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Global dimming and brightening: variations in solar radiation reaching the Earth s surface

The radiation balance of the Earth. Units Wm-2. The radiation balance of the Earth. Units Wm-2. TOA radiation balance determines energy content of climate system. . . . The radiation balance of the Earth. Units Wm-2. TOA radiation balance determines energy content of climate system. . . . . . . Surface radiation balance: driver of surface climate and intensity of hydrological cycle.

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Global dimming and brightening: variations in solar radiation reaching the Earth s surface

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    1. Martin Wild Swiss Federal Institute of Technology ETH Zurich, Switzerland Global dimming and brightening: variations in solar radiation reaching the Earth’s surface The topic of my talk is the state of the radiation balance of the earth, how it varies over time and what this all means in terms of climate changeThe topic of my talk is the state of the radiation balance of the earth, how it varies over time and what this all means in terms of climate change

    2. I would like to start my presentation with s slide that most of you have seen before.I would like to start my presentation with s slide that most of you have seen before.

    3. I would like to start my presentation with s slide that most of you have seen before.I would like to start my presentation with s slide that most of you have seen before.

    4. I would like to start my presentation with s slide that most of you have seen before.I would like to start my presentation with s slide that most of you have seen before.

    5. I would like to start my presentation with s slide that most of you have seen before.I would like to start my presentation with s slide that most of you have seen before.

    6. What do we know on changes in solar fluxes on decadal timescales?

    7. Over the past 15 years we established at ETH two database to reduce these uncertainties.Over the past 15 years we established at ETH two database to reduce these uncertainties.

    8. The data frim the global energy balance archive which go back to the 60ties were analyzed in several studies. The amount of sunlight we received at the earth has not been constant over time, but varies a substantiallyThe data frim the global energy balance archive which go back to the 60ties were analyzed in several studies. The amount of sunlight we received at the earth has not been constant over time, but varies a substantially

    9. Evidence for solar dimming came also from a completely other network. The network of evaporation pans. These very simple measurements measure the evaproative loss from a water filled pan. The decrease in evaporation has been related to the decrease in available solar energy, which is the key driver of evaporation. This is an iondication fro the reduced solar energy, since solar energy is the principal driver of evaporation. This shows the potential/power that solar diming has to modify the surface hydrology and thereby influence the global hydrological cycle. This has also been shown in a modelling study of Ronstan and lohman. They show that solar dimming can significantly alter the precip distribution. They carried out numerical simulations with a climate model with preindustrial and present day aerosol concentration and found The significant changes Independently from the trends in sola radiation, it has been noted that evaporation as measured with evaporation pans has been decreading between the 1960s and 1990s. Evaporation pans are very simple measurements, where the reduction of the water level due to evaporation is measured. Since solar radiation is a key energy input to evaporation, a reduction of solar energy at the surface should reduce the evaporation from these pans. This indicates that the reduction of solar radiation can have a significant impact on the hydrology. This has also been noted in modelling studies and I want here to present the study of Ronstayn and Lohmann. Pan evaporation is a simple measurement of complex meteorological interactions. Although pan evaporation cannot fully represent lake evaporation and is even less indicative of ground evapo-transpiration, analysis of observed trends in pan evaporation can provide considerable insights into current climate change and the impact this change may be having on agriculture and water resources. Peterson and Golubev (1995) derived five regional trends of pan evaporation from data obtained from 746 homogeneous reporting stations in the United States - (1) Eastern US, (2) Western US - and 190 such stations in the former Soviet Union - (3) European, (4) Middle Asian, (5) Siberia - for the last half of the 20th century.  And they found that All of the regions exhibited downward trends in pan evaporation over the last half of the 20th century, with four of them (all but the Middle Asian region) registering significance at the 99% level or better(Fig 2.1). The solid lines are area-average pan evaporation and the smooth curves result from 11-point binomial smoothing. Linear trend estimates for these regions are shown in parentheses. They are significant at the 99% level except for the former Soviet Union Middle Asian region. The largest actual change in pan evaporation is in the western United States, where the area-averaged linear regression slope corresponds to a decrease in pan evaporation of 97 mm per warm season (May – September) during the past 45 years in a region with a mean pan evaporation of 1,130 mm per warm season. The downward trend in pan evaporation over most of the United States and former Soviet Union implies that, for large regions of the globe, the terrestrial evaporation component of the hydrological cycle has been decreasing. One explanation is that increased global cloudiness, especially low cloud cover, would be an expected consequence of higher global temperatures. Some increases in annual mean cloudiness have been observed over Europe, Australia, the Indian sub–continent and North America. When cloudiness over the oceans is also considered it is not possible to be confident that average global cloudiness has really increased. 3.1 Reassessment of evaporation An explanation by Brutasert (1998) is that trends in pan evaporation and actual evaporation may well be opposite. The actual evaporation from a well-watered surface is E= E0=a* Epa. Whenever the land-surface moisture becomes limiting and insufficient to sustain E0, E decreases below E0 and the energy not used up by E manifests itself as an increase in sensible heat flux ?H. (E= E0-?H). This in turn causes a* Epa to exceed E0, or a* Epa = E0 +b*?H, where b is another coefficient slightly larger than one. The main point is that E and Epa exhibit complementary rather than proportional behavior; indeed, for instance in the extreme case of a desert environment E is zero, whereas Epa is at its maximum. In the case of a pan filled with water and placed in a region with less than adequate ground wetness to sustain E0, elimination of ?H in the above yield E=[(1+b)* E0-a* Epa]/b. Because a and b are of order one, this equation indicates how the observed decreases in pan evaporation Epa can be interpreted as evidence for increasing terrestrial evaporation E. Zubenok(1976) shows that over a large part of the globe both the absolute values and the geographical pattern of potential and actual evaporation are quite different. Leaving aside obvious situations in subtropical deserts, over the most of the Northern hemisphere south of 50N the derivatives dE/df and dE0/df have opposite signs and when E is increasing with the latitude, E0 is going up and vise versa (Fig 2.2). Using parallel observations of actual evaporation and pan evaporation at five Russian experimental sites, Golubev et al. (2001) developed a method to estimate actual land surface evaporation from the pan evaporation measurements. By this method, actual evaporation is shown to have increased during the second half of the 20th century over most dry regions of the United States and Russia. Similarly, over humid maritime regions of the eastern United States (and north-eastern Washington state) actual evaporation during the warm season was also found to increase. Only over the heavily forested regions of Russia and the northern United States did actual evaporation decrease. The increase in actual evaporation is related to the greater availability of moisture at the surface, due to increases in precipitation and the higher temperatures. Now we could say the general intensification of the hydrological cycle over northern extra-tropical land areas is confirmed. Considering the increase of SST over last decades, the evaporation rate over ocean may also be intensified. Evidence for solar dimming came also from a completely other network. The network of evaporation pans. These very simple measurements measure the evaproative loss from a water filled pan. The decrease in evaporation has been related to the decrease in available solar energy, which is the key driver of evaporation. This is an iondication fro the reduced solar energy, since solar energy is the principal driver of evaporation. This shows the potential/power that solar diming has to modify the surface hydrology and thereby influence the global hydrological cycle. This has also been shown in a modelling study of Ronstan and lohman. They show that solar dimming can significantly alter the precip distribution. They carried out numerical simulations with a climate model with preindustrial and present day aerosol concentration and found The significant changes Independently from the trends in sola radiation, it has been noted that evaporation as measured with evaporation pans has been decreading between the 1960s and 1990s. Evaporation pans are very simple measurements, where the reduction of the water level due to evaporation is measured. Since solar radiation is a key energy input to evaporation, a reduction of solar energy at the surface should reduce the evaporation from these pans. This indicates that the reduction of solar radiation can have a significant impact on the hydrology. This has also been noted in modelling studies and I want here to present the study of Ronstayn and Lohmann. Pan evaporation is a simple measurement of complex meteorological interactions. Although pan evaporation cannot fully represent lake evaporation and is even less indicative of ground evapo-transpiration, analysis of observed trends in pan evaporation can provide considerable insights into current climate change and the impact this change may be having on agriculture and water resources. Peterson and Golubev (1995) derived five regional trends of pan evaporation from data obtained from 746 homogeneous reporting stations in the United States - (1) Eastern US, (2) Western US - and 190 such stations in the former Soviet Union - (3) European, (4) Middle Asian, (5) Siberia - for the last half of the 20th century.  And they found that All of the regions exhibited downward trends in pan evaporation over the last half of the 20th century, with four of them (all but the Middle Asian region) registering significance at the 99% level or better(Fig 2.1). The solid lines are area-average pan evaporation and the smooth curves result from 11-point binomial smoothing. Linear trend estimates for these regions are shown in parentheses. They are significant at the 99% level except for the former Soviet Union Middle Asian region. The largest actual change in pan evaporation is in the western United States, where the area-averaged linear regression slope corresponds to a decrease in pan evaporation of 97 mm per warm season (May – September) during the past 45 years in a region with a mean pan evaporation of 1,130 mm per warm season. The downward trend in pan evaporation over most of the United States and former Soviet Union implies that, for large regions of the globe, the terrestrial evaporation component of the hydrological cycle has been decreasing. One explanation is that increased global cloudiness, especially low cloud cover, would be an expected consequence of higher global temperatures. Some increases in annual mean cloudiness have been observed over Europe, Australia, the Indian sub–continent and North America. When cloudiness over the oceans is also considered it is not possible to be confident that average global cloudiness has really increased. 3.1 Reassessment of evaporation An explanation by Brutasert (1998) is that trends in pan evaporation and actual evaporation may well be opposite. The actual evaporation from a well-watered surface is E= E0=a* Epa. Whenever the land-surface moisture becomes limiting and insufficient to sustain E0, E decreases below E0 and the energy not used up by E manifests itself as an increase in sensible heat flux ?H. (E= E0-?H). This in turn causes a* Epa to exceed E0, or a* Epa = E0 +b*?H, where b is another coefficient slightly larger than one. The main point is that E and Epa exhibit complementary rather than proportional behavior; indeed, for instance in the extreme case of a desert environment E is zero, whereas Epa is at its maximum. In the case of a pan filled with water and placed in a region with less than adequate ground wetness to sustain E0, elimination of ?H in the above yield E=[(1+b)* E0-a* Epa]/b. Because a and b are of order one, this equation indicates how the observed decreases in pan evaporation Epa can be interpreted as evidence for increasing terrestrial evaporation E. Zubenok(1976) shows that over a large part of the globe both the absolute values and the geographical pattern of potential and actual evaporation are quite different. Leaving aside obvious situations in subtropical deserts, over the most of the Northern hemisphere south of 50N the derivatives dE/df and dE0/df have opposite signs and when E is increasing with the latitude, E0 is going up and vise versa (Fig 2.2). Using parallel observations of actual evaporation and pan evaporation at five Russian experimental sites, Golubev et al. (2001) developed a method to estimate actual land surface evaporation from the pan evaporation measurements. By this method, actual evaporation is shown to have increased during the second half of the 20th century over most dry regions of the United States and Russia. Similarly, over humid maritime regions of the eastern United States (and north-eastern Washington state) actual evaporation during the warm season was also found to increase. Only over the heavily forested regions of Russia and the northern United States did actual evaporation decrease. The increase in actual evaporation is related to the greater availability of moisture at the surface, due to increases in precipitation and the higher temperatures. Now we could say the general intensification of the hydrological cycle over northern extra-tropical land areas is confirmed. Considering the increase of SST over last decades, the evaporation rate over ocean may also be intensified.

    10. Now what could have caused this dimming? Fossil fuel consumption does not only result in increased greenhouse gas levels, but it can also generate a variety of particles in the atmosphere, so called aerosolsNow what could have caused this dimming? Fossil fuel consumption does not only result in increased greenhouse gas levels, but it can also generate a variety of particles in the atmosphere, so called aerosols

    11. To address these questions we have to have a closer look at these sunshine records To go more into this discussion we have to be aware of the fact that alll studies related to dimming use data prior to 1990To address these questions we have to have a closer look at these sunshine records To go more into this discussion we have to be aware of the fact that alll studies related to dimming use data prior to 1990

    18. Trends in anthropogenic SO2 and BC emissions 1980 - 2000

    21. Simulation of observed trends

    22. Simulation of observed trends

    23. Simulation of observed trends

    24. Simulated TOA SW clear sky changes

    25. What are the consequences of the radiative changes for climate change?

    26. 1960s - 1980s Surface solar dimming counter-balances increasing thermal downward radiation Surface radiative heating is not increasing Wild et al. (2004) GRL 32 We can divide 2 phases in past decades.We can divide 2 phases in past decades.

    29. It seems that with the disapearance of solar dimming the greenhouse effect could fully developIt seems that with the disapearance of solar dimming the greenhouse effect could fully develop

    30. Impact on daily temperature range

    31. Impact on mountain glaciers

    32. Impact on pan evaporation

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