Special Issue

Nature Conservation 2009 — 1. 9. 2009 — Special Issue

Current trends in the climate and projections for the future

author: Jan Pretel

The climate is a long-term characteristic weather regime, determined by the energy balance, atmospheric circulation, active surface characters and, recently, also human beings.

Through their activities, humans have directly or indirectly contributed particularly to changes in the energy balance of the whole climate system. This is not simply a matter of greenhouse gas emissions into the atmosphere, but is also related to mankind’s influence on other components of the system the ocean, cryosphere, lithosphere and biosphere. The temperature is an important indicator of the on-going changes, as it is closely connected with changes in the systems energy balance.

Trends in the global climate

Growing trends in the global temperature and their physical consequences are now quite obvious and indisputable. The tempera­ture has increased by 0.74 ^oC over the past century, with a trend in increase over the past 25 years of 0.18 ^oC per decade, which is approximately 2.5 times as great as the average for the whole last century (Solomon et al.2007). While the northern hemisphere has become warmer over the past quarter century by 0.24 ^oC per decade, the trend in the southern hemisphere is only half as large. In the areas above the northern Arctic Circle, the temperature is increasing at a rate of approx. 0.6 ^oC per decade, while the increase in tropical regions is only one quarter of this Đ thus the changes occurring on the planet are very inhomogeneous. The main cause of this lack of homogeneity consists in differences in the distribution of the land and sea and albedo (the ratio of diffusely reflected to incident electromagnetic radiation) on the Earth’s surface.

More than 80 % of anthropogenic heat is absorbed by the oceans and their surface layers are heated rapidly. For example, over the past 40 years, the contribution of the water thermal expansivity to sea level rise has increased four times. The Arctic and continental glaciers and the snow cover are decreasing in extent and are a source of a further increase in the water volume. Warmer water has lower ability to absorb carbon from the atmosphere and the ocean, together with changes in circulation, fundamentally affects the humidity and precipitation regimes all over the planet. From there, it is only a step to rapid changes in the climatic regime over the entire planet.

Observed trends in changes in Europe

The position of the European continent is the main cause of the highly regional varia­bility in the climate. As there is an exceptionally dense network of long-term measuring stations in Europe, supplemented by a number of long-distance measurements, analyses of trends in changes are far more exact there than anywhere else on the pla­net. Over the past century, the temperature on the European continent has increased by an average of 1.2 ^oC, of which the increase over the past 25 years was 0.45 ^oC, almost half as much again as the global average. While the trend in increase over the past half century was approx. 0.1 ^oC per decade, this has increased to twice as much over the past decade. The temperature over the land is increasing faster than over the surrounding ocean Đ most markedly over the Iberian Peninsula, in Central and Northeast Europe and in Alpine regions and also in North Europe in the winter time. As a whole, the greatest warming in Europe has occurred in spring and summer (the effect of the more frequent occurrence of episodes with extremely high temperatures) and least in the autumn months. Extremely low tempera­tures are less frequent. The mean number of summer days has doubled over the past century and the number of tropical days has even tripled. Of the thirteen-year period of 1996Đ2008 nine of the years were among the 12 warmest since 1850 and the war­mest years were 1998 and 2005 (EEA 2008). However, because of the mentioned regional variability of the climate, there can be substantial differences in details among the individual parts of the continent.

Observed trends in changes in the Czech Republic

The trends in observed changes in temperature can be illustrated using data from the Prague-Clementinum station. The mean annual temperature has increased constantly; over the past 15 years, the winter temperatures (December to February) have increased somewhat more rapidly than the summer (June to August) or annual temperatures. This is also confirmed by the linear trends in changes over the past 100, 50, 25 or 10 years (Table 2), which are in good agreement with general information for the European region (EEA l.c.). The changes in mean va­lues are associated with extreme values Đ the number of tropical summer days has recently increased, while the number of frost and ice days has decreased.

Tables 1 and 3 present the linear trends in mean territorial temperatures and precipitation, which are modified values from the general processing of data from the entire network of stations, taking into account the positions of the individual stations, and can therefore, to a certain degree, refine the concept of current trends in the weather over the country. The values also confirm the constantly growing trend in the increase in mean temperatures, which is more pronounced in winter and summer. The differences between the western and eastern half of the country are statistically insignificant and vary at the limits of precision of the estimates. Deviations in the mean annual and seasonal temperatures compared to the 1961Đ1990 normal values have exhibited positive values over the entire past twenty-year period (with the exception of 1996), which have been particularly marked in the warm half of the year (April to September).

Trends in total winter precipitation over the past few years indicate a substantial reduction in total precipitation in all the seasons of the year, with the exception of winter, which are more noticeable in spring and summer. In contrast to temperatures, diffe­rences are apparent between the two parts of the country. In the eastern part of the Czech Republic, reductions in summer precipitation are more conspicuous and cause the decrease in annual totals; in the western part, the increase in winter precipitation is more marked, resulting in an increase in annual total values. The diferences found correspond to the geographic trend in changes in precipitation in Europe in 1961Đ2006 (EEA l.c.). Nonetheless, in comparison with the seasonal normal values, these changes correspond to a maximum of one percent of the total precipitation per decade.

The changes in the zonal and meridional components of the flow are, with a few exceptions, statistically insignificant and spatially very variable. Therefore, changes in the atmosphe­ric circulation in the Czech Republic cannot be unambiguously traced. However, in recent years, it seems that the systematically positive trend in the North Atlantic Oscillation over the past decades and consequent increase in wind speeds that have been appa­rent in West Europe in the past few years is already beco­ming manifested in the country.

Trends in the relative humidity, cloud cover, hours of sunshine, snow cover depth and duration are mutually consistent and correspond well to trends in the temperature and its amplitudes. Winter, spring and summer are characterized by an increase in the number of hours of sunshine, decreasing cloud cover and decreases in the relative humi­dity. In contrast, in autumn, when the temperature and daily amplitude decrease, a reduction in the number of hours of sunshine and an increase in cloud cover and relative humidity can be observed (Moliba et al.2006). The mean number of days with snow cover at the elevation to 600 m a.s.l. has decreased over the past 20 years by an average of approx. 15 % compared to the usual number of days in the middle of the past century (shortening of the season by 12 days); this decrease was approx. half of that at higher altitudes. The maximum snow cover has decreased by 25 % at lower altitudes and by up to 30 % at higher altitudes. Total amounts of new snow in the winter exhibit similar trends (Němec 2005).

Climate change projections for Europe

The mean temperatures in Europe will continue to increase, and this will probably be faster than on any other continent. Regardless of the choice of SRES scenario (Nakicenović et al.2000), temperatures will increase in the first third of this century by more than 0.2 ^oC per decade and the increase in temperature by the end of the century can be expected to equal from 1.0 to 1.5 ^oC compared to the 1961Đ1990 period (EEA l.c.). The temperature will increase more rapidly in East Europe and Scandinavia, including the Arctic areas in the winter and Southwest Europe and the Mediterranean in summer. Extreme heat weaves will continue to be more frequent (the Iberian Peninsula, Central Europe including the Alps, the eastern parts of the Mediterranean and the southern part of Greece).

The movement of humid air masses from the Atlantic Ocean and the Mediterranean will be decisive for the precipitation regime. Together with the increasing temperature, the change in the precipitation regime will be the main cause of more frequent occurrences of floods or periods of drought. The changes will be accompanied by substantial regional differences and seasonal variations resulting from the specific circulation conditions in the particular area. We assume that the total annual precipitation in North Europe will increase by up to 20 %, while it will decrease by 5 to 40 % in South Europe and the Mediterranean. There will be an increase in winter precipitation in Central and North Europe, while the total summer precipitation will decrease in Central and South Europe; much smaller changes can be expected in the spring and especially in the autumn.

Climate change projection for the Czech Republic

The climate change projection for the Czech Republic is currently being updated on the basis of refinement of regional scenarios being developed during the pro­ject of the Ministry of the Environment of the Czech Republic No. SP/1a6/108/07. Preliminary results of the ALADIN CLIMATE CZ simulation model (so far only in characteristic valu­es for the territory of the country) indicate that, to the end of the 2030s, in scenario A1B (Nakicenović et al. l.c.), the mean temperatures will increase compared to the 1961Đ1990 period by the values according to Table 4, which also gives the expected range of values (median Q50, lower and upper quartile Q25 or Q75). The trend in the increase in the mean annual temperatures (0.24 ^oC per decade) corresponds well with the values given by Solomon et al. (l.c.),i.e.0.2^o C per decade.

The mean temperatures should increase more rapidly in autumn and in winter (ma­xima in March and September), while the increase in spring and summer temperatures will be less (minimum in May). Lower trends in increase in temperature in the warm and greater trends in the cooler half of the year indicate that the temperature differences between the seasons will become more equalized, in accordance with EEA (l.c.) and Solomon et al. (l.c.). Maximum and minimum temperatures will change similar to chan­ges in the mean temperatures. Maximum temperatures will exhibit a trend towards a clear increase in winter and summer; the minimum temperatures will tend to increase particularly in summer and partly also in autumn and winter.

In a similar way, simulated changes in total precipitation (Table 5) indicate the possibility of a slight increase in annual totals (on an average by approx. 4 % compared to 1961Đ1990), higher in winter and spring (maximum February to April), lower in summer and autumn (minima in July to November). The difference between the values for the two quartiles in the individual months (4 to 17 %) suggests substantial variabi­lity in mean total precipitation. The value of quartile Q25 in the period from May to October (possible reduction in total precipitation by 2 to 8 %), together with increased evaporation in these months, shows the risk of an increase in soil water deficit.

If the simulated temperature trends (Tab­le 4) are compared with current trends, it seems that temperatures will probably vary at the level of the Q75 quartile by the end of the 2030s. Acceptable correlation of results can be determined from the standpoint of seasonal changes and a truly rapid increase in mean winter and autumn temperatures. Similar comparison of total precipitation (Table 3 and 5) indicates that the agreement of model simulation with the results of current observations is much lower for precipitation. It can be more pro­bably expected that total precipitation will vary around the level of the lower quartile Q25; nonetheless, the probability of an increase in total winter precipitation is high.

However, it has been confirmed that, within the European continent and particularly in Central Europe, model prediction of the precipitation regime is burdened by a much higher level of uncertainty than similar temperature projections (Bates et al.2008, EEA l.c.). This is manifested both in the effect of the relatively complicated topography of the continent, which currently does not permit sufficiently detailed mapping of precipitation processes, and one of the predominant manifestations of climate change in Europe Đ increased temporal and spatial variability in precipitation.

Conclusions

Preliminary simulation of further climate change development in the Czech Republic to the end of the 2030s will be gradually refined for the individual parts of the country. Nonetheless, it is clear from the mentioned preliminary outputs that the present trend in climate change will continue and will most probably become more intense. Thus, it is quite justified for us in the Czech Republic to be increasingly concerned particularly with the consequen­ces of the changing climate and to prepare suitable adaptation measures, such as a set of possible adaptations of the natural or anthropogenic environment to on-going or projected climate changes and their impacts. They must be concerned with the most vulnerable areas of our lives, i.e.not only water management, agriculture, fores­try and landscape protection, but it is also necessary to take into consideration effects on human health, tourism, transport and energy production (Pretel 2007). Reduction of greenhouse gas emissions as one of the phenomena associated with the impact of humans on the increasing greenhouse effect can certainly alleviate the impacts of climate change, but not completely eliminate them. Consequently, it is important to pay more attention and to make greater efforts towards adaptation measures.

The author is Head of the Department of Climate Change at the Czech Hydrometeorological Institute Prague and National Representative in the Intergovernmental Panel on Climate Change (IPCC).