Afterword To The Princeton Science Library Edition

Five years have passed since I wrote Plows, Plagues, and Petroleum (first published in 2005), and this new Princeton Science Library edition gives me an opportunity to look back on the way the science covered in the book has evolved. Because parts 1 and 2 provided fundamental background information, little has changed regarding the issues they discussed. Part 5 largely dealt with modern and future climate, and the most noteworthy shift in the last five years has been the development of an even stronger consensus that humans are the primary explanation for the approximately 0.7oC global warming during the last 125 years.

In contrast, the early anthropogenic hypothesis that was covered in parts 3 and 4 remains the subject of an ongoing debate that has generated several relevant new scientific findings. The dozen or more invited lectures I have given on this topic every year are one of several indications that interest in this issue is still at a high level. To date, the scientific community has not yet come to a consensus on whether the increases in carbon dioxide and methane during the last several thousand years were natural or anthropogenic in origin.

Here, I add a new summary updating the last half-decade of research on topics relevant to this continuing debate, with the new findings grouped into six major topics. Because some of these discussions are at times (unavoidably) a bit technical, they may be best suited for those interested in closely tracking the debate.

For the most part, these six topics track developments related to challenges to the anthropogenic hypothesis covered in chapter 11 of the book. By the time that chapter was written, just a year after the hypothesis first appeared, the major criticisms of the hypothesis had already been published. In the intervening five years, these criticisms have remained the same as before, and most of the critics seem to have moved on to other issues. As a result, the research summarized here explores new information or new thinking that responds to those early challenges. In several instances, these developments strengthen the case for the early anthropogenic hypothesis.

INSOLATION AND GREENHOUSE-GAS TRENDS

The early anthropogenic hypothesis had its origin in a simple observation: greenhouse-gas trends during this interglaciation (the Holocene) were different from those in the three preceding ones. During the first 10,000 years of interglacial stages 5, 7, and 9, the gas trends fell, but during the Holocene they fell for half or less of that interval before reversing direction and turning upward—CO2 near 7,000 years ago and CH4 near 5,000 years ago. Those reversals coincided with early farming activities that would be expected to have produced gas emissions: early deforestation that would have emitted CO2, and early rice irrigation, livestock tending, and biomass burning that would have emitted methane. The centerpiece of the early anthropogenic hypothesis was the claim that early agriculture caused those greenhouse-gas increases.

Several studies soon countered the anthropogenic hypothesis with arguments keyed to interglacial stage 11, near 400,000 years ago. That interglaciation was chosen because its insolation trends were more similar to those of today than the three interglaciations I had selected. Because stage 11 orbital eccentricity values were as low as those in the Holocene, the amplitude of the eccentricity-modulated precession cycle at 22,000 years was more like the Holocene than those in interglacial stages 5, 7, and 9.

Several studies also concluded that gas emissions did not fall during the early part of stage 11, but remained high during the time that was most similar to the last few thousand years. These studies concluded that the warmth of the current interglaciation is likely to last another 16,000 years or so. In chapter 11 of the book, I responded that those studies had erred by neglecting to align stage 11 with the current (Holocene) interglaciation based on matching the insolation trends. Instead, they had aligned the early parts of the two preceding deglaciations and then counted forward in "elapsed time." When this method was tested against astronomically calculated insolation trends, it produced a major mismatch: the modern-day insolation minimum was aligned against a stage 11 insolation maximum. In contrast, when the two interglaciations were aligned based on insolation trends, I found that greenhouse-gas concentrations were falling during the interval in stage 11 that is most similar to recent millennia, consistent with the early anthropogenic hypothesis.

Now, several years later, ice-core drilling by the European Project for Ice Coring in Antarctica (EPICA) has recovered records that extend 800,000 years into the past. The fact that these records penetrate several additional interglaciations gives us several more chances to look at early interglacial greenhouse-gas trends. Two of these early interglaciations qualify for this comparison because they were preceded by rapid deglaciations that ended in a sharply defined early interglacial peak.

Methane and CO2 trends during the six previous interglaciations are compared to those during the Holocene (stage 1) in figure A.1. The trends are plotted

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A.1. Trends of methane (A) and carbon dioxide (B) based on analyses of records from Dome C by the European ice-coring consortium EPICA, aligned according to their EDC3 time scale. Filled circles show trends during the current interglaciation. Other symbols show trends during the early parts of six previous interglaciations.

at a time scale compiled by the EPICA group. The first insolation minimum in each previous interglaciation is used as the "zero point" to align the records with the modern-day insolation minimum.

The CO2 and CH4 trends share several features. In the earlier parts of the records, the gas concentrations rose as the preceding glacial climates gave way to interglacial conditions. The concentrations then reached maximum values some 11,000 years or so prior to the "zero-point" alignment, at the start of the interglacial climate regimes. Following this peak, values then began to fall, some quickly, some slowly. For all six previous interglaciations, the gas concentrations continued to drop through the times equivalent to today. But for the Holocene, and only for the Holocene, the trends reversed direction and rose steadily for the last several thousand years. The Holocene CO2 and CH4 trends are thus unique compared to the previous interglaciations.

These trends pose a major problem for those who favor a natural explanation for the Holocene CH4 and CO2 increases. If the late-Holocene gas increases were caused by the natural operation of the climate system, similar trends would be expected to have occurred early in the previous interglaciations, when the insolation forcing was similar. Yet not a single previous interglaciation in figure A.1 shows a CO2 or CH4 rise during the comparable interval.

Interglacial stage 19 is of particular interest because it is a closer insolation analog to the Holocene than any previous interglaciation, including stage 11. Stage 19 has the same low-amplitude precession signal as stage 1, but also a more similar phasing of the separate insolation contributions from the tilt and precession cycles. In contrast, the tilt and precession cycles in stage 11 have a very different alignment from those in the Holocene. The CO2 trends for stage 19 and the Holocene are very similar, starting at nearly identical glacial values of about 185 parts per million (not shown in figure A.1b), rising to early interglacial peaks in the 260—270 parts per million range, and then beginning similar decreases. As noted earlier, the Holocene trend reversed direction nearly 7,000 years ago and rose to a peak of 280-285 parts per million in late preindustrial time. In contrast, the stage 19 trend continued to fall and reached a value of about 245 parts per million at the time equivalent to the present day. This stage 19 value lies at the top end of the proposed natural range of 240-245 parts per million in the early anthropogenic hypothesis and is 35-40 parts per million lower than the approximately 283 parts per million peak concentration reached in the latest Holocene (around AD 1200).

One mechanism by which science operates is falsification. Hypotheses cannot be proved, but they can be disproved by strong evidence. In this case, we have the results from six tests run by the climate system on the group of hypotheses that invoke natural causes of the Holocene greenhouse-gas increases. Because no comparable increases in gas concentrations occur during any of the six previous interglaciations, the natural hypothesis fails twelve successive tests—six for CO2 and six for methane. Based on these failures, a strong case can be made that natural explanations (of any kind) have been falsified.

CliMAte/CARBon Models

Models that attempt to simulate the interactions between the physical parts of the climate system and the major carbon reservoirs have been used to explore the cause of the Holocene CO2 increase. These simulations, which have started from the assumption that the CO2 increase was natural in origin, have simulated a late-Holocene CO2 increase, but often for different reasons, including a decrease in (loss of) terrestrial biomass, an increase in ocean temperatures, construction of coral reefs, and delayed adjustments of ocean chemistry to imbalances imposed in earlier millennia.

All of these climate/carbon simulations face a difficult challenge that, to date, has not been overcome. The problem is that the models need to simulate not just the rising CO2 trend during the Holocene but also the falling CO2 concentrations during previous interglaciations (fig. A.1b). The challenge this poses, summarized schematically in figure A.2, is linked to the assumption used in these models that insolation forcing is the primary driver of CO2 changes in all cases. If the relatively low-amplitude insolation trends during the Holocene drove the observed CO2 increase, then it would seem only logical that the higher-amplitude insolation trends during previous interglaciations would have driven larger CO2 increases at those times. But any simulated CO2 increases (large or small) would be contrary to the trends found in ice cores.

At this point, only one attempt (by Guy Schurgers and colleagues) has been made to simulate CO2 trends during both the Holocene and any previous interglaciation. This climate/carbon model simulated a 7 parts per million CO2 increase during the Holocene (about a third of the observed amount), but it also simulated a 22 parts per million increase during the isotopic stage 5 interglaciation. Yet the CO2 signal early in stage 5 shows no increase but instead a slight downward trend. This modeling attempt failed to meet the challenge.

Other such attempts to simulate previous interglaciations are underway as this is being written. I find it difficult to see how any model that simulates a natural upward CO2 trend during the Holocene can avoid simulating an even larger upward CO2 trend early in the previous interglaciations, contrary to ice-core observations.

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