have occurred during the modern reference period. Of 33 expedition or supply ships  bound for the western part of Lancaster  Sound between 1819 and 1859, only two  failed due to unfavorable ice conditions.

seagrant.uaf.edu/nosb/2005/resources/arctic-explorers.pdf