Thermal radiation, also known as longwave, terrestrial or far-IR radiation is sensitive to changes in atmospheric GHGs, temperature and humidity. Long-term measurements of the thermal surface components as well as surface net radiation are available at far fewer sites than SSR. Downward thermal radiation observations started to become available during the early 1990s at a limited number of globally distributed terrestrial sites. From these records, Wild et al. (2008) determined an overall increase of 2.6 W m–2 per decade over the 1990s, in line with model projections and the expectations of an increasing greenhouse effect. Wang and Liang (2009) inferred an increase in downward thermal radiation of 2.2 W m–2 per decade over the period 1973–2008 from globally available terrestrial observations of temperature, humidity and cloud fraction. Prata (2008) estimated a slightly lower increase of 1.7 W m–2 per decade for clear sky conditions over the earlier period 1964–1990, based on observed temperature and humidity profiles from globally distributed land-based radiosonde stations and radiative transfer calculations. Philipona et al. (2004; 2005) and Wacker et al. (2011) noted increasing downward thermal fluxes recorded in the Swiss Alpine Surface Radiation Budget (ASRB) network since the mid-1990s, corroborating an increasing greenhouse effect. For mainland Europe, Philipona et al. (2009) estimated an increase of downward thermal radiation of 2.4 to 2.7 W m–2 per decade for the period 1981–2005.
There is limited observational information on changes in surface net radiation, in large part because measurements of upward fluxes at the surface are made at only a few sites and are not spatially representative. Wild et al. (2004, 2008) inferred a decline in land surface net radiation on the order of 2 W m–2 per decade from the 1960s to the 1980s, and an increase at a similar rate from the 1980s to 2000, based on estimated changes of the individual radiative components that constitute the surface net radiation. Philipona et al. (2009) estimated an increase in surface net radiation of 1.3 to 2 W m–2 per decade for central Europe and the Alps between 1981 and 2005.