Southern Hemisphere subtropical drying as a transient response to warming

Climate projections1–3 and observations over recent decades4,5 indicate that precipitation in subtropical latitudes declines in response to anthropogenic warming, with significant implications for food production and population sustainability. However, this conclusion is derived from emissions scenarios with rapidly increasing radiative forcing to the year 21001,2, which may represent very different conditions from both past and future ‘equilibrium’ warmer climates. Here, we examine multi-century future climate simulations and show that in the Southern Hemisphere subtropical drying ceases soon after global temperature stabilizes. Our results suggest that twenty-first century Southern Hemisphere subtropical drying is not a feature of warm climates per se, but is primarily a response to rapidly rising forcing and global temperatures, as tropical sea-surface temperatures rise more than southern subtropical sea-surface temperatures under transient warming. Subtropical drying may therefore be a temporary response to rapid warming: as greenhouse gas concentrations and global temperatures stabilize, Southern Hemisphere subtropical regions may experience positive precipitation trends

Studying climate stabilization at Paris Agreement levels

Following the Paris Agreement, there have been hundreds of studies researching the impacts of 1.5 °C and 2 °C of global warming above pre-industrial levels. Multiple methods have been developed to address the question of how regional climate change and impacts differ between global warming levels (GWLs), including pattern scaling1,2, time-slicing of existing climate projections3, single coupled-model experiments4 and multi-model atmosphere-only experiments5. The problem is that, while the Paris Agreement is not explicit, the intention is that global temperatures will be stabilized well below the 2 °C or, preferably, the 1.5 °C, GWL and will not continue to increase6, but the methods described above are based on transient projections in one form or another (Table 1) that do not reflect stabilized climates. This issue has come to the fore with the use of a time-sampling approach in transient simulations generating GWL-based climate projections in the Sixth Assessment Report of Working Group 1 (AR6 WG1) of the IPCC

Pollen analysis of Australian honey

<div><p>Pollen analysis is widely used to verify the geographic origin of honeys, but has never been employed in Australia. In this study, we analysed the pollen content of 173 unblended honey samples sourced from most of the commercial honey producing regions in southern Australia. Southern Australian vegetation is dominated by <i>Eucalyptus</i> (Myrtaceae) forests and, as expected, most Australian honeys are palynologically dominated by <i>Eucalyptus</i>, while other important components include Myrtaceae taxa such as <i>Corymbia</i>/<i>Angophora</i> and the tribe Leptospermeae; plus Brassicaceae, <i>Echium</i>, <i>Macadamia</i>, and <i>Acacia</i>. An important feature of the honeys is the number of Myrtaceae pollen morphotypes per sample, which is generally high (mean = 4.6) compared to honeys produced outside of Australia, including <i>Eucalyptus</i> honeys produced in the Mediterranean region, and honeys produced in South America, which has its own rich indigenous Myrtaceae flora. In the latter regions, the number of Myrtaceae morphotypes is apparently generally ≤2. A high number of Myrtaceae morphotypes may be a feasible criterion for authenticating the origin of Australian honeys, since most Australian honey is produced by honey bees mainly working indigenous floral resources. Myrtaceae morphotype diversity is a convenient melissopalynological measure that could be applied even where detailed knowledge of the pollen morphology of the many component genera and species is absent. Palynological criteria developed in Europe for authenticating <i>Eucalyptus</i> honeys should not be relied upon for Australian honeys, since those criteria are not based on samples of Australian honey.</p></div

Principal component biplots of pollen types.

<p>Upper panel, numerically dominant types, 42% of variance explained by the first two axes; lower panel, numerically minor types, 31% of variance explained by the first two axes.</p

Myrtaceae pollen diversity observed in the honeys.

<p>36 distinct Myrtaceae morphotypes observed in the 173 honey samples, many unassigned below family level. Morphotypes 1–12, parasyncolporate grains with more or less well developed pore thickenings, broadly consistent with <i>Eucalyptus</i> species; morphotype 15, large grains, weakly oblate, approaching cubic or spheroidal shape, consistent with some <i>Corymbia</i>/<i>Angophora</i> species; morphotype 16, with short colpi not reaching the polar region, consistent with some members of the VACDH clade [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0197545#pone.0197545.ref052" target="_blank">52</a>]; morphotype 29, regulate grains possibly consistent with tribe Myrteae; morphotypes 30–32 and 34–35, consistent with tribes Leptospermeae and Chamelaucieae; morphotype 33, very small grain possibly consistent with <i>Tristania</i>. For brief descriptions, see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0197545#pone.0197545.s001" target="_blank">S1 Appendix</a>.</p

East-west comparison of Myrtaceae and Proteaceae morphotypes diversity.

<p>Histograms of number of Myrtaceae (green) and Proteaceae (blue) pollen morphotypes, for eastern (<b>a,b</b>) and southwestern (<b>c,d</b>) Australian honey samples.</p

Numbers of pollen types and pollen concentrations.

<p><b>a</b>, Histograms of total number of pollen types per sample, and <b>b</b>, pollen concentration, on a log₁₀ scale, with Maurizio’s groups I-V [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0197545#pone.0197545.ref007" target="_blank">7</a>] delineated.</p

Location of the 173 honey samples within Australia.

<p>The data used to produce this figure can be found in the <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0197545#pone.0197545.s003" target="_blank">S1 Table</a>.</p

Percentage pollen diagram illustrating the pollen content of the honey samples.

<p>Samples are arranged in a sequence determined by cluster analysis. Coloured bar indicates whether each sample was produced in eastern (purple) or southwestern (green) regions of Australia, though note that region was not used as a criterion for the cluster analysis; <b>a,</b> the numerically most important taxa; <b>b</b>, minor taxa, arranged according to whether probable source species are primarily indigenous to Australia or primarily introduced/agricultural species.</p