The questions before this committee today are whether press coverage of global warming in this country has portrayed accurately the state of scientific knowledge and whether the press has properly framed the issue for the public and for decision makers like yourselves. I am hesitant to generalize, as reporting on this issue is quite variable. I think it is safe to say that press reports are accurate when they present the strong consensus that exists among climate scientists that global warming is occurring, and when they describe some of the risks we face. When I have taken issue with press coverage of global warming, it is usually because the issue is presented as a debate between “believers” and “skeptics.” Articles often give a voice to extreme views, rarely evaluating credentials or credibility. The public is left trying to decide whether global warming is real based on highly technical arguments, and left uncertain whether corrective action is necessary.

I think the proper framing of this issue is quite different: There is no serious debate about whether the earth will warm as carbon dioxide levels increase over this century – it will. What is difficult to predict is exactly how much warming will occur, and exactly how that will affect human society. The media does not usually explain this distinction very well. I would like to see the press raise the same question used for other issues of national security: Are the risks of severe consequences sufficient to warrant taking preventative action?

Humans are changing the amount of carbon dioxide in the atmosphere, mostly from burning of coal, oil and gas, with deforestation also playing a significant role. The current level, in excess of 380 parts per million (ppm), is higher than it has been for at least the last 650,000 years, and perhaps for tens of millions of years (Fig. 1). To put it differently, we are experiencing higher CO2 levels now than any human being has ever seen in the history of the earth; and over the next 100 years, without substantial changes in the trajectory of energy technology or economic development, we will see atmospheric CO2 rise to 800 to 1000 ppm, roughly triple the pre-industrial level. Carbon dioxide is a greenhouse gas. Its presence in planetary atmospheres causes warming of planetary surfaces; an extreme example is the CO2-rich atmosphere of Venus, which is responsible for its surface temperature in excess of 460 °C.

The question that confronts us now is how the rise of CO2 on this planet will affect our climate, not over millions or even thousands of years but over decades and centuries. We know that, coincident with the unprecedented rise in CO2 over the last century, we have seen a rise in global temperatures. We know from Lonnie Thompson’s work on tropical glaciers that this warming is not part of any natural cycle (Fig. 2). But this does not address the question of what will happen as CO2 levels continue to rise. To answer this question, climate scientists have constructed models that represent the best understanding of the climate system from the last century of observations. These models tell us that climate change in this century may be dramatic, and perhaps even catastrophic. These models are not perfect – but this is not surprising as they are attempting to make predictions about an atmospheric state that no human being has ever seen. They remain an essential tool for exploring future scenarios, but we must also consider evidence for climate change from the geologic past. This is the major area of my research. I cannot cover it today in much detail, but let me simply say that lessons from earth history are surprisingly consistent, whether from warm climates or cold, whether over millions of years or thousands: our climate system is very sensitive to small perturbations (Fig. 3). And human activities represent a large perturbation, sending our atmosphere to a state unlike any seen for millions of years.