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Human Influence On Monsoon Precipitation

Congress: 2015
Author(s): Debbie Polson (Edinburgh, UK), Massimo Bollasina, Gabriele Hegerl, Laura Wilcox
University of Edinburgh1, University of Reading2

Keyword(s): Sub-theme 17: Climate change, impacts and adaptation,
AbstractIntroduction The Northern Hemisphere monsoons are integral to Earth's hydrological cycle. They affect the lives of billions of people and changes to monsoon precipitation has potential consequences for water resources, health, agriculture and ecosystems. Over the latter half of the twentieth century, observations of monsoon precipitation show substantial changes, with decreasing precipitation from the 1950s to mid-1980s (e.g. Held et al., 2005) and increasing precipitation in recent years (e.g. Hsu et al., 2011). Human influence is known to have driven changes to the Earth's hydrological cycle. Greenhouse gas driven atmospheric warming causes global precipitation to increase (Held and Soden, 200) and changes in the zonal mean (Zhang et al, 2007, Polson et al., 2013a) and Northern Hemisphere (Wu et al., 2013) have been shown to have been influenced by human activity. Changes in monsoon precipitation have been linked to increases in emissions of anthropogenic aerosols (Bollasina et al., 2011). Aerosols cause precipitation to decrease as they scatter and absorb incoming solar radiation, causing reduced radiation and cooling at the surface (Ming and Ramaswamy, 2009). Aerosols also influence precipitation by interacting with clouds (Rotstayn and Lohmann, 2002) and by driving changes in atmospheric circulation. As aerosols are predominately emitted in the Northern Hemisphere, they cool the Northern Hemisphere more than the Southern Hemispheres, causing a southward shift of the Intertropical Convergence Zone (e.g. Hwang et al., 2013). Here we investigate the influence of individual forcings on summer monsoon land precipitation in the Northern Hemisphere for 1951-2005. Precipitation in monsoon regions has large seasonal variability, associated with a strong land-sea temperature contrast and seasonal wind reversals. By analysing the Northern Hemisphere monsoon system as a whole, rather than at a regional scale, we can more easily identify how the global-scale external forcings have affected monsoon precipitation. Using large climate model ensembles, we derive "fingerprints" of forcing for all combined external forcings and individual forcings and apply a robust statistical analysis to determine whether observed changes can be explained by the natural climate variability alone or whether they are driven by external factors (such as greenhouse gases or anthropogenic aerosols). Methods Four gridded observational monthly land precipitation datasets are used in this analysis; CRUTS3.21 (Harris et al., 2014), Global Precipitation Climatology Centre (GPCC) (Schneider et al., 2014), VasclimO (Beck et al., 2005) and a dataset updated from Zhang et. al. (Zhang et al., 2007). The mean summer (May-September) precipitation across all Northern Hemisphere Summer monsoon (NHSM) regions is calculated for each. Similarly the NHSM precipitation is calculated for ensembles of climate model simulations. Climate models from the Coupled Model Intercomparision project are used to derive "fingerprints" for all external forcings (anthropogenic and natural), greenhouse gas forcing, anthropogenic aerosol forcing, natural forcing (solar and volcanoes) and anthropogenic forcing (greenhouse gas, anthropogenic aerosols, land use and ozone) for the NHSM region. These are used in a total least squares linear regression detection and attribution analysis (Allen and Stott, 2003) that allows us to determine if the observed changes can be explained by internal variability alone or whether they are attributable to one or more of the individual forcings. Results and discussion Comparing the observed timeseries of monsoon precipitation to the climate model data shows that the models that include anthropogenic aerosol forcing (i.e. the models with all external forcings, anthropogenic forcing and anthropogenic aerosol only forcing) correlate better with observations than models that do not include this forcing (i.e. greenhouse gas only forcing and natural forcing). The results of the total least squares detection and attribution analysis show that the observed temporal changes in monsoon precipitation can only be explained when including the influence of anthropogenic aerosols, even after accounting for internal climate variability. That is the observed changes are attributable to the aerosol forcing, but not to greenhouse or natural forcing. Aerosols have therefore been the dominant influence on monsoon precipitation over the second half of the 20th century. Conclusion During the second half of the 20th century, increasing greenhouse gas concentrations have caused atmosphere to warm. According to climate models, this should have led to an increase in precipitation in the monsoon regions, however observations show that precipitation actually decreased. The results of this work shows that this decrease was caused by anthropogenic aerosols. During the coming century, anthropogenic aerosol emissions are expected to decrease while greenhouse emissions continue to increase, potentially leading to increasing monsoon precipitation in the future. 1. Allen, M. & Stott, P. (2003) Estimating signal amplitudes in optimal fingerprinting, Part I: Theory. Climate Dynamics 21, 477–491. 2. Beck, C., Grieser, J. & Rudolf, B. (2005) A new monthly precipitation climatology for the global land areas for the period 1951 to 2000, in Climate Status Report 2004. German Weather Service, Offenbach. 3. Bollasina, M. A., Ming, Y. & Ramaswamy, V. 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