Figure 11. Median time of emergence (TOE) for TXx and TNn based
on SNR thresholds under SSP5-8.5 and RCP8.5. (a) TOE for TXx under
SSP5-8.5 when SNR > 1; (b) TOE for TXx under SSP5-8.5 when
SNR > 2; (c, d) same as (a) and (b), but for TNn; (e-h)
same as (a-d), but for RCP8.5.
Over some central and tropical parts of Australia, the multi-model
median TOE in TXx for SNR > 1 can occur as early as the
second decade of this century (2010-2020). Generally, the signal emerges
earlier over northwestern region than the southeast for both thresholds
(Fig. 11a, b, e and f), in which the signal emerges in 2020s for SNR
> 1 and 2040s for SNR > 2, as there indicate
relative smaller noise and larger signal (Figs. S17 and S21). Over the
southeast regions, the TOE occurs within 2030-2050 for SNR
> 1. In contrast, for TNn, the signal emerges from the
noise in 2020s over Australia (SNR > 1; Fig. 11c and g);
while for SNR > 2, the TOE is within the fifth decade
(2040-2050) over western and central regions (Fig. 11d and h). Compared
to RCP8.5, the multi-model medians of TOE for TXx and TNn in CMIP6 show
earlier TOE over more regions based on the threshold SNR >
2, implying the larger median SNR in the middle of this century as shown
in Figs. 8 and 9. However, the uncertainty surrounding these TOE
estimates remains large (Figs. 8 and 9). For example, for SNR = 2, the
range (inter-model spread) of TOE for TXx over AUS can be from 2010s to
2060s (Fig. 8a). For lower scenarios, the multi-model medians of TOE
commonly occur later and over smaller regions than the stronger
pathways. For example, TOE (SNR > 1) for TNn under
SSP2-4.5&RCP4.5 usually can be 10 years later over some southeast
regions (Fig. S28g and k) than that shown in SSP5-8.5&RCP4.5 (Fig. 11c
and g), as the signal is lower compared to that in higher pathways.
The analysis on SNR and TOE has useful implications for Australia. Under
the highest-emission scenarios SSP5-8.5&RCP8.5, in which the medians of
the signal for TXx are comparable, the early emergence over northwest
Australia suggests that there is less time for stakeholders and policy
makers to implement effective measures, compared to southeast Australia.
In contrast, if under lower scenarios, the TOE for TXx can be postponed,
especially for southeast regions which exhibit larger variability for
TXx in the extratropics. However, the adaptation policy may change for
different extremes, even under same future pathways. For TNn, the TOE
(SNR > 1) can occur over most regions even under
lower-emission scenarios; while the “unfamiliar” climate (SNR
> 2) can be largely postponed if taking a more sustainable
pathway (lower emission). It is also noted that the large uncertainty in
the estimate of SNR and TOE highlights further challenges for
stakeholders and policy makers.
3.3 Large Ensembles in CMIP6
Previous research has demonstrated the model uncertainty in estimating
the effects of internal variability on the TXx and TNn trends, shown in
LEs during 1950-2014 over Australian regions (Deng et al., 2021).
Therefore, how internal variability influences the projected changes and
TOE/SNR (including signal and the noise) needs further investigation. In
Fig. 12, which represents the boxplots of projected changes in TXx and
TNn for CanESM5-LE and MIROC6-LE over Australian regions under SSP5-8.5
and SSP1-2.6, model uncertainty for representing internal variability
still exists, and the relative magnitudes of the spreads for projected
changes resemble the results in Figs. 12 and 13 in Deng et al. (2021).
The projected changes in TXx for MIROC6-LE span larger ranges than
CanESM5-LE by a factor of ~3 or more over the regions,
which can be larger than that in Fig. 3a. Moreover, there exhibit larger
ranges of the projected changes for TXx over SEA, MEA, and SSA, and
relatively narrower spreads over TA for CanESM5-LE and SWA for
MIROC6-LE. For TNn, the relative magnitude for the two LEs are
comparable over the regions. The different effects of internal
variability for different LEs and regions complicate the assessment of
the uncertainty on projected changes.