He writes:

“The different hemispheric records in the mid-twentieth century have never been convincingly explained. The most likely explanation is atmospheric aerosols…However, there are no aerosol measurements to confirm that interpretation. If there were adequate understanding of the relation between fossil fuel burning and aerosol properties it would be possible to infer the aerosol properties in the past century. But such understanding requires global measurements of aerosols with sufficient detail to define their properties and their effect on clouds, a task that remains elusive”

http://www.columbia.edu/~jeh1/mailings/2009/20091216_TemperatureOfScience.pdf

Also discussed at http://wattsupwiththat.com/2009/12/21/hansen-on-the-surface-temperature-record-climategate-solar-and-el-nino/

It is worth noting that the divergence problem is not restricted to a few sites. Briffa describes it as “widespread” in the Northern Hemisphere.

“Climate scientists are worried in particular about tree-ring data from a few northern sites. By examining temperature measurements from nearby, researchers know that tree growth at these locations tracked atmospheric temperatures for much of the twentieth century and then diverged from the actual temperatures during recent decades. It may be that when temperatures exceed a certain threshold, tree growth responds differently.”

i.e. therefore tree rings are useless as a proxy to show that 20th Century temperature rises are unprecedented, and that there was no Medieval Warm Period, since they apparently don’t respond to increases in temperature.

Funny, the phrase ‘hide the decline’ springs to mind…