The Reliance School

1,400 Words / 6 min. Read

At first glance, 2°C might not sound like much. Humans can survive and thrive in a wide range of temperatures, climates, and biomes; why would a couple of degrees change anything?

But much like the difference between a healthy body temperature and a fever,^1,2 a small increase in energy can wreak havoc in complex systems, especially when those changes happen too rapidly for us to adapt. Our Earth’s atmosphere is highly sensitive to changes in CO2, and right now levels are the highest they’ve been in over 14 million years.³

Heat waves and extreme temperatures are primary effects of climate change, and their frequency could increase by a factor of 100 under 2°C of warming.⁴ Crossing that threshold also greatly increases the threat of secondary effects like sea level rise, hurricanes, floods, droughts, and wildfires, as well as habitat losses, insect and animal extinctions, and declining crop yields.^5,6 Then there are tertiary effects like economic downturns, supply chain disruptions, pandemics, migration, unrest, and international conflicts.^7,8,9,10

Unfortunately, we can’t predict those tertiary effects with much accuracy (that’s why we advocate for a holistic approach to climate resilience). So in this series, we’re going to take a look at some factors of climate change that we do have reliable models for, starting with temperature itself.

Why 2°C?

We’ve chosen to focus on a temperature increase of 2°C (3.4° Fahrenheit) because that’s a change that most of us will see within our lifetimes.¹¹ An analysis of the latest generation of climate models (CMIP6)¹² found that the world will likely exceed 2° of warming by 2052.¹³

(The world has already hit 2°C of warming for short periods,¹⁴ but we don’t consider that threshold met until global temperatures remain above 2° for over a decade.)¹⁵

Given that we breached the 1.5°C limit for an entire year in 2024¹⁶ (and are expected to “officially” cross that threshold by the 2030s),¹⁷ a 2°C increase is a very conservative number. Over the coming years, we’ll begin to see longer periods where temperatures exceed that threshold; first weeks, then months, then years. So the data we’re about to look at paints a picture of a world that we’re likely to experience firsthand.

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.¹⁸

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.¹⁹

Using their county-level data sets, we’ve created a series of interactive maps to help you visualize how climate change will affect America.

The NCA5 has a data atlas as well,²⁰ but their information is displayed in relative terms (i.e. the change in the number of hot days). We’re more interested in absolute terms (i.e. the total number of hot days), so we’ve sourced historical climate data from NOAA to see how current conditions (1991-2020)²¹ will compare to a warmer world.

If you find these maps helpful, we’ve expanded on this data with projections for 3°C (5.4°F) in our free collection of climate risk maps.

Average Temperature

When global temperatures increase, those changes aren't evenly distributed. Instead, temperatures are projected to increase by a greater amount in northern areas of the country. Here’s a map of current average temperatures by county, in degrees Fahrenheit:

As you might expect, the highest temperatures are found in the southern half of the country, increasing as we move towards the equator. There are some notable exceptions; the Appalachian and Rocky Mountains are cooler due to the moderating effect of altitude on temperature,²² while drier regions of the West are cooler due to higher temperature differentials between day and night.²³

Let’s see how temperatures are projected to change in a world that’s 2°C warmer:

As you can see, there’s a big difference between latitudes; southern regions may only see temperatures increase by 2°F, while some northern areas may see temperatures increase by over 4°F. The Pacific Northwest is an exception to this trend, although the region still faces the threat of extreme heat events.²⁴

But while southern areas of the country may see less change in temperature, their baseline temperatures are much higher. So let’s take a look at average temperatures again, but this time with 2°C of warming:

Despite a larger temperature increase in northern areas of the country, the southern half of the country is still projected to experience the highest average temperatures, with a difference of nearly 40°F from the northernmost to southernmost counties. If you’re considering relocating due to climate change, this is why we recommend staying above 40° north when searching for a place to call home.

Days Over 95°F

While average temperatures are an important metric, extreme heat is a more pressing concern. Heat waves are the deadliest weather event in America (more than hurricanes, floods, winter storms, and tornadoes),²⁵ and at 95°F and above, it becomes much harder for us to regulate our internal temperature.²⁶

So to start, let’s look at the average number of days per year over 95°F from 1991 to 2020:

Extreme heat is more common in southern regions, but it’s not directly tied to latitude; instead, it’s concentrated in the central corridor of the country,^27,28 as well as in Arizona, central California, and the Deep South. The eastern half of the country is buffered from extreme heat from Appalachia to Maine & Minnesota, and most western states have been spared as well. The moderating effect of the ocean on coastal temperatures can be clearly seen here.²⁹

Next, let’s look at how a 2°C increase is projected to change those numbers:

Unfortunately for the central and southern US, the greatest increases in extreme heat are concentrated there as well. The Gulf Coast is the hardest-hit, with some counties experiencing an additional month or more of days over 95°F. Here the moderating effect of altitude is more apparent, with the Appalachian and Rocky Mountains showing smaller increases in heat than surrounding regions.

Let’s combine these datasets to show the total number of days over 95°F with a 2°C rise in temperature:

As before, the pattern is clear; the central and southern US will experience the most days of extreme heat in a warming world. The worst counties are those of central California, Arizona, and southwest Texas, with some areas projected to experience temperatures over 95°F for nearly half of the year.

Keep in mind that heat index (a combined measure of heat and humidity) is more important than temperature alone.³⁰ At 40% humidity, 95°F is bearable; at 80% humidity, it could be lethal.³¹ As such, areas with higher humidity (such as the Northeast, Southeast, and the Pacific Northwest) will face amplified risks from extreme heat.

Days Under 32°F

Lastly, let’s take a look at the number of days where the minimum temperature dips below freezing (32°F). While we might not mind seeing our heating bills go down, freezing temperatures are essential to plant, insect, and animal life cycles, as well as for recharging snowpack and sustaining winter economies.³² Warming winters also increase the range and populations of disease-carrying insects like ticks and mosquitos.^33,34

The biggest changes are found in the northeastern and northwestern United States, with the Great Plains being less affected. In the hardest-hit counties, the data projects a loss of 3-4 weeks of freezing temperatures; that’s a huge change, and is likely to have dramatic consequences for local ecosystems.³⁵

(While we’d like to show absolute values here, we weren’t able to source historical data for days under 32°F on a county-level scale. If you happen to have a lead, send us a message!)

Rising temperatures may be the most obvious effect of climate change, but that's just one way in which carbon emissions are impacting our world. In the next post, we’ll take a look at how climate change is projected to affect rainfall across the country. >> Part 2: Precipitation

Footnotes & References

1. Human body temperature (Wikipedia)
2. Fever (Wikipedia)
3. A New 66 Million-Year History of Carbon Dioxide Offers Little Comfort for Today (Kevin Krajick, Columbia Climate School)
4. Here’s how much climate change increases the odds of brutally hot summers (Nikk Ogasa, ScienceNews)
5. The impacts of climate change at 1.5C, 2C and beyond (Robert McSweeney, Rosamund Pearce, Tom Prater, Carbon Brief)
6. Myth-buster: Why two degrees of global warming is worse than it sounds (Daisy Simmons, Yale Climate Connections)
7. Five ways the climate crisis impacts human security (United Nations)
8. Climate Change and International Responses Increasing Challenges to US National Security Through 2040 (National Intelligence Council)
9. Climate Change, Human Health, and Social Stability: Addressing Interlinkages (Samuel Sellers, Kristie Ebi, Jeremy Hess, Environmental Health Perspectives)
10. Global climate change, war, and population decline in recent human history (David Zhang, Peter Brecke, Harry Lee, Yuan-Qing He, Jane Zhang, PNAS)
11. World will not meet 2C warming target, climate change experts agree (David Adam, The Guardian)
12. CMIP6: the next generation of climate models explained (Zeke Hausfather, Carbon Brief)
13. Analysis: When might the world exceed 1.5C and 2C of global warming? (Zeke Hausfather, Carbon Brief)
14. Global temperature exceeds 2°C above pre-industrial average on 17 November (Copernicus)
15. Interpreting the Paris Agreement’s 1.5C temperature limit (Joeri Rogelj, Carl-Friedrich Schleussner, Carbon Brief)
16. World's first year-long breach of key 1.5C warming limit (Mark Poynting, BBC)
17. When will global warming actually hit the landmark 1.5 ºC limit? (Nicola Jones, Nature)
18. Information Quality (Fifth National Climate Assessment)
19. The Fifth National Climate Assessment (Fifth National Climate Assessment)
20. Atlas of the 5th National Climate Assessment (Fifth National Climate Assessment)
21. NOAA Releases Updated Climate Normals (National Weather Service)
22. Change in the Atmosphere with Altitude (UCAR)
23. Why do deserts get so cold at night? (Harry Baker, LiveScience)
24. 2021 Northwest Heat Dome: Causes, Impacts and Future Outlook (USDA Northwest Climate Hub)
25. Number of deaths due to weather conditions in the United States in 2022 (Erick Burgueño Salas, Statista)
26. Human temperature regulation under heat stress in health, disease, and injury (Matthew Cramer, Daniel Gagnon, Orlando Laitano, Craig Crandal, Physiological Reviews)
27. Extreme Heat Proves Relentless in Central States (Andrew Freedman, Climate Central)
28. Hazardous Heat (First Street)
29. How the ocean shapes weather and climate (World Meteorological Organization)
30. What is the heat index? (National Weather Service)
31. How hot is too hot for the human body? (Larry Kenney, Daniel Vecellio, Rachel Cottle, Tony Wolf, The Conversation)
32. Fewer Frigid Nights (Kyri Baker, Climate Central)
33. ‍Climate Change Indicators: Lyme Disease (EPA)
34. As climates change, prepare for more mosquitoes in winter, new study shows (Samantha Murray, University of Florida)
35. Ecosystem Response to Climatic Change: The Importance of the Cold Season (Stef Bokhorst, Jarle Bjerke, Hans Tømmervik, Catherine Preece, Gareth Phoenix, Ambio)