The bias and signal attenuation present in conventional pollen-based climate reconstructions as assessed by early climate data from Minnesota, USA

Abstract

The inference of past temperatures from a sedimentary pollen record depends upon the stationarity of the pollen-climate relationship. However, humans have altered vegetation independent of changes to climate, and consequently modern pollen deposition is a product of landscape disturbance and climate, which is different from the dominance of climate-derived processes in the past. This problem could cause serious signal distortion in pollen-based reconstructions. In the north-central United States, direct human impacts have strongly altered the modern vegetation and hence the pollen rain since Euro-American settlement in the mid-19th century. Using instrumental temperature data from the early 1800 s from Fort Snelling (Minnesota), we assessed the signal distortion and bias introduced by using the conventional method of inferring temperature from pollen assemblages in comparison to a calibration set from pre-settlement pollen assemblages and the earliest instrumental climate data. The early post-settlement calibration set provides more accurate reconstructions of the 19th century instrumental record, with less bias, than the modern set does. When both modern and pre-industrial calibration sets are used to reconstruct past temperatures since AD 1116 from pollen counts from a varve-dated record from Lake Mina, Minnesota, the conventional inference method produces significant low-frequency (centennial-scale) signal attenuation and positive bias of 0.8-1.7 °C, resulting in an overestimation of Little Ice Age temperature and likely an underestimation of the extent and rate of anthropogenic warming in this region. However, high-frequency (annual-scale) signal attenuation exists with both methods. Hence, we conclude that any past pollen spectra from before Euro-American settlement in this region should be interpreted using a pre-Euro-American settlement pollen set, paired to the earliest instrumental climate records. It remains to be explored how widespread this problem is when conventional pollen-based inference methods are used, and consequently how seriously regional manifestations of global warming have been underestimated with traditional pollen-based techniques.

Figure 1. Fort Snelling (Minneapolis-St. Paul), Minnesota, historical record of mean annual temperature (°C) for AD 1820–2012.

Red ordinary-least-squares regression line shows significant increase (t = 7.45, d.f. = 191, p-value = 3.5 × 10^–6).

Figure 2. Comparison of the monthly climate normals used in the two calibration sets.

The differences and standard deviations between the monthly mean temperatures of AD 1961–1990 and those of AD 1895–1924 are shown. Averages calculated over the 133 sites. An (*) denotes a significant change at the 0.05 level as assessed by a paired t-test with 132 degrees of freedom.

Figure 3. Minnesota isopoll maps from the pre-settlement AD 1870 and modern core-top samples.

The nine taxa showing the greatest changes in relative abundance (%) after Euro-American settlement according to St. Jacques et al. [5] are presented. Interpolation based on natural neighbor interpolation (Matlab 2012).

Figure 4. Lake Mina, Minnesota, pollen-inferred climate reconstructions using the pre-settlement 1870 and the modern calibration sets.

(a) unsmoothed February mean temperatures for AD 1820–2002; (b) February mean temperatures for AD 1116–2002, smoothed by a 5-point moving average; (c) unsmoothed May mean temperatures for AD 1820–2002; and (d) May mean temperatures for AD 1116–2002, smoothed by a 5-point moving average. Also shown in (a) and (c) are the extended instrumental February and May mean temperatures for nearby Alexandria, Minnesota, AD 1820–2002, cumulatively averaged as the pollen samples were according to the varve counts for comparison. The black bars denote the AD 1961–1990 monthly climate normals.

Figure 5. Analog analysis.

Diagram showing minimum squared chord distances between the Lake Mina pollen samples (AD 1116–2002) and their nearest neighbors in the pre-settlement 1870 pollen calibration set and the modern pollen calibration set.

Figure 6. Residuals of the reconstructed 1870 pollen sets from hindcasting and cross-validation.

The reconstructions from hindcasting (i.e., the application of the modern calibration set to the pre-settlement 1870 pollen samples for each of the 133 sites) minus their paired observed AD 1895–1924 temperatures (°C) for (a) February and (b) May are shown. Also shown are the reconstructions from leave-one-out cross-validation (i.e., the application of the pre-settlement 1870 calibration set to the 1870 pollen samples for each of the 133 sites) minus their paired observed AD 1895–1924 temperatures (°C) for (c) February and (d) May.