Arctic warming is receiving much scientific scrutiny and public interest, and your News Feature about Alaska's climate “Too hot to handle” (Nature 425, 338–339; 200310.1038/425338a) gives a generally accurate overview of the reasons why the Arctic deserves our continued attention. However, one ongoing event in your list of direct consequences of recent Arctic warming, the rapid retreat of Columbia Glacier, should not be there.

Columbia Glacier is one of many tidewater glaciers — glaciers that terminate in the ocean but do not float — that fringe Alaska's central and southern coasts. They are well-known to advance and retreat out of synchrony with land-terminating glaciers, and even with each other. Climate appears to control tidewater glacier advance and retreat only on long (millennial) timescales; shorter annual, decadal, and even century-to-century comparisons do not show a close relationship between tidewater glacier activity and climate.

Early European explorations of Alaska at the end of the eighteenth century showed that tidewater glaciers in southern Alaska were fully extended, with termini reaching all the way to the coast, but during the nineteenth and early twentieth centuries these glaciers rapidly retreated, exposing such long and deep fjords as Johns Hopkins Inlet and Disenchantment Bay. Columbia Glacier attained its advanced position between the fifteenth and nineteenth centuries, and only since the early 1980s has it started its retreat.

The end of Columbia Glacier has retreated roughly 13 kilometres from its pre-1980 position, and continues to retreat at roughly half a kilometre per year while discharging some 11 cubic kilometres of ice per year into Prince William Sound. But as Columbia Glacier retreats, Hubbard Glacier, 400 kilometres to the southeast in Disenchantment Bay, has been advancing since 1980.

None of this is to say that tidewater glacier behaviour is irrelevant to present-day concerns about climate change. Tidewater glaciers are affected by climate change on long timescales (primarily through accumulation of mass by snowfall and loss by melt), but are even more strongly affected on short timescales by the ocean, through the influence of heat advection and forces of hydrostatic water pressure. These factors add great complexity to tidewater glacier dynamics and result in nonlinear and irreversible responses to climate, which on long time-scales does indeed control all glacier growth and contraction.

Arctic warming is very real, but the complicated interaction between tide-water glaciers, oceans and climate — and the likelihood that tidewater glacier retreat may not be caused directly by the current climate — should be more widely recognized.