Take Aways: 

1. Global land and ocean temperatures are rising rapidly 
2. An intensifying water cycle is a direct consequence of the warming, causing the wet to get wetter and the dry to get dryer. It is also amplifying the warming trend by positive feedback mechanisms 
3. The stronger evaporation leads to stronger “moist convection” which drives more intense rain and wind storms 
4. Unprecedented drought, wildfires, storms, floods, landslides, and airline turbulence are a result of the warming induced intensification of the water cycle 
5. Salient can provide probabilistic decision support tools to mitigate the risks of weather extremes in our increasingly volatile climate 

The past few months have seen unprecedented weather disasters strike the United States. Wildfires have devasted neighborhoods in Los Angeles and parts of the normally wet East Coast. Intense rainfall and flooding has plagued the South from Texas to the Carolinas and intense Atmospheric Rivers have caused landslides and extreme snowfalls in the West. Severe storms with strong tornados and derechos have devastated communities in the Midwest. A late spring snow storm has hit the northern tier and Phoenix reached a new early heat record of 99^oF on March 25. The increasingly severe weather is no random act of nature, rather it is directly physically linked to the remarkable warming that the Earth has experienced in the last few years (Fig1.)

The past two years are in the record books as having the warmest global average temperatures on record (figure 1). The last ten years have been the warmest ten years on record. The remarkable suddenness of the recent up-turn in temperatures is alarming, but it helps us to understand some of the extremes in  weather we are experiencing.  

Warming leads inevitably to an intensifying water cycle. This is due to the basic thermodynamics of the water molecule. The vapor pressure is an important property of water that shows the equilibrium point between the gas and liquid phase. It is an exponentially increasing function of the temperature (Figure 2). This essentially means that the higher the vapor pressure, the more easily a liquid evaporates. This relationship, which controls the moisture holding capacity of the atmosphere, means that even a 1℃ increase in global temperature increases the water vapor in the atmosphere by about 7%.

The combination of the rapid warming and the strong increase in water vapor pressure with temperature shown in these steep upward curves is the fundamental cause of the increasing severity of our weather. Some of the impacts are summarized below:

• Enhanced greenhouse warming due to the increased water vapor in the atmosphere. Water vapor is a strong greenhouse gas that serves as an amplifier on the increasing CO2 induced warming (Figure 3)
• Enhanced Moist Convection - The H₂O molecule is much lighter than the other constituents of the atmosphere (N2 & O2) so the increased evaporation leads to more “moist convection”. This is strong upward motion in the atmosphere that is compensated by similarly strong downward flows. This contributes to the turbulence felt by airplanes and brings down stronger winds from higher in the atmosphere to the surface. Stronger straight-line winds called derrechos as well as more frequent and intense tornados are causing increasingly destructive and life-threatening storms across the US and elsewhere.
• A higher threshold for rainfall. With the greater moisture holding capacity of the atmosphere, the circumstances that allow water vapor to condense into rain are found at higher elevations. This has led to more intense rainfall events in narrower rainbands and atmospheric rivers. More frequent and intense floods and landslides are a direct consequence.
• Diminishing cloud cover. This result has been found in satellite records of the past several decades (Ceppi and Nowack, 2021; Wan et al , 2024) and seems to be a result of the narrowing of the rain bands. It is somewhat of a surprise, as some had argued decades ago that warming would lead to increased cloudiness that would reflect solar energy back into space (Lindzen, 1990). But diminishing cloud cover is actually allowing more solar energy to reach the surface to be absorbed by the ocean or heat the ground and leading directly to another amplifying feedback on global temperatures.
• Expansion of the subtropical high-pressure systems. Complementary to the narrowing but more intensely convecting rainfall zones of the tropics, is an expansion of the dry downwelling regions in the subtropical high-pressure zones. The circuit defined by the rising moist air of the tropics and the sinking dry air of the midlatitude subtropics is known as the Hadley Cell. This expansion of the subtropics (Lu et al, 2007) is leading to reduced precipitation in midlatitudes (He and Soden, 2017). This is the root cause of drought and desertification on land and increasing salinity in the midlatitude oceans (Durack and Wijffels, 2010). This is a major cause of the increased incidence of wildfires in both the east and west coasts of ocean basins as the high pressure zones expand their longitudinal extent (Karnauskas and Ummenhofer, 2014). The ongoing expansion of the North Atlantic Hadley Cell has led to increased wildfire occurrence in both the US east coast and Spain and Portugal.

Thus, the two steeply sloping upwards curves of warming and the water cycle response to that warming are acting to both accelerate the warming and intensifying the extremes of drought and wildfires, rain, flooding and landslides, as well as the severe convective storms that cause so much death, property destruction and more turbulent air travel (Figure 3).

The bad news is that it will only get worse. We are locked into a vicious cycle of accelerated warming, because we have transitioned into a regime where strong amplifying feedbacks are taking over. Salient has developed unmatched forecasting abilities for the risk of extreme weather and a sophisticated suite of decision support tools based on uniquely reliable probabilistic AI weather forecasting model. These can help industries hedge against the increasingly extreme weather that is sure to come. Learn about the superior skills of our new GEM model from the recent webinar.