WASHINGTON — Senator Marco Rubio on Sunday said that he was ready to be president, becoming the second potential Republican candidate recently to drop big hints about 2016 as he vies for early attention in a crowded field of maybes.

In a wide-ranging interview on “This Week With George Stephanopoulos” on ABC, Mr. Rubio, of Florida, also disagreed with accepted scientific wisdom that humans were having an effect on what he called the “always evolving” climate. “I do not believe that human activity is causing these dramatic changes to our climate the way these scientists are portraying it,” he said. “And I do not believe that the laws that they propose we pass will do anything about it, except it will destroy our economy.”

His comments challenged a major scientific report released Tuesday that found the effects of human-induced climate change are being felt in every corner of the United States, from dry regions where water is becoming more scarce to historically wet regions that are seeing increases in torrential rains.

The report, the National Climate Assessment, stated that the sweeping changes have been caused by an average warming of less than 2 degrees Fahrenheit over most land areas of the United States in the past century. If greenhouse gases like carbon dioxide and methane continue to escalate at a rapid pace, the scientists said, the warming could conceivably exceed 10 degrees by the end of this century.