Given:
The heat content of the pizza is Q = 500 kcal
The volume of the cold water is V = 50 L
The mass of a liter of cold water is 1 kg.
Calculate the mass, M, of cold water.
M = (50 L)*(1 kg/L) = 50 kg
The specific heat of water is
c = 4.184 kJ/(kg-K)
Also,
1 kcal = 4184 J, therefore
Q = (500 kcal)*(4184 J/kcal) = 2.092 x 10⁶ J
Let ΔT = increase in temperature of the cold water, °C (same as K).
Then
Q = M*c*ΔT
or
ΔT = Q/(M*c)

Answer: The temperature increases by 10 °C (or 10 K)
One of the factor is the Sun's elevation where the UV reaches us at a much lower frequency when the sun is more distant. Next is our ozone, with our ozone depleting as time goes by, the amount of solar energy it can diffuse is much lesser thus the heat is much more than the previous decades.