For going on 3 decades now, Intergovernmental Panel on Climate Change (IPCC) reports have estimated that the climate’s sensitivity to the doubling of preindustrial levels of CO2 (from 280 ppm to 560 ppm) may range between 1.5°C to 4.5°C due significantly to the assumed “dangerous” warming amplification from positive water vapor feedback. Despite years of analysis, the factor-of-three difference between the lower and higher surface temperature range thresholds has changed little. There apparently have been no breakthroughs in understanding the “basic physics” of water vapor amplification to narrow this range further.
The theoretical conceptualization for the surface temperature change resulting from CO2 doubling alone — without the “dangerous” amplification from water vapor feedback — has also been in use, and unchanged, for decades. Since the 1960s it has been hypothesized that if preindustrial CO2 levels were to be doubled to 560 ppm, the surface temperature change would amount to a warming of a non-alarming 1.2°C in the absence of other feedbacks.
Below are brief summaries from scientific papers (and the Skeptical Science blog) confirming that the IPCC and models claim doubling CO2 only results in 1.2°C of warming.
“[T]he radiative forcing corresponding to a doubling of the CO2 concentration would be 4 Wm-2. To counteract this imbalance, the temperature of the surface-troposphere system would have to increase by 1.2°C (with an accuracy of ±10%), in the absence of other changes”
“We know that if the amount of carbon dioxide (CO2) in the Earth’s atmosphere doubles from the pre-industrial level of 280 parts per million by volume (ppmv) to 560 ppmv, this will cause an energy imbalance by trapping more outgoing thermal radiation in the atmosphere, enough to directly warm the surface approximately 1.2°C.”
“The temperature change at the earth’s surface is ΔT=+1.2°C when the present [CO2] concentration is doubled.”
“The increase of equilibrium surface temperature for doubled atmospheric CO2 is ∼1.2°C. This case is of special interest because it is the purely radiative-convective result, with no feedback effects.”
“The radiative forcing resulting from doubled atmospheric CO2 would increase the surface and tropospheric temperature by 1.2°C if there were no feedbacks in the climate system.”
“An increase in atmospheric CO2 concentration from 275 to 550 ppm is expected to increase radiative forcing by about 4 W m2, which would lead to a direct warming of 1.2°C in the absence of feedbacks or other responses of the climate system”
IPCC: Dangerous future warming levels (3°C and up) are caused mostly by water vapor, not CO2
As mentioned, the IPCC authors have claimed that it is primarily due to the conceptualization of positive feedback with water vapor that the surface temperature response is projected to reach the dangerous warming levels of 3.0°C and up as CO2 doubles to 560 ppm.
“The so-called water vapour feedback, caused by an increase in atmospheric water vapour due to a temperature increase, is the most important feedback responsible for the amplification of the temperature increase [from CO2 alone].”
In their 4th report, the IPCC acknowledged that humans have little influence in determining water vapor levels:
“Water vapour is the most abundant and important greenhouse gas in the atmosphere. However, human activities have only a small direct influence on the amount of atmospheric water vapour.”
The main reason why IPCC authors have asserted that water vapor will do most of the “dangerous” projected warming, while CO2 will contribute a much smaller fraction, is apparently because the greenhouse warming effect from water vapor forcing is “two to three times greater” than that of carbon dioxide:
“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.”
Even NASA agrees that water vapor and clouds together account for 75% of the greenhouse effect, while CO2 only accounts for 20%.
“Carbon dioxide causes about 20 percent of Earth’s greenhouse effect; water vapor accounts for about 50 percent; and clouds account for 25 percent. The rest is caused by small particles (aerosols) and minor greenhouse gases like methane.”
IPCC: Positive water vapor feedbacks are believed to cause dangerous warming
It is curious to note that the insufficiently understood positive water vapor feedback conceptualization is rooted in . . . belief. Literally. In the third report (TAR), the IPCC authors actually used the word “believed” to denote how they reached the conclusion that 1.2°C will somehow morph into 1.5°C to 4.5°C of warming due to amplification from feedbacks.
“If the amount of carbon dioxide were doubled instantaneously, with everything else remaining the same, the outgoing infrared radiation would be reduced by about 4 Wm-2. In other words, the radiative forcing corresponding to a doubling of the CO2 concentration would be 4 Wm-2. To counteract this imbalance, the temperature of the surface-troposphere system would have to increase by 1.2°C (with an accuracy of ±10%), in the absence of other changes. In reality, due to feedbacks, the response of the climate system is much more complex. It is believed that the overall effect of the feedbacks amplifies the temperature increase to 1.5 to 4.5°C. A significant part of this uncertainty range arises from our limited knowledge of clouds and their interactions with radiation.”
IPCC climate sensitivity estimates have been based on hypotheticals, or the belief that water vapor positive feedback will cause another 1.8°C to 3.3°C of “extra” or “dangerous” warming (to reach upwards of 3.0°C to 4.5°C). CO2 alone only causes 1.2°C of warming as it is doubled from 280 ppm to 560 ppm. Since when are modeled beliefs about what may possibly happen to global temperatures at some point in the next 100 years . . . science?
IPCC: Water vapor increased substantially since 1970 — but didn’t cause warming
If water vapor is the primary determinant of the “extra” and “dangerous” warming we are expected to get along with the modest 1.2°C temperature increase as the CO2 concentration reaches 560 ppm, then it is natural to ask: How much of the warming since 1950 has been caused by the additional CO2, and how much has been caused by the water vapor feedback that is believed to cause the extra, “dangerous” warming?
This last question arises because, according to the IPCC, there has been a substantial increase in the potent water vapor greenhouse gas concentration in the last few decades. Specifically, in their 4th report, the IPCC authors claim there has been “an overall increase in water vapour of order 5% over the 20th century and about 4% since 1970“(IPCC ).
Considering its abundance in the atmosphere (~40,000 ppm in the tropics), if water vapor increased by 4% since 1970, that means that water vapor concentrations could potentially have increased by more than 1,500 ppm in the last few decades. The overall magnitude of this water vapor concentration increase is therefore more than 20 times greater than the increase in atmospheric CO2 concentration (~70 ppm) since 1970.
But even though the IPCC claims that (a) water vapor will cause most of the “dangerous” warming in the future, (b) water vapor climate forcing is “two to three” times greater than CO2 forcing within the greenhouse effect, and (c) water vapor concentrations have increased substantially since 1970, the IPCC simultaneously claims that (d) CO2 has caused most — if not all — of the warming since the mid-20th century anyway. In the 5th report, the IPCC’s “consensus” statement reads like this:
“It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together.”
For advocates of dangerous anthropogenic global warming (DAGW) projections, the “more than half” CO2 attribution apparently isn’t quantitatively strong enough. After all, “more than half” could be interpreted as only slightly more than 50%. To rectify this, Gavin Schmidt — a primary overseer of NASA temperature adjustments — has calculated that the anthropogenic impact on climate has not just been “more than half,” but more than 100%. In a recent RealClimate blog entry, Schmidt claims that humans have caused 110% of the global warming since 1950 — and that IPCC analysis (found in Fig. 10.5 in IPCC AR5) also supports an anthropogenic CO2 attribution of “near 100%”.
“The best estimate of the warming due to anthropogenic forcings (ANT) is the orange bar [in Fig. 10.5] (noting the 1𝛔 uncertainties). Reading off the graph, it is 0.7±0.2ºC (5-95%) with the observed warming 0.65±0.06 (5-95%). The attribution then follows as having a mean of ~110%, with a 5-95% range of 80–130%. This easily justifies the IPCC claims of having a mean near 100%, and a very low likelihood of the attribution being less than 50% (p < 0.0001!).”
Conflicting IPCC climate sensitivity feedback suppositions
The IPCC believes that the climate’s overall surface temperature sensitivity to the doubling of preindustrial CO2 ranges between 1.5°C to 4.5°C, with the projected higher warming levels due primarily to amplifying water vapor feedback. This conceptualization appears to be in conflict with other IPCC suppositions.
On one hand, the IPCC reports have claimed that (a) water vapor is much more potent than CO2 within the greenhouse effect, that (b) the bulk of the 3.0°C and up “dangerous” warming that is believed to occur in the future will be forced by positive water vapor feedback, and that (c) water vapor levels have significantly increased in recent decades (by 4% since 1970).
On the other hand, (d) water vapor is claimed to have caused right around 0% of the warming in the last several decades.
Summarily, these conflicting explanations or suppositions about what can happen, what will happen, and what has already happened to the climate due to water vapor feedback beg the questions:
Why hasn’t the “dangerous” water vapor warming found in models “kicked in” during the last several decades, when water vapor levels have increased (according to the IPCC)?
Since it reportedly hasn’t yet, at what point in the future will the “dangerous” water vapor warming projections found in modeling finally show up in the temperature record?
Considering how fundamental climate sensitivity estimates are to climate science, and ultimately to the direction of political policies and energy production and consumption, these questions deserve to be answered . . . with something more substantive than what the IPCC authors have long believed to be true.