Per scientists, removing a greenhouse gas – water vapor – from the atmosphere results in net additional forcing (warming). Irrigation studies also affirm adding water vapor cools the surface.
Per “mainstream” climate science, Earth’s total greenhouse effect radiative forcing from water vapor, clouds, CO2, and trace greenhouse gases (like methane) amounts to 150 W/m² (Lacis, 2018). About 50% of this 150 W/m² total (75 W/m²) greenhouse effect forcing is derived from water vapor, 25% (37 W/m²) is from clouds.
Image Source: Lacis, 2018
But the contradictory “mainstream” science says removing clouds and water vapor from the atmosphere actually has a far greater radiative impact than their influence as greenhouse warming agents.
A cloud-free day at noon results in an extra +251 W/m² of radiative forcing relative to a cloudy day at noon. Removing water vapor , a greenhouse gas, leads to a forcing increase of +131 W/m².
Image Source: van Heerwaarden et al., 2021
Together, that’s +382 W/m² of additional forcing with the absence of these greenhouse effect/greenhouse gas agents, whereas the total greenhouse effect forcing from water vapor and clouds is just +112 W/m² (75 + 37 W/m²).
So the presence of greenhouse gases (water vapor) with clouds combined have a net -270 W/m² cooling effect on Earth’s surface temperatures.
Adding water vapor to surface air leads to cooling
According to a 2018 study by Sherwood and co-authors, the last 100 years of anthropogenic water vapor emissions (mostly from irrigation) have produced a total effective radiative forcing of -0.1 to 0.05 W/m². In other words, adding more greenhouse gases in water vapor form leads to a “near-zero or small cooling effect.”
Sherwood et al. even acknowledge “large increases in anthropogenic water vapor emissions would have negligible warming effects on climate” because “greenhouse-gas warming is outweighed by increases in…humidity-induced low cloud cover.”
Image Source: Sherwood et al., 2018
New studies have continued to verify that adding the water vapor greenhouse gas to the surface air results in cooling.
For example, Kala et al. (2023) report substantial cooling results from adding this greenhouse gas.
“Our results show that irrigation could potentially reduce the mean seasonal maximum temperature during the Angry summer of 2012/2013 by −1.44°C to −2.13°C over irrigated regions.”
Zhang et al. (2023) also report expanding irrigation across regions of China has resulted in verifiable cooling effects.
“We found the space-and-time method can better represent the cooling effects of long-term irrigation expansion than the space-for-time method, despite that they derived similar cooling trends. The cooling effects were most evident in South Xinjiang, followed by North Xinjiang and Hexi Corridor, and varied in different irrigation expansion sources. Specifically, more intensive cooling occurred in new irrigated areas from unused lands (-0.69 ± 0.02 K) than that from grasslands (-0.47 ± 0.05 K) and forests (-0.28 ± 0.04 K). The cooling effects were dominated by marked daytime cooling compared to negligible nighttime warming.”
Other recent studies also affirm the conclusion that adding water vapor, Earth’s main greenhouse gas, cools the surface air.
In the very same regions of India where irrigation has been most prevalent, significant growing-season cooling has been occurring. Land surface temperatures (LST) in the Indo-Gangetic Plain have cooled by -0.8°C since 1979, and this is “associated with the irrigation expansion” in the region.
Image Source: Krishnankutty Ambika and Mishra 2021
The same phenomenon has been observed in China. Scientists (Yang et al., 2020) even refer to adding anthropogenic water vapor emissions to the atmosphere as the irrigation cooling effect (ICE), which is “closely linked to the surface energy balance.”
Irrigation “cools daytime LST by 1.15 K, and cools nighttime LST by 0.13 K, on average, across the irrigated areas of China.” During the growing season, pumping Earth’s most powerful greenhouse gas into the atmosphere results in a cooling effect of more than 6 K in irrigated (ICE) regions compared to adjacent locations not subjected to greenhouse gas cooling.
Image Source: Yang et al., 2020
Image Source: Zhang et al., 2022
These observational results would appear to contradict the modeled claims that water vapor is a positive feedback leading to enhanced warming.