A leading climate scientist warned a local audience Sunday that Texas and the world are approaching a turning point in the planet’s environmental future.

“The decisions that we make now in the next few decades will essentially determine the climate for the next ten- or twenty-thousand years,” Andrew Dessler, a professor of atmospheric sciences at Texas A&M University, told about 40 people at a lecture at the San Antonio Botanical Garden.

His predictions about the future, under a scenario of greenhouse gas emissions remaining high, were at times ominous: a seven-degree Fahrenheit rise in the average global temperature by 2100; cities such as Miami, perhaps Houston, ravaged by hurricanes and sea level rise; families and businesses struggling to pay higher utility bills; and intense flooding turning neighborhoods into blighted areas.

But if local, state and federal governments work to decrease greenhouse gas emissions, the temperature rise can remain limited to a few degrees, minimizing the other impacts, he said.

“It took us 10,000 years to come out of the last ice age. We’re going to do this in 100 years. So the disruption to ecosystems and the disruption to the world is going to be immense if we actually follow this upper-limit estimate” on emissions, Dessler said.

Dessler, co-author of “The Science and Politics of Global Climate Change: A Guide to the Debate,” explained that natural factors such as changes in the earth’s orbit, increased solar output, volcanic activity or atmospheric oscillation patterns, have been all but completely ruled out as causes of global warming that has already been documented.

“The scientific community has concluded that humans are now the dominant driver of the climate system,” he said. “We’re in the driver’s seat. We’ve got our foot on the gas. We don’t have our hands on the wheel, unfortunately.”

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Dessler predicted that in 1,000 years, if there is 20 to 30 feet of sea level rise, “Florida will be gone.”

“These (outcomes) are 100 percent certain. There is no question these things are going to happen. And a lot of them will happen in the lifetime of younger people.”

The reason the issue has become so politicized is because the choices in what to do are limited, Dessler said during the lecture and panel discussion sponsored by the San Antonio Water System, and held in partnership with William R. Sinkin Eco Centro.

“You can have government regulations, or you can have climate change. That is the choice. There is no third choice,” he said. “If there’s a playbook out there, if you want to oppose environmental regulations, you say the science is uncertain, and you also say it’s going to bankrupt you.”

San Antonio is one of a number of U.S. cities that have climate mitigation plans adopted or in the works. People have until Tuesday to comment and take an online survey on the proposed Climate Action and Adaptation Plan that is posted at saclimateready.org. The City Council is scheduled to vote on the plan on May 16, but it could be delayed for more public input — a decision likely to be made this week.

Councilwoman Ana Sandoval, who also spoke Sunday, said the plan is a framework that, once adopted, will launch efforts to pursue specific strategies regarding industrial emissions, transportation and building energy use. But she noted that the plan, which would be reviewed and updated every few years, already is getting some pushback from the business community and city leaders about the cost of trying to eliminate all local greenhouse gas emissions by 2050.

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Sandoval, a key proponent of the climate plan, said she should have gone to large energy companies last year to explain the intent of the plan and why it is needed.

“I would fault us, or me, for not having gone out to Valero and NuStar at the beginning of this process and saying, ‘Let’s sit down and talk. You’re a big employer in San Antonio, but we are going to have to do something about climate. And let’s talk about how we’re going to do that together,’” she said.

Sandoval, who faces three challengers in the May 4 election, said she is trying to balance economic stability with climate change.

“That’s something that we have to come to agreement on,” Sandoval said. “Is this city going to be much more than jobs in the fossil fuel industry? There are other types of jobs and there are other concerns for our future.”

One of the plan’s initiatives includes efforts by the city to promote the use of electric vehicles, such as adding charging stations. Julia Murphy, sustainable special projects manager with the city’s Office of Sustainability, said electric vehicles are expected to become more affordable.

“I think within the next few years, the scene on the electric car marketplace is going to be totally transformed,” Murphy said.

Other changes that will come about from global warming and more extreme weather will affect employment.

“For people working outside, it’s going to be harder. … people who work in fields, roofers, people who do lawns,” Dessler said in an interview last week. Those who don’t work outside will be inside running their air-conditioners more, he said.

He was responding to a recent analysis of temperatures across the U.S. that provided more evidence of climate change. During the last 20 years, chart-topping daily high temperatures are being set at twice the pace of cold temperature records.

The Associated Press analyzed daily highs and lows from 424 weather stations across the continental U.S. that had consistent temperature records since 1920, counting the times daily high records were tied or broken and the times daily cold records were set.

In a stable climate, the numbers should be roughly equal. Since 1999, the ratio has been two hot records set or broken for every cold one.

In Texas, the pattern is similar: hot records have surged since 2000 compared to earlier decades. From then through 2009, they outpaced cold records 1.8 to 1. Since 2010, there have been more than 2.1 hot records for every cold one.

“A few degrees doesn’t sound like very much, but it really has an enormous impact,” Dessler said. “If people really understood how bad a few degrees of warming is, they’d take notice and say this is something we need to deal with.”

After the lecture, he said Texas still has a big oil-and-gas industry but will need to rely on wind, solar and other renewable energy sources to keep up with worldwide trends.

“You don’t have to do it in one year, or five years or 10 years,” he said. “It’s sort of a multi-decade process. But you’ve got to realize that the rest of the world is going to do this.”

“Fossil fuels are on their way out, whether we like it or not,” Dessler said. “People will be using fossil fuels for decades to come. But the goal will be to transition off onto other things. Do it in a way that doesn’t cause economic disruption.”

Staff Writer Josh Baugh contributed to this story.

Scott Huddleston covers Bexar County government and the Alamo for the San Antonio Express-News. Read him on our free site, mySA.com, and on our subscriber site, ExpressNews.com. | shuddleston@express-news.net | Twitter: @shuddlestonSA