Hot summers ahead? Multi-decadal spring season warming precedes sudden summer temperature rise in pre-anthropogenic climate change

Abstract

Waning annual seasonality is documented in an up to one-month advance in spring onset since the 1980's in northern latitudes, perturbing ecosystems and socioeconomic performance. Summer temperatures, in contrast, have been rising only recently, indicating an offset in seasonal warming. The limited time span of this observational data makes the asynchronous pattern difficult to quantify, hindering projections of intra-annual dynamics. We explore temporal phase relations of seasonal warming over the Late Pleniglacial/Bølling and the Younger Dryas/Holocene climate transitions that preceded present anthropogenic warming. We determine past spring onset and thermal properties from dwarf birch paleo-phenology. Reconstructed spring warming led maximum summer warming by about a century during both transitions. Long-term reconstruction of intra-annual temperature regimes provides the perspective required for seasonal response analysis. Our results document that multidecadal spring season warming precedes sudden summer temperature rise also during natural climate change. The rapidity of present seasonality changes, however, is unprecedented.

Effects of experimental warming on Betula nana epidermal cell growth tested over its maximum climatological growth range

“…The experiments in Finland and Poland consistently show the expected response of higher UI in the warming treatment compared to the control treatment in both individual years studied (Fig 4). This temperature response is in line with results from previous timeseries data for B. nana collected in Kevo, Finland [12,17] and studies of B. pubescens and mountain birch hybrids over a latitudinal temperature gradient in Scandinavia that both document the strong positive correlation between GDD and UI [15]. Further experimental evidence of the generality of the observed temperature response comes from climate chamber trials, where B. pendula UI values increased significantly with increasing chamber temperatures of 12˚C, 20˚C and 30˚C in multiple weeks treatments [43].…”

“…This temperature response is in line with results from previous time-series data for B . nana collected in Kevo, Finland [ 12 , 17 ] and studies of B . pubescens and mountain birch hybrids over a latitudinal temperature gradient in Scandinavia that both document the strong positive correlation between GDD and UI [ 15 ].…”

“…Analysis of epidermal cell properties is commonly done by microscopic analysis of the cuticle, which enables a detailed determination of cell size and shape [12,15]. A very indicative feature of lateral epidermal cell ontogeny is the size of epidermal cells and the degree of sinuosity of the epidermal cell walls, whole relation is quantified as the undulation index (UI) [12,[15][16][17]. This microphenological trait is closely related to the prevailing air temperature during the growing season, as has been shown for dwarf birch, Betula nana (L.) in long-term single-site studies [12] and for downy birch, B. pubescens (Ehrh.)…”

“…The final stage of development reached thus depends on the length and warmth of the available growth period: long and warm seasons lead to large and highly undulated epidermal cells, while short and cold seasons suppress full maturation [12]. The effect of GDD on UI was originally quantified as a 'paleothermometer' where UI of (sub-) fossil B. nana epidermis material from sedimentary archives provided growing season temperature reconstructions for episodes of past climate change [12,17,19]. Apart from GDD influence on UI, differences in light availability has also been documented to affect UI [20][21][22][23].…”

“…The effect of GDD on UI was originally quantified as a ‘paleothermometer’ where UI of (sub-) fossil B . nana epidermis material from sedimentary archives provided growing season temperature reconstructions for episodes of past climate change [ 12 , 17 , 19 ]. Apart from GDD influence on UI, differences in light availability has also been documented to affect UI [ 20 – 23 ].…”