JUDY WOODRUFF: The past 12 months are the warmest ever recorded in the United States since record-keeping began in 1895. That word comes as a new report from the National Oceanic and Atmospheric Administration says climate change, including human factors, has increased the odds of extreme weather.
The severe storms that finally broke the deadly heat wave in the United States blew in with their own set of dangers this week. In Greensboro, North Carolina, residents are struggling to recover from flooding and power outages brought on by slow-moving storms yesterday. In Fredericksburg, Va., violent thunderstorms pummeled a cheerleading facility.
GIRL: We were scared, and we were just, like, praying to God and hoping that we weren't going to die.
GIRL: And we saw it. It just came in on us.
JUDY WOODRUFF: And in Houston, Texas, they sent ball players scrambling for cover.
The scares come after high temperatures are being blamed for at least 46 deaths and loss of power for close to a million people last week. For over 11 consecutive days, temperatures exceeded 100 degrees across much of the country.
Meanwhile, out West, wildfires fueled by near-record droughts have raged for weeks in Colorado, forcing residents to leave their homes. Nationwide, fires burned 1.3 million acres in June alone, the second highest acreage burned in June of any year.
Now the National Oceanic and Atmospheric Administration, NOAA, is reporting the first half of this year was in fact the hottest on record, with 170 all-time heat records matched or broken.
NOAA has issued a report attempting to assess the role climate change, including human factors, played, if any, in six global extreme weather events in 2011. About one of those, the report asked if the human influence on climate made the 2011 Texas drought more probable. It concluded that it did.
The report also examined climate change's role in last year's drought in East Africa, heat wave across Europe and floods in Thailand. Regarding Thailand, the report said climate change cannot be shown to have played any role in the excess rain and flooding.
For more on all this, we turn to Thomas Karl, director of NOAA's National Climatic Data Center, which oversaw the studies.
I spoke with him this afternoon.
Thomas Karl, thank you for joining us.
THOMAS KARL, director, National Climatic Data Center, National Oceanic and Atmospheric Administration: Thank you, Judy.
JUDY WOODRUFF: Let's start with the news. Temperatures, the first half of 2012 this year were the hottest on record. What's known about why that's happening?
THOMAS KARL: That is true.
The temperatures the first six months of this year in the U.S. are the warmest on record. And, in fact, the last 12 months of the period May through June have been the warmest on record. Why? We believe there is an important human component explaining these record-breaking temperatures, and that's the increase of greenhouse gases in the atmosphere.
JUDY WOODRUFF: So, how does that compare with what has happened historically? Can you put it in the some context for us?
THOMAS KARL: Well, to give you an example, we have had warm conditions in the past.
1930s, many people are familiar with the Dust Bowl and the heat associated with the 1930s. What we're seeing today is equivalent or even greater than the temperature records that fell in the 1930s. And what we're seeing more frequently is record-breaking high temperatures.
Again, the more recent record even exceeds the heat that we saw in the 1930s. That is the warmth of the last year, the warmth of the past spring, last winter, last fall and, if you remember, the record heat last summer, particularly in the Southern part of the U.S., where Texas and Oklahoma had such severe heat and drought.
JUDY WOODRUFF: Now, you also released a study today looking at what last year's conditions were like and the relationship between greenhouse gases and human activity. What did you find?
THOMAS KARL: Well, in that report, there was a series of scientific teams across the world that contributed to trying to look at a number of selected extreme weather and climate events, and to do an analysis to see if they could understand whether these events would have occurred all by themselves, without human contribution, or whether or not we could say humans clearly made the events stronger than what they might otherwise have been.
And, in most cases, we can actually say with some confidence that these events wouldn't have been as strong or intense if it were not for the increase of greenhouse gases in the atmosphere.
JUDY WOODRUFF: And, as I understand it, there was some disparity. In the Texas heat wave, you found more of a connection, but with flooding in Thailand, it was less clear.
THOMAS KARL: Yes. And it's clear every extreme weather and climate event cannot be attributed to human activity or greenhouse gases.
But there's an increasing number of these where they can. And one specific example was the heat wave and drought in Texas and Oklahoma last year. The analysis that just completed suggested that that event would have occurred normally with the kind of La Nina conditions that occurred last year, but the severity of it made it much more likely.
In fact, the statistics suggest it was 20 times more likely to occur because of the current conditions we have with the increasing temperatures related to increasing greenhouse gases.
JUDY WOODRUFF: La Nina, referring to ocean currents.
But help us understand for the layperson watching the connection between human activity, climate and severe weather.
THOMAS KARL: Well, the best way we can describe it, it's sort of like a baseball player on steroids.
Now, if you're going to break records, home run records, you're likely going to have to be a home run hitter to break home run records. With someone on steroids, the likelihood of hitting a ball over the fence and hitting a home run increases. And that is what we're seeing. The increase of greenhouse gases in the atmosphere leads to warmer global temperatures.
Those then break global temperature records. They also have other impacts, like increases in precipitation intensity, more intense droughts. These are the kinds of things we're seeing, more records with greater severity and intensity than they would -- might have otherwise been.
JUDY WOODRUFF: And what do you say to those climate skeptics who say that some of these changes are simply part of natural cycles that have been around all the time?
THOMAS KARL: Well, I would say part of that answer is correct.
Some of the events are part of natural variability. In fact, natural variability has always been around with it. What we mean by natural variability is, you're going to have a drought, you're going to have a heat wave even if you didn't have humans adding greenhouse gases to the atmosphere.
But the scientific evidence, the analysis done, the look at climate models, the look at the observation of data leads one to believe that humans in fact are making these events more intense and stronger than they would otherwise have been.
JUDY WOODRUFF: Thomas Karl of NOAA, thank you for joining us.
THOMAS KARL: Well, thank you, Judy. Appreciate it.