Climate change impacts and vulnerability in Europe 2016

Changes in the climate system
Box 3.2
Climate reanalysis
Reanalysis is the name given to the use of a modern data assimilation system to analyse comprehensive sets of
observations that extend back in time over multiple decades. It employs a model of the atmosphere, ocean or coupled
climate system to spread observational information in space and time, and from one variable to another. Reanalysis
produces gap-free global datasets for numerous climate variables, at higher spatial or temporal resolution than is usually
provided by directly analysing a single type of observation. This makes the datasets valuable for studying short-term
climatic processes and extreme events. The value of the information provided for longer term trends depends on the
quality and changes over time of the global observing system, on the realism of the assimilating model, and on how well
observations are combined with background information from the model, which takes into account the varying biases and
random errors of both the observations and the background information.
Smoothed time series of global and European average surface air temperatures are presented in Figure 3.5, starting from
1979, a year that followed a significant upgrade of the observing system. Differences from the 1981–2010 average are
shown for the ERA-Interim reanalysis (Dee et al., 2011). Uncertainty bars show the spread of values provided by ERA-Interim,
the Japanese 55-year Reanalysis (JRA-55) (Kobayashi et al., 2015), HadCRUT4 (Morice et al., 2012) and US National Oceanic
and Atmospheric Administration (NOAA) Global Temp (Karl et al., 2015) datasets that combine monthly temperature data
from land stations with measurements of sea-surface temperature. All datasets indicate that global temperature has risen
since the 1970s to reach values that are the highest on record.
Shorter-period fluctuations in global values are more uncertain. This is largely because the effects of sea ice changes are
better represented in the reanalyses and because sea-surface temperature analyses differ. The datasets agree quite well
across Europe, where average temperatures were high from mid-2006 to mid-2007 and have recently been high for a
prolonged spell since 2014.
Figure 3.5 Global and European average surface air temperatures from 1979 to 2015
o
C
0.4 Globe
0.2
0
– 0.2
– 0.4
o
C
1980
1985
1990
1995
2000
2005
2010
2015
1.5
Europe
1.0
0.5
0
– 0.5
– 1.0
– 1.5
1980
1985
1990
1995
2000
2005
2010
2015
Note:
Running 12-month averages from the ERA-Interim reanalysis are shown, with solid red and blue shading indicating when
temperature is above and below, respectively, the average for 1981–2010. The short, darker bars provide an indication of the
uncertainty in the estimates provided by different datasets.
Source: Copernicus Climate Change Service, ECMWF ( 42 ).
( 42 ) For further details on the time series and maps, see http://climate.copernicus.eu/resources/data-analysis/average-surface-air-temperatureanalysis.
72 Climate change, impacts and vulnerability in Europe 2016 | An indicator-based report