The IPCC Fourth Assessment Report (AR4) (Solomon et al., 2007) summarizes publications showing global surface temperature remaining constant or slightly decreasing from the 1950s to the 1970s, but drastically increasing thereafter. Surface temperature is shown to increase more over continents than the oceans and rising more in the Northern- than the Southern hemisphere (Brohan et al., 2006). Very strong increases are observed in the inner parts of continents such as in Central Europe, where the temperature rise of 1.3 °C since 1981 is about twice as large as the average increase on the Northern Hemisphere (Philipona et al., 2004; Rebetez and Reinhard, 2007). Variations in total solar irradiance measured from spacecraft since 1979 are very small and are unlikely to have appreciably contributed to the rapid temperature rise observed over the last three decades (Foukal et al., 2006). Surface albedo may well have declined with an observed step-like decrease in snow days in Central Europe at the end of the 1980s (Marty, 2008), but long-term records of the radiation change involved are not available. Surface solar radiation measured at various regions around the globe shows rather decreasing irradiances after the mid-1950s and increasing trends since the 1980s (Ohmura, 2006). In Europe, this dimming (Ohmura and Lang, 1989; Russak, 1990; Gilgen et al., 1998; Stanhill and Cohen, 2001; Liepert, 2002; Power, 2003) and brightening of surface solar irradiance (Wild et al., 2005; Norris and Wild, 2007; Stjern et al., 2009; Wild, 2009) were related to cloud changes and changes of the aerosol load in the troposphere, which strongly increased after the Second World War, and drastically decreased due to enormous efforts to curb emissions and air pollution since the mid-1980s (Mylona, 1996; Sliggers and Kakebeeke, 2004; Vestreng et al., 2007). Aerosol optical depth (AOD) records from six remote locations in Europe confirmed a strong decline of aerosol concentrations since the 1980s and concurrent rising of solar irradiance measured at a large number of stations confirmed solar brightening over continental Europe (Ruckstuhl et al., 2008). Trends of first rising and then declining emissions of atmospheric pollutants over recent decades have been reported from different regions in the world (Stern, 2006; Streets et al., 2006). Satellite measurements have been reported with respect to trends in aerosol load (Mishchenko et al., 2007; Wang et al., 2009) but the results are uncertain (Anderson et al., 2003).
In this paper, we show that despite solar dimming and brightening temperature in the Alps increased primarily due to rising greenhouse gases. We use measurements from the Alps and the surrounding Lowland, which show different temperature, humidity and surface radiation changes, to demonstrate the climate forcing of greenhouse gases, aerosols and clouds at different elevations in Central Europe.
2. Temperature, humidity and solar radiation trends
2.1. Measurements from 25 lowland and 10 alpine stations
We contrast surface temperature, humidity and solar radiation measurements from 1981 to 2005 at 25 stations in the Swiss Lowlands to measurements made at ten stations in the Alps (Figure 1). The measurements are homogenized (Begert et al., 2005) and are from identical stations of the MeteoSwiss Automatic Network (ANETZ). Monthly means of the individual parameters are averaged over the 25 and the 10 stations respectively, and annual means are calculated for the mean elevation at 500 m a.s.l. in the Swiss Lowlands and 2200 m a.s.l. in the Alps.
2.2. Long-term statistics and trends from 1981 to 2005
Temperature (T), relative humidity (Urh), absolute humidity (Uabs) and shortwave downward radiation (SDR), all measured 2 m above ground, are shown in Figure 2. Annual means from 1981 to 2005 are averaged over the 25 Lowland (left graphs) and the ten Alpine stations (right graphs). Linear regression lines through the 25 years (dashed green lines) and trend values (green numbers) are given per decade with the 95% confidence interval in square brackets [ ± 2 stdev] (trends shown later in the text have the same notation). In order to show that the observed temperature rise is not merely due to the extreme radiation increase in summer 2003, linear regression lines and trends are added without 2003 to all parameters (red lines and red numbers). Since the interest here is rather to investigate gradual ‘long-term’ changes than extremes, we concentrate in the following on trends without the year 2003. Average values over the 25 year period of the individual components (without 2003) are shown in the upper left corner.
T increases less in the Alps than at Lowlands, but even without 2003 trends are statistically significant at both elevations. The lower temperature increase in the Alps is interesting because model calculations rather expect larger greenhouse warming at higher altitudes (Manabe et al., 1991). Urh also shows an interesting behaviour with a small non-significant negative trend in the Lowlands but a statistically significant positive trend of 1.3% decade−1 in the Alps. Uabs shows statistically significant positive trends at both elevations. SDR increases are non-significant but considerably larger in the Lowlands than the Alps. Details on the radiation increase in relation with declining aerosols and changing cloud amount since the 1980s were shown in a recent study (Ruckstuhl et al., 2008). This study reveals that AOD is much larger at low elevations and consequently decreased much more at the Lowland station Payerne (490 m a.s.l.) than at alpine stations such as Davos (1610 m a.s.l.) and Jungfraujoch (3580 m a.s.l.). Hence, the lower increase of solar radiation observed in the Alps is likely due to a reduced aerosols effect on solar radiation at higher altitudes.
3. Surface radiation- and energy budget
3.1. Shortwave net radiation
The individual radiation components and decadal trends and 95% confidence intervals are shown in Figure 3 for annual means averaged over the 25 lowland (left) and the 10 alpine (right) stations. In the radiation budget solar radiation only appears as shortwave net radiation (SNR), with the reflected part due to the surface albedo (A) being subtracted by multiplying SDR by (1 − A). Because continuous albedo measurements are not available over the full measuring period, no assumptions are made on possible climate forcings due to changing albedo. Instead, albedo measurements at Weissfluhjoch (WFJ 2690 m a.s.l.) and Payerne (PAY 490 m a.s.l.) were used to calculate average monthly means, and these values were then used for alpine lowland stations respectively.
3.2. Longwave downward radiation
The longwave downward radiation (LDR) is derived from monthly values of absolute humidity (Uabs) measured at all stations and an empirical relation determined with monthly values of LDR and Uabs measured from 2001 to 2005, at two Lowland reference stations Locarno-Monti (388 m a.s.l.) and PAY (498 m a.s.l.), as well as two mountain reference stations Davos (1598 m a.s.l.) and Jungfraujoch (3584 m a.s.l.). The used relation is: LDR = 189.36*Uabs∧0.26 (Ruckstuhl et al., 2007). This empirical relation is based on humidity and related temperature changes, and does therefore not account for LDR increases due to anthropogenic greenhouse gases (aGHG), nor does it account for changes related to changing cloud amount and optical thickness. At the tropopause the radiative forcing of all the long-lived, well-mixed greenhouse gases increased from 1.7 W m−2 in 1979 to 2.65 W m−2 in 2004 (Hofmann et al., 2006). A similar anthropogenic greenhouse forcing was found by surface measurements in the Alps (Philipona et al., 2005). An estimated surface forcing due to aGHG increases of + 0.35 [+0.30 to + 0.40] W m−2 decade−1 is therefore added. Hence, LDR shown in Figure 3 represents annual averages of LDR that increases due to changes of temperature and all greenhouse gases, but possible changes of cloudiness are not included here.
3.3. Longwave upward radiation
Thermal- or longwave upward radiation (LUR) is emitted from the earth's surface with an average emissivity of about 0.96. However, since more than 80% of the LUR is emitted back as LDR (Philipona et al., 2005), and surface absorption is equal to emission, LDR is partly reflected on the ground, adding to LUR and leading to an apparent emissivity of about 99% (Marty et al., 2002). LUR is negative in the radiation budget and is calculated using the Stefan-Boltzmann law and monthly mean temperature values measured at the 2 m level at the individual radiation stations. LUR trends are therefore directly related to temperature trends.
3.4. Total net radiation
The radiation budget at the surface is the balance between downward and upward radiation fluxes, or the sum of SNR, LDR and LUR, which result in the total net radiation (TNR). TNR is positive and the largest part of this energy is used in the phase change of water, which results in the latent heat flux. A smaller part is changing the temperature of the air, which results in the sensible heat flux, and a minor part changes the subsurface temperature, which results in the ground heat flux. Hence, with the increase of TNR over the investigated period an increase of the latent and sensible heat fluxes, and therefore an increase of water vapour in the atmosphere is expected.
4. Humidity and total net radiation increase with Clausius–Clapeyron
4.1. Humidity and total net radiation change with altitude
From the Lowlands (500 m a.s.l.) to the Alps (2200 m a.s.l.) average temperature over the investigated period decreases by 9.1 K, whereas absolute humidity decreases by 3.3 g m−3 (Figure 4). This results in an altitude dependence rate of water vapour (ADRUabs) of 6.4% per K−1. (The percentage is found by dividing 3.3 g m−3 by 9.1 K and again dividing by 5.65 g m−3, which is the mean of the absolute humidity between the Lowland and the Alps). A water vapour change of 6–7% K−1 is expected by the law of Clausius–Clapeyron. Figure 4 also shows that TNR decreases with a similar altitude dependence rate (ADRTNR) of 6.8% K−1. This shows that with respect to altitude water vapour at the surface in fact changes by a similar percentage as TNR.
4.2. Humidity and total net radiation change with climate change
TNR in the Lowland increases by + 1.38 [+0.10 to + 2.65] W m−2 decade−1 or 2.5% decade−1 with respect to the average TNR of 55 W m−2 over the investigated period. Absolute humidity Uabs at the surface, which for monthly means is proportional to the atmospheric column integrated water vapour (IWV) (Ruckstuhl et al., 2007), increased by 2.9% decade−1. Dividing the percentage change of TNR and Uabs by the observed decadal temperature rise of + 0.49 °C decade−1 results in climate change rates for the TNR (CCRTNR) of 5.1% K−1 and for absolute humidity (CCRUabs) of 5.9% K−1 in the Lowland. Hence, the rate of change of TNR and Uabs due to climate forcing is also close to 6% (Clausius–Clapeyron) and comparable to the altitude dependence rate.
In the Alps, TNR increased by 6.9% decade−1and absolute humidity by 4.5% decade−1. With respect to the temperature rise in the Alps of + 0.31 °C decade−1 the CCRTNR is 22.2% K−1 and the CCRUabs is 14.5% K−1. The strong absolute humidity increase in the Alps is related to the observed 1.3% decade−1 increase of relative humidity, which represents a temperature-specific Urh increase of 4.1% K−1. This large increase of water vapour at high altitude is not fully understood but the strong increase of TNR demonstrates a very large water vapour feedback and more energy available in the Alps to increase the sensible and latent heat fluxes. Similar large Urh increases (0.5%–2.0% decade−1) were observed over the Central and Eastern United States, India and Western China, resulting from large Uabs increases coupled with moderate warming (Dai, 2006). In contrast, at Lowlands in Switzerland, Urh rather decreased as observed in other investigations made in continental Europe (Willet et al., 2008).
5. Radiative forcing
5.1. Shortwave forcing
Individual forcings of shortwave SNR and longwave LDR components from changing anthropogenic aerosols, greenhouse gases and cloudiness, as well as radiative flux changes that are due to changing surface temperature and humidity are shown in Figure 5. In their analysis Ruckstuhl et al., (2008) showed that the strong AOD decline in Europe had a large impact on SNR through the direct aerosol effect and only a marginal impact from cloud-mediated indirect aerosol effects. Figure 5(a) shows the shortwave forcing (SNRTotal) which is the sum of the forcing due to declining aerosols (SNRAOD) and due to declining cloud amounts (SNRCloud) in the Lowlands and the Alps (SNRAOD and SNRCloud from Philipona et al., 2009). In the Alps the SNR forcings are small.
5.2. Longwave forcing
The longwave radiation LDR (Figure 5(b)) increased due to rising anthropogenic greenhouse gases and water vapour and due to the temperature increase itself. To assess the individual LDR changes that are due to temperature-, humidity- and anthropogenic greenhouse gas increases, the cloud compensation part (LDRCloud), which is due to the slight decrease of cloud amount and which was manifested on the shortwave side, needs to be subtracted on the longwave side. On the other hand, the effect that is due to the rising temperature at the surface and in the atmosphere (LDRΔT) is calculated by multiplying the LUR increase by the average apparent longwave sky emissivity 0.82 for Europe (Marty et al., 2002). Hence, the LDRWV forcing that is due to increasing water vapour (water vapour feedback) is calculated by subtracting LDRaGHG, LDRCloud and LDRΔT from the total increase LDRTotal (shown also in Figure 3) and is found to be + 0.64 [−0.49 to + 1.80] W m−2 decade−1 in the Lowlands and + 1.82 [+0.55 to + 3.11] W m−2 decade−1 in the Alps. Changing cloud amount and changing humidity may be influenced by large-scale circulation. However, recent analyses of circulation effects on climate change in Europe (Vautard and Yiou, 2009; Ceppi et al., 2010) have shown some evidence for seasonal effects but no evidence was found for the overall rapid temperature rise. Also, in our analysis we show annual means of the surface radiation components and compare their changes or radiative forcings to annual means of temperature and humidity and their respective changes. Hence, we assume that our measurements take into account all the changes involved over and within the investigated region.
5.3. Total atmospheric forcing
In the study by Philipona et al. (2009) it was shown that the observed slight decrease of cloud amount was balanced between increasing shortwave and decreasing longwave radiation. It is also mentioned above that a large part of the LDR increase is simply due to the rising surface temperature. Hence, the total of the atmospheric forcing (TAF) (Figure 5(c)), which actually increases the surface temperature and raises the atmospheric water vapour, results from changing aerosols (SNRAOD), increasing anthropogenic greenhouse gases (LDRaGHG) and the water vapour feedback (LDRWV). The radiation budget (Figure 3) in contrast showed that TNR is available to sustain the sensible and latent heat fluxes, and that an increasing TNR represents forcing energy that is available to increase water vapour in the atmosphere. In other words, the TAF causes surface temperature and water vapour to increase and the total net radiative forcing TNR is responsible for the water vapour increase. Hence, by making the difference between TAF and TNR we find the energy that was used to increase the surface temperature. At Lowlands, this surface forcing energy (TAF–TNR) is + 0.46 [−0.28 to + 1.21] W m−2 decade−1, whereas in the Alps (TAF–TNR) is + 0.23 [−0.74 to + 1.22] W m−2 decade−1. Even though these forcings are not statistically significant they indicate that in the Lowland about tree quarter of the total forcing energy is used to increase water vapour and only one quarter is used to rise the surface temperature.
6. Discussion and summary
The above analysis reveals that with strong temperature increase in Central Europe over the last three decades, absolute humidity increased by about 6% K−1 in the Lowlands, while the relative humidity showed a non-significant minor negative trend. The radiation budget shows that TNR at the surface also increased by nearly 6% K−1. This close relation between TNR increase at the surface and water vapour increase in the atmosphere was also observed in the altitude dependence, where the two parameters increased by 6–7% K−1 conform to the law of Clausius–Clapeyron. Figure 4 also shows that the three parameters, T, Uabs and TNR, which have been measured separately and are related by physical laws, all show statistically significant increase from 1981 to 2005. This shows that in Central Europe sufficient surface water and soil moister were available to allow atmospheric water vapour to increase under normal conditions.
In the Alps TNR increased by 22% and absolute humidity by 14% K−1 temperature rise. This large absolute humidity increase is related to a statistically significant relative humidity increase of 4.1% K−1. However, why the relative humidity increased is unclear and the question may be asked whether the strong water vapour increase in the Alps is due to circulation changes, maybe due to increased south-westerly winds at certain altitudes.
This leads to the main question: What is the reason for the temperature increase in Central Europe? The numbers in Figure 5 suggest that the rapid temperature rise in Central Europe over the last three decades is strongly related to shortwave and longwave transmission changes through the atmosphere. In the Lowland about half of the radiative forcing + 0.85 [+0.01 to + 1.66] W m−2 decade−1 is due to declining anthropogenic aerosols and a statistically significant solar brightening. The large part of the solar radiation increase was found to be due to the direct aerosol effect (Ruckstuhl et al., 2008). The cloud amount slightly decreased over the last three decades. However, this cloud change, which may be related to the indirect aerosol effect or to circulation changes, had only a minor climate forcing effect since the related solar radiation increase was partly compensated by LDR decrease. The other half of the radiative forcing is due to rising greenhouse gases and a related increase of LDR. In the Lowland about one third of the longwave increase is due to anthropogenic greenhouse gases + 0.35 [+0.30 to + 0.40] W m−2 decade−1 and two thirds are due to a statistically non-significant water vapour feedback + 0.64 [−0.49 to + 1.80] W m−2 decade−1. In the Alps, however, solar brightening is small and here temperature increased almost exclusively due to a rise of the greenhouse effect, manifested by a much larger and statistically significant water vapour feedback forcing of + 1.82 [+0.55 to + 3.11] W m−2 decade−1 than in the Lowlands, which may in part be due to circulation changes. The analysis also clearly shows that much more forcing energy is needed to increase water vapour in the atmosphere than to increase the temperature at the surface.
In the Lowland of Central Europe the climate forcing from 1981 to 2005 is primarily due to shortwave and longwave transmission changes in the atmosphere. Almost half of the forcing is due to declining aerosols and consequent solar brightening. The other half is due to rising anthropogenic greenhouse gases in the atmosphere and a Clausius–Clapeyron conform increase of water vapour, which further increased climate forcing and hence the surface temperature in the Lowland.
In the Alps, the percentage increase of water vapour is much larger and it likely increased due to the strong warming and thermal advection in the Lowlands and may also have increased due to atmospheric circulation changes. However, with almost no solar brightening in the Alps temperature increased less than in the Lowland, despite the much larger water vapour feedback greenhouse warming.
This study was completed as part of the National Center of Competence in Research on Climate (NCCR Climate), an initiative funded by the Swiss National Science Foundation (NSF). The author would also like to thank an anonymous reviewer of the manuscript for very helpful and important comments.