Even though the Earth is warming, some years are still cooler than the year before.

Heat can affect things without causing a temperature rise.
Extra heat can be used in ‘changing state’ instead of raising temperature.
A change of state could be … a solid melting to a liquid. 
Or a liquid evaporating to a gas.
So heat is needed to change ice at zero C to water at zero C.
And to change water into water vapour….. without raising the temperature.
Scientists call heat used to change state latent heat.
Also there are natural variations in the climate, the El Nino events being the ones that affect world temperature the most.
The opposite to 'El Nino' is 'La Nina', a cooling effect.
If global temperatures are plotted on a graph in a way that shows these variations, it makes the overall warming trend very obvious.


Every La Nina year since 1998 has been warmer than every El Nino year before 1995.


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