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Najcieplejszy klimat od około 100tys lat

13.08.23, 19:25
The upcoming global warming will be driven by continued emissions of greenhouse gases and will be exacerbated regionally as slow-moving components of the climate system continue to react to the greenhouse gases already in the atmosphere for centuries to come (Fox-Kemper et al., 2021). In AR6, climate model projections out to 2300 use new emissions scenario extensions (Meinshausen et al., 2020) and an updated emissions-driven emulator, which is calibrated to the extensively modeled 21st century climate (Lee et al., 2021). These projections show that, even under a low-emissions scenario,3 global temperature of at least 1 ∘C warmer than the late 1800s is nearly certain to persist for centuries (Fig. 1). Even for the low-emissions scenario (SSP1-2.6), the average temperature over the 400-year-long period from 1900 to 2300, which is a temporal resolution on par with proxy records, is estimated at 1.2 ∘C [0.9, 1.7 ∘C] warmer than 1850–1900 (mean [5 %, 95 % range]) (Lee et al., 2021). Assuming CO2 emissions reach net zero, climate models indicate that global warming might still further increase or decrease during subsequent centuries (MacDougall et al., 2020). In contrast, under a high-emissions scenario (SSP3-7.0), the 400-year average temperature is estimated at 4.1 ∘C [2.1, 4.7 ∘C] relative to 1850–1900.

This multi-century, approximately 1.2 to 4.1 ∘C level of global warming can be reasonably compared with global temperature over a similar time horizon: that is, the warmest multi-century interval of the Holocene (Fig. 1).AR6 determined, with medium confidence, that peak Holocene global temperature was between 0.2 and 1 ∘C warmer than the late 1800s (Gulev et al., 2021). This assessed temperature range was based on several published studies of proxy-based temperature reconstructions. These include a global compilation of quality-controlled marine and terrestrial multi-proxy records (Temp12k, Kaufman et al., 2020a), which was used to generate a multi-method ensemble global temperature reconstruction (Kaufman et al., 2020b). The median reconstruction shows that the warmest 200-year-long interval prior to industrialization took place around 6500 years ago, when global temperature is estimated to have been 0.7 ∘C [0.3, 1.8 ∘C] warmer than the 19th century. The Temp12k reconstruction is similar to the only other available Holocene global temperature reconstruction based on both terrestrial and marine proxy data (Marcott et al., 2013). That study used a much smaller dataset and different procedures to estimate maximum warmth of 0.8 ± 0.3 ∘C (2σ) relative to 1850–1900 at around 7 ka (adjusted by adding 0.3 ∘C to account for different reference periods). In contrast, AR5-generation reconstructions based on selected proxy types indicate that mid-Holocene land and ocean surface temperatures were indistinguishable from preindustrial climate (Harrison et al., 2015). This and other evidence for relatively low mid-Holocene global temperatures led to the AR6-assessed value that favors the lower estimates within the Temp12k ensemble reconstruction (Fig. 1b).

Climate models generally simulate Holocene global temperature that correlates with greenhouse gas concentrations. More specifically, global temperature estimated from 16 climate models programmed to simulate climate for the mid-Holocene, 6000 years ago, averages around −0.3 ± 0.1 ∘C (±1σ) colder than preindustrial control runs (Brierley et al., 2020). A recent global temperature reconstruction based on data assimilation, which was published after AR6, blends climate model simulations with sea-surface temperature proxies and shows essentially no multi-millennial change in global temperature following 8000 years ago and prior to industrialization (Osman et al., 2021a). While the discrepancy between the observational and model results is the subject of ongoing research, the lower temperatures simulated by models are consistent with the conclusion that a global warming level of at least 1 ∘C is unprecedented during the preindustrial Holocene.

We need to look much further back for a time when temperature might have exceeded the 1 ∘C global warming level (Fig. 1). There is no evidence that global temperature higher than Holocene occurred during the last major ice age (MISs 4–2; MIS: marine isotope stage). For the last interglacial (MIS 5) around 125 000 years ago, AR6 (Gulev et al., 2021) has medium confidence that global temperature averaged over multiple centuries was between 0.5 and 1.5 ∘C higher than the late 1800s, which overlaps with the 1 ∘C warming level. This led to the AR6 statement that the last decade was more likely than not warmer than any multi-century period after the last interglacial (Gulev et al., 2021). The statement was simplified in Fig. SPM.1 of the Summary for Policy Makers (IPCC, 2021), which stated that global temperature during the mid-Holocene was the warmest in at least the last 100 000 years and that the last interglacial, around 125 000 years ago, is the next most recent candidate for a period of higher temperature. No attempt was made in AR6 to assess temperatures of other warm substages of MIS 5.

W skrócie obecny przyrost globalnej temperatury o co najmniej 1∘C jest niemal pewny, więc możemy porównać średnią z 1900-2300r z porównywalnymi pod względem długości okresami wcześniejszymi, czyli z danymi na podstawie rekonstrukcji czy symulacji.

Średnia dla 1900 - 2300 rok
1.2 ∘C [0.9, 1.7 ∘C], SSP1-2.6, Lee et al., 2021

Dla środkowego holocenu

Rekonstrukcje proxy
0.7 ∘C [0.3, 1.8 ∘C], Kaufman et al., 2020b
0.8 ± 0.3 ∘C (2σ), Marcott et al., 2013
0.2 -1 ∘C, Gulev et al., 2021

−0.3 ± 0.1 ∘C (±1σ) Brierley et al., 2020

Asymilacje nie pokazują znacznego ocieplenia środkowego holocenu, tj. między innymi cytowany Osman et al., 2021a
Przed holocenem był MIS 4–2, więc kolejny kandydat do najcieplejszego wielowiekowego okresu jest w MIS 5, czyli 125 000lat wstecz. Ilustracja ładnie to wszystko obrazuje.
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