Trends in air temperature in Estonia and in water temperature of Estonian large lakes in 1961-2004,possible consequences on water quality

摘要

According to the Intergovernmental Panel on Climate Change, Fourth Assessment Report (AR4), global mean surface air temperatures (AT) have risen by 0.74 ± 0.18 ℃ when estimated by a linear trend over the last 100 years (1906- 2005). The rate of warming over the last 50 years is almost double of that over the last 100 years (0.13 ± 0.03 ℃ vs. 0.07 ± 0.02 ℃ per decade). Since 1979, warming has been strongest over western North America, northern Europe and China in winter; Europe and northern and eastern Asia in spring; Europe and North Africa in summer; and northern North America, Greenland and eastern Asia in autumn (Tren-berth et al. 2007). In Estonia, annual mean AT increased during 1951-2000 by 1.0-1.7 ℃. The biggest change was observed in March, when the monthly mean temperature increased by 3-5 ℃ (Jaagus 2006). Water temperature and ice cover are most directly affected by climate forcing; the existence of consistent trends has been demonstrated in river water temperatures (Hari et al. 2006), lake water temperatures (Livingstone 2003, Straile et al. 2003, Arhonditsis et al. 2004, Coates et al. 2006, Dokulil et al. 2006) and ice phenology (Weyhenmeyer et al. 2005). The physical response to local meteorological forcing is strongest in the uppermost levels of the water column and is most apparent in lake surface water temperatures (SWT). Long-term water temperature data from the Central European lakes have been analyzed extensively, while only few data sets from Northen European lakes have been published so far. Lakes in northern Europe have shown few significant trends in SWT compared to lakes elsewhere in Europe. No trends were found in lakes in Wisconsin in the northern USA between 1981 and 2001 (Magnuson et al. 2006). Only 1 of the 8 long data series (1961-2000) for lakes in Finland showed a significant increase in SWT (Korho-nen 2002). In 8 Lithuanian lakes, the average trend of SWT was 0.5 ℃ dec~(-1) (Pernaraviciute 2004). In Estonian lakes Vortsjarv and Peipsi, a SWT trend of 0.2-0.3 ℃ dec~(-1) in April and May 1948-2000 was reported by Noges & Jarvet (2005). Hari et al. (2006) showed an abrupt increase of air and river water temperature in winter, spring and summer in 1987/1988. A step jump up of water temperature took place in lakes in Austria, UK, Finland, and Sweden between 1981 and 1993, most commonly in 1988 (L. Arvola, unpubl. data). Our study aimed to reveal the recent trends and the step jump up periods in the AT in Estonia and in SWT of Estonian large shallow lakes.