A woman uses an umbrella on East Houston Street to provide shade from the sun in the heat.
A woman uses an umbrella on East Houston Street to provide shade from the sun in the heat. Credit: Bonnie Arbittier / Rivard Report

San Antonio could sweat through up to four solid months per year of 100-degree-plus days by the end of this century if humanity does not act to slow the rate of global warming.

That’s according to climate modeling by University of Texas at San Antonio professor Hatim Sharif. Even in a possible future where greenhouse gas emissions decrease by the year 2100, the models on average project 55 days per year with temperatures over 100 degrees.

That’s compared to an average of seven triple-digit days per year from 1971 through 2000, used as a baseline in Sharif’s report, which relied on averages from multiple different climate models.

“It’s going to be significantly hotter,” Sharif told the Rivard Report. “All models agree, and it seems for some scenarios, these increases will be significant.”

Sharif’s report is part of San Antonio’s Climate Action and Adaptation Plan, a partnership among the City, CPS Energy, Navigant, and UTSA. His findings are consistent with a previous climate modeling report for San Antonio by Texas Tech University professor Katharine Hayhoe.

Studies like these can never predict the future with absolute certainty, but they are useful guides for what to expect as the 21st century unfolds, said Doug Melnick, the City’s chief sustainability officer.

“There’s really nothing shocking about it,” Melnick said. “It’s going to get hotter, it’s going to get drier, and when we do get precipitation, it’s going to get more extreme.”

The report comes at a time of record-breaking heat around the world, including in Texas. Last Monday as a high temperature hit 105 degrees, the need for air conditioning pushed San Antonio’s electricity demand to an all-time high.

“The projections are showing that that’s going to be more common and a regular occurrence,” Melnick said. “What conversations do we need to have over something as simple as affordability for residents that have challenges paying their utility bills?”

Globally, average temperatures increased 2 degrees Fahrenheit during the 20th century, according to NASA. An overwhelming majority of climate scientists agree that this rapid rate of warming is due to greenhouse gas emissions from the burning of fossil fuels.

Reliable temperature data for San Antonio date back to around the 1880s. Since then, the city’s average temperatures have climbed 2.4 degrees, according to National Oceanic and Atmospheric Administration data.

The city has also seen a steady increase in the number of hot days per year, a trend that will likely continue over the next century.

Under a worst-case scenario, the climate models Sharif used predict that San Antonio will have between 75 and more than 125 days per year – two-and-a-half to four whole months– above 100 degrees by the 2071-to-2100 time period.

Also in a worst-case scenario, the city could have roughly eight days per year on average of 110 degrees or more by the end of this century. San Antonio’s highest-ever recorded temperature was 111 degrees on Sept. 5, 2000.

To arrive at these projections, Sharif used the outputs of 21 different global climate models, which run climate simulations on a worldwide scale.

“They all focus on different things,” Sharif said. “To reduce the uncertainty, you have to run all of them and then average the models.”

He then took these outputs through a standardized “downscaling” process to make predictions at a local level.

The outputs are based on two possible futures – one where greenhouse gas emissions hit a peak and then decline towards the end of the century, and another business-as-usual scenario where fossil fuel dependence continues and emissions keep increasing.

The models predict San Antonio’s summers will become even hotter by several measures. Maximum temperatures, average temperatures, and the number of hot days will all increase, while the number of cool days and nighttime freezes will go down.

For rainfall, the future is less certain, though Sharif’s modeling indicates that droughts and floods could become more severe.

“One of the things about climate change is that it will maybe increase variability,” Sharif said. “We can go from extreme to extreme.”

Extremes are already the norm for Texas and especially San Antonio. The city is prone to frequent droughts but also part of a region known as Flash Flood Alley.

Under both low- and high-emission scenarios, heavy downpours are expected to get even heavier.

For example, a current 100-year storm (a storm with a 1-in-100 chance of happening each year) would drop about 10 inches of rain over 24 hours. By 2071 to 2100, it would drop between 12 and 15.2 inches.

Information like this could help local authorities plan for more severe floods. Currently, city codes require most drainage structures to handle 100-year storms, but that may not be sufficient in a future where 100-year storms are more intense than they are now.

San Antonio has already seen an increase in peak flooding along its urban streams and rivers, according to U.S. Geological Survey data. That’s also likely tied to an increase in asphalt and concrete.

Scientists frequently publish studies looking at how climate change is already affecting Texas and will continue to in the future. Recent papers have covered warmer temperatures’ influence on Hurricane Harvey’s rainfall, crop yields, the spread of dengue fever, and the effects of climate change on Texas groundwater.

It will be up to local officials and the roughly 90 volunteers serving on various climate plan committees to figure out how San Antonio can best adapt to this changing climate.

They expect to finish a draft climate plan in January, with a final version ready for City Council approval in April.

Brendan Gibbons

Brendan Gibbons

Brendan Gibbons is the San Antonio Report's environment and energy reporter.