Challenges And Responses

Most new scientific ideas follow a typical sequence. After the "thesis" (publication of the new hypothesis) comes the "antithesis" (evaluation and criticism by the scientific community), and later, for those hypotheses that survive close scrutiny, the "synthesis" (refining or reshaping of the hypothesis into a form that addresses the criticisms and satisfies a wider range of observations). At this point, usually years later, the hypothesis may come to be called a "theory."

In the case of my hypothesis, the thesis stage occurred the first week of December 2003, when my paper on early human impacts on climate was published in the journal Climatic Change and when I gave a lecture at the annual meeting of the American Geophysical Union in San Francisco. Almost 10,000 scientists attended that meeting, and more than 800 of them were in the lecture hall or spilling out into the nearby hallways on that Tuesday evening. The initial impact of the hypothesis seemed very positive, although this "honeymoon phase" would prove to be brief. In the next few days, reports of the talk and article appeared in major media outlets, including the New York Times, London Times, Economist, Associated Press, and BBC radio. Dozens of people came up to me at the meeting over the next few days voicing favorable opinions, often with obvious enthusiasm. Near the end of the meeting, a climate-modeler friend and self-described skeptic told me that he had talked to other skeptics in related fields and that all of them had the same reaction to my talk: "Damn, why didn't I see that?"

This initially favorable reception lingered into the winter of 2004, with feature articles in two prestigious scientific journals, Science and Nature. Both articles had the tone of cautious objectivity appropriate to commentary on a newly published idea, but both also cited noticeably positive comments from prominent scientists, and little by way of direct criticism. Later in the winter I received word that a joint proposal I had submitted to the National Science Foundation with colleagues at the University of Wisconsin to explore this hypothesis had been ranked highly by external reviewers and a panel of experts, and that it would be funded.

With all this good news, I began to wonder if the hypothesis might be accepted more quickly than I had expected. In the spring of 2004, however, two published papers raised serious challenges that needed to be addressed. The standard antithesis phase of the science process had begun. Those challenges, and my responses to them, are the subject of this chapter.

One challenge was that I had not chosen the optimal part of Earth's climatic record as an analogy for the natural trend that climate would have taken during the last few thousand years in the absence of human impacts. As explained in chapters 8 and 9, I had compared greenhouse-gas trends in recent millennia with those during the early stages of the last three interglaciations because those were also times (like today) when ice sheets had melted from Northern Hemisphere continents, and when changes in solar radiation were moving in the same direction as now. During those intervals, the natural concentrations of methane and CO2 had dropped, whereas levels of both gases had risen in the last few thousand years (fig. 11.1). I had concluded that only human activities can explain the rise in gas levels in recent millennia compared to the natural drops in earlier times.

The criticism of my method was that I should have looked farther back in the sequence of ice-age cycles at evidence from an interglaciation that occurred 400,000 years ago. Solar radiation trends at that time were the closest available analog to the trends during the last few thousand years, whereas the radiation trends during the times I had used for comparison were larger in magnitude, even though moving in the same direction.

This criticism was valid; the earlier interglaciation 400,000 years ago is indeed the closest analog to the recent radiation trends. Figure 11.2 shows the summer solar radiation trends at 65oN for the earlier interval and for the last few millennia (and several thousand years into the future). The amplitudes of the two trends differ a little: the amount of radiation during the earlier interglaciation is a bit lower than that during the last 10,000 years, but the differences compared to the recent trends are a factor of 2 or more smaller than during the intervals I had used in my paper.

I had previously avoided attempting a comparison with this earlier interval because the one long ice-core record available from Antarctica bottomed out in the very interglacial interval of interest, and I was concerned that it did not quite reach the level that was the best analog to the last few thousand years. But now that I was hearing this criticism (from several directions), it was clear that I needed to take a closer look at that older part of the signal.

As it turned out, the ice core did reach far enough back in time. Three independent methods of estimating the age of the deeper ice converged on the same time scale, and the interval most similar to today was indeed present.

Here was a moment of truth: the answer would come quickly and would likely be decisive. If the greenhouse-gas concentrations in the ice during this earlier interglacial analog dropped more or less as I had predicted (fig. 11.1), the implication would be that the gas increases of the last few millennia are indeed anomalous, and thus the result of human activities. But if the earlier trends followed

cd 500

B 490

cd 500

B 490

Solar Radiation

Observed CH4 ;

V_

' V

-

if v >

t 1

X 250 ppb -

V

-

r _

1 1

1 1 1

1 1 1 1 1

600 T

5,000 Years Ago

SB ZJ CD

600 T

10,000

5,000 Years Ago

B 290

E 270

E 270

5000 Years Ago

10,000

5000 Years Ago

11.1. Concentrations of methane (A) and CO2 (B) should have fallen during the last several thousand years but instead rose because of human activity.

more or less the same rising trend as has occurred during the last few millennia, the implication would be that the recent trends are natural, and my hypothesis would be invalid.

The answer was clear, and it was decisive: concentrations of both methane and CO2 fell during the earlier interglaciation to levels very near those I had proposed (fig. 11.3). The methane value fell to a level slightly lower than predicted, while the CO2 value fell to a value slightly higher, but in each case both the fundamental downward direction of the trends and the levels reached vindicated the

Years Ago 10,000

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