Vegetation-climate interactions in the warm mid-Cretaceous

J Zhou,B Briegleb,C J Poulsen,C Shields,N Rosenbloom

doi:10.5194/cp-8-565-2012

Abstract

Abstract. Vegetation-climate interactions are thought to have amplified polar warmth during past warm periods. Here, we explore the vegetation-climate interactions in the mid-Cretaceous using a fully coupled ocean-atmosphere general circulation model with a dynamic vegetation component. We run simulations with 1x, 10x and 16x pre-industrial atmospheric CO2. Results show that forests expand from mid-latitudes to high latitudes as CO2 increases from 1x to 10x and 16x, mainly due to the CO2-induced warming. This expansion of mid-to-high latitude forests are largely supported by the distribution of mid-Cretaceous fossil woods and coal deposits. Globally, the presence of vegetation increases mean annual temperature and precipitation by 0.9 °C and 0.11 mm day−1 relative to bare ground. High-latitude warming induced by the presence of vegetation (∼1.9 °C) is less than half of that reported in previous studies. The weaker warming here is mainly due to less pronounced albedo feedbacks, and to a less extent, reduced poleward heat transport via weakening of the meridional overturning circulation. Our results suggest that other mechanisms in addition to high atmospheric CO2 and high-latitude vegetation are required to maintain the polar warmth.

Highlights

The mid-Cretaceous (∼100 Ma) was a period of extreme polar warmth
Global net primary production (NPP), which is equal to the net gain of carbon through photosynthesis and respiration, is estimated to be 61.8 PgC yr−1 in the 1xDGVM experiment, comparable to observational estimates of present-day NPP (Schlesinger, 1997)
The weaker warming here is linked to the decrease in poleward oceanic heat transport resulting from the weakening of meridional overturning circulation (MOC), which is not resolved in the earlier studies

Summary

Introduction

The mid-Cretaceous (∼100 Ma) was a period of extreme polar warmth. Paleoclimate proxies indicate that mean annual temperature were as high as 20 ◦C and 13 ◦C over polar ocean and land (Bice et al, 2003; Jenkyns et al, 2004; Spicer et al, 2002, 2008). Several hypotheses have been proposed to reconcile this high-latitude model-data discrepancy including enhanced poleward heat transport via strengthening of atmospheric or oceanic circulation (e.g., Covey and Barron, 1988; Farrel, 1990; Korty et al, 2008), increased local radiative forcing through high-latitude cloud feedbacks (e.g., Abbot and Tziperman, 2008; Sloan and Pollard, 1998; Kump and Pollard, 2008), high atmospheric methane levels (Bice et al, 2006) and high-latitude forests (Deconto et al, 2000; Otto-Bliesner and Upchurch, 1997; Upchurch et al, 1998). Of these mechanisms only the existence of high-latitude forests is directly supported by observations in the geological records (e.g., Herman and Spicer, 1996; Falcon-Lang et al, 2001; Spicer and Parrish, 1986; Spicer et al, 1993)

Methods

Results

Discussion

Conclusion