In a recently published study on spring leaf unfolding appearing in Nature a team of scientists led by Fu et al found that spring leaves on trees in Europe are in fact not unfolding much earlier – as we should expect in these times of “global warming”.
Hat-tip: Der Standard
The abstract excerpt of the paper (emphasis added):
Using long-term in situ observations of leaf unfolding for seven dominant European tree species at 1,245 sites, here we show that the apparent response of leaf unfolding to climate warming (ST, expressed in days advance of leaf unfolding per °C warming) has significantly decreased from 1980 to 2013 in all monitored tree species. Averaged across all species and sites, ST decreased by 40% from 4.0 ± 1.8 days °C−1 during 1980–1994 to 2.3 ± 1.6 days °C−1 during 1999–2013. The declining ST was also simulated by chilling-based phenology models, albeit with a weaker decline (24–30%) than observed in situ. The reduction in ST is likely to be partly attributable to reduced chilling. Nonetheless, other mechanisms may also have a role, such as ‘photoperiod limitation’ mechanisms that may become ultimately limiting when leaf unfolding dates occur too early in the season. Our results provide empirical evidence for a declining ST, but also suggest that the predicted strong winter warming in the future may further reduce ST and therefore result in a slowdown in the advance of tree spring phenology.
Strong winter warming?
So why is leaf unfolding not happening as early as expected? The authors here appear to be baffled and thus are left speculating and offering adventurous explanations. Maybe they ought just take a look at the temperature trends for spring and winter in Europe. If they did so, they would find that winters and early springtime have gotten COLDER over the past couple of decades, and not warmer as the authors seem to believe.
For example an analysis by Kowatsch and Kämpfe using data from Germany’s DWD national weather services for the month of March in Germany over the past 26 years shows a pronounced cooling trend:
March mean temperature for Germany has fallen more than a degree Celsius over the last 27 years.
That would hardly promote earlier leaf unfolding. The same is true for the February trend over the past 22 years:
The 22-year February trend for Germany also shows a marked decline in temperature. Cold weather acts to delay the onset of spring.
Little wonder leaves aren’t unfolding earlier. The Kowatsch and Kämpfe summarize:
Winter and pre-spring have gotten somewhat cooler since the late 1980s, especially February. The temperature trend lines are negative. Therefore the start of spring is currently being delayed and is coming later than the relatively warm 1990s.”
Moreover the European Alps have gotten “considerably colder” over the last 26 years. Also read here. It’s truly stunning that the authors seemingly never bothered to look at the temperature trends.