A Meteosat look at budgets
January 2004-October 2020
Meteosat Second Generation (MSG) spans 18 years of an atmosphere in evolution.
21, December 2020
By José Prieto and Vesa Nietosvaara (EUMETSAT) and Ivan Smiljanic (CGI)
The Earth manages its temperature and keeps it roughly constant by emitting infrared (IR) radiation to space, in exchange for the solar radiation absorbed by atmosphere and surfaces. The increase in surface and air temperatures since pre-industrial times, intensifying in recent decades, modifies the radiative flows. As CO2 gets more abundant in the atmosphere, weaker radiation is expected in satellite channels around 15 µm. Other infrared (IR) spectral regions supply a stronger satellite signal based on hotter surfaces, or perhaps based on less cloud blocking. Also the solar reflection, mainly on cloud, contributes to the new balance, as vaguely stated by the IPCC in its Fourth Assessment Report: "The cloud feedback is likely positive but its quantification remains difficult".
This case looks at solar and IR radiation flows for the period from January 2004 to October 2020, with weekly imagery for day and night from MSG satellites (Meteosat-8 to 11).
Figure 1 shows the variations in an effective period of 12.7 years, based on radiance averages for the four initial and four final years. The outgoing longwave radiation (OLR) has increased under Meteosat by 0.6 Wm-2 in that period. The solar reflected radiation has decreased, in turn, by a similar amount. Since the solar reflection decrease is strong in the southern Atlantic, without land or known changes in aerosol, a cloud cover reduction can be assumed.
Figure 2 specifies the respective values for regions in the Meteosat-0° field of view. Variability can be expected from different soils, cloud frequency and ocean influence in the areas. For instance, the increase of sea surface temperature (SST), proxied by 10.8 µm, is moderate in the southern Atlantic compared with land scenes, with a higher increase in IR values.
On Figure 3, the average year cycle is presented for the early and the late periods, to illustrate natural variability. Oceans fill 71% of the Earth surface, so its analysis in the future (for instance with IASI) can greatly clarify the amount of warming in oceans. The area of southern Africa and Mozambique (yellow square) tends to lose energy to space in the balance. The tropical area south of Lake Victoria hardly changed in 13 years.
The global ERA-5 atmospheric reanalysis was used in this study to subjectively compare the changes seen in satellite measurements and, respectively, the changes in model 12:00 UTC mean 2-metre-temperatures during the same periods of time.
In Figure 4 (left) we can see that most of the areas within the Meteosat disc area were slightly (in the order of 0.5-1 K) warmer towards 2020 compared to the period 2004-2007. Over Europe and many parts in Africa the difference is more pronounced, in the order of 1-2 K. However, some areas, such as the northern Atlantic, show negative difference. The areas with the highest increases in air temperature, e.g. the Mediterranean area or southern Africa, bear resemblance to those with the highest increase in IR radiative flux.
Surface air temperature
Figure 4: Difference in mean 12:00 UTC 2-metre-temperature between periods 2016-2020 and 2004-2007. ECMWF ERA-5 reanalysis (left). Full two decades change: from 1994-1999 and 2014-2019. Similar patterns, a bit more pronounced warming (right).
The average images comparison between the two subperiods (2004-2007 and 2016-2020) (Figure 5) shows the specific areas of highest variation in radiative fluxes, on the example of the south of Africa.
Change areas in southern Africa
Figure 5: Less solar reflection from the satellite perspective in 0.8 µm shown in red (left frame), more reflection in blue. Increased brightness temperature on channel 10.8 µm also in red for the south of Africa (right frame).
