Hot summers (and balmier winters) may simply be the new normal, thanks to carbon dioxide lingering in the atmosphere for centuries.

This trend reaches back further than a couple of years. There have been exactly zero months, since February 1985, with average temperatures below those for the entire 20th century. (And those numbers are not as dramatic as they could be, because the last 15 years of the 20th century included in this period raised its average temperature, thereby lessening the century-long heat differential.) That streak—304 months and counting—was certainly not broken in June 2010, according to the U.S. National Oceanic and Atmospheric Administration (NOAA). Last month saw average global surface temperatures 0.68 degree Celsius warmer than the 20th-century average of 15.5 degrees C for June—making it the warmest June at ground level since record-keeping began in 1880.

Not only that, June continued another streak—this year, it was the fourth warmest month on record in a row globally, with average combined land and sea surface temperatures for the period at 16.2 degrees C. The high heat in much of Asia and Europe as well as North and South America more than counterbalanced some local cooling in southern China, Scandinavia and the northwestern U.S.—putting 2010 on track to surpass 2005 as the warmest year on record. Even in the higher reaches of the atmosphere—where cooling of the upper levels generally continues thanks to climate change below—June was the second warmest month since satellite record-keeping began in 1978, trailing only 1998.

"Warmer than average global temperatures have become the new normal," says Jay Lawrimore, chief of climate analysis at NOAA's National Climatic Data Center, which tracks these numbers. "The global temperature has increased more than 1 degree Fahrenheit [0.7 degree C] since 1900 and the rate of warming since the late 1970s has been about three times greater than the century-scale trend."

So what does the near future hold in terms of heat waves and record-breaking highs? Depending on how quickly La Niña conditions strengthen in the Pacific Ocean (and a host of other factors), this year could surpass previous records or at least take its place as one of the warmer years on record.

The short-term is fairly clear to climatologist James Hansen of the NASA Goddard Institute for Space Studies. "A new record global temperature, for the period with instrumental measurements, should be set within the next few months," he wrote in an unpublished paper in March.

But record global temperature for the calendar year might not occur if El Niño conditions deteriorate rapidly by mid-2010 into La Niña conditions, Hansen added.

That is exactly what is happening right now, according to NOAA. Sea surface temperatures in the Pacific Ocean—which play a key role in determining global temperatures—continued to decline in June, transforming from the heated condition known as El Niño to the cooler condition known as La Niña. A similar change in 2007 ended that year's chance to surpass 2005 as the warmest year on record. "La Niña typically contributes to a lower global average temperature than do neutral or El Niño conditions," Lawrimore explains. "The forecasted development of La Niña has the potential to bring the annual 2010 global average temperature below a record for the year as a whole."

Nevertheless, record highs are already being recorded across the globe: Pakistan set Asia's record for highest temperature on record, notching up 53.5 degrees C on May 26—one of nine countries to set high heat records so far this year. There are now roughly twice as many days with record highs than days with record lows, according to research from the National Center for Atmospheric Research. And warmer than usual temperatures in the ocean have prompted coral bleaching in the Indian Ocean and offshore of Southeast Asia; NOAA also warns there is "high potential" for such bleaching to develop in the Caribbean this summer.

All this heat comes at a time when the sun—despite a recent uptick in solar storm activity, much of it associated with sunspots, since late 2008—continues to pump out slightly less energy. This diminished solar radiation should be promoting a slight cooling but is apparently outweighed by the ongoing accumulation of atmospheric greenhouse gases, particularly carbon dioxide, as scientists have predicted for more than a century. Of course year to year variations in weather cannot be conclusively tied to climate change, which is best measured by a multiyear trend, such as the long-term trend of warming into which this year fits—2000 to 2010 is already the warmest decade since records have been kept and the 10 warmest average annual surface temperatures have all occurred in the past 15 years.

Precipitation patterns are changing as well—southern regions in China, Europe and India, along with the U.S. Midwest and Northwest all saw more rainfall than usual in June. Similar factors resulted in Nashville being drowned this spring, among other impacts. Meanwhile, drought afflicted Australia, eastern Asia, northern India and northeastern South America. The U.K. is experiencing the lowest rainfall since 1929.

Regardless, the long-term trend is clearly continued warming, which is "very likely" (with more than 90 percent certainty) caused by greenhouse gases emitted in the course of human activity, according to climate scientists. And climate model research from Stanford University set to be published in Geophysical Research Letters shows that extended heat waves are likely to become more commonplace in the U.S. by 2040—and predicts hotter and drier seasons over the next decade.

"Frankly, I was expecting that we'd see large temperature increases later this century with higher greenhouse gas levels and global warming," Stanford climate scientist Noah Diffenbaugh, who headed up the research, said in a prepared statement. "I did not expect to see anything this large within the next three decades."

"Heat waves are expected to become more frequent and intense in the 21st century," NOAA's Lawrimore adds. "Today's rare heat waves will likely become the typical weather conditions by the last half of the century."