2. Greenhouse Gases and Climatic Change

See, for example, http://​www.​esrl.​noaa.​gov/​gmd/​ccgg/​trends/​.

Recent estimates show that global fossil and cement emissions equal 9.5 +/− 0.5 GtC/year (that is +54 % from 1990) while global land-use change emissions equal 0.9 +/− 0.5 GtC/year in 2011 (Le Quéré et al. 2012).

This quantity equals the solar “constant” (1,366 W/m² = solar energy intercepted by the earth disk) divided by 4, for geometry constraint, multiplied by 1 minus albedo (= 0.3), the albedo being the fraction of radiation reflected by the earth’s system, that is, 1366/4 × 0.7~239 W/m². The earth’s surface absorbs 50 % of total solar radiation and the atmosphere absorbs 20 % of it.

The emissivity of land, ocean surface and thicker clouds than cirrus is close to 1, that is, the one of a black body. The clear-sky atmosphere has an emissivity of 0.4–0.8, and cirrus clouds have a typical emissivity of 0.2.

See http://​www.​realclimate.​org/​index.​php/​archives/​2010/​07/​a-simple-recipe-for-ghe/​ for a simple explanation of how the greenhouse effect works. For a more comprehensive review, see, for example, Danny Harvey (2000).

One of the last estimates of the relative contribution of atmospheric long-wave absorbers to the current-day greenhouse effect is: 50 % for water vapor, 25 % for clouds, and 20 % for CO₂. (Schmidt et al. 2010; available at http://​pubs.​giss.​nasa.​gov/​docs/​2010/​2010_​Schmidt_​etal_​1.​pdf).

At glacial–interglacial scale (i.e., between 10,000 and 100,000 years), CO₂ variations tend to follow temperature variations in Antarctica by ~200–1,000 years at the glacial termination. At this scale, temperature variations are mostly driven by orbital changes (Milankovitch theory). The time lag between CO₂ and temperature seems at least partly due to the adjustment of the ocean deep circulation that releases some CO₂ into the atmosphere when the earth warms. This relationship does not invalidate the current one because the increased concentration of atmospheric CO₂ released by human activities drives the current warming whereas glacial termination was initiated by orbital changes 20,000 years ago. Moreover, it is assumed that GHG variations at glacial–interglacial scale had exerted a positive feedback on temperature variations with other processes as the ice–albedo–temperature feedback, that is, the fact that deglaced areas decrease the mean earth albedo, increasing the amount of absorbed solar radiation (Lorius et al. 1990). Lastly, recent analyses (Shakun et al. 2012) demonstrate that global temperatures mostly lag CO₂ variations in Antarctica during the last deglaciation.

Since 1750 the total radiative forcing related to the increase of atmospheric GHG concentrations due to human activities equals +2.9 W/m². The net anthropogenic effect including cooling effect mostly due to sulfur emissions equals +1.6 W/m². The direct cooling effect of anthropogenic sulfur associated with the aerosol veil, that increase albedo at a regional scale is complicated by its indirect effect through the modification of the optical properties of clouds. The cooling effect is less certain than the one associated with GHG increase (IPCC 2007).

There is a debate about the sense of a “global” (in the sense of planetary) mean of surface temperature. Everybody could experience very large temperature variations on small time and spatial scales, for example, simply moving from shade to sunlight in a summer day. It seems then unreasonable to compute a spatial mean from a few samples. But, the range of temperature variations strongly decreases when time means (instantaneous record to annual mean) are considered, especially when raw temperatures are scaled to the local mean annual cycle (theoretically estimated with at least 30 years of data). It is because the drivers of temperature variations at this timescale are from regional (e.g., atmospheric Rossby waves, which are giant meanders in the atmosphere. There are typically 4–6 such Rossby waves around the globe between subtropical and subpolar latitudes, that are the main factor determining the spatial scale of monthly or seasonal temperature anomalies at the extratropical latitudes) to zonal/near-global (as El Niño Southern Oscillation phenomenon) or even planetary scales (e.g., variations of the solar constant or GHG concentrations, large volcanic eruptions, etc.). The anomalies of temperatures relative to the annual cycle at monthly and moreover annual timescales have thus a far larger spatial coherence and less amplitude than localized records. In that way, it is possible, and physically plausible, because of the link between temperature variations and the change in radiative balance, to compute the spatial mean of surface temperature at continental or even planetary scales.

See http://​www.​ipcc.​ch/​pdf/​special-reports/​spm/​sres-en.​pdf

Note that the thermal difference between glacial and interglacial periods during the quaternary equals 6–10 °C in Antarctica and Greenland.

This hypothesis also excludes engineering solutions able either to remove massive amounts of carbon from the atmosphere or to increase the earth’s albedo.

See http://​unfccc.​int/​kyoto_​protocol/​items/​2830.​php.

This ability should decrease with time—and perhaps saturate—inasmuch as the ocean acidifies itself as it absorbs more and more carbon. The recent estimates show that sinks of Carbon averaged since 1959 equal respectively: atmosphere (44 % of total Carbon anthropogenic emissions), land (28 %), ocean (28 %) (Le Quéré et al. 2012).