Critical insolation-CO2 relation for diagnosing past and future glacial inception

Area of Science:

• Paleoclimatology
• Climate modeling
• Earth system science

Background:

• Northern Hemisphere ice sheet growth historically followed reduced boreal summer insolation, ending interglacial periods.
• Current low summer insolation suggests conditions ripe for glaciation, yet no ice age is imminent, challenging existing theories.
• Understanding glacial inception mechanisms is crucial for predicting future climate states.

Purpose of the Study:

• To investigate the relationship between boreal summer insolation and global carbon dioxide (CO2) concentrations in driving glacial cycles.
• To explain the onset of the past eight glacial cycles and anticipate future glacial inception.
• To assess the impact of anthropogenic CO2 emissions on the timing of the next glacial period.

Main Methods:

• Utilized an Earth system model of intermediate complexity.
• Constrained model simulations with paleoclimatic data.
• Performed ensemble simulations to analyze climate dynamics.

Main Results:

• A critical relationship between boreal summer insolation and CO2 concentration was identified, explaining past glacial cycles.
• Glacial inception was narrowly averted before the Industrial Revolution due to high late-Holocene CO2 and low orbital eccentricity.
• Without human influence, the current interglacial would likely persist for 50,000 years, with minimal ice sheet growth.

Conclusions:

• Anthropogenic CO2 emissions of 1,000-1,500 GtC will postpone the next glacial inception by over 100,000 years.
• The Earth system naturally tends towards a stable interglacial state, avoiding extreme glaciation or deglaciation.
• Current interglacial conditions are unusually prolonged due to a delicate balance, significantly extended by human CO2 emissions.