Scientists Exclude Water Vapour & Clouds
As Contributors To Recent Climate Change
The IPCC has long acknowledged that water vapour is the Earth’s most influential greenhouse gas.
“Water vapour is the primary greenhouse gas in the Earth’s atmosphere. The contribution of water vapour to the natural greenhouse effect relative to that of carbon dioxide (CO2) depends on the accounting method, but can be considered to be approximately two to three times greater.”
— IPCC AR5 Chapter 8, page 666
In the 2007 report, the IPCC reported an increasing water vapour trend during the 1980s to 2000s.
Utilizing the IPCC’s own figures and models (from the 4th report), water vapour’s proportional contribution to the total planetary greenhouse effect reaches a dominating 96%.
In contrast, CO2’s proportional radiative forcing contribution to the greenhouse effect is just 2.7% regardless of the CO2 concentration level.
“Calculation of Radiative Forcing (Warming Effect)
of Carbon Dioxide At Any Concentration”
“Figure 7 is FAQ 1.1 Figure 1 from page 96 of AR4 [IPCC, 2007]. It shows the radiation balance for the earth and that the back radiation of all of the greenhouse gases is 324 W m-2. This is the value used to calculate the RF [radiative forcing] of CO2 at 378 ppmv as (8.67/324)/100 = 2.7% back radiation of the total of all of the greenhouse gases. … From Table 1, CO2 accounts for 2.7% of the global warming while all of the other gases [i.e., methane] account for approximately 0.7% for a total of approximately 3.4%. It becomes evident that, on average, water vapour accounts for approximately 96% of the current global [greenhouse effect] warming. This is an important finding because it leads to the conclusion that the factors controlling the average level of water vapour in the atmosphere also control atmospheric temperature.”
“[O]n average, each molecule of CO2 is surrounded by approximately 23 molecules of water vapour at ground level [i.e., water vapour is 23 times more abundant in the atmosphere than CO2]. … If the warming effect of water molecules and CO2 molecules were the same, then the contribution of CO2 would be (1/22.7) = 4.4% of that of water vapour. But from the previous section, water molecules are 1.6 times more effective at warming than CO2 molecules. Using this value and the ratio of 22.7:1, the contribution of CO2 to warming of the atmosphere is approximately (1/22.7)/1.6 = 2.8% of that of water vapour. As water vapour is approximately 96% of the total RF of all of the GHG, the contribution of CO2 is approximately 4% less than this, i.e., 2.69%. If the average RH were 60%, the contribution of CO2 would be ((1/27.4)/1.32) x 0.96 = 2.65%. For practical purposes, these values are the same as the 2.7% obtained by the quadratic model.”
Cloud Radiative Forcing
In a seminal paper (currently 1,440 citations) published in the journal Science entitled “Cloud-radiative forcing and climate…”, Ramanathan et al. (1989) conclude that the cloud contribution to radiative forcing (both shortwave and longwave) is ten times greater than doubled CO2.
Ramanathan et al., 1989 “The size of the observed net cloud forcing is about four times as large as the expected value of radiative forcing from a doubling of CO2. The shortwave and longwave components of cloud forcing are about ten times as large as those for a CO2 doubling.”
Cloud cover changes are significant determinants of the Earth’s top-of-atmosphere (TOA) radiation imbalance, or how much solar radiative forcing is absorbed by the Earth’s surface (oceans).
Wielicki et al., 2002 “It is widely assumed that variations in Earth’s radiative energy budget at large time and space scales are small. We present new evidence from a compilation of over two decades of accurate satellite data that the top-of-atmosphere (TOA) tropical radiative energy budget is much more dynamic and variable than previously thought. Results indicate that the radiation budget changes are caused by changes in tropical mean cloudiness.”
More cloud cover on a net global scale means less solar radiation penetrates the surface, which leads to a net cooling, and less cloud cover means more solar radiation penetrates into the (ocean) surface, which ultimately leads to net warming trend. In other words, clouds significantly determine global warming (or cooling) trends.
Graph from Page, 2017
McLean, 2014 “The reduction in total cloud cover of 6.8% [between 1984 – 2009] means that 5.4 Wm−2 (6.8% of 79) is no longer being reflected but acts instead as an extra forcing into the atmosphere… To put this [5.4 Wm-2 of solar radiative forcing via cloud cover reduction between 1984-2009] into context, the IPCC Fifth Assessment Report…states that the total anthropogenic radiative forcing for 2011 relative to 1750 is 2.29 Wm−2 for all greenhouse gases and for carbon dioxide alone is 1.68 Wm−2. The increase in radiative forcing caused by the reduction in total cloud cover over 10 years is therefore more than double the IPCC’s estimated radiative forcing for all greenhouse gases and more than three times greater than the forcing by carbon dioxide alone [from 1750 to present].”
Goode and Palle, 2007 “The decrease in Earth’s reflectance from 1984 to 2000 … translates into a Bond albedo decrease of 0.02 (out of the nominal value of about 0.30) or an additional global shortwave forcing of 6.8 Wm2. To put that in perspective, the latest IPCC report (IPCC, 2001) argues for a 2.4 Wm2 increase in CO2 longwave forcing since 1850. The temporal variations in the albedo are closely associated with changes in the cloud cover.”
New Paper Excludes Water Vapour, Clouds As Climate Contributors
Despite the widely recognized prominence of water vapour and clouds in models of greenhouse forcing, a new paper published in Climate Dynamics fails to even identify either factor as a contributor to global temperature changes during the 1984-2005 period.
Instead, the 5 authors have decided that carbon dioxide (24%) and methane (19%) are the dominant greenhouse drivers of recent temperature variations, and water vapour and clouds apparently no longer assume any role. Meanwhile, the authors do recognize natural solar forcing factors (17% combined), natural ENSO variability (12%), and volcanic aerosols (23%) as assuming slightly more than half of the responsibility for temperature changes combined since the mid-1980s.
“Quantitative Assessment of Drivers of
Recent Global Temperature Variability”
“Quantitative assessment of drivers of recent [1984-2005] global temperature variability … Measurements of greenhouse gases: CO2, CH4 and N2O; volcanic aerosols; solar activity: UV radiation, total solar irradiance (TSI) and cosmic ray flux (CR); El Niño Southern Oscillation (ENSO) and Global Mean Temperature Anomaly (GMTA) made during 1984–2005 are utilized to distinguish driving and responding signals of global temperature variability.”
“Estimates of their relative contributions reveal that CO2 (∼24%), CH4 (∼19%) and volcanic aerosols (∼23%) are the primary contributors to the observed variations in GMTA [Global Mean Temperature Anomaly]. While UV (∼9%) and ENSO (∼12%) act as secondary drivers of variations in the GMTA, the remaining play a marginal role in the observed recent global temperature variability. Interestingly, ENSO and GMTA mutually drive each other at varied time lags. … [A]ll the constituents of natural forcings together seem to make contributions equal to the greenhouse gases in the context of recent global temperature variability.”
Methane: A Primary Driver Of Recent Temperature Change?
It would appear rather odd that Bhaskar et al. (2017) would wish to claim, for example, that methane gas has been a significant driver of warming, but at the same time reject water vapour and cloud cover changes as factors affecting global temperatures.
It is likely, though, that the main reason why these authors chose to aggrandize the influence of methane emissions on climate is because it is not currently acceptable to claim that human activity plays only a small role (<25%) in temperature variations. Therefore, adding methane emissions to the list of anthropogenic dangerous “greenhouse gases” could bolster the perceived human influence percentage to within “acceptable” ranges (near 50%).
However, recently published scientific papers contravene this approach and indicate that (a) we just don’t have substantial observational evidence yet to attribute cause to a the rise in methane, (b) methane emissions may not have increased recently after all, and (c) human activity (fossil fuel consumption) has not been the “dominant factor” driving the (assumed) increase in methane.
Turner et al., 2017 [press release] ” We conclude that the current surface observing system does not allow unambiguous attribution of the decadal trends in methane without robust constraints on OH variability, which currently rely purely on methyl chloroform data and its uncertain emissions estimates. … [M]ethane emissions might not have increased dramatically in 2007 after all. … When atmospheric concentrations of methane increase, it may not be correct to chalk it up solely to an increase in methane emissions”
Ruppel and Kessler, 2017 “[T]here is no conclusive proof that the released methane is entering the atmosphere at a level that is detectable against the [natural] background of ~555 Tg yr−1 CH4 emissions. The IPCC estimates are not based on direct measurements of methane fluxes from dissociating gas hydrates, and many numerical models adopt simplifications that do not fully account for sinks, the actual distribution of gas hydrates, or other factors, resulting in probable overestimation of emissions to the ocean-atmosphere system.”
Nisbet et al., 2016 “The isotopic evidence presented here suggests that the methane rise was dominated by significant increases in [naturally occurring] biogenic methane emissions … [T]he sustained shift to more 13C-depleted values and its significant interannual variability, and the tropical and Southern Hemisphere loci of post-2007 growth, both indicate that fossil fuel emissions have not been the dominant factor driving the increase.”
A Skeptical Conclusion
CO2’s alleged 24% contribution to temperature changes since the 1980s (Bhaskar et al., 2017) is substantially smaller than most estimates. NASA’s Dr. Gavin Schmidt, for example, claims that the human contribution to climate changes since the mid-20th century is 110%.
As suggested above, the dubious inclusion of methane as a driver of planetary temperatures may have been added to expand the (perceived) anthropogenic contribution to climate change.
Likewise, Bhaskar et al. (2017) may be tendentiously excluding variations in natural factors like clouds and water vapour in their attribution analysis because they wish to avoid having to explain why the contribution from human activity is modest to negligible for recent decades.
When the results don’t fit the narrative, change the results.