Wetenschap - 28 maart 2013

Is this cold normal?

Who isn't fed up with the cold? The 2012-2013 winter seems to be lasting an exceptionally long time. It is indeed cold, confirms biologist Arnold van Vliet. But exceptionally cold? No. It is more a question of short memories, climate change and definitions.

What is normal now?  
'Phenologists use the period 1940-1968 as a reference. That period is representative for the previous century and we have a lot of data. Phenologists are interested in what is changing and you need a fixed reference point for that. The normal temperature for climatologists is the average temperature over the past 30 years, so they are not using a fixed reference. Temperatures hardly increased at all up to 1988 but started to rise after that because of climate change. The KNMI says this is a cold winter with temperatures that are half a degree lower than the norm for 1980-2010.'
But is there nothing exceptional about this winter for phenologists?
'No. Fifty years ago, the average temperature from 1 January to 18 March was 2.0 degrees. Now it is 2.3 degrees. So nature is actually slightly above what is phenologically normal. But compared with the past 12 years it is 1.5 degrees colder. That is a difficult message to get across to the general public. I am always getting questions from journalists.'
But the first half of the winter was really warm?
'That is the interesting thing about this winter. Witch hazel, for example, has never flowered so early. The early species have benefited from this warmth. The question is now what impact the subsequent cold spell has had. Butterflies are very sensitive to temperature. Some of the butterflies became active during the warm spell and now have to go back into hibernation. I'm afraid they will be hit hard by the cold.'
Are there winners too?
'It might turn out a good thing for migrating birds. Spring is starting increasingly early due to climate change. That has led to a mismatch with the arrival of migrating birds. The long winter will eliminate that mismatch this year.'