Carbon Dioxide Is the Biggest Climate Temperature Control Knob, Not Water Vapor
by Daniel Bailey
(Michigan, United States)
Radiative Forcing by Emissions and Drivers
As for water vapor vs CO2, the Earth's climate doesn't change significantly without a change in factors capable of forcing it to change (see larger image).
When climate is at an equilibrium, seasons come and go at their appointed times and polar ice cover stays within range of natural variations. As does ocean pH and global temps.
If global temps and ocean pH are changing, which we can empirically measure and verify that they are (indeed, right now the RATE of change is without parallel in the paleo record), then there must be a change in the composition of those gross factors which can effect climate.
Factors Affecting Climate
The gross factors affecting climate are: Milankovich cycles (orbital factors), solar output, volcanoes (typically a negative forcing), aerosols, surface albedo and non-condensible greenhouse gases (water vapor plays the role of feedback).
Orbital forcing has been negative for the past 5,000 years (since the end of the Holocene Climate Optimum), solar output during the past 40+ years has been flat/negative, volcanoes exert a short-term (up to several years) negative forcing (but none of note since Pinatubo), aerosols (natural and manmade) are a net negative forcing over that time period.
Albedo is a net positive forcing due to the ongoing loss of Arctic sea ice; cloud albedo effects are thought to be general a net zero forcing, but some of the latest research shows a slight positive feedback effect (Dessler 2010).
Which leaves the non-condensible greenhouse gases, primary of which are carbon dioxide (CO2) and methane (CH4). Atmospheric concentration levels of both are rising, and have been for literally centuries now, so they are a net positive forcing.
While the concentration of CH4 is rising, and it is a potent GHG, the radiative forcing from it is overall less than that of CO2 due to the much more massive injection of previously-sequestered, fossil-fuel-derived bolus of CO2 humans are re-introducing back into the carbon cycle.
We know from research that water vapor is a feedback to temperature changes and not a forcing of temperature change (due to water vapor being a condensible GHG). Per Lacis et al 2010:
"Ample physical evidence shows that carbon dioxide (CO2) is the single most important climate-relevant greenhouse gas in Earth's atmosphere. This is because CO2, like ozone, N2O, CH4, and chlorofluorocarbons, does not condense and precipitate from the atmosphere at current climate temperatures, whereas water vapor can, and does.
Non-condensing greenhouse gases, which account for 25% of the total terrestrial greenhouse effect, thus serve to provide the stable temperature structure that sustains the current levels of atmospheric water vapor and clouds via feedback processes that account for the remaining 75% of the greenhouse effect.
Without the radiative forcing supplied by CO2 and the other non-condensing greenhouse gases, the terrestrial greenhouse would collapse, plunging the global climate into an icebound Earth state."
Per Lacis et al 2013:
The climate system of the Earth is endowed with a moderately strong greenhouse effect that is characterized by non-condensing greenhouse gases (GHGs) that provide the core radiative forcing. Of these, the most important is atmospheric CO2. There is a strong feedback contribution to the greenhouse effect by water vapor and clouds that is unique in the solar system, exceeding the core radiative forcing due to the non-condensing GHGs by a factor of three. The significance of the non-condensing GHGs is that once they have been injected into the atmosphere, they remain there virtually indefinitely because they do not condense and precipitate from the atmosphere, their chemical removal time ranging from decades to millennia. Water vapor and clouds have only a short lifespan, with their distribution determined by the locally prevailing meteorological conditions, subject to Clausius-Clapeyron constraint.
Although solar irradiance is the ultimate energy source that powers the terrestrial greenhouse effect, there has been no discernible long-term trend in solar irradiance since precise monitoring began in the late 1970s. This leaves atmospheric CO2 as the effective control knob driving the current global warming trend.
Over geologic time scales, volcanoes are the principal source of atmospheric CO2, and the weathering of rocks is the principal sink, with the biosphere particpating as both a source and a sink. The problem at hand is that human industrial activity is causing atmospheric CO2 to increase by 2 ppm/yr, whereas the interglacial rate has been 0.005 ppm/yr. This is a geologically unprecedented rate to turn the CO2 climate control knob. This is causing the global warming that threatens the global environment.
Lastly, the atmospheric composition of water vapor has increased by about 5% since 1970 (Trenberth & Fasullo, 2009; pp 317).
As a result, the atmosphere now holds the equivalent of an extra volume of Lake Erie in it, spread throughout. Driven by the warming of the atmosphere engendered by the human burning of fossil fuels.