Climate change, impacts and vulnerability in Europe ... - MemoFin.fr

Changes in the climate systemBox 2.2Climate reanalysesReanalysis is a method to reconstruct the past state of the atmosphere and oceans in a coherent way bycombining available observations with numerical models. These reconstructions are created with model-baseddata assimilation methods which are similar to those used for numerical weather prediction. Using this approachit is possible to extract useful information about rainfall using temperature and humidity observation fromsatellites, or to infer large-scale features of the global circulation in the early 20th century using only surfacepressure observations available at that time (Compo et al., 2011). Reanalysis is a rapidly evolving field, andnew reanalysis products benefit from recent modelling capabilities, improved techniques in data assimilation, inthe latest observation techniques (i.e. from satellite measurements), and newly digitalised historical datasets(Blunden et al., 2011). Reanalyses also allow the user to estimate temperature, humidity, wind and precipitationover regions where in situ observations are not available (e.g. Polar regions, or large areas of Africa). In recentyears, datasets from reanalyses have been widely used for research in the atmospheric and ocean sciences and inclimate services.Global average surface temperature anomalies from in situ temperature measurements and from variousreanalyses are presented in Figure 2.6. In situ temperature anomalies use temperature measurements over landand sea. While they differ in their methods, which can lead to differences in ranking years, all time series are closein agreement. Global average temperature datasets from reanalyses have larger spread than those obtained fromin situ observations due to complex construction of the reanalyses.Reanalyses are explicitly and implicitly used in various sections of this report.Figure 2.6Global average surface temperature anomalies from in situ observations andfrom reanalysisSurface temperature anomaly ( o C)0.5A)0.0– 0.5– 1.00.5B)0.0– 0.5– 1.01880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010A) In situ land and oceanB) Reanalysis land and oceanNOAA/NCDCNASA/GISSERA-InterimHadCRUT3 JMA MERRAJRA-25/JCDASNote:For the in situ datasets near-surface (2 m) air temperature is used over land and sea surface temperature over theoceans. For the reanalyses a 2 m temperature is used over the whole globe. In situ datasets use the 1961–1990 baseperiod whereas all other reanalysis datasets use the 1989–2008 base period. However, to aid comparison, all time serieshave been adjusted such that they give a mean of zero over the common period 1989–2001.Source: Blunden et al., 2011.58 Climate change, impacts and vulnerability in Europe 2012