Six passages, 24 questions

For most of the twentieth century the age of European cave paintings was inferred from style. Sequences were built on the assumption that representational technique developed in one direction — from simple outline toward modelled form and the suggestion of depth — so that a panel’s sophistication indicated its place in the chronology. The framework had the virtue of requiring nothing but the images themselves, which was convenient, since pigment could not then be dated directly.

Uranium-thorium dating of the calcite crusts that form over painted surfaces has undone much of this. A crust overlying a painting yields a minimum age for the paint beneath it, and the minimum ages returned from several Iberian caves are older than the stylistic sequence permits. In one chamber, a panel of simple hand stencils and a panel of modelled animals, separated in the stylistic scheme by some twenty millennia, returned crust ages within a few thousand years of each other.

The response has divided. Some argue that the calcite ages are unreliable, since a crust can incorporate carbonate weathered from older rock and so return an age greater than its own. Others accept the dates and abandon the stylistic sequence, holding that technique varied with the purpose of an image and the constraints of the surface rather than with period.

What neither position examines is the assumption beneath the original framework: that a single trajectory of development can be recovered from images made across forty thousand years by populations who left no other record of contact with one another. That assumption was never argued for. It was inherited from the study of classical sculpture, where the sequence is fixed by external evidence, and applied to a body of material for which no such evidence exists.

Between 1980 and 2010, while nearly every glacier system in the world lost mass, a substantial number of glaciers in the Karakoram advanced. The anomaly was noticed early and has been difficult to explain, since the region has warmed at roughly the rate of the surrounding mountains.

The explanation now generally accepted involves the seasonal distribution of precipitation rather than its total. Most Himalayan glaciers are fed by the summer monsoon, which delivers snow at temperatures close to melting, so that warming converts a portion of the accumulation to rain that runs off rather than consolidating. The Karakoram receives the greater part of its precipitation in winter, from westerly systems, at temperatures far below freezing. Warming of two degrees moves summer snowfall across a threshold; it does not move winter snowfall across one.

The account is consistent with the pattern of advance, which is concentrated in the basins with the highest proportion of winter accumulation. It is also consistent with an observed increase in winter precipitation over the period, which some have attributed to expanded irrigation in the plains to the south and the moisture it releases into the atmosphere.

Whether the anomaly will persist is a separate question, and one the explanation answers less well. The threshold that protects winter accumulation is not infinitely far away, and the same models that reproduce the advance under observed conditions produce retreat under three degrees of additional warming. The Karakoram is not exempt from the process affecting the other systems. It is further from the point at which the process becomes visible.

Grave goods have been read by archaeologists in two broadly incompatible ways. The first treats them as a record of the deceased’s position in life: a weapon-rich burial indicates a warrior, an assemblage of tools a craftsman, and the quantity of metal a measure of wealth or rank. On this reading the cemetery is a census, and variation across graves maps onto variation across the living population.

The second holds that grave goods record not the dead but the choices of the living who buried them. A funeral is a performance staged by survivors with interests of their own, and the assemblage may assert a claim — to inheritance, to status, to continuity with a lineage — that the deceased’s actual position did not support. On this reading the cemetery is not a census but an argument, and one made by people whose motives are not recoverable.

The dispute is not symmetrical. The first reading generates propositions that can be tested against other evidence: skeletal wear patterns consistent with particular occupations, isotopic signatures indicating diet or origin, settlement data indicating the scale of the community. Where such tests have been run, the results have been mixed, and in several well-studied cemeteries the weapon burials belong to individuals whose skeletons show no trace of the injuries or asymmetries that combat produces.

The second reading generates fewer testable propositions, which its critics count against it and its proponents regard as an accurate reflection of what the evidence can bear. Both responses have force. A reading that cannot be tested is of limited use; a reading that can be tested only by assuming what it sets out to establish is of less.

The width of a tree’s annual growth ring varies with the conditions of the season in which it formed, and this dependence is what makes ring sequences usable as records of past climate. A chronology assembled from many trees at a site, cross-dated so that the same year is identified in each, can extend several thousand years and yields an annual resolution that few other proxies match.

The dependence is not simple, however, and the complications are of a kind that reconstruction must address rather than assume away. A ring is narrow when the growing season was cold and also when it was dry; at most sites one of the two limits growth more often than the other, and sites are selected accordingly, but the selection is a judgement rather than a measurement. Trees also respond to the preceding season, since a poor year depletes the reserves available for the next, so that a sequence of widths is not a sequence of independent observations.

The most discussed complication is the divergence problem: at a number of high-latitude sites, ring widths tracked measured temperature closely until around 1960 and less closely thereafter, generally underestimating the observed warming. If the relationship between width and temperature has changed within the instrumental period, its stability across the pre-instrumental period cannot be assumed, and the assumption of stability is what licenses the reconstruction.

Proposed explanations for divergence are numerous — drought stress induced by warming, reduced solar radiation at the surface, changes in the seasonal window of growth — and none has been established for more than a subset of sites. The problem is at present a limit on what the method can claim rather than a refutation of it, and treating it as either less or more than that has caused difficulty in both directions.

That migratory birds navigate by the earth’s magnetic field has been established for half a century, but the mechanism has proved unusually resistant to identification. Two candidates have been pursued. The first locates the receptor in deposits of magnetite in the upper beak, which would respond to field intensity in the manner of a compass needle. The second locates it in the retina, where a light-dependent reaction in a class of proteins called cryptochromes produces a pair of radicals whose recombination rate varies with the orientation of the field.

Evidence has accumulated on both sides and has not converged. Birds with anaesthetised beaks retain the ability to orient, which tells against the first candidate; birds deprived of light in the blue part of the spectrum lose it, which tells for the second. But magnetite has been located histologically, and the cryptochrome mechanism, though demonstrated in solution, has not been shown to operate at the field strengths a bird encounters, which are some fifty times weaker than those used in the laboratory demonstrations.

A resolution proposed by several groups is that both are present and serve different functions: the retinal mechanism providing directional information, a compass, and the magnetite providing intensity information, a component of a map. The proposal accommodates the evidence, which is a virtue, and it does so by postulating exactly as many mechanisms as there are bodies of evidence to accommodate, which is not.

The difficulty of the problem is worth noting, because it is not the difficulty of an obscure question. A migrating warbler weighs twelve grams and crosses two continents. Whatever it uses is small enough to fit in that animal and robust enough to work through weather, and neither candidate mechanism has yet been shown to be both.

Common land in early modern England is often described as unregulated, a characterisation that survives largely because the alternative required reading a different class of document. Manorial court rolls, which record the routine business of local administration, show a dense body of restriction: the number of animals a commoner might pasture, the dates between which the meadow was closed, the quantity of turf that might be cut, the obligation to maintain a share of the fencing. Infringements were presented at the court and fined, and the fines were collected.

The regulation was local in a strict sense. It was made by the commoners themselves, sitting as the court, and it varied between manors that were otherwise similar, in ways that track differences in soil, drainage and the balance between arable and pasture. Where the meadow flooded early, the closing date was earlier. Where the waste was extensive relative to the number of holdings, stinting was looser or absent.

This has been taken to show that common-field agriculture was not the tragedy that later writers assumed, and in the narrow sense it does. But the inference sometimes drawn from it — that the system was therefore efficient, and that enclosure was accordingly unnecessary — does not follow from the same evidence. A system may be well governed and still produce less than an alternative would. The court rolls establish that the commons were managed; they say nothing about what the land would have yielded under a different arrangement, and they cannot, since that arrangement did not exist in the manors whose rolls survive.

The question of what enclosure achieved therefore remains open, and it is worth being clear about why. It is not open because the evidence is thin. It is open because the evidence that survives is evidence about how a system worked, and the question is about how it compared with a system that did not exist alongside it.