From studying climate patterns over time, Dr. Oppenheimer says global warming is not a natural shift. "The last 50 years stick out like a sore thumb," he says. "The temperature's gone up and up and up. It bears the imprint of human activity."
The 1990s alone were the warmest decade of the last century. Temperatures rose one degree Fahrenheit. That may not sound like a lot, but in scientific terms, that's a big deal. "Over the next 10 to 30 years, Earth could be as much as three degrees warmer than today," says Dr. Oppenheimer. "By 50 years from now, Earth could be five degrees or so warmer than today."