A Meteosat look at budgets

By José Prieto, Vesa Nietosvaara and Rob Roebeling (EUMETSAT) and Ivan Smiljanic (CGI)

The Earth manages its temperature, and keeps it roughly constant, by emitting thermal radiation to space and absorbing solar radiation in the atmosphere and Earth surfaces. Since pre-industrial times, surface and air temperatures have increased, faster since the 1970s. The observed warming of the Earth is attributed to the imbalance in incoming shortwave and outgoing longwave radiative flows of energy, caused by anthropogenic emissions (Stocker et al. 2013). These emissions change concentrations of well-mixed greenhouse gases, as CO₂ and CH4, leading to a direct, or indirect, impact on the energy balance of the Earth, and driving climate change. As CO₂ becomes more abundant in the atmosphere, weaker radiation is expected in CO₂-absorption satellite channels (around 15 µm). At the same time more radiation is expected at the infrared (IR) satellite window channels (e.g. around 10 and 12 µm), due to warmer surfaces or lower cloudiness. Changes in the amount of solar reflection (shortwave outgoing radiation, SOR) result from the feedback of clouds or snow, and will affect the Earth’s radiative flows of energy contributing to a new balance. These changes, however, are more difficult to quantify, as the IPCC states in its IPCC in its Fourth Assessment Report: "The cloud feedback is likely positive but its quantification remains difficult".

The scheme in Figure 1 (right panel) shows the individual energy fluxes of the Global Mean Radiation Budget. Among others, satellite observations are particularly well suited to monitor changes in the atmospheric radiation budget at the Top of the Atmosphere (TOA), a sum of the Outgoing Longwave Radiation (OLR) and the Outgoing Shortwave Radiation (OSR). IPCC (2007) reports that, during the 1980s and 1990s, the OLR decreased (~0.7 W/m2) and OSR increased (~2.1 W/m2) over the tropics. During the first decade of the 21st century no noticeable trends in either the tropical or global radiation budget were observed (Hartmann et al., 2013).

We try here an approximation based on radiances by using four shortwave (or solar, including 3.9 µm where sun energy predominates during the day) and four longwave (infrared) channels of the SEVIRI instrument on board of Meteosat Second Generation (MSG) satellites, and look at changes in shortwave and longwave radiation flows over the observation period 2004 to 2020. To do so, we compared changes in average channel radiances (Figure 2) between the first four years (2004-2007) and the last four years (2016-2020) of the observation period for five geographical regions, including land and ocean surfaces over Africa and Europe.

The results show (Figure 1, left panel) that the Outgoing Longwave Radiation (OLR) has increased in most Meteosat longwave channels between 1% and 3%, except at 13.4 µm, with a decrease around 3%. In turn, the shortwave (solar) reflected radiation (Outgoing Shortwave Radiation, OSR) has decreased by 2-3%. Since the decrease in solar reflected radiation is strong in the southern Atlantic, without land or known changes in aerosol, a cloud cover reduction is suggested.

Figure 2 specifies the values for regions in the Meteosat-0° field of view. Variability can be the result of different soils, cloud frequency and ocean influence in the areas. For instance, the increase of sea surface temperature (SST), related to 10.8 µm, is moderate in the southern Atlantic compared with land scenes, with a higher increase in IR values. 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.

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 the IASI interferometer on board of Metop polar satellites) can greatly clarify the amount of warming in oceans.

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 (right) we can see that most of the areas within the Meteosat disc area were slightly (in the order of 0.5 K) warmer towards 2020 compared to the period 2004-2007. Over Europe, and many parts in Africa, the difference is more pronounced. 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.