In an article released in Nature Geoscience recently, authors Putnam et al. present their latest work on glacial retreat in New Zealand, and how this can inform our broader understanding of global climate fluctuations over the past ~11,500 years. Putnam, a stellar up-and-coming glaciologist and paleoclimatologist, and colleagues are able to demonstrate that previously the Northern and Southern hemisphere glaciers advanced and retreated out of phase with one another; a phenomenon that is to be expected when thinking about orbital cycles where one hemisphere tilts closer to the sun at the expense of the other. HOWEVER, this new study demonstrated that widespread glacial retreat is no longer a regional event, with concurrent, rapid melting observed in both hemispheres. The most logical cause? The rapid, and global, warming unique to the planet since the industrial revolution that overwhelms regional climate and oceanographic variability.
The team used moraine geomorphology and beryllium-10 surface exposure dating to date rock faces that were once at the glaciers terminal edge, thereby tracking their advance and retreat. A geometrical method was also used to reconstruct the height of the equilibrium line, or snowline, that demarcates where the a glacier transitions from gaining to losing ice. Both of these parameters are sensitive to atmospheric temperature and can therefore be used to track regional climate change. The study’s results provide a sucker punch to climate skeptics who point to the Medieval Warm Period as an analogue to today’s warming climate. While there was warming in Europe, the Southern Hemisphere cooled, as demonstrated by advancing glaciers. Along with findings from previous studies, the authors point toward the movement of the Inter Tropical Convergence Zone (ITCZ) and strength of the East Australian Current as responsible for fluctuations in the Southern Hemisphere glacial behaviour on submillennial timescales.
The article is an excellent read. The science is fascinating, the piece beautifully written and the thinking elegant. In the New York Times Aaron says “The idea that the modern global warming is part of a natural cycle is a valid, testable hypothesis, and merits attention,” he said. “Thus, we approached the idea as a testable hypothesis.” Yes, yes you did and you were very successful at demonstrating the current rate of melting is not natural.
I happen to be terribly fond of New Zealand glaciers, as a wonderful colleague of mine put it, nature worked for 10 million years to provide the backdrop for my wedding (see the Isobel Glacier below). And speaking of weddings, I was very privileged to have attended Mr Putnam’s wedding just a few weeks ago on the coast of Maine in the U.S. I do believe he proposed to his girlfriend, and my very dear friend, Kat (who is the other turkey diving with me in Puerto Rico) in the New Zealand Alps when on field work. Who says you can’t mix business with pleasure?