"Let's begin informally. Tell us what you know about the evidence for global warming."
"Well," Evans said, "I know that temperatures around the globe have risen dramatically in the last twenty or thirty years as a result of increases in carbon dioxide that is released by industry when fossil fuels are burned."
"Okay. And by a dramatic rise in temperature, you mean how much?"
"I think about a degree."
"Fahrenheit or Celsius?"
"Fahrenheit."
"And this rise has occurred over twenty years?"
"Twenty or thirty, yes."
"And earlier in the twentieth century?"
"Temperatures went up then, too, but not as fast."
"Okay," she said. "Now I am going to show you a graph..." She pulled out a graph*on foam core backing:
Global Temperature 1880-2003
"Does this look familiar to you?" she asked.
"I've seen it before," Evans said.
"It's taken from the NASA-Goddard data set used by the UN and other organizations. Do you consider the UN a trustworthy source?"
"Yes."
"So we can regard this graph as accurate? Unbiased? No monkey business?"
"Yes."
"Good. Do you know what this graph represents?"
Evans could read that much. He said, "It's the mean global temperature from all the weather stations around the world for the last hundred years or so."
"That's right," she said. "And how do you interpret this graph?"
"Well," he said, "it shows what I was describing." He pointed to the red line. "World temperatures have been rising since about 1890, but they start to go up steeply around 1970, when industrialization is most intense, which is the real proof of global warming."
"Okay," she said. "So the rapid increase in temperature since 1970 was caused by what?"
"Rising carbon dioxide levels from industrialization."
"Good. In other words, as the carbon dioxide goes up, the temperature goes up."
"Yes."
"All right. Now you mentioned the temperature started to rise from 1890, up to about 1940. And we see here that it did. What caused that rise? Carbon dioxide?"
"Um...I'm not sure."
"Because there was much less industrialization back in 1890, and yet look how temperatures go up. Was carbon dioxide rising in 1890?"
"I'm not sure."
"Actually, it was. Here is a graph showing carbon dioxide levels and temperature."
Global Temperature 1880-2003
"Okay," Evans said. "Just what you would expect. Carbon dioxide goes up, and makes temperatures go up."
"Good," she said. "Now I want to direct your attention to the period from 1940 to 1970. As you see, during that period the global temperature actually went down. You see that?"
"Yes..."
Global Temperature vs CO2 1940-1970
"Let me show you a closeup of that period." She took out another chart.
"This is a thirty-year period. One third of a century during which temperatures declined. Crops were damaged by frost in summer, glaciers in Europe advanced. What caused the decline?"
"I don't know."
"Was carbon dioxide rising during that period?"
"Yes."
"So, if rising carbon dioxide is the cause of rising temperatures, why didn't it cause temperatures to rise from 1940 to 1970?"
"I don't know," Evans said. "There must have been another factor. Or it could be an anomaly. There are anomalies within broad secular trends. Just look at the stock market."
"Does the stock market have anomalies that last thirty years?"
He shrugged. "Or it could have been soot. Or particulate matter in the air. There were a lot of particulates back then, before environmental laws took effect. Or maybe some other factor."
"These graphs show that carbon dioxide rose continuously, but temperature did not. It rose, then fell, then rose again. Even so, I take it you remain convinced that carbon dioxide has caused the most recent temperature rise?"
"Yes. Everybody knows that's the cause."
"Does this graph trouble you at all?"
"No," Evans said. "I admit it raises some questions, but then not everything is known about the climate. So, no. The graph doesn't trouble me."
"Okay, good. I'm glad to hear it. Let's move on. You said this graph was the average of weather stations around the world. How reliable is that weather data, do you think?"
"I have no idea."
"Well, for example, in the late nineteenth century, the data were generated by people going out to a little box and writing down the temperature twice a day. Maybe they forgot for a few days. Maybe somebody in their family was sick. They had to fill it in later."
"That was back then."
"Right. But how accurate do you think weather records are from Poland in the 1930s? Or Russian provinces since 1990?"
"Not very good, I would guess."
"And I would agree. So over the last hundred years, a fair number of reporting stations around the world may not have provided high-quality, reliable data."
"That could be," Evans said.
"Over the years, which country do you imagine has the best-maintained network of weather stations over a large area?"
"The US?"
"Right. I think there is no dispute about that. Here is another graph."
US Temperature 1880-2000
"Does this graph look like the first one we saw of world temperatures?"