No abstract

In order to evaluate how much Total Solar Irradiance (TSI) has influenced Northern Hemisphere surface air temperature trends, it is important to have reliable estimates of both quantities. Sixteen different estimates of the changes in TSI since at least the 19th century were compiled from the literature. Half of these estimates are “low variability” and half are “high variability”. Meanwhile, five largely-independent methods for estimating Northern Hemisphere 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 widths as temperature proxies; 5) glacier length records as temperature proxies. The standard estimates which use urban as well as rural stations were somewhat anomalous as they implied a much greater warming in recent decades than the other estimates, suggesting that urbanization bias might still be a problem in current global temperature datasets – despite the conclusions of some earlier studies. Nonetheless, all five estimates confirm that it is currently warmer than the late 19th century, i.e., there has been some “global warming” since the 19th century. For each of the five estimates of Northern Hemisphere temperatures, the contribution from direct solar forcing for all sixteen estimates of TSI was evaluated using simple linear least-squares fitting. The role of human activity on recent warming was then calculated by fitting the residuals to the UN IPCC’s recommended “anthropogenic forcings” time series. For all five Northern Hemisphere temperature series, different TSI estimates suggest everything from no role for the Sun in recent decades (implying that recent global warming is mostly human-caused) to most of the recent global warming being due to changes in solar activity (that is, that recent global warming is mostly natural). It appears that previous studies (including the most recent IPCC reports) which had prematurely concluded the former, had done so because they failed to adequately consider all the relevant estimates of TSI and/or to satisfactorily address the uncertainties still associated with Northern Hemisphere temperature trend estimates. Therefore, several recommendations on how the scientific community can more satisfactorily resolve these issues are provided.

Long‐term trends in solar quiet geomagnetic field variation (Sq) are studied in connection to the Earth's magnetic field secular variations and increasing greenhouse gas concentrations. Sq is mainly caused by ionospheric current systems that flow in the E region and depends, among other variables, on the ionospheric conductivities. These conductivities in turn depend on the Earth's main magnetic field (B) and the electron concentration in the E region, for which foE is a measure of its peak value. Since B shows secular variation, induced long‐term changes in Sq might be expected. Another possible mechanism that would be able to induce Sq trends is the increasing concentrations of greenhouse gases that produce a cooling effect in the upper atmosphere and, according to model predictions and experimental results, an increasing trend in foE. To detect if both mechanisms mentioned are able to induce trends in Sq, the Sq variation of the horizontal intensity (H) of three magnetic observatories (Apia, Fredericksburg, and Hermanus), for which B is decreasing, is analyzed for the period 1960–2001. We find significant increasing trends (6.6%, 5.4%, and 9.9%, respectively) which may be partially accounted for by B secular variations in the respective sites. The Sq trend expected from the theoretically predicted foE increase is low (∼0.5%), although positive, as is the observed trend.