Free Field Notes From a Catastrophe: Man, Nature, and Climate Change by Elizabeth Kolbert Page A

had shifted their range northward in recent decades, while only one had shifted south.
    After a while, the sun emerged, and we went back outside. Thomas’s wolfhound, Rex, a dog the size of a small horse, trailed behind us, panting heavily. Within about five minutes, Thomas had identified a Meadow Brown ( Maniola jurtina ), a Small Tortoiseshell ( Aglais urticae ), and a Green-veined White ( Pieris napi ), all species that have been flitting around Yorkshire since butterfly record-keeping began. Thomas also spotted a Gatekeeper ( Pyronia tithonus ) and a Small Skipper ( Thymelicus sylvestris ), which until recently had been confined to a region well south of where we were standing. “So far, two out of the five species of butterflies that we’ve seen are northward invaders,” he told me. “Sometime within the last thirty years they have spread into this area.” A few minutes later, he pointed out another invader sunning itself in the grass—a Polygonia c-album . With its wings closed, the Comma was a dull, dead-leaf brown, but with them open, it was a brilliant orange.
    That life on earth changes with the climate has been assumed to be the case for a long time, indeed for very nearly as long as the climate has been known to be capable of changing. Louis Agassiz published Études sur les glaciers , the work in which he laid out his theory of the ice ages, in 1840. By 1859, Charles Darwin had incorporated Agassiz’s theory into his own theory of evolution. Toward the end of On the Origin of Species , in a chapter titled “Geographical Distribution,” Darwin describes the vast migrations that he supposes the advance and retreat of the glaciers must have necessitated:

    As the cold came on, and as each more southern zone became fitted for arctic beings and ill-fitted for their former more temperate inhabitants, the latter would be supplanted and arctic productions would take their places. The inhabitants of the more temperate regions would at the same time travel southward … As the warmth returned, the arctic forms would retreat northward, closely followed up in their retreat by the productions of the more temperate regions. And as the snow melted from the bases of the mountains, the arctic forms would seize on the cleared and thawed ground, always ascending higher and higher as the warmth increased, whilst their brethren were pursuing their northern journey.
    For Darwin and his contemporaries such a narrative was necessarily speculative. Much as the existence of ice ages had had to be inferred from the signs they left behind—erratics, moraines, and striated bedrock—so, too, the succession and redistribution of species on Earth could only be reconstructed from fragmentary traces: scattered bones, fossilized insect casings, ancient pollen deposits. Even as paleontologists and paleobotanists found more and more evidence of how species had responded to climate change in the past, it was taken for granted that the process was not something that could be observed in real time, an assumption that has now been proven false.
    Almost anywhere you go in the world today, except perhaps for the urban areas where most of us live, it is possible to observe biological changes comparable to the northern expansion of the Comma. A recent study of common frogs living near Ithaca, New York, for example, found that four out of six species were calling—which is to say, mating—at least ten days earlier than they used to, while at the Arnold Arboretum, in Boston, the date of peak blooming for spring-flowering shrubs has advanced, on average, by eight days. In Costa Rica, birds like the keel-billed toucan ( Ramphastos sulfuratus ), once confined to the lowlands, have started to nest on mountain slopes; in the Alps, plants like purple saxifrage ( Saxifraga oppositifolia ) and Austrian draba ( Draba fladnizensis ) have been creeping up toward the summits; and in the Sierra Nevada of California, the average Edith’s Checkerspot butterfly