Regional and global forcing of glacier retreat during the last deglaciation

Abstract

The ongoing retreat of glaciers globally is one of the clearest manifestations of recent global warming associated with rising greenhouse gas concentrations. By comparison, the importance of greenhouse gases in driving glacier retreat during the most recent deglaciation, the last major interval of global warming, is unclear due to uncertainties in the timing of retreat around the world. Here we use recently improved cosmogenic-nuclide production-rate calibrations to recalculate the ages of 1,116 glacial boulders from 195 moraines that provide broad coverage of retreat in mid-to-low-latitude regions. This revised history, in conjunction with transient climate model simulations, suggests that while several regional-scale forcings, including insolation, ice sheets and ocean circulation, modulated glacier responses regionally, they are unable to account for global-scale retreat, which is most likely related to increasing greenhouse gas concentrations.

Figure 1. Study area locations.

The locations of the 195 moraines and 30 glaciated bedrock surfaces.

Figure 2. Glacier fluctuations and climate forcings.

(a and b) Normalized moraine elevations and positions for the past 30 kyr. Closed symbols represent the mean of boulder surface-exposure ages on a moraine, and error bars (1σ) give the s.d. of the boulder ages plus the production-rate uncertainty, added in quadrature. Moraines are grouped by region. Open-colored symbols represent individual bedrock-exposure ages and 1σ external (analytical plus production rate) uncertainties. Red error bars in the Holocene at a length of 0 are radiocarbon and dendrochronologic ages from the Alps, and the blue error bar at 14.7 ka at length of 0 is a radiocarbon age for final deglaciation of Mauna Kea, Hawaii (Supplementary Data 1). (c) Proxy global temperature reconstruction (red). (d) Atmospheric CO₂ from ice cores (blue and purple). (e) Global sea level (green). (f) Local summer insolation for 45°N (June-July-August, blue) and 45°S (December-January-February, red) and mean annual insolation for the equator (yellow). Gray vertical band highlights the interval of deglacial CO₂ rise.

Figure 3. Modeled climate and reconstructed glacier fluctuations.

Modeled temperatures from the single-forcing (colored lines) and ALL (black line) simulations, as well as normalized moraine positions (black dots) for (b) Hawaii, (c) tropical South America, (d) subtropical South America, (e) Australia and New Zealand, (f) Patagonia, (g) the western United States, and (h) the Alps. Error bars (1σ) give the s.d. of the boulder ages plus the production-rate uncertainty, added in quadrature. Local summer temperatures are shown for mid-latitude sites (June-July-August in Northern Hemisphere, December-January-February in Southern Hemisphere) and annual mean temperatures are shown for low-latitude sites. Modeled precipitation from the ALL simulation (gray dashed line) has been scaled to temperature as −25%=1 °C (ref. 1), and is shown for local winter at the mid-latitude sites and mean annual at the low-latitude sites. All model time series are 500-year moving averages and given as anomalies from 19 ka. The first panel (a) shows large-scale stacks for each simulation derived by averaging the regions together; the time series for each region were first normalized by the variance of their ALL simulation temperatures to give them equal weight in the stacks. y-axis on left of each graph is temperature and y-axis on right is normalized moraine position, which have been scaled to align maximum (1) and minimum (0) glacier extent with simulated Last Glacial Maximum and modern temperatures, respectively. See Supplementary Fig. 9 for analogous figure showing normalized moraine elevations.

Figure 4. Ice volume-CO₂ gain function.

This plot shows the ratio of spectral power of an ice-volume reconstruction to the ice-core CO₂ record over the past 800 kyr after normalizing each series to mean zero, unit variance. The periods of eccentricity (100 kyr), obliquity (41 kyr) and precession (19 and 23 kyr) are shown.