People who leave dogs or cats inside cars on hot summer day will face fines and jail sentences, and law enforcement officers will be authorized to break into their cars to let the pets out, under legislation signed into law over the weekend by Gov. Arnold Schwarzenegger.

Authored by Sen. Liz Figueroa, D-Fremont, the legislation, which takes effect Jan. 1, makes it a misdemeanor crime for someone to carelessly leave a pet unattended in a locked vehicle. The law is Senate Bill 1806.

A conviction will carry a fine of up to $100 if the pet is unharmed, and up to $500 and a jail term of up to six months if there is injury or death.

In all subsequent violations, regardless of whether there is injury, the penalty could be a $500 fine and six-month jail sentence.

California already has laws on the books that make clear it is a crime to leave a child unattended in a hot car. Escondido Police Sgt. Doug Sams said those laws allow prosecutors to charge a violator with felony and seek a prison term of two, four or six years, or a one-year county jail sentence.

In signing the pets bill on Friday, Schwarzenegger said every year dogs die needlessly because pet owners don't realize how fast their cars become heat boxes.

"People go on errands, they run into a friend and they either forget about their dogs, or they think rolling down the window a bit will keep them cool," Figueroa said, in a statement. "On a hot day, that's not enough."

Area animal advocates praised the new law, but The Animal Council, a Millbrae-based nonprofit that monitors state and federal legislation and advocates on behalf of animal owners, disagreed.

"It's an invitation to harass animal owners," said Sharon Coleman, president of The Animal Council and an attorney. "You've got a person dragged into the criminal process. They get into serious trouble. And maybe there has been no harm at all."

Coleman said in a telephone interview that pet owners who are convicted of such misdemeanors could suffer serious consequences, such as being stripped of a professional license or being fired from a job that entails working with animals.

"This is criminalizing negligence," she said.

Jane Cartmill, an Encinitas resident and past director of San Diego Animal Advocates, a nonprofit animal rights organization, said she believes it should be a criminal offense to accidentally leave behind a pet in a car.

"Even with the windows cracked and parked in the shade, a car can heat up very fast," Cartmill said.

Figueroa said that, with windows cracked, even a relatively mild outside air temperature of 85 degrees can trigger a temperature of 120 inside a car within a half hour.

Kris Anderson, board president for Animal Friends of the Valleys, which operates an animal shelter in Lake Elsinore where it can get a lot hotter than 85, said that, if anything careless pet owners should be fined more.

"It's like putting an animal in an oven," Anderson said.

Although state law already deals with children left in hot cars, Cartmill said it is about time California address the problem of pets being left in cars unattended.

Besides holding people responsible for their carelessness, Cartmill said, the possibility of serious consequences will make people think twice about leaving a dog or cat behind.

And it will make them more aware of the danger, said Florence Lambert, a local member of the Humane Society of the United States in La Jolla.

"They think, 'I'm only going to be gone for a short time. The air conditioning has been on. The car is cool. The car won't warm up that fast,'" Lambert said.

But the reality is, it will, she said.

Animal rights advocates also support the part of the law giving police and animal control officers the green light to break into a car if they can't find the owner.

"It's unfortunate," Cartmill said. "But are you willing to let an animal die for a piece of glass? I'm not."

Coleman, however, said the threat of a fine or jail term won't make anyone more aware than they are now of the dangers. And she maintained that an officer already can enter a vehicle under existing laws if an animal is in distress.

"I don't think we needed this," Coleman said.

- Contact staff writer Dave Downey at (760) 740-5442 or ddowney@nctimes.com.