Water vapor is a greenhouse gas, but it is not a major driver of global warming

CLAIM

Water vapor is the most abundant greenhouse gas so it is the main cause of recent global warming

VERDICT more about the rating framework

SOURCE: Facebook users, Climate Change is Crap, 18 Nov. 2023  

DETAILS

Flawed Reasoning: Water vapor, which lasts only days in the atmosphere, is ultimately controlled by air temperature and pressure according to thermodynamics. Long-term changes in temperature driven by other greenhouse gasses supersedes water vapor feedbacks.
Misleading: Water is only relevant for maintaining the natural greenhouse effect. It is a negligible contributor to the enhanced greenhouse effect which is responsible for global warming.

KEY TAKE AWAY

Water vapor is fundamentally different from the other greenhouse gasses which directly cause global warming. It leaves the atmosphere in a matter of days and its atmospheric concentration is ultimately governed by temperature. It is critical for the natural greenhouse effect, but it is a negligible contributor to the enhanced greenhouse effect. We already know, beyond any reasonable doubt, that human emissions of the other greenhouse gasses is the primary cause of global warming.

REVIEW

CLAIM: Water vapor is the main cause of recent global warming because it is the biggest contributor to the greenhouse effect and the most abundant greenhouse gas by weight and volume. Water vapor is natural and the greenhouse effect is natural, so recent global warming is natural.

Although water vapor is the most abundant greenhouse gas (GHG) by weight and volume, it is not a major driver of long-term climate change and the recent global warming we are experiencing. The opposite is often claimed as a challenge to anthropogenic climate change because it is the most important gas maintaining the greenhouse effect and it is naturally occurring. So, as this claim goes, human emissions of GHGs are not to blame. For example, in this Facebook post from 18 November 2023 a table of data is presented that misleads viewers towards this conclusion. We address that post specifically in Box 1 at the bottom of this article. In the next few sections, we will review more generally why this claim is based on flawed reasoning.

Water vapor is different than the other greenhouse gasses

There are a few differences between GHGs that are driving global warming, like carbon dioxide (CO₂) and those that do not, like water vapor. The ability of a gas to add to or reduce global warming is represented by its effective radiative forcing value (ERF, measured in units of watts per square meter (W.m^–2)). The ERF is the energy added (heating) or subtracted (cooling) from the Earth system due to a change in that gasses’ concentration and also their global warming potential, which represents the gasses’ ability to trap heat in the atmosphere compared to CO₂ as the standard. The ERF is based on radiative transfer models that account for the gasses’ specific absorption and emission properties. The concept implies a change from the norm; a change in radiative forcing from the conditions that we should normally be experiencing under the natural greenhouse effect which makes Earth liveable.

We can estimate the ERF of the main greenhouse gasses, including water vapor, over the industrial era to get an idea of the main drivers of the recent global warming we are experiencing (Fig. 1). From the years 1750 to 2019, out of a total anthropogenic ERF value of 2.72 W.m^–2, atmospheric-CO₂ has been the most important single driver, contributing 2.16 W.m^{–2 [1]}. The next largest drivers are methane contributing 0.54, ozone at 0.42, nitrous oxide at 0.21, and halogens at 0.41. Stratospheric water vapor, resulting from the oxidation of methane emitted from humans, has an ERF of only 0.05 W.m^–2. It is among the least powerful greenhouse gasses humans directly or indirectly emit. As a reminder, because these ERF values are all positive, they represent energy added to the Earth system and increased global warming.

Figure 1 – Change in effective radiative forcing from 1750 to 2019 by greenhouse gasses, ozone, stratospheric water vapor, surface albedo, contrails and aviation-induced cirrus, aerosols, and solar. The anthropogenic total category represents the combined positive and negative effects of human activities (everything except solar). The solid bars represent best estimates with very likely (5–95%) ranges represented by the error bars (source).

Near-surface water vapor has a similarly low ERF and negligible effect on driving global warming as stratospheric water vapor^[2]. Because any increase in water vapor near the surface does not reach the upper-troposphere, the potential positive contribution to global warming is negated by the increased reflectance from humidity-induced low cloud cover. This results in a net-zero or even cooling effect from increased water vapor.

In addition to the ERF of GHGs (which considers concentration and global warming potential), we also need to consider how long they stay in the atmosphere contributing to global warming. This is known as the atmospheric residence time, or the average time a molecule of that gas remains in the atmosphere before changing or being removed by natural processes. Gasses with longer residence times will have more time to let their ERF impact global climate conditions, whether they increase (e.g., like CO₂) or decrease (e.g., like aerosols) global warming. In a sense, we can assume that the longer the residence time in the atmosphere, the greater the cumulative effect of the gas on the greenhouse effect.

The atmospheric residence time of water vapor is extremely short compared to the other GHGs. Water vapor cycles through the atmosphere through evaporation and precipitation within 8–10 days on average (median residence time of 4–5 days)^[3]. The average residence time of methane is around 10–12 years, nitrous oxide lasts over 100 years, while fluorinated gasses last weeks to thousands of years. Fluorinated gasses, like hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride, are not only the longest lasting, they are also the most powerful GHGs emitted by human activities (thousands of times greater global warming potential than CO₂).

For CO₂, one molecule may leave the atmosphere after about 5 years, but it is replaced by another CO₂ molecule from the ocean or biosphere. The global carbon cycle, with its tight budget of sources and sinks (or stocks), is such that it can take hundreds to thousands of years to really “lock away” the excess CO₂ that humans have emitted (primarily through burning fossil fuels which was carbon previously locked away). Therefore, atmospheric-CO₂ has a much more lasting impact as a GHG than methane and even nitrous oxide because its increase in the atmosphere is independent of its actual residence time^[4]. Excess water vapor molecules are gone in the blink of an eye comparatively. This is why water vapor is not a major driver of global warming, even if it is the most abundant GHG.

The enhanced greenhouse effect and water vapor feedbacks

The previous section discussed the ERF of GHGs and estimated negligible values for stratospheric and near-surface water vapor. This water vapor comes from the oxidation of methane emitted from human activities or from irrigation practices, meaning it is water vapor that we ultimately added to the atmosphere (i.e., a change from the norm). But what about all the water vapor that exists naturally in the atmosphere, evaporating from the land and oceans, condensing as clouds and returning as rain and snow?

We’ve known for decades that natural water vapor returns much more infrared radiation to the Earth’s surface than other gasses like CO₂^[5]. Water vapor is what keeps this planet habitable and keeps us warm; it alone causes around half of the natural greenhouse effect^[6]. While fluorinated gasses may have the highest global warming potential and CO₂ is driving global warming the most, naturally occurring water vapor is indeed the most important greenhouse gas in the atmosphere.

The properties of water vapor have also helped keep the Earth’s temperature stable. For at least two thousand years, average global temperatures barely fluctuated more than a few tenths of a degree Celsius before the industrial revolution (based on proxy data like ice cores). But now there is no question that global temperatures have rapidly risen over the last few decades, resulting from the increased anthropogenic greenhouse gas emissions. The planet is currently more than 1°C warmer than the pre-Industrial average with CO₂ alone increasing by 50% over this period. This is known as the enhanced greenhouse effect (Fig. 2). When it comes to understanding recent global warming, it is solely the enhanced greenhouse effect which matters.

Figure 2 – Illustration of the global energy balance as affected by the natural greenhouse effect (left) and the enhanced greenhouse effect (right). Water vapor (H2O) plays an important role in maintaining the natural greenhouse effect, not driving the enhanced greenhouse effect. Incoming solar shortwave radiation is represented by the yellow arrows and outgoing terrestrial longwave radiation is represented by the red arrows (source).

The claim that because both water vapor and the greenhouse effect are natural, global warming is natural is an example of flawed reasoning. It is an oversimplification which disregards the current understanding that human activities, particularly the burning of fossil fuels, have significantly enhanced the natural greenhouse effect by adding more GHGs to the atmosphere. The enhanced greenhouse effect caused by human activities is impacting global temperatures more than what would have occurred naturally with the pre-Industrial concentrations of GHGs.

Water vapor is not relevant when it comes to creating the enhanced greenhouse effect because it is a byproduct of temperature change and not a driver, governed by the Clausius–Clapeyron relation. While the other GHGs remain as gasses in the atmosphere, water vapor is easily condensable. It condenses into precipitation and evaporates back into water vapor as a function of the current temperature and air pressure. So any excess water vapor that should not be there precipitates quickly, and any deficit in water vapor is restored by evaporation as soon as possible. Atmospheric-CO₂, on the other hand, does not disappear under normal climate conditions in a matter of days like water vapor. In fact, atmospheric-CO₂ has been repeatedly shown to be the primary factor controlling global temperature anomalies (changes). These relationships illustrate that it is not water vapor that drives temperature; it is the reverse.

So, unlike the other GHGs, human activities do not significantly increase water vapor directly. Any water vapor that is emitted directly or indirectly (e.g., from irrigation or methane oxidation) is not long-lasting, as described above. Human activities cannot directly control how much water vapor is in the atmosphere because ocean and air temperature does. This is also unlike the other GHGs. Warmer temperatures will create warmer surfaces which promote more evaporation and increases in atmospheric water vapor. Colder air temperatures hold less water vapor, resulting in more precipitation and decreases in atmospheric water vapor. Human activities can influence atmospheric water vapor very indirectly by continuing to drive global warming, raising the atmosphere’s capacity to hold water vapor.

There is a clear feedback loop between water vapor and global temperature, where an increase in one leads to an increase in the other. It is one of the most dominant climate feedbacks, explaining why global temperatures are so sensitive to changes in the long-lived greenhouse gasses. As the oceans warm because of the enhanced greenhouse effect resulting from increases in atmospheric-CO₂, for example, there will be more water evaporation and more water vapor released to the atmosphere. This excess water vapor can trap even more heat and encourage more evaporation. According to the laws of thermodynamics, water vapor concentration should increase by roughly 7% in the atmosphere with every degree Celsius rise in temperature. So, water vapor does indeed contribute to global warming by reinforcing the enhanced greenhouse effect. But it was the initial contributions of the other anthropogenic GHGs that started this feedback.

The reduction of GHGs like CO₂ can also reverse the water vapor-climate feedback loop by reducing air temperatures which reduces the amount of water vapor in the atmosphere. Earth’s air temperature does not directly affect the concentrations of the other GHGs; their feedback loops with global warming are different. Ultimately, water vapor is very important for maintaining habitable temperatures and it can have a reinforcing feedback with rising temperatures, but it is simply not in the driver’s seat when it comes to creating and maintaining the enhanced greenhouse effect.

Conclusion

Water vapor may be the most abundant GHG keeping the planet habitable, but it is not a major driver of long-term climate change and the recent global warming we are experiencing. It is incorrect to claim that because water vapor is natural and the greenhouse effect is natural, recent global warming must be natural. Water vapor is fundamentally different from the other GHGs which directly cause global warming. It has a negligible ERF, it leaves the atmosphere in a matter of days, and it is ultimately governed by temperature rather than human emissions. It is more of a consequence of global warming than a cause. We already know beyond any reasonable doubt that anthropogenic emissions of the other GHGs like CO₂, methane, and nitrous oxide, is the primary cause of global warming. Water vapor is not relevant when it comes to the enhanced greenhouse effect in the context of recent global warming.

REFERENCES

• 1 – IPCC (2021) The Earth’s Energy Budget, Climate Feedbacks, and Climate Sensitivity. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change.
• 2 – Sherwood et al. (2018) The global warming potential of near-surface emitted water vapour. Environmental Research Letters.
• 3 – Gimeno et al. (2021) The residence time of water vapour in the atmosphere. Nature Reviews Earth & Environment.
• 4 – Cawley (2011) On the atmospheric residence time of anthropogenically sourced carbon dioxide. Energy & fuels.
• 5 – Kiehl & Trenberth (1997) Earth’s annual global mean energy budget. Bulletin of the American meteorological society.
• 6 – Soden & Held (2006) An assessment of climate feedbacks in coupled ocean–atmosphere models. Journal of climate.
• 7 – Anbar et al. (2016) Addressing the Anthropocene. Environmental Chemistry.
• 8 – Broecker (2012) The carbon cycle and climate change: memoirs of my 60 years in science. Geochemical Perspectives.9 – Kunzig & Broecker. (2009) Carbon scrubbers: taking CO₂ out of the air. New Scientist.