Water vapor in the atmosphere is expected to rise with warming because a warmer atmosphere can hold more moisture. However, over the last four decades, near-surface water vapor has not increased over arid and semi-arid regions. This is contrary to all climate model simulations in which it rises at a rate close to theoretical expectations, even over dry regions. This may indicate a major model misrepresentation of hydroclimate-related processes; models increase water vapor to satisfy the increased atmospheric demand, while this has not happened in reality. Given close links between water vapor and wildfire, ecosystem functioning, and temperature extremes, this issue must be resolved in order to provide more reliable climate projections for arid and semi-arid regions of the world.
Abstract
Arid and semi-arid regions of the world are particularly vulnerable to greenhouse gas–driven hydroclimate change. Climate models are our primary tool for projecting the future hydroclimate that society in these regions must adapt to, but here, we present a concerning discrepancy between observed and model-based historical hydroclimate trends. Over the arid/semi-arid regions of the world, the predominant signal in all model simulations is an increase in atmospheric water vapor, on average, over the last four decades, in association with the increased water vapor–holding capacity of a warmer atmosphere. In observations, this increase in atmospheric water vapor has not happened, suggesting that the availability of moisture to satisfy the increased atmospheric demand is lower in reality than in models in arid/semi-arid regions. This discrepancy is most clear in locations that are arid/semi-arid year round, but it is also apparent in more humid regions during the most arid months of the year. It indicates a major gap in our understanding and modeling capabilities which could have severe implications for hydroclimate projections, including fire hazard, moving forward.
The Earth’s greenhouse effect is much larger than suggested so far. If surface radiation and the greenhouse effect set surface temperatures, our oceans would be boiling. Fortunately, they don’t. Water Earth has a strong water-vapor-based evaporative surface cooling mechanism that effectively sets and stabilizes surface temperatures at a much lower level than cooling by surface radiation emissions can do. Thanks to water vapor our temperature system is far more stable than admitted by the consensus, and thanks to water, water vapor, and clouds surface temperatures are favorable for present life.
Introduction
Early Earth consisted of hot molten lava covered by an extreme greenhouse atmosphere: hardly any surface radiation could reach space, if any. Nevertheless, its surface cooled. Upward convection brought sensible and latent heat from hot surfaces to elevations on the very edge of the atmosphere from where energy effectively could be radiated into space. Despite the near maximal greenhouse effect the surface of Early Earth cooled down and at a certain moment the first oceans developed. Those boiling oceans still resulted in a huge upward convective transport of energy, further cooling the surface. Until now, convective upward transport of energy plays the main role in surface cooling. Convection sets and regulates surface temperatures at actual level. Without evaporative-convective-cloud-cooling, our actual greenhouse atmosphere would theoretically result in a surface temperature of 202.3°C. On the real Earth the greenhouse effect warms the surface, but greenhouse warming does not set and control final surface temperatures. Earth’s H2O-based cooling system does.
Theoretical greenhouse effect
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Conclusions
The Earth’s greenhouse effect is huge, much higher than normally assumed. If cooled by ‘surface radiation only’ the surface of a theoretical planet would have had a surface temperature of 202.3°C. But the Earth’s surface temperatures are not set by the strength of Earth’s greenhouse effect. Additional H2O-based cooling systems keep the surface at a much lower temperature, balancing rising surface radiation uptake. At present, that balance is reached at a yearly average of 15 degrees Celsius.
Thanks to H2O-related surface cooling the Earth’s surface temperatures are bound to a narrow range, at a temperature level well suited for life on Earth. Due to its stability, life developed over many hundreds of millions of years.
Temperature regulates the cooling system; the cooling system regulates temperature.
This looping video shows an umbrella cloud generated by the underwater eruption of the Hunga Tonga-Hunga Ha’apai volcano on Jan. 15, 2022. The GOES-17 satellite captured the series of images that also show crescent-shaped shock waves and lightning strikes.
Credit: NASA Earth Observatory image by Joshua Stevens using GOES imagery courtesy of NOAA and NESDIS
The huge amount of water vapor hurled into the atmosphere, as detected by NASA’s Microwave Limb Sounder, could end up temporarily warming Earth’s surface.
When the Hunga Tonga-Hunga Ha’apai volcano erupted on Jan. 15, it sent a tsunami racing around the world and set off a sonic boom that circled the globe twice. The underwater eruption in the South Pacific Ocean also blasted an enormous plume of water vapor into Earth’s stratosphere – enough to fill more than 58,000 Olympic-size swimming pools. The sheer amount of water vapor could be enough to temporarily affect Earth’s global average temperature.
“We’ve never seen anything like it,” said Luis Millán, an atmospheric scientist at NASA’s Jet Propulsion Laboratory in Southern California. He led a new study examining the amount of water vapor that the Tonga volcano injected into the stratosphere, the layer of the atmosphere between about 8 and 33 miles (12 and 53 kilometers) above Earth’s surface.
Water, H2O, determines the ‘General Background Temperature’ for the Earth, resulting in Hothouse and Ice House Climate States. During geological periods the movement of continents changes the position of
continents, oceans and seas. Because of the different configurations, a dominant warm or a dominant cold deep-water production configuration ‘sets’ average temperatures for the deep oceans. Changing vertical oceanic circulation changes surface temperatures, especially in the higher latitudes. During a Hot House State, higher temperatures in the high latitudes result in a high water-vapor concentration that prevents a rapid loss of thermal energy by the Earth.
These three processes, plate tectonics (continental drift), vertical oceanic circulation variability and variations in atmospheric water vapor concentration and distribution, caused previous Hot House and Warm House Climate States. A change in the working of those mechanisms resulted in a transition from the previous Hot House Climate State to the very cold ‘Ice House State’ that we live in now. That change was set in motion by the changing configuration of continents, oceans and seas.
Some have speculated that the distribution of relative humidity would remain roughly constant as climate changes (Allen and Ingram 2002). Specific humidity can be thought of as “absolute” humidity or the total amount of water vapor in the atmosphere. We will call this amount “TPW” or total precipitable water with units of kg/m2. As temperatures rise, the Clausius-Clapeyron relationship states that the equilibrium vapor pressure above the oceans should increase and thus, if relative humidity stays the same, the total water vapor or specific humidity will increase. The precise relationship between specific humidity and temperature in the real world is unknown but is estimated to be between 0.6 to 18% (10-90%ile range) per degree Celsius from global climate model results (Allen and Ingram 2002) …
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