When I reached Heidi Cullen, chief climatologist for Climate Central, she remarked on the glorious weather we were enjoying. But underneath our pleasure at the sunshine and blossoms, we were both feeling uneasy about the warmth. So far all 11 years of the 21st century have been among the 13 warmest years on record, according to the National Oceanic and Atmospheric Administration.

Cullen is in the habit of keeping a close eye on the weather: Climate Central is a nonprofit science research organization headquartered in Princeton, N.J. Before she joined them, Cullen, who holds a doctorate from Columbia University, was the Weather Channel’s first on-air climate expert.

We had just been through a March of record-shattering heat, and we were roasting through mid-April days. On top of that, according to the U.S. Drought Monitor, most of the U.S. is experiencing an unusually dry period, with southern Florida bone dry. More than 63 percent of Georgia is in the worst two levels of drought, the highest of any state.

Because of the dry, windy conditions, wildfires and brush fires have been raging along the East Coast from New England to Florida; billowing black clouds from New Jersey Meadowlands fires have been visible from midtown Manhattan. And in much of New England, stream flow levels were at record lows—with Vermont, though still reeling from last summer’s disastrous floods, abnormally dry.

“We may have just broken another record,” Cullen says of the recent heat wave. “That’s what we do these days. We break records.”

Cullen noted that her phone didn’t stop ringing during the March heat wave. Whenever there is an unusual weather event, journalists want to know if it is caused by climate change. “In fact, I was just talking to someone at NPR who facetiously asked, ‘How’s your summer going so far?’”

It seemed an appropriate atmosphere in which to ask a climate scientist what’s going on with the weather. And what weather has to do with climate change.

Let’s start with the basics, Heidi. What’s the difference between climate and weather?

When I was at the Weather Channel, I got this question all the time. I think Mark Twain said it best: “Climate is what we expect; weather is what we get.”

Climate is a statistical concept, so it can be hard for people to wrap their minds around it. When we talk about climate, we’re talking about the average of weather. We have an intuitive feel for climate “forecasts” in our own lives—we expect, in January, that July will be much warmer. And that’s accurate. Where things get tricky, of course, is over long time periods.

I think of climate as being like an orchestra. It has so many elements, the way an orchestra has many sounds. The climate system is made up of components such as our atmosphere, the oceans, ice sheets, and the land surface. We experience natural climate variations, such as el Niño (the periodic warming of the eastern equatorial Pacific Ocean), which has an inherent time scale of three to seven years. In fact, we just came out of a la Niña episode (the opposite phase of el Niño, in which there is a cooling of the eastern equatorial Pacific Ocean), which is generally associated with drought along the southern portion of United States (including Texas, which saw its hottest, driest summer on record in 2011). Other natural climate variations include solar activity and volcanoes.

So think of all these instruments, all these natural variations, at play in our climate. Now add to that a steady drumbeat of warming in our climate system caused by us.

How do we know we are causing the recent warming trend? Didn’t we just come out of an Ice Age?

Yes, there is tremendous natural climate variability. I studied the period known as the Holocene when I was working on my Ph.D. at Lamont-Doherty Earth Observatory at Columbia University. That’s the past 10,000 years. The Holocene is known as having had a relatively stable climate compared with other time periods. The start of the Holocene also corresponds with the rise of complex human civilizations. It would seem we require this rather small envelope of climate stability to thrive.

This doesn’t mean there weren’t catastrophic climate events during the Holocene. Various civilizations have collapsed because of significant changes in our environment; for example, a multidecade drought played an important role in the collapse of the Akkadian Empire in Mesopotamia roughly 4,200 years ago. Jared Diamond describes several of these historical events quite beautifully in his book Collapse. But comparatively speaking, we’ve had 10,000 years of a comfortable, life-enhancing climate on earth.

So how do scientists sort out what is “natural” warming from the climate change human activity is causing?

This is directly measurable, and there are two lines of response.

First, we can measure chemical isotopes of CO 2 in the atmosphere. We know where CO 2 comes from—because these molecules have chemical fingerprints. CO 2 from different sources has different mixes of carbon isotopes. Scientists use instruments called spectrometers that chemically separate the kinds of carbon found in fossil fuels from the kind normally found in air and water.

We know which CO 2 molecules were put into the atmosphere there by us. For instance, there is no C-14 in CO 2 molecules that come from fossil fuels. Roughly, one out of every four molecules of CO 2 in our atmosphere comes from human activity. It adds up. The increase in CO 2 since the start of the Industrial Revolution has mostly been from burning fossil fuels—the remainder comes mainly from clearing and burning forests. Today global atmospheric carbon dioxide concentrations are at 395 parts per million (ppm). Prior to the Industrial Revolution of the late 19th and early 20th centuries, the carbon dioxide level was about 280 ppm.

The second way that scientists sort out human contribution to global warming is that we run climate models. Think of creating twin earths—one is an earth without human activity, an earth managed exclusively by Mother Nature, with all those orchestral elements I mentioned earlier. We run that out for a hundred years, using data that excludes human-generated carbon emissions, and we compare that to the sibling earth, which includes human activity.

We have seen approximately 1.4 degrees Fahrenheit of warming in the past century. We are seeing CO 2 going up—it is a measurable trend, not a cyclical phenomenon. On her own, Mother Nature’s earth cannot reproduce the observed global temperature record we have measured over the past century. Solar activity, volcanoes, and variability of other types are important, but on their own they simply cannot produce the significant warming trend we are now experiencing. Our climate models prove this.

Whenever there is an extreme weather event, people wonder: is this because of climate change? Is that a legitimate way to think about climate change?

That’s natural. People look to weather for cues about what’s going on. Climate change stacks the deck for certain types of extreme weather events. For example, we expect heat waves to become more frequent, longer lasting, and more intense. This has tremendous implications for water resources and agriculture. When we break records now—and we are breaking thousands of them—we break them by a lot.

Let’s talk about the Russian heat wave of 2010—the hottest summer temperatures ever experienced in the region. It resulted in thousands of deaths, damaged crops, and serious forest fires. Did we increase the likelihood of that happening?

You have to do a “weather autopsy” to understand events like heat waves. Scientists use a lot of the same techniques that epidemiologists turn to in understanding the cause of death. We look at specific risks. Did someone smoke? Or were they obese? Climate scientists study the risks present within our climate system—was there an el Niño, for example? But also they look at how human actions may have increased the risk.

A recent study suggests the devastating and deadly Russian heat wave of 2010 was made three times more likely because of global warming caused by human activity.

One of the most important research studies to come out along these lines—it was a beautifully done study—had to do with the European heat wave of 2003. Climate scientists reported that human activity doubled the chance of such a heat wave happening. The report predicted that by 2040, the European heat wave will be happening every other year. And by 2070, the summer of 2003 will look relatively cool.

So much of this science is about where we are going if we don’t reduce the amount of heat-trapping pollution released into our atmosphere. All weather is now born into an environment that is warmer and moister because of man-made greenhouse-gas pollution. But we don’t always know what influences (man-made or natural) will win out on any given day.

What about people who argue that we don’t know enough about the weather patterns yet to make a judgment about what will happen?

The “wait and see” approach—let’s see how bad it gets—may seem rational, but in fact it is incredibly risky. There’s a time lag between what we do and what we see in our climate and weather. By the time we’re really seeing the full impact of global warming, it is too late to fix it.

It is so much cheaper, and safer, to take steps in advance of catastrophe. Fix the problem upfront. Two thousand and eleven set the record for the most billion-dollar weather disasters—14 of them, in one year. We can’t afford these problems.

Have scientists been studying this problem long enough to achieve certainty about what’s going on?

People think climate change is something that scientists have only been focused on for a few decades. This isn’t the case. Global warming is not a new problem. (See more in my book or here.) In 1896 a Swedish scientist named Svante Arrhenius published a new idea. As humanity burned fossil fuels such as coal, carbon dioxide was added to the earth’s atmosphere. As a result, we were literally raising the planet’s average temperature. Arrhenius proposed this “greenhouse effect” as an explanation for climate change, and scientists have been working on the problem ever since. Arrhenius was awarded the Nobel Prize in Chemistry in 1903.

Do you think people are beginning to understand that something serious is going on?

Recent polls suggest that belief in climate change, and concern about it, is picking back up. Those numbers dropped between 2008 and 2010. Why? Well, the economic downturn certainly played a role. A recent paper by sociologist Robert Brulle at Drexel University suggests the lack of federal leadership on climate change also makes a difference. Fortunately, we are seeing stronger leadership at the state and local level these days, in places like California and New York City.

But some argue that the weather we saw over that period also played a role. We had colder winters those years—and people stopped worrying about climate change. Now we’ve just gone through a record-breaking winter in December, January, and February. We’ve had the warmest winter on record—that’s since 1895. And March shattered all kinds of records. Every state in the nation experienced a record warm daily temperature during March. According to preliminary data, there were 15,272 warm temperature records broken (7,755 daytime records, 7,517 nighttime records). Hundreds of locations across the country broke their all-time March records. There were 21 instances of the nighttime temperatures being as warm, or warmer, than the existing record daytime temperature for a given date.

Let’s go back to your point about people taking their cues from the weather—what are scientists watching the weather for now?

We look at the Big Three:

Heat and “mega–heat” waves. And we’re watching the air-pollution issues that are related to higher temperatures, like ozone levels. Those are known to cause health problems.

Heavy rainfall. As we warm the planet, we also increase the risk of heavy downpours. A warmer atmosphere holds more moisture, and as a result, we’re seeing an increase in very-heavy-rainfall events. Here’s a map of these observed increases from the 2009 “Global Climate Change Impacts in the United States” report.

Drought. We’re watching Florida and Georgia with great concern, as they head into a prolonged drought. Texas may finally be pulling out of this drought. But of course, drought will continue to be an intensifying problem for the Southwest. According to the U.S. Drought Monitor, parts of every state except for Alaska and Ohio are either abnormally dry or in some form of drought.

What about tornadoes?

There are still lots of unanswered questions about tornado activity. Will tornadoes become more frequent, more intense? Will Tornado Alley shift or get bigger? Will the season last longer? The short answer is that there are two key ingredients that go into forming a tornado. Both are expected to change as a result of global warming.

First, there is water vapor (moisture in the atmosphere). Thanks in part to warmer oceans, water vapor has already increased about 4 percent, and it will continue to increase as the planet warms—providing more fuel for storms.

Second is wind shear—that’s the name we give to changing wind speed and direction with height. Wind shear may decrease in a warmer world and that could mean fewer tornadoes. So which influence wins out—increasing water vapor or decreasing wind shear? We simply don't know yet.

Should scientists be involved in the public conversation about climate change?

Yes. I think it is part of our responsibility as scientists to help people understand the research we’ve done. But we have to do that carefully with integrity so that we don’t lose the public’s trust. Parts of this research are enormously complex and in some cases uncertain. We need to do a better job of communicating that uncertainty—and we need to do a better job of communicating our certainty about what is happening because of greenhouse gases.

We need real leadership on solving this problem—both in terms of adaptation (things like infrastructure to decrease our vulnerability) and mitigation (reducing our emissions by investment in alternative sources of energy). That takes a serious commitment.

I strongly believe that scientists have a moral responsibility to help people understand what we are facing.