  To evaluate the role of Total Solar Irradiance (TSI) on Northern Hemisphere
(NH) surface air temperature trends it is important to have reliable estimates
of both quantities. 16 different TSI estimates were compiled from the
literature. 1/2 of these estimates are low variability and 1/2 are high
variability. 5 largely-independent methods for estimating NH temperature trends
were evaluated using: 1) only rural weather stations; 2) all available stations
whether urban or rural (the standard approach); 3) only sea surface
temperatures; 4) tree-ring temperature proxies; 5) glacier length temperature
proxies. The standard estimates using urban as well as rural stations were
anomalous as they implied a much greater warming in recent decades than the
other estimates. This suggests urbanization bias might still be a problem in
current global temperature datasets despite the conclusions of some earlier
studies. Still, all 5 estimates confirm it is currently warmer than the late
19th century, i.e., there has been some global warming since 1850. For the 5
estimates of NH temperatures, the contribution from direct solar forcing for
all 16 estimates of TSI was evaluated using simple linear least-squares
fitting. The role of human activity in recent warming was then calculated by
fitting the residuals to the UN IPCC's recommended anthropogenic forcings time
series. For all 5 NH temperature series, different TSI estimates implied
everything from recent global warming being mostly human-caused to it being
mostly natural. It seems previous studies (including the most recent IPCC
reports) that had prematurely concluded the former failed to adequately
consider all the relevant estimates of TSI and/or to satisfactorily address the
uncertainties still associated with NH temperature trend estimates. Several
recommendations are provided on how future research could more satisfactorily
resolve these issues.
