One of the effects of global warming is rising sea level over a period of time. This phenomenon occurs due to the melting of ice caps and glaciers, as well as the thermal expansion of seawater as it warms.
The other options you mentioned—increasing carbon dioxide in the atmosphere and increasing greenhouse gas concentration in air—are contributors to global warming rather than direct effects. Changing seasons every year can occur, but it's more about climate variability than a definitive effect of global warming.
So, the correct answer focusing on a direct effect would be the rising sea level.