The warming trend for the entire globe (1850 to 2005) is 0.04°C decade–1. A specific warming period started around 1980 and continues until the present. This warming also occurred in the Baltic Sea catchment, which lies between maritime temperate and continental subarctic climate zones. A detailed study of climate variability and the associated impact on the Baltic Sea area for the period 1958 to 2009 revealed that the recent changes in the warming trend are associated with changes in large-scale atmospheric circulation over the North Atlantic. The number and pathways of deep cyclones changed considerably in line with an eastward shift of the North Atlantic Oscillation centers of action. There is a seasonal shift of strong wind events from autumn to winter and early spring. Since the late 1980s, the winter season (DJFM, i.e. December to March) of the Baltic Sea area has tended to be warmer, with less ice coverage and warmer sea surface temperatures, especially pronounced in the northern parts of the Baltic Sea. There is a tendency for increased cloud cover and precipitation in regions that are exposed to westerlies and less cloud coverage at the leeward side of the Scandinavian Mountains and over the Baltic Sea Basin.

Detailed assessment of climate variability of the Baltic Sea area for the period 1958-2009 (solicited)

Quasi-decadal variability in solar irradiance has been suggested to exert a substantial effect on Earth's regional climate. In the North Atlantic sector, the 11-year solar signal has been proposed to project onto a pattern resembling the North Atlantic Oscillation (NAO), with a lag of a few years due to ocean-atmosphere interactions. The solar/NAO relationship is, however, highly misrepresented in climate model simulations with realistic observed forcings. In addition, its detection is particularly complicated since NAO quasi-decadal fluctuations can be intrinsically generated by the coupled ocean-atmosphere system. Here we compare two multi-decadal ocean-atmosphere chemistry-climate simulations with and without solar forcing variability. While the experiment including solar variability simulates a 1–2-year lagged solar/NAO relationship, comparison of both experiments suggests that the 11-year solar cycle synchronizes quasi-decadal NAO variability intrinsic to the model. The synchronization is consistent with the downward propagation of the solar signal from the stratosphere to the surface.

Solar forcing synchronizes decadal North Atlantic climate variability

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.

Richard Cina

Papers

Decadal and multidecadal natural variability in European temperature