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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 StatementThe 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.
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Technical Note: Past and future warming – direct comparison on multi-century timescales
Abstract. In 2013, the Intergovernmental Panel on Climate Changeconcluded that Northern Hemisphere temperatures had reached levelsunprecedented in at least 1400 years. The 2021 report now sees global meantemperatures rising to levels unprecedented in over 100 000 years. ThisTechnical Note briefly explains the reasons behind this major change.Namely, the new assessment reflects additional global warming that occurredbetween the two reports and improved paleotemperature reconstructions thatextend further back in time. In addition to past and recent warming, theconclusion also considers multi-century future warming, which therebyenables a direct comparison with paleotemperature reconstructions onmulti-century time scales.
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- Award ID(s):
- 1929460
- PAR ID:
- 10337244
- Date Published:
- Journal Name:
- Climate of the Past
- Volume:
- 18
- Issue:
- 4
- ISSN:
- 1814-9332
- Page Range / eLocation ID:
- 911 to 917
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/cp-18-911-2022
