August 10, 2023

Climate Shift Index® alert

New analysis by Climate Central shows that human-caused climate change made the prolonged extreme heat in Florida and Puerto Rico from June 15 to August 8, 2023 much more likely. The climate-fueled heat exposed millions of people to potential heat-related health impacts as well as the economic burden of increased energy costs. 

CSI Heat Alert FL/PR Graphic (EN)

The analysis uses Climate Central’s daily attribution system, the Climate Shift Index®, or CSI, which applies the latest peer-reviewed methodology to map the influence of climate change on temperatures across the globe, every day. 

From June 15 to August 8, 2023, the Climate Shift Index indicates that over 16.5 million people in five major Florida cities (Miami, West Palm Beach, Sarasota, Fort Myers, and Pensacola) and San Juan, Puerto Rico experienced between 24 to 49 days with high temperatures at a CSI level 3 or higher, indicating these conditions were made at least three times more likely due to human-caused climate change. 

The data reflects an even stronger influence of climate change at night. Out of the 10 Florida cities analyzed, 8 of them experienced between 21 and 47 nights with low temperatures at a CSI level 5 — the highest level on the scale — indicating the temperatures on those nights were made at least five times more likely due to human-caused climate change. In San Juan, Puerto Rico, 54 of the 55 nights evaluated reached a CSI level 5.

These strong climate fingerprints show that the persistent, extreme heat in Florida and Puerto Rico is now much more likely because of carbon pollution. At the time of the release of this report, the heat continues to reach record levels.

Daily Records and Heat Highlights

  • The persistent, extreme heat was bolstered by the unprecedented marine heat wave in the waters surrounding Florida.

  • High tropical humidity values enhanced the intensity of the extreme heat, pushing daytime heat index values into the 105°F to 115°F range. Miami and Fort Lauderale registered their all-time hottest heat index values of 113°F and 114°F, respectively, on August 8.

  • 39 Heat Advisories (37 above average) and 14 Excessive Heat Warnings (13 above average) were issued for Puerto Rico during the highlighted time frame.

  • 12 records for high temperatures were set in Miami (the highest was a record of 98°F on July 23)

Extremely warm nighttime conditions led to a number of new daily warm low temperature records:

CityNumber of record warm overnight temperaturesWarmest overnight tempertureDates of highest record warm overnight temperatures
Sarasota1385°FJuly 8
Tampa1283°FJune 28, July 9, August 7, August 8
Orlando1080°FAugust 8
Fort Myers982°FAugust 8
Miami884°FJuly 20
San Juan682°FJuly 11, July 19

Attribution Science Analysis

  • We analyzed the temperature statistics for 10 cities in Florida and San Juan, Puerto Rico, over the period from June 15 through August 8, 2023. The analysis considered the daily high, nighttime low, and daily average temperatures. For each day, we calculated the strength of the climate change influence using the Climate Shift Index.

  • While we see a clear influence of climate change on both highs and lows, the influence is much stronger at night. For example, Orlando's high temperatures had an average CSI of 0.1 and they had no days at a CSI level 3 or higher. However, their nighttime temperatures had an average CSI of 4.3 and they had 49 out of 55 days at a CSI level 3 or higher, indicating that the temperatures on those nights were at least three times more likely because of climate change. This pattern of stronger attribution at night holds in all cities considered except Pensacola.

San Juan and Tampa stand out in the analysis. Every night in San Juan was at a CSI level 3 or higher and only one night did not hit level 5 (more than five times more likely because of climate change). San Juan also had 40 daily highs at a CSI level 5. Tampa had 50 nights at a CSI level 3 or higher and 47 nights at CSI level 5.

CityDays of high temperatures at CSI = 3 or higherDays of high temperatures at CSI = 5 or higherDays of low temperatures at CSI = 3 or higherDays of low temperatures at CSI = 5 or higherAverage daily temperature anomaly (°F)Percentage increase of cooling degree days
Fort Myers261545401.414.4%
San Juan494055542.77.9%
West Palm Beach291444381.98.2%


Cooling degree days are a way of estimating the energy required to keep homes and buildings cool. Each of the cities in our analysis had at least an 8% increase in cooling degree days, with Sarasota increasing by 15% relative to the 1991-2020 average. This means that the cost of staying cool has increased for everyone in the region, and this summer continues the trend of rising cooling degree days. It is worth noting that the humidity in this region is very high, so temperature alone will underestimate the actual cost of cooling. Higher energy costs associated with increased cooling needs can place economic stress on households and communities, and can force low-income households into energy vulnerability

Extreme heat contributes to a rise in heat-related illnesses and fatalities. During heat waves, unusually warm overnight lows enhance the risk of heat illness by not allowing sufficient time for our bodies to cool off. Some groups of people are more vulnerable to extreme heat as a result of increased exposure, greater sensitivity, and/or reduced adaptive capacity. Vulnerable populations include older adults, members of low-income communities, pregnant people, outdoor workers, and those with limited English proficiency. 


Andrew Pershing, Ph.D., VP of Science at Climate Central, said: 

“The persistently warm conditions in Florida and the Caribbean are strongly connected to climate change. The uncomfortable conditions this summer are a direct result of decades of burning coal, oil, and natural gas.”

Daniel Gilford, Ph.D., climate scientist at Climate Central, said:

“As a Floridian, I thought I was used to hot weather, but this summer has been something else. The impacts of this summer's heat are clear: the relentless stretch of warmer overnight temperatures has been adding up, taking physical, economic, and mental tolls on our families and communities. The persistence of this heat—over an entire summer season—is another clue that human-caused global warming is indelibly changing Florida's day-to-day way of life. To stem the heat, we need lasting policies that reduce carbon pollution.”


To request an interview with a Climate Central scientist, please contact Peter Girard at

Reporting resources as this heat event unfolds:

Health & Safety

  • Populations most vulnerable to high temperatures include older adults, young children, pregnant people, individuals with chronic conditions, members of low income and historically marginalized communities, athletes, and outdoor workers.

  • Exposure to risky heat can trigger heat-related illnesses, including heat exhaustion and heat stroke, a life-threatening condition. For more information on heat safety, visit the National Integrated Heat Health Information System’s Planning and Preparing guide.

  • For more information on heat and the related human health implications refer to the follow  Climate Matters briefs

Attribution Science

About the Climate Shift Index® 

Climate Central’s Climate Shift Index, grounded in the latest peer-reviewed science, maps the influence of climate change on temperatures across the globe, every day. 

Climate Shift Index levels indicate how much human-caused climate change has altered the frequency of daily temperatures at a particular location. Level 1 indicates that climate change is detectable in that day’s temperature. Level 2 means that climate change made exceptionally warm temperatures in a given location at least twice as likely. Level 5 is the maximum and indicates temperatures at least 5 times more likely because of climate change.

For this analysis, temperatures come from NOAA’s Global Forecast System model.

Previous Climate Shift Index alerts