Species migration and global warming

Jim Hansen in the NY Review of Books:

Studies of more than one thousand species of plants, animals, and insects, including butterfly ranges charted by members of the public, found an average migration rate toward the North and South Poles of about four miles per decade in the second half of the twentieth century. That is not fast enough. During the past thirty years the lines marking the regions in which a given average temperature prevails ("isotherms") have been moving poleward at a rate of about thirty-five miles per decade. That is the size of a county in Iowa. Each decade the range of a given species is moving one row of counties northward.

As long as the total movement of isotherms toward the poles is much smaller than the size of the habitat, or the ranges in which the animals live, the effect on species is limited. But now the movement is inexorably toward the poles and totals more than a hundred miles over the past several decades. If emissions of greenhouse gases continue to increase at the current rate—"business as usual"—then the rate of isotherm movement will double in this century to at least seventy miles per decade. If we continue on this path, a large fraction of the species on Earth, as many as 50 percent or more, may become extinct.