Groundbreaking AccuWeather climate study reveals profoundclimate trends with far-reaching impacts

If the climate trends continue, they are likely to have major impacts on the U.S. economy, our food and water supply, human comfort levels, energy needs and the health and welfare of citizens over the coming decades.  

AccuWeather has released a new and important study of climate trends for the contiguous United States. If the climate trends we have identified continue, they are likely to have major impacts on the U.S. economy, our food and water supply, human comfort levels, energy needs and the health and welfare of citizens over the coming decades.

This AccuWeather study has found:  

•Temperatures have increased an average of 0.5°F (0.28°C) per decade over the past 70 years 
•Dew point temperatures have increased an average of 0.3°F (0.17°C) per decade over the same period, though most of this increase occurred before 1995 
•Relative humidity remained more or less steady until 1995, but then decreased by a significant 5.3%, or an average of 1.7% per decade 
•Average annual rainfall has declined 2.7% since 1995, or on average 0.9% per decade, yet the frequency of rainfall amounts greater than 4 inches in a 24-hour period have actually increased by 70%. Likewise, heavy rainfall amounts greater than 2 inches within a 24-hour period have increased by 23%. 

This comprehensive study is based on a detailed and error-corrected database of 44 official, geographically dispersed weather stations in AccuWeather’s Data Suite.

“If these trends continue, the well-known climate models may not be capturing all of the important changes our studies have revealed,” said Dr. Joel Myers, Founder and Executive Chair of AccuWeather.

“What has been surprising in our detailed analysis is the significant decline of rainfall amounts, especially since 1995. Over the last 30 years, average rainfall has been declining by an average of 1% every 11 years, and the rain that does fall is less effective for crop production and maintaining our water supply because it tends to come in shorter bursts. Furthermore, the average relative humidity during the same 30-year period has declined by a very significant 5.3%, which is 1.7% per decade,” said Dr. Myers.

“This means that the rain that does fall is evaporating more quickly and it is less effective for crop production, water supply, and for filling reservoirs, lakes, and other water sources. When you combine these two factors, the “effective” average rainfall across the United States seems to be declining by 2% to 3% per decade. This is quite significant,” said Dr. Myers.

“If these trends continue, we expect to see accelerating harmful impacts on crop production, more frequent wildfires, and less available water due to greater drought. Furthermore, the crop-growing areas in the United States may shrink as soil becomes more arid. Even in productive growing areas, crop yields may decline. These effects could become more obvious over the next decade or two,” Dr. Myers added. 

Dr. Myers explained the physical reasoning behind these trends. “Warm air can hold more water vapor
than cooler air. So, as the temperature rises, there is the potential for more water vapor in the atmosphere. When atmospheric mechanisms to transform that water vapor into liquid water are
present, such as in thunderstorms, the rainfall amounts in those events tends to be higher,” Dr. Myers
said.

“What is seemingly happening here is the average temperature and dew point increased together up until 1995, and so the average relative humidity more or less remained constant. Since 1995 however, the average temperature continued to increase, but the dew point leveled off, and that caused a significant drop in relative humidity. This suggests we reached a tipping point around 1995, as the temperature continued to increase, evaporation from the ground and vegetation could not keep up with the increase in temperature, and so the dew point has not been able to increase as quickly as the temperature over the last 30 years, resulting in a decline of relative humidity. If this trend continues, desert areas will expand,” Dr. Myers explained. 

“To be more specific, the capacity of the atmosphere to hold water vapor approximately doubles for
every 20-degree Fahrenheit increase in temperature. A common measure of the absolute amount of
water vapor in the atmosphere is the dew point, which is the temperature at which saturation occurs when the air is cooled,” Dr. Myers said. “This means if the temperature and dew point are both 50°F
(10°C), the relative humidity is 100%. If you increase the temperature to 70°F (21.1°C) and do not
increase the amount of water vapor in the air, the relative humidity falls to 50%. If you then raise the
temperature even further to 90°F (32.2°C), the relative humidity falls to 25%, and so on.”

The average temperatures from 1955 to 2024 show a clear warming trend. To smooth out year-to-year fluctuations on the annual datasets, we created averages over a 10-year period, which shows an increase of 3°F (1.66°C), or an average of 0.5°F (0.28°C) per decade, reflected in Chart 2. 

Yet, the average dew point temperature over the same period only increased by 1.8°F (1.0°C), or 0.3°F
(0.17°C) per decade, reflected in Chart 3. Notice since 1995, the dew point temperature has not changed much. So, nearly all the increase occurred in a period from 1960 to 2000.

If these trends continue, there could be serious implications for many industries including agriculture,
those impacted by a decline in water supply and the increasing frequency of wildfires because the drying will accelerate. There will be less and less moisture in the soil to dampen the rise in temperature
because of the heat needed to evaporate moisture from the soil and this could also mean that the
heating will accelerate.

“A stark example of this occurred during the Dust Bowl in the 1930s. Despite the overall increase in
average temperature across the United States over the last 100 years, the all-time record high
temperatures in 23 states and Washington D.C. still stand from the Dust Bowl years. That was also a
decade that featured extraordinary drought. Dry soil increases the risk of extreme heat,” Dr. Myers said.

Myers highlighted another impact that is not widely discussed but could be important.

“As the soil and the ground becomes drier, the warming of the air and the ground could accelerate
beyond what the climate models are currently predicting. The reason for that is moisture in the soil
provides a major dampening of heating. Evaporation of water utilizes a great deal of heat, as you
learned in physics, 540 calories per gram of water. That means the same amount of heat is used to
evaporate one cubic centimeter of water as is needed to raise the temperature of an equal amount
of soil by scores of degrees or more,” Dr. Myers explained.

Lack of moisture in the soil also decreases the conductivity of heat deeper into the ground, so the
heating from the sun is concentrated at the top level of the ground, therefore causing greater increases
in air temperature. This connection between dry soil and extremely hot temperatures has been well
known for a long time but perhaps is not being fully considered in climate predictions we have heard.

“Furthermore, when the soil is drier, there tends to be more dust and soil in the air,” said Dr.
Myers. “Heat then leads to increased drought. You have more soil particles in the atmosphere, more dust competing for nuclei for droplets, so you get more water droplets that are smaller and have trouble
falling out of the clouds, so you get less rain. Less rain means drier and hotter soil which brings drought
and drought brings heat and you have a cycle that is running away. That is the concern.”

It is important to note climate fluctuations and trends over 10, 20 or 30 years can reverse. There are no
assurances these trends AccuWeather has found will continue, slow down, or possibly even reverse. It is
also important to realize that these are averages for the nation as a whole and different regions may
have more dramatic changes and other areas much less dramatic changes.

To highlight long-term climate signals that can be hidden by year-to-year variability, AccuWeather®
averaged the data over 10- and 20-year periods. This smoothing method filters out short-term
fluctuations and reveals the underlying climate signals.

The Earth’s climate has undergone significant natural variations throughout history. For example, we
were in the last major ice age only 120 centuries ago. More recently, the period known as the Little Ice
Age ended only 175 years ago. These examples underscore that climate conditions can evolve in ways
that are not always linear or predictable. Even though the meteorological community uses 30 years to
define normals, some cycles operate on longer time periods.

AccuWeather’s new study has been compiled based on official government weather
stations in 44 regions of the United States. This report is part of AccuWeather's ongoing climate analysis.

Additional research and insights will continue to be released, focusing on regional impacts across the
U.S., disaster potential, public and business impacts and other key findings.

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