Regional climate control of glaciers in New Zealand and Europe during the pre-industrial Holocene

Abstract

Mountain glaciers worldwide have undergone net recession over the past century in response to atmospheric warming 1 , but the extent to which this warming reflects natural versus anthropogenic climate change remains uncertain 2,3 . Between about 11,500 years ago and the nineteenth century, progressive atmospheric cooling over the European Alps induced glacier expansion 2,4-6 , culminating with several large-scale advances during the seventeen to nineteenth centuries 3 . However, it is unclear whether this glacier behaviour reflects global or a more regional forcing. Here we reconstruct glacier fluctuations in the Southern Alps of New Zealand for the past 11,000 years using 10 Be exposure ages. We use those fluctuations to estimate the associated temperature variations. On orbital to submillennial timescales, changes in glacier snowlines in New Zealand were linked to regional climate and oceanographic variability and were asynchronous with snowline variations in European glaciers. We attribute this asynchrony to the migration of the intertropical convergence zone. In light of this persistent asynchrony, we suggest that the net glacier recession and atmospheric warming in both regions over the past century is anomalous in the context of earlier Holocene variability and corresponds with anthropogenic emissions of greenhouse gases.The Southern Alps are well positioned to record Holocene climate changes 7 . Situated on New Zealand's South Island in the southwestern Pacific Ocean, this mountain range lies in the zone of interaction between the belts of subtropical highs and subpolar lows that demarcate the westerly wind belt, whereas South Island inflects the subtropical front where subtropical and subantarctic water masses converge (Fig. 1). Located in a highprecipitation maritime environment, glaciers of the Southern Alps are highly sensitive monitors of atmospheric temperature because of rapid ice throughflow from accumulation to melting zones 8 . Therefore, moraine ridges can be used to reconstruct the former geometries of glaciers, enabling estimation of former equilibriumline (snowline) altitudes that reflect atmospheric temperatures. Here we combine moraine geomorphology with high-precision 10 Be surface-exposure dating 7,9 , to reconstruct glacier fluctuations and atmospheric temperatures in the Southern Alps during the Holocene interglaciation. We then compare this reconstruction LETTERS NATURE GEOSCIENCE

The timing and cause of glacial advances in the southern mid-latitudes during the last glacial cycle based on a synthesis of exposure ages from Patagonia and New Zealand

“…Cobb Valley -41 173 12 (9) (Shulmeister et al, 2005) Taramakau -43 171/172 34 (29) (Barrows et al, 2013) Arthur's Pass -43 172 5 (4) (Ivy-Ochs et al, 1999) Waimakariri -43 172 31 (29) (Rother et al, 2015) Rakaia Valley -43/-44 171/172 55 (46) (Shulmeister et al, 2010;Putnam et al, 2013a) Cameron glacier -43 171 10 (10) (Putnam et al, 2012) Franz Josef -43/-44 170 6 (6) (Barrows et al, 2007b) Rangitata Valley -43/-44 171 56 (51) (Rother et al, 2014) Pukaki -44 170/171 169 (159) (Schaefer et al, 2006;Putnam et al, 2010a;Kelley et al, 2014;Doughty et al, 2015;Schaefer et al, 2015) Ohau -44 170 91 (84) (Kaplan et al, 2013;Putnam et al, 2013b) Irishman Stream -44 170 33 (31) Cascade Plateau -44 168 19 (14) (Sutherland et al, 2007) Boundary Stream Tarn -44 170 10 (10) (Putnam et al, 2010b) Total within Last Glacial Cycle 531 (482) 37 Table 2. The timing of culminations in glacial advances identified from relative cumulative 4 probability density functions for New Zealand and Patagonia using the New Zealand 5 production rate of , and using the Patagonian production rate (PPR) of 6 Kaplan et al (2011) for Patagonia.…”

Hydrologic impacts of past shifts of Earth’s thermal equator offer insight into those to be produced by fossil fuel CO 2

“…Although tree line, snowline, sea ice, and ice core records make clear that the northern cap of our planet underwent a warm-cold-warm cycle of amplitude of about 1°C, indications from the Southern Hemisphere are murky. However, recent glacier reconstructions from the New Zealand Southern Alps (31,32) hint that southern glacier snowlines may have been out of step with those in the north. For example, Southern Alps snowlines registered colder than present conditions during Medieval times and gradual warming during the northern Little Ice Age (31,32).…”

“…However, recent glacier reconstructions from the New Zealand Southern Alps (31,32) hint that southern glacier snowlines may have been out of step with those in the north. For example, Southern Alps snowlines registered colder than present conditions during Medieval times and gradual warming during the northern Little Ice Age (31,32). If this is correct, then interhemispheric asynchrony in glacier extent is consistent with Earth's thermal equator having maintained a northern position during Medieval time and a southern position during Little Ice Age time.…”

A review of topographic controls on moraine distribution

“…Glacier-to-glacier variations in total direct snowfall and superimposed ice are largely governed by climate (including topoclimatic factors) (Oerlemans et al, 1998;Dyurgerov and Meier, 2000;Wadham and Nuttall, 2002;Putnam et al, 2012), whilst the redistribution of snow and ice generally occurs through snowblow and avalanching and is controlled by accumulation area topography. Specifically, the volume of redistributed material accumulating on a glacier's surface is determined by the area, slope, and aspect of surrounding topography (Benn and Lehmkuhl, 2000).…”

“…Sedimentological and morphometric analyses of moraines provide information about former glacier flow dynamics, thermal regime, and debris content (amongst other factors) (see Boulton, 1986;Lukas, 2005;Evans, 2009), but moraines are most commonly used simply as indicators of former ice margin positions (e.g., Svendsen et al, 2004;Fredin et al, 2012). On the assumption that fluctuations in ice margins are driven by variations in climate (Oerlemans et al, 1998;Dyurgerov and Meier, 2000;Putnam et al, 2012), this approach potentially allows moraines to be utilised as proxies for palaeoclimate (e.g., Benn and Ballantyne, 2005;Ballantyne et al, 2007). This link between moraines and palaeoclimate is usually made in one of three ways: (i) moraine positions are used to infer variations in the areal extent of glaciers, and to provide a qualitative understanding of palaeoclimatic variation between periods (e.g., Lasalle and Elson, 1975).…”