Climatologists study ocean currents because they play a major role in global climate.
with which two parts of the earth system is a climatologist involved?
a.
atmosphere and lithosphere
b.
atmosphere and hydrosphere
c.
hydrosphere and biosphere
d.
atmosphere and magnetosphere

Based upon these answers and the initial description of climatologists focusing on the ocean currents as an important aspect of climate change it would be fair to state the the two "spheres" that climatologists focus on are the atmosphere and the hydrosphere. The atmosphere is concerned with the airspace above the earth and is related to greenhouse gas emissions and other factors that impact climate science. The hydrosphere is the name given to all the water on the planets surface including the oceans, rivers, and lakes. This region is important in regards to climate science because it has an enormous impact on the climate of the globe and has a direct relationship with the atmosphere as well. 

How is this even a question?