Calcifying Phytoplankton Demonstrate an Enhanced Role in Greenhouse Atmospheric CO2 Regulation

Karin Kvale,Nadine Mengis,Wolfgang Koeve

doi:10.3389/fmars.2020.583989

Karin Kvale, Nadine Mengis + Show 1 more

Open Access

https://doi.org/10.3389/fmars.2020.583989

Copy DOI

Abstract

The impact of calcifying phytoplankton on atmospheric CO2 concentration is determined by a number of factors, including their degree of ecological success as well as the buffering capacity of the ocean/marine sediment system. The relative importance of these factors has changed over Earth's history and this has implications for atmospheric CO2 and climate regulation. We explore some of these implications with four “Strangelove” experiments: two in which soft-tissue production and calcification is stopped, and two in which only calcite production is forced to stop, in idealized icehouse and greenhouse climates. We find that in the icehouse climate the loss of calcifiers compensates the atmospheric CO2 impact of the loss of all phytoplankton by roughly one-sixth. But in the greenhouse climate the loss of calcifiers compensates the loss of all phytoplankton by about half. This increased impact on atmospheric CO2 concentration is due to the combination of higher rates of pelagic calcification due to warmer temperatures and weaker buffering due to widespread acidification in the greenhouse ocean. However, the greenhouse atmospheric temperature response per unit of CO2 change to removing ocean soft-tissue production and calcification is only one-fourth that in an icehouse climate, owing to the logarithmic radiative forcing dependency on atmospheric CO2 thereby reducing the climate feedback of mass extinction. This decoupling of carbon cycle and temperature sensitivities offers a mechanism to explain the dichotomy of both enhanced climate stability and destabilization of the carbonate compensation depth in greenhouse climates.

Highlights

The status of greenhouse climates as the apparent default state of our earth system is an emerging view
We explore the strength of marine soft-tissue production and calcification in regulating climate in two highly idealized icehouse and greenhouse states at thousand-year timescales, as well as the carbon response of the earth system to their perturbation
Northern hemisphere differences are dominated by the shoaled Atlantic Meridional Overturning Circulation (AMOC), but southern hemisphere differences are dominated by a strengthened anticlockwise overturning in the Southern Ocean

Summary

Introduction

The status of greenhouse climates as the apparent default state of our earth system (as opposed to icehouse or hothouse states; Kidder and Worsley, 2010) is an emerging view. Greenhouse climates are warm, with little continental sea ice and global overturning driven by weak deep water formation near the poles. The deep ocean is poorly ventilated; water column oxygen and nitrate inventories are lower than in an icehouse climate. Icehouse (to which category both modern and glacial climates belong) oceans have greater inventories of nitrate and oxygen because enhanced polar ice promotes better deep ocean ventilation via a stronger global overturning circulation and cooler global temperatures. Hothouse climates are rare in the geological record and are characterized by very warm, ice-free conditions that promote a shift toward global ventilation reliant on mid-latitude deep water formation driven by haline, rather than thermal, gradients. The specifics of climate state transitions and the interplay between endogenic (to a geologist, e.g., volcanic) and exogenic (e.g., ocean) processes is an area of active research (McKenzie and Jiang, 2019)

Methods

Results

Conclusion