Which of the following is NOT considered a climatic forcing for Earth?

Climatic forcing refers to factors that can cause a change in the climate system. Among the options provided, changes in Earth's orbit and changes in the sun's strength are known climatic forcings. These factors can significantly influence the Earth's climate over long periods.

Changes in plate tectonics, while not directly related to short-term climate variations, can impact climate on geological timescales by altering ocean currents and creating or extinguishing landmasses.

On the other hand, changes in the sun's orbit are not generally referenced in the context of climatic forcing. The sun itself is fixed in its orbit and does not change in a manner that affects Earth’s climate on significant timescales. This is why the answer identifying changes in the sun’s orbit as not being a climatic forcing is accurate, as it does not align with recognized factors that influence the Earth’s climate system.