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11.2. Changes in summer solar radiation nearly 400,000 years ago are the closest analog to July radiation changes during the last several millennia.

hypothesis. The greenhouse-gas increases during the last few thousand years have indeed been anomalous compared to the natural behavior of the climate system. And if these increases were not natural, they must have originated from human activities.

Yet at first this evidence appeared to conflict with other analyses of this same interglaciation 400,000 years ago. Research on marine sediments had indicated that the interval of interglacial warmth at that time had lasted for an unusually long time, and results from a newly drilled Antarctic ice core had confirmed that conclusion. These two lines of evidence made a convincing argument that the earlier interglaciation had lasted much longer than the present interglaciation has to date.

Based on this evidence, some scientists had concluded that the current interglaciation still has thousands of years to run until the "natural" shift toward glacial conditions begins. This interpretation seemed reasonable, but it directly conflicted with my hypothesis that a significant climatic cooling and new glaciation are now overdue. The next glaciation can't be both overdue and also far in the future. One of these interpretations must be incorrect, but which one?

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11.3. During the interglaciation nearly 400,000 years ago, methane and CO2 concentrations fell to natural values much lower than those reached during recent millennia.

A closer look at the evidence pointed me to the answer. During the same interval in the Vostok ice core nearly 400,000 years ago when the CO2 and methane values dropped (fig. 11.3), Antarctic temperatures had also plummeted from relatively warm interglacial levels to the colder values typical of glacial conditions (much colder than the temperatures in that already frigid region today). This evidence meant that the warmer interglacial conditions in Antarctica had ended at a time that was fully consistent with my hypothesis—during the interval just after 400,000 years ago when solar radiation changes were most similar to those today.

This additional information meant that the long interval of interglacial warmth 400,000 years ago must have begun earlier (in a relative sense) than the current interglaciation. An earlier start to the warm interval would permit that interglaciation to last for a long time and yet come to an end at the time when solar radiation trends were most analogous to the present day. The apparent conflict with my hypothesis was not a real one.

The evidence from marine sediments presented a similar challenge. As in Antarctica, the North Atlantic Ocean had warmed early at the start of the older interglacial interval 400,000 years ago, and it had also remained warm for an unusually long time. But in this case, the ocean south of Iceland had stayed warm through the interval when my hypothesis predicted new ice growth in the Northern

Hemisphere. Because a warm ocean seemed to imply an interglacial world, some scientists had concluded that the world must have remained free of ice sheets through an interval when my hypothesis predicted they should be growing.

I rejected the interpretation that this ocean warmth meant that ice sheets were not growing. More than 25 years ago, a colleague, Andrew McIntyre, and I had shown that much of the subpolar North Atlantic Ocean south of Iceland stays warm at times when ice sheets are growing on nearby continents. Why it does this is not clear, but oceans are not simply passive responders to climate change. Each region has its own characteristic dynamic response that can allow it to react to local changes in winds and other factors. In the North Atlantic, that local response for some reason tends to create "lagging ocean warmth" during ice growth.

Other important evidence comes from the Atlantic Ocean north of Iceland. A study of ocean sediments in that area found that icebergs began to drop large quantities of coarse debris to the sea floor just after 400,000 years ago, after an interval free of such deposition. To generate icebergs that reached the ocean and dropped debris, ice sheets must already have been growing for several thousand years. If the estimated timing of this iceberg influx is correct, ice sheets must have been growing at the same time that the North Atlantic south of Iceland was still warm, in agreement with the hypothesis.

The amount of ice at this time was probably not very large, perhaps no more than 10 percent of the volume of ice typically present when each 100,000-year cycle of ice growth culminates in glacial-maximum conditions (chapter 4), and possibly less. This modest amount of ice apparently was not enough to make its presence felt by sending cold winds into the more temperate latitudes of the North Atlantic. Yet it would likely have been a larger volume of ice than the present-day Greenland ice sheet.

In summary, the evidence for a long interval of interglacial warmth in the North Atlantic did not disprove my hypothesis, and other evidence from the marine record supported it. During the earlier interglaciation 400,000 years ago, at a time when solar radiation trends were most similar to those during recent millennia, a significant natural cooling had occurred in many regions, and a new glaciation (probably small in scale) had begun in the Northern Hemisphere. In contrast, during the last few thousand years, few regions on Earth have cooled, and no new ice has appeared in northern lands. The only plausible reason for the differences in climatic response during these two intervals is that emissions of greenhouse gases by human activities averted most of a natural cooling that would otherwise have occurred and thereby prevented a glaciation from getting underway. A glaciation is now overdue, and we are the reason.

Another major challenge to my hypothesis appeared in a paper whose authors claimed that humans could not possibly have cleared and burned enough forest vegetation in preindustrial times to account for the 40 parts per million size I had estimated for the CO2 anomaly (fig. 11.1). This criticism also appeared to have considerable merit.

My calculations of forest clearance had suggested that more than 200 billion tons of carbon could have been emitted by human activities, an estimate that seemed reasonably close to the amount needed to explain the 40-ppm CO2 anomaly. But now a model simulation had been run that required a much larger amount of carbon to explain the CO2 anomaly—somewhere in the range of 550 to 700 billion tons. Because humans could not possibly have released that much carbon into the atmosphere in the last few thousand years, the authors of this paper concluded that human activities could not account for a 40-ppm CO2 anomaly. If so, my hypothesis must be flawed.

Here was another impasse—two lines of evidence that again seemed to be in direct conflict. On the one hand, my new investigations had seemingly confirmed that the CO2 anomaly that developed in recent millennia could only be the result of a "new" process that had not been present during the four previous interglaciations, and forest clearance for agriculture was the most obvious candidate. Yet the model results indicated that human biomass burning could not account for the 40-ppm anomaly. How could these seemingly conflicting lines of evidence be resolved?

I looked again at my original definition of the "CO2 anomaly." I had defined it as the difference between (1) the observed rise in CO2 concentration during the last few thousand years and (2) the observed drops in CO2 concentrations during previous interglaciations (fig. 11.1). Until this point, I had been focusing only on biomass burning that added CO2 to the atmosphere, but now I realized that I had been ignoring the other side of the ledger: the natural drops in atmospheric CO2 concentrations of 25 to 45 parts per million that had occurred during the four previous interglaciations. During the current interglaciation, a similar-looking drop had begun 10,500 years ago, but after 8,000 years ago the CO2 trend had reversed direction and begun the anomalous rise. It occurred to me at this point that the prevention of a natural CO2 drop should also count as a contribution to the size of the CO2 anomaly. If part of the anomaly was explained by the lack of such a drop during recent millennia, then the part of the anomaly produced by direct carbon emissions from biomass burning would not have to be as large.

What were the mechanisms that had caused the natural CO2 decreases during the earlier interglaciations but had been prevented from doing so during the present one? Two possibilities came to mind, both based on well-known hypotheses published in the early 1990s, and both consistent with the evidence from ice cores and marine sediments.

One potential mechanism is that advances of Antarctic sea ice cut off carbon exchanges between the ocean and the atmosphere and cause decreases in atmospheric

CH4 and CO2 -Emissions

Warming Counters Most of Natural Cooling Trend

No Northern -►Ice Sheets; -Smaller Southern Sea-ice Advance

CO2 Prevented



11.4. The CO2 anomaly caused by humans prior to the industrial era was due in part to burning of forests (direct CO2 emissions) and in part to the prevention of a natural CO2 decrease.

CO2 concentrations. The evidence cited earlier from Antarctic ice cores fits this explanation nicely: major decreases in Antarctic temperature occurred early in previous interglaciations, but only a small cooling has taken place during the current one. Sea ice would have advanced and CO2 levels would have fallen during the previous interglaciations for this reason but would have failed to do so during this one.

The other mechanism is related to CO2 changes caused by ice sheets in the North. Scientists have hypothesized that ice sheets create their own positive CO2 feedback, forcing CO2 levels lower as the ice grows, and letting concentrations rise as it melts. The means by which the ice sheets affect CO2 levels is still debated, but two promising candidates have been identified. One hypothesis is that the dust generated on the continents by the ice sheets is blown to the ocean and fertilizes algae in the surface layers of the ocean. As the algae die, their carbon-rich tissue sinks to the sea floor, removing CO2 from the surface ocean and the atmosphere. The other hypothesis calls on large ice sheets to alter the circulation and chemistry of the deep ocean in such a way as to change the CO2 content of the atmosphere. Again the evidence cited earlier in this chapter is consistent with this explanation: ice sheets that grew during the early stages of previous interglaciations presumably helped to drive CO2 levels down, but the failure of ice to appear during the current interglaciation should have kept CO2 levels higher.

My proposed answer to the carbon-budget CO2 dilemma is shown in figure 11.4. Emissions of carbon from human activities accounted for most of the observed methane anomaly, but only a fraction of the CO2 anomaly (probably about a third). These "direct" emissions of methane and CO2 caused climate to warm, and the warming suppressed a natural advance of sea ice in the South and prevented new ice sheets from beginning to grow in the North. A natural drop in CO2 was thereby averted, and the absence of this decrease is the "indirect" part of the CO2 anomaly. This explanation, if correct, resolves the impasse over the CO2

budget. Because these indirect mechanisms are also entirely the result of human activities, humans are responsible for the CO2 anomaly, as originally proposed.

In summary, my original hypothesis (the thesis) has already been adjusted in response to the challenges directed at it (the antithesis). The evidence that at first seemed inconsistent with the hypothesis turned out not to be so. In my opinion, the issues are now clearer and the hypothesis stronger because of these challenges.

As I write this chapter (a late addition to the book), I have just submitted a paper to a scientific journal summarizing all of these challenges and responses, and the community at large is still unaware of this work. It may take years for my arguments to persuade the scientific community. One reason for the gradual pace of acceptance is the reluctance on the part of scientists to abandon concepts that they have accepted and lived with for a long time. For decades, scientists have "known" several basic "truths." One is that climate has been warm for a little over 10,000 years, the length of the present interglaciation. Another is that this warmth has occurred for a natural reason: the forces that drive climate into new glaciations at regular cycles have not yet become strong enough to do so. A third "truth" is that human emissions of greenhouse gases became important only a few hundred years ago.

The first of these "truths" really is true. Climate has remained warm, cooling a little only in polar regions. But my hypothesis says that the second and third truths are not true. The warmth of the last several thousand years stems from a colossal coincidence: a natural cooling that was almost completely offset by a human-induced warming tied to greenhouse-gas emissions from agriculture.

A related reason that new ideas are resisted is simply time. At major research universities in the United States, scientists prepare and deliver lectures, and they counsel and advise undergraduate and graduate students. They serve on committees at departmental, university, national, and international levels. They write proposals for funding, and they manage and oversee labs, assistants, and students working on funded grants. They review proposals for funding agencies and papers for scientific journals. Week after week, two endeavors are shoved to the tail end of this long list of priorities: (1) reading and carefully scrutinizing new papers, especially in areas not relevant to their immediate research focus, and (2) thinking— the long, slow process of digesting and weighing enough information to allow new ideas to germinate. In a sense, the very structure of our work lives seems designed to keep us from achieving this most important goal of our research.

So it will take time for this hypothesis to be considered (by those who have the time) and for the evidence to overcome the inertia of "truths" scientists already "know." I feel confident that this acceptance will come.


Disease Enters the Picture

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