IPCC ARG6: Long-Term Implications

IPCC ARG6: Long-Term Implications

John W. Purdie,  31 August 2021

The IPCC ARG6 WG1* contains a massive amount of data related to climate change. The validity of the data is carefully assessed, and the level of confidence in it stated. The report states that: ‘It is unequivocal that human influence has warmed the atmosphere, ocean and land’.  Furthermore, the report points out that reaching net zero anthropogenic CO₂ emissions is a requirement to stabilize human-induced global temperature increase at any level.

More ominous, however, are the long-term implications of the climate futures discussed.  Global Mean Sea Level (GMSL) is particularly significant as it is a comprehensive indicator of global changes due to accumulation or loss of energy.

IPCC ARG6 states that sea-level will continue to rise over centuries and millennia following cessation of emissions (high confidence).  For example, over 2000 years committed GMSL rise is projected to be about 4 – 10 m with 2°C of peak warming.

This is consistent with my estimate in Biosphere: Long-Term Risk.   (www.johnwpurdie.ca).  Based on the current rate of increase, sea level will rise by 8.5 m in 2500 yrs.  That is significant as 8.5 that was the maximum reached during the previous interglacial warm epoch, 120 thousand years ago (USGS).  That peak was followed by the quaternary ice age, referred to as the Wisconsin glaciation in North America.  If the same pattern prevails in the current interglacial phase, the Earth will reach a sea level and temperature peak in about 2500 years, before cooling off and moving into an ice age.  Historically, after peaking in the warm phase, the temperature drops over several thousand years, but it may be accelerated by the unusually high level of CO₂ in the atmosphere at present.

According to the Milankovich theory, ice ages are caused by variation in the Earth’s orbit which causes periodic reductions in solar insolation.  However, studies of Antarctic ice cores revealed that ice ages are more closely related to the level of CO₂ in the atmosphere.  Thus, an alternative theory may be more applicable: that change in reflectivity of Earth due to loss of snow and ice cause it to be pushed to a higher orbit.  Significantly less solar energy would be received as it is inversely proportional to the distance from the sun.

The correlation between CO₂ and ice ages suggests that it may be possible to prevent one by reducing the level of CO₂ in the atmosphere; Carbon Dioxide Removal (CDR).   As stated in IPCC ARG6, the reduction in global surface temperature is approximately linearly related to cumulative CO₂ removal (high confidence).

It follows that reducing the CO₂ in the atmosphere on a scale that allows sea ice and polar ice caps to expand would enable Earth to avoid an ice age.  However, if that is the case, there are immense and far-reaching implications.  Not only would the catastrophic effects of an ice age be avoided, but it would mean that the Earth could be maintained in the “Goldilocks” zone, not too hot and not too cold, indefinitely by regulating the level of CO₂ in the atmosphere.  In other words, it would break the cycle of ice ages and interglacial warm spells that has been a feature of Earth’s climate for a million years.

*IPCC ARG6 WG1: Intergovernmental Panel on Climate Change; Sixth Assessment Report; Working Group 1.