The IPCC acknowledge the hemispheres have been out of sync by stating this has even caused many drastic climate shifts in the past as a result of ocean currents transporting heat between hemispheres. These changes were from internal forcing not a change in the global radiation budget or CO2. The IPCC goes on to tells us “changes are not synchronous in Greenland and Antarctica, and they are in opposite direction in the South and North Atlantic.” (AR4 FAQ 6.1 Page 449)

The IPCC explains surface air warms faster over land compared to over the oceans. They note most of the land is in the NH, not SH. They also claim there is a greater uptake of heat from the Southern Ocean compared to Northern Ocean. They conclude the NH warms faster than the SH. Plus the Antarctic is made of land that warms slower than the Artic ocean. (AR5 Section 12.4.1.1 Page 1055)

The IPCC goes on to tells us proxy data shows when the NH warmed the SH cooled and vice versa many times indicating the oceans transport heat between hemispheres. (AR4 Section 6.4.2.2 Page 456)

They note ‘Arctic Amplification’ where the Arctic surface warming is a faster rate than rest of planet. (AR5 Section 10.5.1.1 Page 906)

The IPCC goes on in AR4 stating the “warming is smaller in SH than in the NH”. (AR4 Section 9.4.2 Page 693)

Here in AR4 the IPCC admits there are few proxy records for the Southern Hemisphere compared to the Northern Hemisphere and as a result the uncertainties are much higher. They go on to state that today it is better to consider regional indicators of temperature change. (AR4 Section 6.6.2 Page 474)

For example, in AR4 the IPCC notes during the last 100 years or up to 2005 the “areas of south of Greenland and parts of North and South America” have not been warming. They go on, “Warming has been the strongest over the continental interiors of Asia and northwestern North America and some mid-latitude ocean regions of the SH as well as south-eastern Brazil.” (AR4 Section 9.4.2 Page 693)

AR5 goes on to state some regions have seen a cooling like “Pacific coast of South America”. (AR5 Section 2.4.1.2 Page 188)

Despite this AR5 states “with high confidence, that the mean NH temperature of the last 30 or 50 years very likely exceeded and previous 30 to 50 year mean during the past 800 years”. (AR5 Section 5.3.5.1 Page 409)  

AR5 notes the eastern pacific and Southern Oceans cooled from 1979-2010. They note models in the future must handle these areas differently, or not as hot. (AR5 Section 10.3.1.1.2 Page 881)

As you can see there is more understanding today of how heat can transfer from one hemisphere to the other via ocean currents, but much more needs to be learned.