The deadly heat that has killed people in national parks this summer is a reminder of the human toll of climate change. Heat kills more people in the US than hurricanes, floods, and tornadoes combined.
Scientists know that a warmer atmosphere holds more water vapor, which enhances moisture convergence and rainfall rates in storm systems like hurricanes. Two studies (Knutson and Tuleya 1998) and (Guzman and Jiang 2021) found that anthropogenic climate warming has increased extreme rainfall rates over land, including those in hurricanes.
Temperature
Temperature is a measure of the average kinetic energy of particles in matter. Higher temperatures mean the particles have more kinetic energy and are moving faster. This is why warmer air can hold more moisture than cooler air. During a hurricane, this extra moisture raises the storm’s potential for extreme rain events. Anthropogenic climate change has supercharged our hurricanes by increasing the amount of water vapor in their clouds and raising the atmospheric temperature.
Heat can also be defined as the difference in the internal energy of a system (which includes kinetic, potential, and other forms of energy) at two different times. The concept of heat as a transfer of internal energy between two physically separate regions of the same system is the theoretical basis for the concept of temperature, which describes the rate of flow of thermal energy among those parts of a physical system.
The most commonly used temperature scales are the Celsius and Fahrenheit scales, both of which have 100 increments between their freezing and boiling points. However, the International System of Units has another temperature scale known as Kelvin, with 0 Kelvin being absolute zero.
For most of the history of thermodynamics, there was a tight link between heat and temperature, with heat being thought to be a form of energy that was transferred from one region of the system to another via a transfer process. This idea of heat as an inexorable transfer of energy between two parts of a system was developed by Benjamin Thompson, Humphry Davy, Sadi Carnot, and others during the eighteenth century.
In 1848, James Prescott Joule reorganized this thinking by describing latent and sensible heat as two components of heat that impacted distinct physical phenomena. He distinguished between potential energy, possessed via a distancing of particles where attraction was over a longer distance and a form of potential energy, and kinetic energy, possessed by the vibrating and clumping molecules in a substance and a form of kinetic energy. This separation was a key advance in our modern understanding of heat. Despite this distinction, heat and temperature continue to be closely related concepts, and both can affect the motion of particles within a material.
Moisture
Water vapor, the gaseous form of H2O, is playing an outsized role in fueling destructive hurricanes and accelerating climate change. The secret energy source of these large weather engines is a massive reservoir of latent heat trapped in water molecules. When water evaporates into air in the warm tropical oceans, that energy is released, but only slowly, and only when the vapor condenses to form raindrops. The energy release is what makes storms so potent, and it’s why the amount of vapor in the atmosphere has increased globally by 4 percent since the mid-1990s.
A lot of this extra vapor is being supplied by the warming oceans. As they absorb more and more of the heat-trapping greenhouse gases we’re pumping into them, the seas are getting hotter. That boosts their surface temperature, and when the warmer water rises into the atmosphere, it boosts the strength of hurricanes forming over it.
This is true even for storms that don’t move over the warmest parts of the ocean. The same effect is occurring in the northern Atlantic, where some of the strongest hurricanes on record have formed and strengthened over waters that are abnormally warm for this time of year.
As the water vapor concentrations in these storms increase, their wind speeds and rainfall rates also speed up. This is because warmer air can hold more vapor than cool air, so it can carry heavier clouds and heavier rains.
The increased vapor also contributes to slower decay times for hurricanes as they approach land. This means that a storm can linger over a region, delivering more intense rainfall and causing devastating floods.
These impacts can be felt far inland as well, in places like Tampa and southwest Florida, where heavy rains can cause widespread flooding and destruction. And, though scientists aren’t sure what’s causing it, the recent spate of monster storms — like Hurricane Ian, which became a Category 4 storm in less than 24 hours and is now barreling toward Tampa Bay — seems to be being turbocharged by climate change. That is, the build-up of heat-trapping greenhouse gases is making these storms slower and wetter, and boosting their intensity and enhancing the deadly effects of storm surge and freshwater flooding.
Wind
In the case of hurricanes, wind is the energy source that powers them through the atmosphere. In the early 1840s, James Prescott Joule defined two forms of energy: latent heat (energy possessed by particles through attraction over a greater distance, or potential energy) and sensible heat (energy associated with the motion of those particles, or kinetic energy).
The distinction between these energies helps scientists understand the physics behind how wind is generated and the ways it can change as temperatures and moisture levels change, especially in a hurricane’s eyewall. The physics also has important implications for the role that climate change may play in future hurricanes.
A warming ocean produces more evaporation, which means more energy is released into the atmosphere to power storms and generate rain. This basic understanding, along with computer simulations of hurricanes in current and future climates, leads to high confidence that rainfall rates during hurricanes will increase by about 7% per degree of global warming.
There is medium confidence that the proportion of hurricanes that make U.S. landfall will increase with global warming, but there is no consensus among studies on the exact size of this effect. It is important to note, however, that hurricanes are driven by a range of factors besides temperature and moisture, including the speed at which they develop and their interaction with other weather systems and topography.
Some experts have used their understanding of the physics of hurricanes to develop attribution research, which seeks to identify the extent to which climate change influences the number of hurricanes that form and how many of them reach land. Detailed attribution work is needed to understand the full extent of the impacts from climate change on hurricanes, and it is still too soon to know whether there will be any detectable human-caused increases in the frequency of hurricanes and major hurricanes overall, or the number that make landfall on US shores. However, there is high confidence that if the number of Atlantic hurricanes continues to increase as projected, their intensity will rise by about one-half category on the Saffir-Simpson scale, with the same percentage increase in near-storm rainfall.
Pressure
Pressure is the force per unit area exerted on a surface by a liquid. It is inversely proportional to the distance from the surface, and it is always equal to the gravitational acceleration (g) of the fluid at that point. When a liquid is squeezed, the pressure increases. When a liquid expands, the pressure decreases. This is because the force exerted by a liquid on any given area is a function of its temperature and volume, and the density of the fluid.
The simplest physical example is water rushing out of a hole in a bucket. As the water reaches the hole, it is forced against the bottom of the bucket and against other parts of the rim in an attempt to maintain its shape. The force exerted on each part of the rim is different, and therefore, the total force exerted on the bucket is less. Similarly, as a hurricane gains strength and moves towards the coastline, the atmospheric pressure exerted on it is greater.
When a tropical storm becomes a hurricane, the atmospheric pressure is greater than the oceanic surface tension, which causes the center of the storm to rise above the surrounding area. This results in a depression (or low pressure) on the lower side of the storm, and a high pressure region on the upper side. The difference between these two regions produces wind.
An increase in the atmospheric pressure also increases the amount of energy a hurricane has. This additional energy allows a hurricane to maintain its size, and it can help the storm move faster.
In addition, a warmer planet holds more water vapor than a cooler one. This extra moisture can increase the amount of rain a hurricane brings with it when it moves inland. A study of the rainiest hurricanes on record found a medium confidence that anthropogenic climate change has contributed to a greater than normal contribution to these rainfall extremes (van Oldenborgh et al. 2017; Risser and Wehner, 2017).
In 1847, James Prescott Joule characterized the concepts of latent heat and sensible heat as distinct physical phenomena. He defined latent heat as the energy possessed by a system by virtue of its physical position, and sensible heat as the energy of moving particles.
