This value matches the
observed annual increase in air temperature observed in
De Bilt. This implies that over the whole period of 46
years the lakes have on average become almost 2 °C
warmer. An important outcome of our analysis is that the
increase in temperature is not equally spread over the
seasons. On the contrary, analysis of the rate of warming
for each day of the year shows a strong fluctuating
pattern with maximum warming rates of 0.08 °C yr− 1
during some days and no warming at all during other
days (Fig. 5a). Warming during the considered period
thus seems to have occurred in cycles with strong
increases in temperature in early April, in May, in late
July and August and again during the winter months.
Little or no warming occurred in late April, late June and
again in mid October. This pattern can still be found
when calculated on a weekly basis (Fig. 5b). However,
when looking at monthly averages or even quarters of the
year this pattern disappears (Fig. 5c).