Carbon dioxide and climate impulse response functions for the computation of greenhouse gas metrics: a multi-model analysis

Abstract

The responses of carbon dioxide (CO₂) and other climate variables to an emission pulse of CO₂ into the atmosphere are often used to compute the Global Warming Potential (GWP) and Global Temperature change Potential (GTP), to characterize the response timescales of Earth System models, and to build reduced-form models. In this carbon cycle-climate model intercomparison project, which spans the full model hierarchy, we quantify responses to emission pulses of different magnitudes injected under different conditions. The CO₂ response shows the known rapid decline in the first few decades followed by a millennium-scale tail. For a 100 Gt-C emission pulse added to a constant CO₂ concentration of 389 ppm, 25 ± 9% is still found in the atmosphere after 1000 yr; the ocean has absorbed 59 ± 12% and the land the remainder (16 ± 14%). The response in global mean surface air temperature is an increase by 0.20 ± 0.12 °C within the first twenty years; thereafter and until year 1000, temperature decreases only slightly, whereas ocean heat content and sea level continue to rise. Our best estimate for the Absolute Global Warming Potential, given by the time-integrated response in CO₂ at year 100 multiplied by its radiative efficiency, is 92.5 × 10^−15 yr W m⁻² per kg-CO₂. This value very likely (5 to 95% confidence) lies within the range of (68 to 117) × 10^−15 yr W m⁻² per kg-CO₂. Estimates for time-integrated response in CO₂ published in the IPCC First, Second, and Fourth Assessment and our multi-model best estimate all agree within 15% during the first 100 yr. The integrated CO₂ response, normalized by the pulse size, is lower for pre-industrial conditions, compared to present day, and lower for smaller pulses than larger pulses. In contrast, the response in temperature, sea level and ocean heat content is less sensitive to these choices. Although, choices in pulse size, background concentration, and model lead to uncertainties, the most important and subjective choice to determine AGWP of CO₂ and GWP is the time horizon

Transient Earth system responses to cumulative carbon dioxide emissions: linearities, uncertainties, and probabilities in an observation-constrained model ensemble

“…IRFs for atmospheric CO 2 , SAT, SSLR, and ocean and land carbon uptake are given elsewhere and we refer the reader to the literature for a general discussion on IRFs, underlying carboncycle and climate processes, and timescales (e.g. Archer et al, 1998;Joos et al, 2013;Maier-Reimer and Hasselmann, 1987;Shine et al, 2005). A main goal of this section on IRF is to discuss to which extent one may expect a close-to-linear relationship between cumulative CO 2 emissions and a climate variable of interest.…”

“…The emission pulse simulations are conducted as described by Joos et al (2013). A pulse input of 100 Gt C is added to a constant background atmospheric CO 2 concentration of 389 ppm in year 2010, while all other forcings are held constant at 2010 levels.…”

The declining uptake rate of atmospheric CO<sub>2</sub> by land and ocean sinks

“…Describing these processes is a fundamental challenge for carbon cycle modelling. A linearised, multi-pool carbon cycle model is equivalent to a pulse response function for atmospheric CO 2 (the airborne fraction after time t of a pulse of CO 2 into the atmosphere) of the form of a sum of exponentials: G(t) = a m exp(−λ m t), where the sum is over a set of modes m with turnover rates λ m and weights a m Joos et al, 2013;. The modes are a set of independent carbon pools z m (t), superpositions of physical carbon pools, that sum to the atmospheric excess carbon c A (t).…”

Time-Dynamic Effects on the Global Temperature When Harvesting Logging Residues for Bioenergy

“…The baselines for the atmospheric GHG concentrations were set according to the latest IPCC report (CO 2 390 ppm, N 2 O 324 ppb and CH 4 1803 ppb), which shows the annual global mean during 2011 [44]. To model the atmospheric decay of CO 2 , the Bern carbon cycle model was used [45,43,46]. For N 2 O and CH 4 , a simple exponential decay function was used based on the perturbation lifetime of the specific gas.…”