The water cycle with climate change

Abstract

Climate change causes temperatures to rise worldwide. Up until now it is unclear what effect this has on the global water cycle. In this study the output of two GFDL model experiments were thoroughly investigated and used in the moisture tracking model WAM-2layers, accounting for model runs in a past case in the end-20th century and future case in the end-21st century, based on RCP8.5. This is done in order to acquire knowledge on what will happen to the global as well as regional water cycle in the future and to find out what processes provide climate change to have an effect on the water cycle. Changes in precipitation and evaporation rates are spatially highly dissimilar, therefore regional differences can be distinguished. Past studies have suggested that a DDWW paradigm - where dry regions dry out further and wet regions get wetter - will take place with climate change. Even though in some regions this might happen, results show that on land this is not necessarily the case - at least, if precipitation rate is the benchmark for a region to get drier or wetter. Processed GFDL data concludes that dry region Western Sahara gets wetter while dry region Middle East gets drier and wet region Indonesia gets wetter while wet region Amazonia gets drier. The DDWW paradigm covers another aspect as well, that there will be a larger spatial variability for variables characterising a region to be wet or dry. However, while everywhere on the globe the temperatures will rise according to the GFDL data, the spread of the yearly mean temperatures over the globe will actually decrease, meaning that mean yearly temperature in the coldest place will lie closer to the mean yearly temperature in the warmest place. This is also the case for mean yearly precipitation and evaporation rates. The mean yearly precipitation and evaporation rate in the driest place will lie closer to the mean yearly precipitation and evaporation rate in the wettest place in the future case. Continents show divergent effects in water cycle due to climate change. The continents with relatively larger sources of terrestrial moisture - North America, Europe and Asia - have an increased water cycle with higher precipitation and evaporation rates in the future. The increases in precipitation rates on these continents will originate from terrestrial evaporation and a higher percentage of evaporation will return as precipitation on land. South America shows a distinct effect, different than all other continents. This continent shows a decreased water cycle with lower precipitation and evaporation rates in the future. Of this precipitation a lower percentage comes from land and the evaporated moisture will return less on land. Africa and Oceania show another pattern. Both these continents will experience more precipitation in the future case, but this moisture will come from oceanic evaporation. Two case study areas are examined in more depth by looking at seasonality effects. A case study in the Amazon forest shows a distinct dry season in the future case, the effect of possible land use change in the Amazon forest. Less evaporation that is an effect of deforestation of the rain forest gives less moisture for clouds to form and precipitate. Moisture evaporates from the oceans on the East of Amazon region but moves over the region before it can precipitate. A case study in Western Africa shows a magnified rain season. The analyses show that during the wet season in the future case there in an increased amount of moisture coming from the oceans West and South of Western Africa and meanwhile there was still moisture coming in from the tropical rain forest East of the area.