At Salient Predictions, we understand that our oceans are retaining more solar energy and growing warmer — and that is the primary driver behind more frequent and more intense storms. Here’s why.

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This year has brought unprecedented heat to the central U.S., devastating wildfires from tropical islands to Canadian forests, and extraordinary flooding events from Libya to New York City. In many ways, the rate of change in the weather has been surprisingly rapid. It is not a coincidence that these extraordinary events occurred while the ocean reached its highest average sea surface temperature (SST) ever recorded on July 31. Ocean temperatures exceeded 100 ^oF in the Florida Keys in late July. A direct line can be drawn from the warming seas to the extreme weather events of this past summer. The connection between high SST and extreme weather relies on the basic physics of the water molecule. And that property is the strong tendency of water to evaporate more readily with increasing temperature, also known as the “saturation vapor pressure of water.”  

The saturation vapor pressure of water is an exponential function of temperature. This means that a small increase in temperature can lead to a large increase in humidity. And the increases get even larger as average temperatures increase. Figure 1 is a plot of this non-linear dependence.  

The strong increase in vapor pressure with rising temperature is due to the thermal energy of water molecules becoming sufficient to break the hydrogen bonds keeping it in the liquid phase. The curve continues to rise to one bar (i.e., one atmosphere or ~ 15 pounds/square inch) at the boiling point of water (212^oF). While the global average surface temperature of the Earth is around 60^oF, it is the variation in vapor pressure over the range of surface air temperatures that makes the relation so important to weather and climate. 

For instance, Phoenix, Arizona reached a high temperature over 110^oF every day in July, had 17 days over 115^oF and three days over 119^oF. The highest temperatures on Earth can exceed 134^oF in places like Death Valley. That means water molecules are 17 times more likely to take flight into the gas phase on a warm afternoon in Phoenix than over 32^oF water, and 28 times more likely on a hot day in Death Valley.  

The reasons behind extensive droughts and extraordinary rainfalls

The stronger evaporation that occurs with higher temperatures is a strong positive feedback on local surface temperatures. This is because any moisture is quickly evaporated, and the sun’s energy goes into further heating the land surface rather than evaporating water. This positive feedback amplified the persistent heat dome that occupied much of the U.S. this past summer and helped lead to wildfires in places as diverse as Hawaii, Greece, and the Canadian forests. The non-linear dependence of vapor pressure on temperature means that warm air can hold much more water than cold air, leading to intense drying when water sources are limited. But it also means that air over lakes, rivers, and the ocean can carry much more water as temperatures rise. And by converting the sun’s energy into the latent heat of evaporation, the temperature increases experienced by the ocean or very moist soils are much more modest than those over dry land.

This process is key to the intensification of the water cycle expected with global warming. It is the main reason we are seeing a great increase in both extensive droughts and extraordinary rainfall events. They are the consequence of an enhanced water source in the warming ocean feeding intense rain events and a much greater drying sink in those places with a hot and dry atmosphere. 

The further we go up the ever-steepening vapor pressure curve, the more intense the water cycle will become. We have already experienced a seven percent increase in the saturation vapor pressure of water with the average global temperature increase of 2^oF over pre-industrial times, but it is the greater range of temperature variations like we saw in Phoenix this summer that will make the impacts of an intensifying water cycle become even more stressful in the future. None of our infrastructure has been built to handle the extremes of rainfall we witnessed this summer, and it is bound to get much worse very quickly as we push the water cycle up that ever steepening exponential curve.

Finally, putting more water vapor into the atmosphere is a positive feedback for global average temperatures. Since water vapor is a strong greenhouse gas, this effectively doubles the warming influence of the increasing CO2.  

Water truly is the key driver of our weather and climate  

Clearly, water is the driver of our weather and climate. At Salient, we are monitoring ocean heat content and changes in salinity that are indicative of energy and moisture transfers to the atmosphere. We have incorporated newly discovered teleconnections between the state of the ocean and future weather on land into our proprietary models alongside dozens of other scientific- and AI-driven insights. We have architected the most sophisticated Machine Learning engine for superior weather forecasts with lead times of weeks to months.  

And because weather impacts nearly every aspect of the economy, Salient’s customers use our forecasts to navigate increasingly volatile weather, avoid catastrophes, and optimize performance across a range of applications in agriculture, energy, commodities, supply chain, insurance, and beyond.