What does the elliptical shape of Earth's orbit cause?
The ellipticity of the Earth's orbit has surprisingly little effect.
The Earth is about 5,000,000km closer to the sun at perihelion than it is at aphelion. Perihelion currently occurs around 3 January. The Earth is actually a few degrees warmer at aphelion in July. The reason for this is that the southern hemisphere is mainly ocean. Water retains heat much better than land and retains heat in the southern winter months.
Perihelion is getting later by about a day every 70 years due to precession. In thousands of years time, perihelion will occur in the northern hemisphere's summer months and will cause northern hemisphere summers to be warmer.
One interesting effect of the Earth's ellipticity is on the time of solar noon. Solar noon is when the sun is at its highest in the sky. This differs from clock noon by up to 18 minutes. The difference between clock noon and solar noon is called the equation of time. It has two components, one due to eccentricity of the elliptical orbit and the other due to the obliquity of the Earth.
The graph shows the equation of time and its two components. Solar and clock noon are only the same on four days per year.