South American (SA) societies are highly vulnerable to droughts and pluvials, but lack of long-term climate observations severely limits our understanding of the global processes driving climatic variability in the region. The number and quality of SA climate-sensitive tree ring chronologies have significantly increased in recent decades, now providing a robust network of 286 records for characterizing hydroclimate variability since 1400 CE. We combine this network with a self-calibrated Palmer Drought Severity Index (scPDSI) dataset to derive the South American Drought Atlas (SADA) over the continent south of 12°S. The gridded annual reconstruction of austral summer scPDSI is the most spatially complete estimate of SA hydroclimate to date, and well matches past historical dry/wet events. Relating the SADA to the Australia–New Zealand Drought Atlas, sea surface temperatures and atmospheric pressure fields, we determine that the El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) are strongly associated with spatially extended droughts and pluvials over the SADA domain during the past several centuries. SADA also exhibits more extended severe droughts and extreme pluvials since the mid-20th century. Extensive droughts are consistent with the observed 20th-century trend toward positive SAM anomalies concomitant with the weakening of midlatitude Westerlies, while low-level moisture transport intensified by global warming has favored extreme rainfall across the subtropics. The SADA thus provides a long-term context for observed hydroclimatic changes and for 21st-century Intergovernmental Panel on Climate Change (IPCC) projections that suggest SA will experience more frequent/severe droughts and rainfall events as a consequence of increasing greenhouse gas emissions.
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An assessment of early 20th century Antarctic pressure reconstructions using historical observations
While gridded seasonal pressure reconstructions poleward of 60°S extending back to 1905 have been recently completed, their skill has not been assessed prior to 1958. To provide a more thorough evaluation of the skill and performance in the early 20th century, these reconstructions are compared to other gridded datasets, historical data from early Antarctic expeditions, ship records, and temporary bases. Overall, the comparison confirms that the reconstruction uncertainty of 2–4 hPa (evaluated after 1979) over the Southern Ocean is a valid estimate of the reconstruction error in the early 20th century. Over the interior and near the coast of Antarctica, direct comparisons with historical data are challenged by elevation‐based reductions to sea level pressure. In a few cases, a simple linear adjustment of the reconstruction to sea level matches the historical data well, but in other cases, the differences remain greater than 10 hPa. Despite these large errors, comparisons with continuous multi‐season observations demonstrate that aspects of the interannual variability are often still captured, suggesting that the reconstructions have skill representing variations on this timescale, even if it is difficult to determine how well they capture the mean pressure at these higher elevations. Additional comparisons with various 20th‐century reanalysis products demonstrate the value of assimilating the historical observations in these datasets, which acts to substantially reduce the reanalysis ensemble spread, and bring the reanalysis ensemble mean within the reconstruction and observational uncertainty.
more » « less- Award ID(s):
- 1744998
- NSF-PAR ID:
- 10454611
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- International Journal of Climatology
- Volume:
- 41
- Issue:
- S1
- ISSN:
- 0899-8418
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Since the Paris Agreement, climate policy has focused on 1.5° and 2°C maximum global warming targets. However, the agreement lacks a formal definition of the nineteenth-century “pre-industrial” temperature baseline for these targets. If global warming is estimated with respect to the 1850–1900 mean, as in the latest IPCC reports, uncertainty in early instrumental temperatures affects the quantification of total warming. Here, we analyze gridded datasets of instrumental observations together with large-scale climate reconstructions from tree rings to evaluate nineteenth-century baseline temperatures. From 1851 to 1900 warm season temperatures of the Northern Hemisphere extratropical landmasses were 0.20°C cooler than the twentieth-century mean, with a range of 0.14°–0.26°C among three instrumental datasets. At the same time, proxy-based temperature reconstructions show on average 0.39°C colder conditions with a range of 0.19°–0.55°C among six records. We show that anomalously low reconstructed temperatures at high latitudes are underrepresented in the instrumental fields, likely due to the lack of station records in these remote regions. The nineteenth-century offset between warmer instrumental and colder reconstructed temperatures is reduced by one-third if spatial coverage is reduced to those grid cells that overlap between the different temperature fields. The instrumental dataset from Berkeley Earth shows the smallest offset to the reconstructions indicating that additional stations included in this product, due to more liberal data selection, lead to cooler baseline temperatures. The limited early instrumental records and comparison with reconstructions suggest an overestimation of nineteenth-century temperatures, which in turn further reduces the probability of achieving the Paris targets.
Significance Statement The warming targets formulated in the Paris Agreement use a “pre-industrial” temperature baseline that is affected by significant uncertainty in the instrumental temperature record. During the second half of the nineteenth century, much of the continental landmasses were not yet covered by the observational station network and existing records were often subject to inhomogeneities and biases, thus resulting in uncertainty regarding the large-scale mean temperature estimate. By analyzing summer temperature reconstructions from tree-rings for the Northern Hemisphere extratropical land areas, we examine an independent climate archive with a typically broader and more continuous spatial extent during the “pre-industrial” period. Despite the additional uncertainty when using climate reconstructions instead of direct observations, there is evidence for an overestimation of land temperature during the summer season in early instrumental data. Colder early instrumental temperatures would reduce the probability of reaching the Paris targets.
