In AR4 the IPCC expressed great confidence in their ability to predict the earth’s future temperatures trends believing temperature would continue to rise at an unprecedent rate.

For example, in AR4 or 2007 the IPCC’s models showed temperature would continue to rise at the current rate. You can see this data here in a graph found in AR5, this includes all of the IPCC model predictions since the second assessment report. (AR5 Page 131 Figure 1.4)

The IPCC in AR4 claimed the current rate of temperature increase would be the fastest increase in the Earth’s average temperature in the last 50 million years. (AR4 FAQ 6.2 Page 465) This was before AR5 with the realization the Earth was experiencing a warming hiatus.

AR4 even went on to express they had the climate so well understood it was easier to predict than the weather. Here the IPCC notes “the chaotic nature of weather making it unpredictable beyond a few days”, but believe predicting changes in climate is “very different and much more manageable issue”. They made the analogy that even though we know the average age a person may die we cannot predict an individual death. (AR4 FAQ 1.2 Page 96)

However, seven years later this changed drastically as seen in AR5 where the IPCC admits “It is not possible to make deterministic, definitive predictions of how climate will evolve over the next century and beyond as it is with short term weather forecasts.’ They note two reasons here, “first because they are dependent primarily on scenarios of future anthropogenic and natural forcings that are uncertain, second because of incomplete understanding and imprecise models of the climate system and finally because of the existence of internal climate variability.” (AR5 Section 12.1 Page 1034)

This is a drastic change in confidence from AR4 where they seen the climate as simpler and more predictable in 2007, but why? Here are possibilities mentioned by the IPCC.

In AR5 the IPCC admits not all of the processes are understood, stating “underlying physical processes are not yet completely understood, making them difficult to model.” (AR5 FAQ 1.1 Page 140)

Among those the IPCC admits “Almost all CMIP5 historical simulations do not reproduce the observed recent warming hiatus.” (AR5 Box 9.2 Page 772)

Let us go back to the graph of the IPCC’s models showing temperature would continue to rise at the current rate. (AR5 Page 131 Figure 1.4). Notice the line of what actually happened as the temperature did not increase at the predicted rate, instead it went flat. This was the warming hiatus from 1998-2012.

One of the big questions is what caused the warming hiatus? Today the IPCC does not know the answer but offers some theories.

For example, IPCC states here they have “medium confidence” the warming hiatus was caused by changes in both internal variability and external forcing. (AR5 Chapter 10 Exec Summary Page 870)

They go on to note with “medium confidence” internal variability played a big role. Other possible errors are “forcing errors and models overestimating the response to increasing GHG and other anthropogenic forcing”. (AR5 Box 9.2 Page 772)

The IPCC explains for climate analysis they need data on internal variability on time scales longer than a decade. They will remove external RF to “estimate internal variability”. However, they explain an estimate is not reliable because of the lack of data today. (AR4 Section 9.1.2 page 668)

One of the most important questions centers around how long will the current warming last? Or maybe more important, when will the next ice age begin?

AR4 informs us 20% of the average glacial-interglacial cycle experiences a warm interglacial mode which normally had lasted for 10k to 30k years. Today warming period or what is called the Holocene period or since the last ice age about 11,500 years old today which is past the 10k minimum. (AR4 Section 6.4.1 Page 444)ans