Seasonal changes

• Evapotranspiration is highest in the summer months and lowest in winter. In the driest parts of lowland England up to 80% of precipitation may be lost to evapotranspiration. With large losses of precipitation to evapotranspiration and the exhaustion of soil moisture, river flows in England are normally at their lowest in late summer
• Seasonal variations in the carbon cycle are shown by month-month changes in the net primary productivity of vegetation (NPP). In middle and high latitudes, day length or photoperiod, and temperature drive seasonal changes in NPP. Similar seasonal variations also occur in the tropics, though there the main cause is water availability
• During the northern hemisphere summer, when trees are in full foliage, there is a net global flow of CO2 from the atmosphere to the biosphere. This causes atmospheric CO2 levels to fall by 2 ppm. At the end of summer, as photosynthesis end, the flow is reversed with natural decomposition releasing CO2 back to the atmosphere
• Seasonal fluctuations in the global CO2 flux are explained by the concentration of continental land masses in the northern hemisphere. During the growing season, ecosystems such as the boreal and temperate forests extract huge amounts of CO2 from the atmosphere, which has a global impact
• In the oceans phytoplankton are stimulated into photosynthetic activity by rising water temperatures, more intense sunlight and the lengthening photoperiod