If you keep hearing a rumble in the distance this year, it may be the footsteps of people crossing over the line to become global warming converts.

This year brought the hottest March on record for the lower 48 states, according to the National Oceanic and Atmospheric Administration. And July was the “all-time warmest month (for the contiguous states) since national records began in 1895,” according to NOAA.

Higher temperatures are being recorded this year all over the planet. The average combined ocean and land temperature for July was the fourth warmest on record, NOAA said. The same global average for the first six months of the year was the 10th warmest on record.

In Nebraska the obvious worry is that the impact of a warming globe might be less rainfall for the state. The worst drought in generations continues. Weekend rains improved conditions in only three counties, and they improved only from extreme drought to severe drought, according to the U.S. Drought Monitor at the University of Nebraska-Lincoln.

During any bout of extreme weather, doomsday scenarios become more popular, and they are in fulsome supply at the moment.

Sea ice in the Arctic shrunk to its lowest level since satellite measurements began, according to the National Snow and Ice Data Center. “We are on track to see an ice-free summer by 2030,” Director Mark Serreze said.

Some scientists believe that Arctic warming might speed melting of the Greenland ice cap. Arctic ice floats; ocean levels won’t rise when it melts. But if the Greenland ice sheet, which is almost two miles thick, were to entirely melt, global sea levels would rise by more than 20 feet.

An alarming study released earlier this summer estimated that there might be significant amounts of methane trapped beneath the Antarctic ice sheet. Methane is a powerful greenhouse gas more than 20 times as effective as carbon dioxide in trapping heat in the atmosphere. Release of the methane would speed global warming.

Prominent skeptics of global warming still are being heard. Columnist George Will earlier this summer had this to say: “How do we explain the heat? One word: summer.”

It’s always useful to remember that one heat wave is not evidence of global warming. Weather is short-term. Climate is long-term.

In the summer of 2012, however, the short-term evidence of a warming world was difficult to ignore.