Can North American "megadroughts", most recently observed in tree ring records from across western North America in the late 16th century, be driven by prolonged ENSO cold phase conditions? If more recent long-term droughts are driven by a different mechanism, there may be potential for even stronger droughts in a greenhouse world. We have applied an ensemble climate field reconstruction technique to the estimation of Pacific sea surface temperatures from a sparse multiproxy network of high resolution marine and terrestrial proxy records. The ensemble conserves the sum of error plus signal variance, despite dramatic changes in the number of proxy datasets available over time. The results may be used to develop forced ensemble atmospheric general circulation modeling experiments designed to test the motivating hypothesis, given realistic error estimates in the reconstructed forcing fields.