Enhanced chemistry-climate feedbacks in past greenhouse worlds

Abstract

Trace greenhouse gases are a fundamentally important component of Earth's global climate system sensitive to global change. However, their concentration in the pre-Pleistocene atmosphere during past warm greenhouse climates is highly uncertain because we lack suitable geochemical or biological proxies. This long-standing issue hinders assessment of their contribution to past global warmth and the equilibrium climate sensitivity of the Earth system (E(ss)) to CO(2). Here we report results from a series of three-dimensional Earth system modeling simulations indicating that the greenhouse worlds of the early Eocene (55 Ma) and late Cretaceous (90 Ma) maintained high concentrations of methane, tropospheric ozone, and nitrous oxide. Modeled methane concentrations were four- to fivefold higher than the preindustrial value typically adopted in modeling investigations of these intervals, even after accounting for the possible high CO(2)-suppression of biogenic isoprene emissions on hydroxyl radical abundance. Higher concentrations of trace greenhouse gases exerted marked planetary heating (> 2 K), amplified in the high latitudes (> 6 K) by lower surface albedo feedbacks, and increased E(ss) in the Eocene by 1 K. Our analyses indicate the requirement for including non-CO(2) greenhouse gases in model-based E(ss) estimates for comparison with empirical paleoclimate assessments, and point to chemistry-climate feedbacks as possible amplifiers of climate sensitivity in the Anthropocene.

Fig. 1.

Terrestrial ecosystem emissions of reactive trace gases in the Eocene and Cretaceous. Simulated equilibrium global-scale (A) reactive biogenic trace gas emissions to the atmosphere for the 2 × CO₂ (560 ppm) and 4 × CO₂ (1,120 ppm) Eocene, and 4 × CO₂ Cretaceous climates, relative to the preindustrial (PI) era. Panels B, C, and D, respectively, show zonal total fluxes for methane (CH₄) from all biogenic sources (wetlands, biomass burning, oceans, and termites), isoprene released from forested regions, and oxides of nitrogen (NO_x = NO + NO₂) released from soils, and generated by biomass burning and lightning (see SI Text, SI Methods).

Fig. 2.

Elevated surface ozone concentrations in the Eocene compared to the preindustrial world. Simulated increases in (A) mean annual surface level ozone for the 4 × CO₂ Eocene simulation and (B) the cross-sectional zonal average profile through the atmosphere. Ozone increases in (A) and (B) are relative to the preindustrial control simulation. Note in (A) changes in ozone fields are plotted over the present-day continental geography.

Fig. 3.

Enhanced chemistry-climate feedbacks in past greenhouse worlds. Simulated equilibrium surface temperature increases (ΔT) by elevated trace greenhouse gas concentrations (Table 1) on the 4 × CO₂ Eocene (A, B) and 4 × CO₂ Cretaceous (C, D) during northern hemisphere winter (DJF) and northern hemisphere summer [June July August (JJA)] climates respectively. Feedbacks were quantified with the coupled ocean-atmosphere GCM HadCM3L by prescribing PI and elevated levels of trace GHGs (Table 1). Only temperature differences exceeding 95% confidence limits are displayed. (E) and (F) indicate the relative contribution of each greenhouse gas to land surface mean annual temperature increases in the Eocene and Cretaceous respectively.

Fig. 4.

Simulated latitudinal temperature gradients in the Eocene and Cretaceous. (A) Latitudinally averaged mean annual land surface temperature in the Eocene (4 × CO₂ concentration) for the control runs (preindustrial concentrations of trace greenhouse gases, black line) and after inclusion of the positive feedback of elevated trace greenhouse gases (red line). (B) Changes in latitudinally averaged seasonal mean annual land surface temperature due to forcing by elevated trace greenhouse gas concentrations relative to the control simulation. JJA; June, July, August; DJF; December, January, February. (C) and (D) as (A) and (B) but for sea-surface temperature. (E–H) Equivalent plots as for A–D, but for the 4 × CO₂ Cretaceous simulations. Proxy data sources for panels A, C, E and G are given in SI Text.