Changes in the annual life cycles of plants and animals are a good footprint of climate change in middle latitudes. But it is a footprint that is less easy to detecet than melting sea ice or glacier retreat.
Eye-observations by millions of gardeners and nature lovers suggest that plants green and flower earlier in spring, and that leaves begin to colour and fall off later in autumn, than they did just a few decades ago. So where will we end then? Will pollen soon be in the air all year round – a nightmare for allergic persons?
Scientists such as Annette Wenzel, an eco-climatologist at the Technical University of Munich, use remote sensing technologies, models, and in-situ observations to study the response of plants to warming temperatures. But the corelation between warming and ‘phenology’ is less clear than one might expect.
One reason, says Wenzel, is that the European phenology network of around 8,000 observation sites is a rather unevenly distributed affair. The vast majority of sites are located in Germany and its neighboring countries, whereas the whole of Scandinavia has just a handful of sites, and Italy not a single one. To make matters worse, the species composition differs from site to site, making it difficult for scientists to define a Euroean-wide ‘green-up’ index.
Even extreme climatic events don’t provide a clear picture. Warm spells have become significantly more frequent in Europe during all seasons, and cold spells less fequent, from 1990 onward than prior to 1990. Wenzel has looked at how recent warm and cold spells affected the flowering of cherry and apple trees, and the sowing and harvest time of winter wheat. Fruit trees and wheat farmers do respond to climate anomalies, she found. But, again, the correlations where not excitingly strong.
Weather-related crop failures are not included at all in the European phenology database. Scientists trying to establish connections between crop failures and climate change rely mostly on anecdotal evidence and media reports.