Between the years 1992 and 2014, anthropogenic CO2 emissions rates grew by 65%, or from a yearly average of 6.1 gigatons of carbon (GtC) in 1992 to a rate of 10.1 GtC by 2014 (Global Carbon Budget, 2014).
However, according to a new Nature paper entitled “A Hiatus of the Greenhouse Effect” by Song, Wang, & Tang (2016), there has been an overall hiatus to slight decline in the influence of the greenhouse warming effect on temperature beginning in 1992, which has coincided with the well-established pause in global warming since the early 2000s.
Below is the encapsulating graph from the paper, illustrating that the global-scale atmospheric and surface greenhouse effect (Gaa and Gsa) has remained flat/slightly declined between 1992 and 2014, during the same period time that anthropogenic CO2 emissions were rapidly rising (from 6.1 GtC/yr to 10.1 GtC/yr). This strongly suggests that anthropogenic CO2 emissions retain a very modest role in affecting variations in the overall greenhouse effect.
“Monthly variations of the areal averaged atmospheric and surface greenhouse effect parameter anomalies (Gaa and Gsa) from 1979 to 2014 for the (a) globe”
A Hiatus of the Greenhouse Effect
[T]he primary goal of this study is to investigate the spatiotemporal evolution of the greenhouse effect to better evaluate its potential impact.
Because of the shorter period of the CERES EBAF product, the areal averaged Gsa [surface greenhouse effect] is represented only between 2003 and 2014 in Fig. 2 but shows no notable trend over the globe, sea or land. Thus, the surface greenhouse effect has not been strengthened in the last decade.
In the 1980s, a significant increasing Gaa [atmospheric greenhouse effect] tendency exists with a linear estimate of 0.19 W m−2 yr−1. However, this uprising trend pauses starting in circa 1992, when Gaa [atmospheric greenhouse effect] begins to slightly decrease at a rate of −0.01 W m−2 yr−1. This statistically non-significant trend indicates that the enhancing global atmospheric greenhouse effect is slowed down. Moreover, the atmospheric greenhouse effect hiatus can be found over both sea and land.
The oceanic Gaa [atmospheric greenhouse effect] exhibits a notable increasing trend with a rate of 0.21 W m−2 yr−1 in 1979–1991, whereas its rate of change (−0.04 W m−2 yr−1) during 1992–2014 is not statistically significant. By contrast, although a sudden change in the Gaa tendency is observed overland, the breakpoint is approximately 5 years later than that of the oceanic Gaa. The terrestrial Gaa trends are 0.12 W m−2 yr−1 and 0.05 W m−2 yr−1 before and after 1997, respectively.
In the last subperiod [2003-2014], the global averaged SULR [surface upwelling longwave radiation/greenhouse effect] anomaly remains trendless (0.02 W m−2 yr−1) because Ts [global temperatures] stop rising. Meanwhile, the long-term change of the global averaged OLR anomaly (−0.01 W m−2 yr−1) is also not statistically significant. Thus, these two phenomena result in a trendless Gaa [atmospheric greenhouse effect].
[A]remarkably decreasing Gaa trend (−0.27 W m−2 yr−1) exists over the central tropical Pacific, indicating a weakened atmospheric greenhouse effect in this area, which largely offsets the warming effect in the aforementioned surrounding regions. As a result, a trendless global averaged Gaa [atmospheric greenhouse effect] is displayed between 1991 and 2002 (Fig. 2).
Again, no significant trend of the global averaged Gaa [atmospheric greenhouse effect] is found from 2003 to 2014 (Fig. 2) because the enhanced warming effect over the western tropical Pacific is largely counteracted by the weakened warming influence on the central tropical Pacific.
Explanation: Cloud Variations, Water Vapor Dominate Greenhouse Effect, With Insignificant CO2 Influence
As scientists Song, Tang, & Wang (2016) point out, during the 2003 to 2014 period, global temperatures “stop rising”. In fact, temperatures declined slightly during this period according to HadCRUT and RSS.
Song, Wang, & Tang ultimately provide an explanation for the hiatus in the global-scale greenhouse effect (from the early 1990s to present) and temperature rise (from the early 2000s to present). They insinuate that water vapor and clouds are the dominant contributors to greenhouse warming, and the CO2 contribution is quite modest by comparison. Small variations in water vapor and cloud can therefore supersede large variations in CO2 in determining trends in the overall greenhouse effect. Below are the key points from the rest of the paper detailing CO2’s small contribution to the overall greenhouse effect (relative to water vapor, clouds).
[T]he influences of water vapor and clouds … contribute approximately 75% of the total [greenhouse] effect.
The results above indicate that the notably downward Gaa tendency over the central tropical Pacific indeed plays an important role in inducing the greenhouse effect hiatus since the 1990s. What causes this decreasing Gaa [atmospheric greenhouse effect]? The variation of the greenhouse effect is substantially influenced by its contributors, including water vapor, clouds, and GHGs. GHG concentrations have risen steadily during recent decades. The variations of metrics related to the other two contributors are given in Fig. 4a and are based on the CERES-EBAF products between 2003 and 2014. The total column precipitable water (TCPW) anomaly significantly increases at a rate of 0.44 cm yr−1. However, the cloud area fraction (CAF) anomaly is reduced by −0.60% yr−1, which is consistent with the decreasing cloud activity described in previous publications. Therefore, although the greenhouse effect can be enhanced by increasing GHGs and water vapor in the atmosphere, it can be weakened by decreasing clouds. If these two actions offset each other, a hiatus of the global greenhouse effect will result.
[T]his pause in the greenhouse effect is mostly caused by the high number of La Niña events between 1991 and 2014. A strong La Niña indicates suppressed convection in the tropical central Pacific that reduces atmospheric water vapor content and cloud volume.
[T]he atmospheric and surface greenhouse effect parameters both become trendless when clouds are considered. … Overall, the downward tendency of clouds is the dominant contributor to the greenhouse effect hiatus.
In conclusion, the pause of the greenhouse effect since the 1990s may be one of the reasons for the global warming hiatus starting in the early 2000s.
Scientific Literature Substantiates Insignificant CO2 Influence
The explanation that water vapor and clouds are the most dominant contributors to the overall greenhouse effect — and that the CO2 contribution is insignificant by comparison — has been well known and documented in the scientific literature for decades. For example:
“[R]ecent increases of atmospheric carbon dioxide have contributed much less than 5% of the recent changes of atmospheric temperature, and will contribute no more than that in the foreseeable future.”
“[T]he measured increase in carbon dioxide in the atmosphere, according to the most recent computations, would not be enough to have any measurable climatic effect. Rasool and Schneider (1971) conclude that an increase in the carbon dioxide content of eight times the present level would produce an increase in surface temperature of less than 2°C, and that if the concentration were to increase from the present level of 320 parts per million to about 400 by the year 2000, the predicted increase in surface global temperature would be about 0.1°C.”
“Water vapour and cloud are the dominant regulators of the radiative heating of the planet. ..The greenhouse effect of clouds may be larger than that resulting from a hundredfold increase in the CO2 concentration of the atmosphere. … 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.”
“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. The results of several current climate model simulations fail to predict this large observed variation in tropical energy budget. The missing variability in the models highlights the critical need to improve cloud modeling in the tropics so that prediction of tropical climate on interannual and decadal time scales can be improved.”
“We will show that changes of relative humidity or low cloud cover explain the major changes in the global mean temperature. We will present the evidence of this argument using the observed relative humidity between years 1970 and 2011 and the observed low cloud cover between years 1983 and 2008. One percent increase in relative humidity or in low cloud cover decreases the temperature by 0.15 °C and 0.11 °C, respectively. In the time periods mentioned before the contribution of the CO2 increase was less than 10% to the total temperature change.”
“The analysis showed that the main atmospheric parameters that affect the amount of global radiation received on earth’s surface are cloud cover and relative humidity. Global radiation correlates negatively with both variables. Linear models are excellent approximations for the relationship between atmospheric parameters and global radiation. A linear model with the predictors total cloud cover, relative humidity, and extraterrestrial radiation is able to explain around 98% of the variability in global radiation.”
Summarizing Question: Why is there so much preoccupation with atmospheric CO2 concentrations and reducing anthropogenic CO2 emissions when it is well documented in the peer-reviewed scientific literature that the CO2 contribution to the overall greenhouse effect is so weak that it can be easily supplanted by small changes in clouds and water vapor, or natural climate-changing constituents?