Greenhouse effect and climate change
Greenhouse gases trap heat and keep Earth liveable; adding more of them by burning fuels traps extra heat and warms the climate.
A worked example
A warming trend of 0.02 °C per year adds 50 x 0.02 = 1.0 °C over fifty years, so a mean summer temperature of 15.0 °C becomes 16.0 °C.
The mistake people actually make
The greenhouse effect is not the problem, since without it Earth would be frozen. Melting land ice raises sea level, while sea ice that is already floating adds very little.