Investigating vegetation–climate feedbacks during the early Eocene

Abstract

Abstract. Evidence suggests that the early Eocene was a time of extreme global warmth. However, there are discrepancies between the results of many previous modelling studies and the proxy data at high latitudes, with models struggling to simulate the shallow temperature gradients of this time period to the same extent as the proxies indicate. Vegetation-climate feedbacks play an important role in the present day, but are often neglected in these palaeoclimate modelling studies, and this may be a contributing factor to resolving the model-data discrepancy.Here we investigate these vegetation-climate feedbacks by carrying out simulations of the early Eocene climate at 2 × and 4 × pre-industrial atmospheric CO 2 with fixed vegetation (homogeneous shrubs everywhere) and dynamic vegetation.The results show that the simulations with dynamic vegetation are warmer in the global annual mean than the simulations with fixed shrubs by 0.9 • C at 2 × and 1.8 • C at 4 ×. Consequently, the warming when CO 2 is doubled from 2 × to 4 × is 1 • C higher (in the global annual mean) with dynamic vegetation than with fixed shrubs. This corresponds to an increase in climate sensitivity of 26 %. This difference in warming is enhanced at high latitudes, with temperatures increasing by over 50 % in some regions of Antarctica. In the Arctic, ice-albedo feedbacks are responsible for the majority of this warming. On a global scale, energy balance analysis shows that the enhanced warming with dynamic vegetation is mainly associated with an increase in atmospheric water vapour but changes in clouds also contribute to the temperature increase. It is likely that changes in surface albedo due to changes in vegetation cover resulted in an initial warming which triggered these water vapour feedbacks.In conclusion, dynamic vegetation goes some way to resolving the discrepancy, but our modelled temperatures cannot reach the same warmth as the data suggest in the Arctic. This suggests that there are additional mechanisms, not included in this modelling framework, behind the polar warmth or that the proxies have been misinterpreted.

Multiple Proxy Estimates of Atmospheric CO2 From an Early Paleocene Rainforest

“…By comparison, the fast-feedback climate sensitivities associated with most climate models tuned to the early Cenozoic are~3°C (Lunt et al, 2012). Our minimum constraints on ESS thus reinforce the notion that some additional positive feedback (either fast or slow) are probably needed to explain early Cenozoic warmth; these missing feedback factors may be related to clouds (Abbot et al, 2009;Caballero & Huber, 2013;Hansen et al, 2013;Sagoo et al, 2013), aerosols (Kiehl & Shields, 2013;Kump & Pollard, 2008;Upchurch et al, 2015), or vegetation Loptson et al, 2014).…”

Transitivity of the climate–vegetation system in a warm climate

“…Only in central Asia do sediments and flora fossils indicate semi-dry to dry conditions with desert vegetation (Wang et al, 2013;Quan et al, 2012). Like previous simulations by Huber and Caballero (2011) and Loptson et al (2014), our simulation reproduces the Asian desert because a monsoon climate causes a seasonally dry climate in this region. In subtropical Africa and America, further small deserts and semi-deserts evolve in the DE d simulation.…”

Influence of ocean tides on the general ocean circulation in the early Eocene

“…Lunt et al, 2012]. In order to solve the discrepancies between models and proxy data, a number of atmospheric and oceanic mechanisms that could increase polar temperatures have been suggested [Bice and Marotzke, 2002;Tripati and Elderfield, 2005;Abbot and Tziperman, 2008;Kump and Pollard, 2008;Shellito et al, 2009;Kiehl and Shields, 2013], such as polar stratospheric clouds [Sloan and Pollard, 1998], vegetation-climate feedbacks [Loptson et al, 2014], or the opening of the Arctic Ocean to its surrounding oceans [Shellito et al, 2009]. However, none of these hypotheses has been able to effectively reduce the meridional temperature gradient to the values suggested by proxy data.…”