Natural and Anthropogenic influence on Precipitation and EXtreme events Naturlige og antropogene påvirkninger på nedbør og ekstremnedbør

Precipitation changes and especially changes in extreme precipitation are some of the most important challenges for future climate change. Differences among climate models for future precipitation changes are high. NAPEX tries to understand why there are so big differences between different climate models. Within NAPEX a large comparative project has been initiated with 11 climate models from model groups in Europe, USA and Japan. This Intercomparison project, Precipitation Driver Response Model Intercomparison Project (PDRMIP), http://www.cicero.uio.no/en/PDRMIP, compares changes in precipitation from various greenhouse gases, aerosols and changes in solar radiation. The calculations are made so that it is possible to analyze precipitation changes that occur on a time scale of a few days to a few years as well as changes occurring over a decade scale. The PDRMIP results clearly show that all 11 models have a rapid precipitation change that is controlled by total absorption in the atmosphere and a slow precipitation change driven by surface temperature changes. There are clear differences between different greenhouse gases and aerosols regarding whether fast or slow precipitation dominates the total precipitation. The precipitation changes from black carbon differ significantly from greenhouse gases and other types of atmospheric aerosols, because the fast precipitation change with reduced precipitation generally dominates over the slow precipitation changes. PDRMIP has given a detailed knowledge of the causes of the relatively low global surface temperature change by black carbon. An important reason is that black carbon changes increase the low and decrease high clouds that overall give a cooling. These changes in the clouds come as a result of temperature rise in the atmosphere and that it varies with height. The temperature increase itself in the atmosphere also contributes to the weak temperature change. The PDRMIP results have a substantial difference in average precipitation change for the various drivers of climate change. For extreme precipitation, there appears to be some smaller differences between the different drivers of climate change and the increase in extreme rainfall is greater than the average precipitation, which is consistent with previous findings. Black carbon contributes relatively more to longer drought periods than greenhouse gas changes for a similar global temperature change. The NAPEX project, and especially through the international collaboration PDRMIP, has resulted in many scientific publications. Several of the studies in PDRMIP have utilized the calculations to understand the various climate drivers contributions in historical and future complex climate simulations used for previous IPCC reports. We hope that many research groups and projects will apply the available PDRMIP data set to understand the upcoming climate model simulations that will be used for the next major IPCC report.

En ny generasjon forskere har fått deltatt og ledet oppgaver og sentrale deler av aktiviteter i et stort modellsammenligningsinitiativ med mulighet for å bygge opp nye internasjonalt nettverk. NAPEX og da særlig PDRMIP har vært viktig for deltakelse i EU H2020-prosjektet CONSTRAIN som starter opp senere i 2019. Deltakerne i NAPEX har sammen med de internasjonale forskerne i PDRMIP fått en detaljert kunnskap om hvordan nedbørsendringer henger sammen med endringer i jordas energibalanse. Videre har deltakerne en mye dypere forståelse av likheter og ulikheter hvordan drivhusgasser og atmosfæriske aerosoler påvirker klimasystemet.

Precipitation is arguably the most direct link between the climate and human society. We depend upon existing precipitation patterns for fresh water and food production, and global infrastructure is designed to withstand current precipitation extremes. At the same time rainfall patterns are naturally highly variable. Their future trends depend upon many climate factors that are currently changing, such as global air and sea surface temperatures, water evaporation rates, atmospheric stability and wind patt erns. Beyond changes in global and local mean precipitation, changes in rates and magnitudes of extreme rainfall events are also of high importance. The NAPEX project brings together a core group of international climate scientists to perform a set of de dicated unified experiments using state of the art climate models. The future scenarios of precipitation changes from global climate models presently do not agree very well on global nor regional scale. However, recent studies from groups participating in NAPEX have shown that precipitation changes from particular drivers of climate change (such as greenhouse gases and various aerosol types) have striking similarities. A main hypothesis of NAPEX is that differences in anthropogenic and natural drivers of climate change, both distribution and treatment, are crucial factors for differences in precipitation response in current global climate models. NAPEX partners have extensive experience with model intercomparisons. Results will be made available to the s cientific community and a common protocol established to encourage participation from further international modelling groups. The experiments will provide answers to the key question of whether idealized and simpler climate drivers give more similar and r obust precipitation changes compared to the mixture of drivers adopted in regular scenarios. A further innovative part of NAPEX is investigation of extreme events related to the various climate drivers.