De onderzoekers hebben de variabiliteit van de atmosferische patronen verwijderd. Je houdt dan de thermodynamische component over en de geobserveerde neerslag. De thermodynamische component komt goed overeen met de resultaten van de klimaatmodellen.
Een van hun vondsten is dat de de thermodynamische component meer zonaliteit toont. Meer neerslag ten noorden van 35N, minder ten zuiden daarvan.
It is difficult to isolate the anthropogenic influence on long?term precipitation trends due to confounding effects from internal variability. Here we remove the influence of atmospheric circulation variability, which is primarily unforced, from observed precipitation trends using an empirical approach called “dynamical adjustment.” This removal isolates the thermodynamic component of observed precipitation trends as a residual. We find that this thermodynamic component is in good agreement with the anthropogenic component determined from historical simulations from climate models. Thus, we conclude that we are able to identify a human influence on observed century?scale precipitation trends over North America and Eurasia.
To the best of our knowledge, this is the first use of dynamical adjustment to identify an anthropogenic influence on observed long?term (95?year) precipitation trends. In particular, we have applied a CCA?based procedure to estimate the dynamically induced portion of winter (November–March) precipitation trends during 1921–2015 over North America and Eurasia. Subtracting this dynamical component from the total trend yields the thermodynamic residual precipitation trend. The geographical pattern and amplitude of this observed thermodynamic residual trend are in good agreement with anthropogenically forced trends obtained from two different climate model large ensembles: the 37?model CMIP5 archive and the 40?member CESM1. In particular, the observed thermodynamic residual trends over Eurasia show a more zonally uniform structure compared to the total trends, with precipitation increases (decreases) north (south) of approximately 35°N, similar to the models' forced (ensemble?mean) patterns. In addition, the magnitudes of the residual wetting and drying trends are in better agreement with the models' forced responses than are the total trends. Over North America, the thermodynamic residual trends show enhanced wetting over the eastern third of the continent, similar to the forced responses in the model ensembles. In observations, this thermodynamic wetting trend is nearly completely offset by a dynamically induced drying trend.
The fact that the forced circulation (SLP) trends in both model ensembles are close to zero over the time period considered leads us to conclude that the dynamically induced portion of the observed precipitation trends is almost entirely internally generated. This, in turn, leads us to the interpretation that the observed residual precipitation trends are externally forced. The close similarity between the models' forced trends and the observed thermodynamic residual trend lends strong support to our inference and helps to reconcile models and observations. We emphasize that the dynamical adjustment procedure we have applied to observations is entirely independent of information from climate models, unlike other approaches such as those used in formal detection and attribution studies.
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Discussie over klimaat en verandering daarvan, broeikaseffect, zeespiegelstijging, etc.
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