The climatic changes on Earth are mostly driven by the oceanic currents. These currents tend to confine themselves to either sides of the Equator. When waters from North and South meet at the Equator, they combine lazily and create an utter lack of wind known as the doldrums.
However, a new study (published in the journal Science) finds certain connections between the climates in the two hemispheres in the long run. According to researchers from Universitat Autonoma de Barcelona in Spain and the Cardiff University in the UK, ocean circulations in the Southern hemisphere have adapted to sudden changes in the North.

Great Ocean Conveyor Belt (Courtesy: NASA)Interestingly enough, the researchers found that several times in history, an increase in the temperature in the Northern hemisphere precipitated a cooling period in the South. Conversely, a cooler Northern hemisphere resulted in a warmer Southern hemisphere! Computer models have predicted this behavior, but this is the first time this has been discovered in the weather records.
The cause seems to be linked to how the oceanic streams transport heat from one part of the ocean to another. For example, the climate of the Northern Atlantics is greatly influenced by the warm waters carried by the Gulf stream (from the Gulf of Mexico). But the strength of this stream depends on the salinity of the southbound water; if salinity decreases, the current weakens.
From time to time, a melting of the Greenland ice-sheets would introduce fresh water into the Northern Atlantic, thus reducing the salinity of the Northern ocean. This in turn would weaken the gulf-stream, thus slowing the heat transport from the Southern hemisphere. Therefore, as the North cools, the South warms up.
Conversely, when a warmer and more saline North (due to more evaporation of sea-water) keeps the gulf stream strong, it results in more heat transfer from the South (and thereby cooling it). As the North heats up, the melting of Greenland ice slowly cools the North, and the cycle starts all over again.