The IPCC reports highlight the exploration of historical temperature questions, which originated during the 19th and 20th centuries through studies in geomorphology and paleontology. These studies have revealed that the Earth’s past climates have experienced both warmer and cooler periods than the present era (AR4 Section 1.4.2, Page 106).

In AR4, an example provided is the Mid-Pliocene period (approximately 3.3 to 3.0 million years ago), which exhibited a climate similar to today but with temperatures 2°C to 3°C higher, despite CO2 levels being similar to current levels, ranging between 360ppm to 400ppm. Additionally, the sea levels during that time were noted to be 12 to 25 meters higher than today (AR4 Section 6.3.2, Page 440).

In AR5, an update was made regarding the Mid-Pliocene, stating with “medium confidence” that the period was 2°C to 3.5°C warmer than the 1800s, but there is “low agreement on how high the temperature got” (AR5 Section 13.2.1.1, Page 1145).

Furthermore, according to newer models presented in AR5, the Last Interglacial Period, occurring approximately 129,000 years ago, had an average temperature about 2°C higher than the present (AR5 Section 13.2.1.3, Page 1146).

In AR6 things get worse as the IPCC points out here during the Pliocene Epoch or 3 million years ago the temperature was higher at “2.5°C–4°C higher than 1850-1900” and the sea levels were 5–25 m higher than 1850-1900 with CO2 close to what it is today. They believe this is evidence of what today’s increase in CO2 will cause once “Earth system that has fully-adjusted to changes in natural drivers”. (AR6 FAQ 1.3 Page 119) 

In AR6 the IPCC points out some past abrupt changes of the past or now what they are referring to as “tipping points” For example, 2.5 and 55.5 million years ago changing the climate for tens to hundreds of thousands of years, but claim “at a rate that is actually much slower than projected anthropogenic climate change over this century, even in the absence of tipping points” (AR6 Section 1.4.4.3 Page 66)

The IPCC believes from reconstructions that there have been two extended warmer periods in the past 800,000 years where the sea level was “at least six metres higher than today”. (AR6 Section 1.2.1.2 Page 17) 

The IPCC admits here that the “LIG (129–116 ka) is the next most recent candidate for a period of higher global temperature.” As AR6 admits warmer past periods than today. (AR6 Section 2.3.1.1.2 Page 34 (214) 

Here the IPCC states past simulations are challenged by lack of data on past emissions and radiative forcings. (AR6 Section 1.4.3.1 Page 59)

The IPCC admits with “high confidence” that “including all relevant forcings is a prerequisite for reproducing historical trends”. (AR6 Chapter 10 Section 10.3.3.8 Page 54) 

The IPCC points out there is a “need for climate models to represent the observed behaviour of past climate as a necessary condition to be considered a viable tool for future projections. This does not, however, provide an answer to the much more difficult question of determining how well a model must agree with observations before projections made with it can be deemed reliable (AR5 Section 9.2.3 Page 755) 

The IPCC claims “the inclusion of ‘long-term’ simulations has allowed incorporation of information from paleoclimate data to inform projections. Within uncertainties associated with reconstructions of past climate variables from proxy record and forcings, paleoclimate information from the Mid Holocene, Last Glacial Maximum, and Last Millennium have been used to test the ability of models to simulate realistically the magnitude and large-scale patterns of past changes. (AR5 Executive Summary Page 122)

In AR4 the IPCC states “throughout the 19th and 20th centuries, a wide range of geomorphology and palaeontology studies has provided new insight into the Earth’s past climates, covering periods of hundreds of millions of years. The Palaeozoic Era, beginning 600 Ma, displayed evidence of both warmer and colder climatic conditions than the present; the Tertiary Period (65 to 2.6 Ma) was generally warmer; and the Quaternary Period (2.6 Ma to the present – the ice ages) showed oscillations between glacial and interglacial conditions”. (AR4 Section 1.4.2 Page 106)

The IPCC claims in AR4 that “changes in Earth’s radiation balance were the principal driver of past climate changes, but the causes of such changes are varied. For each case – be it the Ice Ages, the warmth at the time of the dinosaurs or the fluctuations of the past millennium – the specific causes must be established individually. In many cases, this can now be done with good confidence, and many past climate changes can be reproduced with quantitative models. (AR4 FAQ 6.1 Page 449)