According to the authors, "For the dry season (May-October), [solar irradiation] statistically significant (p ≤0.05) positive trends of 9–18 W m-2 (3–6%) per decade were found at all four high elevation stations tested." By way of comparison, the IPCC alleges a 3.7 W/m2 forcing from a doubling of CO2 levels, 3.6 times less than the mean change of 13.5 W/m2 found per decade in this paper. The 2.4 decade long study thus indicates a 13.5*2.4 = 54 W/m2 mean change in solar irradiation from 1988-2012. Per the IPCC formula for CO2 forcing, the change in CO2 levels from 1988-2012 allegedly produced 0.6 W/m2 forcing*, 90 times less than the change in dry season surface solar irradiation found by this study. The paper joins many other peer-reviewed publications describing global brightening in the latter 20th century.
As Dr. Roy Spencer notes,
"The most obvious way for warming to be caused naturally is for small, natural fluctuations in the circulation patterns of the atmosphere and ocean to result in a 1% or 2% decrease in global cloud cover. Clouds are the Earth’s sunshade, and if cloud cover changes for any reason, you have global warming — or global cooling."*5.35*ln(393.3/351.56) = 0.6 W/m2 CO2 forcing increase 1988-2012
Temporal Solar Radiation Change at High Elevations in Hawai‘i
Ryan J. Longman et al
Trends in downwelling global solar irradiance were evaluated at high elevation sites on the island of Maui, Hawai‘i. Departures from monthly means were assessed for the 6-month Hawaiian wet and dry seasons over the period 1988 to 2012. Linear regression analysis was used to characterize trends in each season. For the dry season (May-October), statistically significant (p ≤0.05) positive trends of 9–18 W m-2 (3–6%) per decade were found at all four high elevation stations tested. Wet season trends were not significant, except at the highest elevation station, which had a significant negative trend. No consistent trends in aerosol concentrations have been observed at high elevations in Hawai‘i, therefore, the observed dry-season brightening is most likely the result of decreasing cloud cover. Supporting this hypothesis, analysis of 15 years (1997-2012) of high temporal resolution Geostationary Operational Environmental Satellite (GOES) imagery over the Hawaiian Islands showed a statistically significant decrease in leeward cloud cover amounting to 5–11% per decade over the stations. In addition, analysis of Moderate Resolution Imaging Spectroradiometer (MODIS) data were in general agreement with the GOES trends, although statistically significant dry-season trends were found at only one of the four stations.