How will 2° of warming affect America? Part 2: Precipitation

1,000 Words / 4 min. Read

In the previous post, we took a look at how 2°C of climate change will affect America in terms of temperature and extreme heat. In this article, we’re going to look at how that same increase is projected to affect rainfall and precipitation across the country.

Water is essential to life; the average American uses over 80 gallons every day,¹ while farms depend on massive amounts of ground and surface water for irrigation.² Our biomes are also dependent on this key resource, and in areas with abundant rainfall, ecosystems are generally more resilient.³

While climate change is generally projected to increase precipitation in America,⁴ those changes aren’t evenly distributed. Higher temperatures can also accelerate soil moisture evaporation, exacerbating drought and water stress in drier regions.⁵

Why 2°C?

We covered this in greater depth in the previous post, but in short, we’ve chosen to look at a temperature increase of 2°C (3.4°F) because that’s a change that most of us will see within our lifetimes.⁶ We’ve already hit 2°C of warming for short periods of time,⁷ and an analysis of the latest generation of climate models (CMIP6) found that the world is likely to “officially” exceed 2° of warming by 2052.⁸

Data Sources

The climate projections we’re using come from the 5th National Climate Assessment (or NCA5), which was released in 2023. This assessment has been rigorously peer-reviewed, and is some of the most recent and comprehensive data available on climate change in America.¹⁰

The Fifth National Climate Assessment is the US Government’s preeminent report on climate change impacts, risks, and responses. It is a congressionally mandated interagency effort that provides the scientific foundation to support informed decision-making across the United States.¹¹

Using their county-level data sets, we’ve created a series of interactive maps to help you visualize how climate change will affect you at a local level. These maps are best viewed on a tablet, laptop, or desktop computer, and you can move your cursor over the maps to display the values for each county.

The NCA5 has a data atlas as well,¹² but most of that data is expressed in relative terms (i.e. the change in inches of rainfall). We’re more interested in absolute terms (i.e. the total inches of rainfall), so we’ve sourced historical climate data from NOAA to see how current conditions (1991-2020)¹³ will compare to a warmer world.

Average Rainfall

The average US county receives about 30” of precipitation per year.¹⁴ Here’s a map of current rainfall averages from 1991-2020; red areas of the map denote counties that are below that average, while blue areas denote counties that are above it:

As you can see, there’s a stark divide between these two halves of the country, with rainfall dropping off dramatically west of the 100th meridian.¹⁵ The southeastern US receives the most rainfall (over 50” per year), while the southwest receives the least (under 10” in some cases). The Pacific Northwest is a notable exception, due to the Cascade mountain range forcing moist coastal air to rise and condense into rain clouds.¹⁶

Next, let’s look at how average rainfall is projected to change with 2°C of warming:

The good news is that most of the country is projected to experience a slight increase in precipitation, especially along the Eastern Seaboard. Unfortunately, most of the areas projected to receive less rainfall are already at risk of drought and water stress. California is especially vulnerable due to the high water demands of agriculture¹⁷ and its dependence on the Colorado River watershed.¹⁸ Arizona and New Mexico are similarly dependent,¹⁹ while Texas is facing dwindling groundwater supplies across the state.^20,21

Let’s apply these percent changes to the total rainfall, and see how that affects averages:

This map looks very similar to the first one; abundant rainfall in the eastern half of the country, and low rainfall in the west. But if you hover your cursor over the map, you’ll see that many southwestern counties have lost an inch or more of rainfall per year. In areas that receive less than 10” of rainfall to begin with, that decrease may stretch wells and aquifers beyond their limits.^22,23Utah, Nevada, Arizona, New Mexico, west Texas, southern California, and parts of inland Washington will fare the worst in a warming world.

Extreme Rainfall

While total rainfall is an important factor, it’s also important to consider how that rainfall will arrive. Extreme rainfall is harder for soils and ecosystems to absorb, increasing the risk of flash floods.²⁴ In cities, impermeable surfaces greatly increase the risk of flooding from extreme weather, as drainage systems may not be able to handle inflow from larger volumes of rainfall.²⁵

This map shows the change in days with extreme precipitation (in the top 1% of historic events)²⁶ per year:

While these changes aren’t evenly distributed, they’re primarily concentrated in the southeastern United States, which already receives the most rainfall on average. Maine, Arkansas, Mississippi, Tennessee, Alabama, Georgia, Kentucky, Virginia, and the Carolinas are most affected, with many regions projected to receive 30% more days of extreme precipitation per year. Michigan, Illinois, Utah, Idaho, and the eastern regions of Oregon and Washington are also looking at significant increases.

Due to higher levels of variation in this data, we would recommend zooming in on the county you live in (or a county you might be considering moving to) and seeing what changes are projected for your area. While we’d like to display absolute values as well, we haven’t yet been able to source historic averages for extreme rainfall (if you have a lead, feel free to send us a message).

Given that water is essential to life, we believe that regions with abundant rainfall will prove to be more resilient in a warming world. As flooding can be a significant threat in those areas, we would recommend consulting FEMA flood maps and avoiding floodplains, river banks, and coastlines to lower your risk exposure.

This concludes our analysis of county-level data from the 5th National Climate Assessment; if you’re interested in exploring more risk factors, we’ve compiled over 50 maps of climate risks in our collection, which you can access for free here. You can also explore the full text of the NCA5 on the official website.

Footnotes & References

WaterSense Statistics and Facts (EPA)
Irrigation & Water Use (USDA)
Global vegetation resilience linked to water availability and variability (Taylor Smith, Niklas Boers, Nature Communications)
Climate Change Indicators: U.S. and Global Precipitation (EPA)
How does climate change affect precipitation? (NASA)
World will not meet 2C warming target, climate change experts agree (David Adam, The Guardian)
Global temperature exceeds 2°C above pre-industrial average on 17 November (Copernicus)
CMIP6: the next generation of climate models explained (Zeke Hausfather, Carbon Brief)
Analysis: When might the world exceed 1.5C and 2C of global warming? (Zeke Hausfather, Carbon Brief)
Information Quality (Fifth National Climate Assessment)
The Fifth National Climate Assessment (Fifth National Climate Assessment)
Atlas of the 5th National Climate Assessment (Fifth National Climate Assessment)
NOAA Releases Updated Climate Normals (National Weather Service)
Climate at a Glance County Mapping (NOAA)
Dividing line: The past, present and future of the 100th Meridian (Harvey Leifert, Earth Magazine)
Why Is the Pacific Northwest So Rainy? (Dennis Mersereau, Mental Floss)
Without Enough Water To Go Around, Farmers In California Are Exhausting Aquifers (Dan Charles, NPR)
Colorado River water cut back (Rachel Becker, Cal Matters)
The Southwest’s most important river is drying up (Drew Kann, Renée Rigdon, Daniel Wolfe, CNN)
Texas Groundwater Supplies Are in Danger, Reports Say (Ronna Kelly, Vanessa Puig-Williams, Environmental Defense Fund)
Heat, drought and population growth have stressed aquifers that supply water to millions of Texans (Dylan Baddour, Texas Tribune)
Arizona is using up its groundwater, researchers warn (Ian James, Arizona Republic)
New threats to New Mexico’s rivers also endanger its groundwater resources (Jonathan Seefeldt, Maurice Hall, Vanessa Puig-Williams, Environmental Defense Fund)
Why has it been raining so hard? How climate change is causing heavier downpours (Tiffany Means, TED)
Cities are scrambling to prevent flooding (Casey Crownhart, MIT)
About the Interactive Atlas (Fifth National Climate Assessment)