Scrutinizing the carbon cycle and CO₂ residence time in the atmosphere

Highlights

• We present a carbon cycle with an uptake proportional to the CO₂ concentration.

• Temperature dependent natural emission and absorption rates are considered.

• The average residence time of CO₂ in the atmosphere is found to be 4 years.

• Paleoclimatic CO₂ variations and the actual CO₂ growth rate are well reproduced.

• Human emissions only contribute 15 % to the CO₂ increase over the Industrial Era.

Abstract

Climate scientists presume that the carbon cycle has come out of balance due to the increasing anthropogenic emissions from fossil fuel combustion and land use change. This is made responsible for the rapidly increasing atmospheric CO₂ concentrations over recent years, and it is estimated that the removal of the additional emissions from the atmosphere will take a few hundred thousand years. Since this goes along with an increasing greenhouse effect and a further global warming, a better understanding of the carbon cycle is of great importance for all future climate change predictions. We have critically scrutinized this cycle and present an alternative concept, for which the uptake of CO₂ by natural sinks scales proportional with the CO₂ concentration. In addition, we consider temperature dependent natural emission and absorption rates, by which the paleoclimatic CO₂ variations and the actual CO₂ growth rate can well be explained. The anthropogenic contribution to the actual CO₂ concentration is found to be 4.3%, its fraction to the CO₂ increase over the Industrial Era is 15% and the average residence time 4 years.

Introduction

The carbon cycle can be understood as a series of carbon reservoirs in the Earth-Atmosphere-System (EASy), which are connected to each other by exchange fluxes of carbon and its main bio-chemical compounds. For climate considerations especially atmospheric CO₂ as the main atmospheric phase of the global carbon cycle is of great importance due to its infrared active properties and its classification as the most dangerous greenhouse gas. Therefore, particularly the increase of CO₂ in the atmosphere, which climate scientists mainly trace back to growing anthropogenic emissions as well as a reduced uptake of CO₂ by oceans and land vegetation, are in the focus of many investigations.

In the 5th Assessment Report (AR5, 2013) of the Intergovernmental Panel on Climate Change (IPCC) we can read (AR5-Chap.12-FAQ 12.3, p. 1107): “Global temperature would not respond quickly to the greenhouse gas concentration changes... Eliminating CO₂ emissions only would lead to near constant temperature for many centuries (commitment from past emissions)... As a consequence of the large inertia in the climate and carbon cycle, the long-term global temperature is largely controlled by total CO₂ emissions that have accumulated over time, irrespective of the time when they were emitted.”

So, the IPCC assumes that not only the Earth as a large heat storage but also the atmosphere as a big storage for CO₂, cumulating this greenhouse gas over many centuries, is responsible for a slow response of the global temperature. But obviously this response is assumed to work only in one direction. While the CO₂ increase of 100 ppm over the last century is made liable for a relatively fast increase of the temperature of about 0.8 °C over this period, eliminating further emissions are expected to lead to near constant temperatures for many centuries. The IPCC explains this with ‘extremely long time scale processes involved into the removal of anthropogenic CO₂ emissions into the atmosphere, which makes the concept of a single, characteristic atmospheric lifetime not applicable to CO₂’ (AR5-Chap.6-Box-6.1).

Because of the IPCC's interpretation of an extremely long atmospheric residence time together with a high climate sensitivity CO₂ is supposed to be the most dangerous component destabilizing our climate. Our own assessment of global warming by CO₂ (Harde, 2013, Harde, 2014, Harde, 2016) shows a less dramatic influence of CO₂ on the climate, yielding an equilibrium climate sensitivity (temperature increase at doubled CO₂) almost a factor of five smaller than published in AR5, and also a closer inspection of the residence time gives significantly different values than presented by the IPCC.

Therefore, it seemed worthwhile to scrutinize the carbon cycle and the different accounting schemes for the residence time with their individual assumptions in more detail and to identify the fundamental distinctions of these concepts. For a better comparison and deeper understanding we have tried to reproduce the IPCC's accounting scheme for the carbon cycle (see AR5-Chap.6) as far as possible, only supplemented by some own contemplations. This is presented in Section 2, while in Section 3 we contrast this to an alternative description, which is based on the balance equation and the empirical evidence that uptake rates scale proportional with the CO₂ concentration, in agreement with the observed exponential decay of ¹⁴C in the atmosphere. The balance equation is a fundamental law that must be obeyed by any legitimate model of CO₂. In addition, we consider temperature dependent natural emission and absorption rates, by which the paleoclimatic CO₂ variations and the actual CO₂ growth rate can well be explained. For these studies we have applied the IPCC's own estimates of natural absorption and emission, not because they are necessarily correct, but to demonstrate that, with those estimates, governing physical laws, lead to an explanation of increased CO₂ entirely different to the one advocated by the IPCC.

Previous critical analyses facing the IPCC's favored interpretation of the carbon cycle and residence time have been published, e.g., by Jaworowski et al. (1992), Segalstad (1998), Dietze (2001), Rörsch et al. (2005) or Essenhigh (2009), and more recently by Humlum et al. (2013), or Salby, 2013, Salby, 2016. Although most of these analyses are based on different observations and methods, they all derive residence times (in some cases also differentiated between turnover and adjustment times) in part several orders of magnitude shorter than specified in AR5. As a consequence of these analyses also a much smaller anthropogenic influence on the climate than propagated by the IPCC can be expected.