Simulating late Holocene climate changes caused by competing forcing of the Earth’s orbital and greenhouse gas changes

Abstract:

Decreases in orbitally-forced summer insolation in the Northern Hemisphere have coincided with downward trends in greenhouse gases (GHG) during the late stages of previous interglacials, with both climate forcings acting collectively to reduce the global temperature. In the last several thousand years of the current (Holocene) interglacial, however, while summer insolation has decreased, there was a reversal of the downward trends in CH₄ and CO₂ concentration around 5,000 to 7,000 years ago. Because this anomalous behavior was presumably due to anthropogenic carbon emissions from early agriculture, the Holocene provides a unique opportunity to evaluate the relative strength of competing climate forcings of orbital and greenhouse gas changes. Here we use the 1-degree, fully coupled Community Climate System Model version 4 (CCSM4) to investigate the climatic response to combined forcing from orbital and GHG changes since the mid-Holocene. Six equilibrium CCSM4 runs, 1,000 years apart, were performed to simulate the climate changes of the past 6,000 years.

Our coupled CCSM4 simulations with combined forcing show that the simulated global mean annual surface temperature increases gradually during the past 6,000 years, following the gradual increase of GHG since the middle Holocene. Despite this global warming trend, the simulated Northern Hemisphere annual snow depths increase by over 20%. Orbital forcing causes a pronounced reduction of seasonality in the Northern Hemisphere mid-latitude regions, with increasingly cooler summers and shorter growing seasons likely presenting a challenge for the development of early agriculture. The simulated annual and summer precipitation decreases significantly over the Sahara region, with major reductions occurring between 5,000 and 4,000 years ago. Changes in hydrologic budgets also occur in parts of Europe, the Middle East, and Asia. The mechanisms of these simulated Holocene climate changes due to the competing orbital and greenhouse gas changes will be discussed with the comparison to PMIP simulations and TraCE-21K transient simulations.