THIS POST IS A CRITICAL REVIEW OF THE RESEARCH PAPER CITED BELOW. PICTURED ABOVE ARE WELL KNOWN CLIMATE SCIENTISTS RETO KNUTTI, JOERL ROGELJ, AND NATHAN GILLETT WHO ARE THREE OF THE EIGHT CO-AUTHORS OF THE PAPER BEING REVIEWED.

Uncertainty in carbon budget estimates due to internal climate variability
Katarzyna B Tokarska1, Vivek K Arora2, Nathan P Gillett2, Flavio Lehner1, Joeri Rogelj, Carl-Friedrich Schleussner4, Roland Séférian and Reto Knutti1, Accepted Manuscript online 13 August 2020 • © 2020 The Author(s). Published by IOP Publishing Ltd.

ABSTRACT: Remaining carbon budget specifies the cap on global cumulative CO2 emissions from the present-day onwards that would be in line with limiting global warming to a specific maximum level. In the context of the Paris Agreement, global warming is usually interpreted as the externally-forced response to anthropogenic activities and emissions, but it excludes the natural fluctuations of the climate system known as internal variability. A remaining carbon budget can be calculated from an estimate of the anthropogenic warming to date, and either (i) the ratio of CO2-induced warming to cumulative emissions, known as the Transient Climate Response to Emissions (TCRE), in addition to information on the temperature response to the future evolution of non-CO2 emissions; or (ii) climate model scenario simulations that reach a given temperature threshold. Here we quantify the impact of internal variability on the carbon budgets consistent with the Paris Agreement derived using either approach, and on the TCRE diagnosed from individual models. Our results show that internal variability contributes approximately ±0.09 °C to the overall uncertainty range of the human-induced warming to-date, leading to a spread in the remaining carbon budgets as large as ±50 PgC, when using approach (i). Differences in diagnosed TCRE due to internal variability in individual models can be as large as ±0.1 °C/1000 PgC (5-95% range). Alternatively, spread in the remaining carbon budgets calculated from (ii) using future concentration-driven simulations of large ensembles of CMIP6 and CMIP5 models is estimated at ± 30 PgC and ± 40 PgC (5-95% range). These results are important for model evaluation and imply that caution is needed when interpreting small remaining budgets in policy discussions. We do not question the validity of a carbon budget approach in determining mitigation requirements. However, due to intrinsic uncertainty arising from internal variability, it may only be possible to determine the exact year when a budget is exceeded in hindsight, highlighting the importance of a precautionary approach. FULL TEXT PDF: https://iopscience.iop.org/article/10.1088/1748-9326/abaf1b/pdf

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THE TCRE IS A SPURIOUS CORRELATION

THE TCRE CORRELATION DERIVES NOT FROM RESPONSIVENESS OF TEMPERATURE TO EMISSIONS BUT FROM A BIAS FOR POSITIVE VALUES IN THE TWO TIME SERIES AS FOLLOWS:

(1) EMISSIONS ARE ALWAYS POSITIVE, AND (2) DURING A TIME OF WARMING, ANNUAL CHANGES IN TEMPERATURE ARE MOSTLY POSITIVE.

IT IS THESE BIASES AND NOT A RESPONSIVENESS OF TEMPERATURE TO EMISSIONS THAT CREATES THE FAUX CORRELATION THAT HAS BEEN INTERPRETED AS A TEMPERATURE RESPONSE TO EMISSIONS DESCRIBED AS A „CLIMATE RESPONSE TO CUMULATIVE EMISSIONS“ IN THE TCRE.

THE CREATION OF THE SPURIOUS TCRE CORRELATION IS DEMONSTRATED BELOW IN TWO GIF IMAGES.

EACH GIF IMAGE CYCLES THROUGH SEVEN RANDOM EMISSIONS AND TEMPERATURE DATA. IN THE FIRST GIF ANIMATION THERE ARE NO BIASES AND THE DATA ARE ACTUALLY RANDOM. THERE WE FIND NO CORRELATION AND A RANDOM TCRE OVER A WIDE RANGE OF VALUES BOTH POSITIVE AND NEGATIVE.

IN THE SECOND GIF ANIMATION, BIASES ARE INSERTED TO MIMIC THE ANNUAL EMISSIONS AND ANNUAL TEMPERATURE CHANGE DATA THAT ARE USED BY CLIMATE SCIENTISTS TO CONSTRUCT THE TCRE. THERE, ANNUAL EMISSIONS ARE ALWAYS POSITIVE AND DURING A TIME OF WARMING, THERE ARE MORE POSITIVE ANNUAL TEMPERATURE CHANGES THAN NEGATIVE. THERE WE FIND A STRONG CORRELATION AND THE TCRE METRIC THAT CLIMATE SCIENCE HAS MISTAKEN FOR A REAL CAUSE AND EFFECT PHENOMENON.

A FURTHER CONSIDERATION FOR THE SPURIOUSNESS OF THIS APPARENT CORRELATION RELATIONSHIP IS THAT A TIME SERIES OF THE CUMULATIVE VALUES OF ANOTHER TIME SERIES HAS NEITHER TIME SCALE NOR DEGREES OF FREEDOM. THIS TIME SERIES AND ITS CORRELATIONS HAVE NO INTERPRETATION IN TERMS OF THE EMISSIONS AND CLIMATE PHENOMENA THEY APPEAR TO REPRESENT.

THERE IS NO USEFUL INFORMATION IN THIS FAUX CORRELATION AND IT IS NOT POSSIBLE TO INTERPRET THIS CORRELATION AS EVIDENCE THAT EMISSIONS CAUSE WARMING OR AS A TOOL FOR CONSTRUCTING CLIMATE ACTION CARBON BUDGETS. THE REMAINING CARBON BUDGET ANOMALY IS A CREATION OF THIS FAUX CORRELATION AND NOT A REAL WORLD PHENOMENON THAT CAN BE UNDERSTOOD IN TERMS OF CLIMATE VARIABLES OR IN TERMS OF EARTH SYSTEM MODELS OF INCREASING LEVELS OF COMPLEXITY.

THE REMAINING CARBON BUDGET PUZZLE AND THE OTHER VEXING ISSUES IN THE TCRE CARBON BUDGET IS BEST UNDERSTOOD IN THESE TERMS AND NOT IN TERMS OF CLIMATE SCIENCE ISSUES THAT CAN BE SOLVED WITH CLIMATE SCIENCE OR EMS MODELS OF GREATER AND GREATER COMPLEXITY. THE PROBLEM IS A SPURIOUS CORRELATION. THE SOLUTION IS TO STOP INTERPRETING AND RELYING ON SPURIOUS CORRELATIONS TO UNDERSTAND HOW EMISSIONS CAUSE WARMING AND HOW TO TAKE CLIMATE ACTION TO MITIGATE THE RATE OF WARMING.

GIF IMAGE #1: NO BIAS FOR POSITIVE NUMBERS IN ANNUAL EMISSIONS OR IN ANNUAL TEMPERATURE CHANGE.

GIF IMAGE #2: WITH BIAS INSERTED. POSITIVE BIAS FOR ANNUAL TEMPERATURE CHANGE AND EMISSIONS ALWAYS POSITIVE.

TCRE CARBON BUDGET BIBLIOGRAPHY

1. Matthews, H. Damon, et al. “The proportionality of global warming to cumulative carbon emissions.” Nature 459.7248 (2009): 829.  The global temperature response to increasing atmospheric CO₂ is often quantified by metrics such as equilibrium climate sensitivity and transient climate response¹. These approaches, however, do not account for carbon cycle feedbacks and therefore do not fully represent the net response of the Earth system to anthropogenic CO₂ emissions. Climate–carbon modelling experiments have shown that: (1) the warming per unit CO₂ emitted does not depend on the background CO₂ concentration²; (2) the total allowable emissions for climate stabilization do not depend on the timing of those emissions^3,4,5; and (3) the temperature response to a pulse of CO₂ is approximately constant on timescales of decades to centuries^3,6,7,8. Here we generalize these results and show that the carbon–climate response (CCR), defined as the ratio of temperature change to cumulative carbon emissions, is approximately independent of both the atmospheric CO₂ concentration and its rate of change on these timescales. From observational constraints, we estimate CCR to be in the range 1.0–2.1 °C per trillion tonnes of carbon (Tt C) emitted (5th to 95th percentiles), consistent with twenty-first-century CCR values simulated by climate–carbon models. Uncertainty in land-use CO₂ emissions and aerosol forcing, however, means that higher observationally constrained values cannot be excluded. The CCR, when evaluated from climate–carbon models under idealized conditions, represents a simple yet robust metric for comparing models, which aggregates both climate feedbacks and carbon cycle feedbacks. CCR is also likely to be a useful concept for climate change mitigation and policy; by combining the uncertainties associated with climate sensitivity, carbon sinks and climate–carbon feedbacks into a single quantity, the CCR allows CO₂-induced global mean temperature change to be inferred directly from cumulative carbon emissions.
2. Allen, Myles R., et al. “Warming caused by cumulative carbon emissions towards the trillionth tonne.” Nature 458.7242 (2009): 1163.  Global efforts to mitigate climate change are guided by projections of future temperatures¹. But the eventual equilibrium global mean temperature associated with a given stabilization level of atmospheric greenhouse gas concentrations remains uncertain^1,2,3, complicating the setting of stabilization targets to avoid potentially dangerous levels of global warming^4,5,6,7,8. Similar problems apply to the carbon cycle: observations currently provide only a weak constraint on the response to future emissions^9,10,11. Here we use ensemble simulations of simple climate-carbon-cycle models constrained by observations and projections from more comprehensive models to simulate the temperature response to a broad range of carbon dioxide emission pathways. We find that the peak warming caused by a given cumulative carbon dioxide emission is better constrained than the warming response to a stabilization scenario. Furthermore, the relationship between cumulative emissions and peak warming is remarkably insensitive to the emission pathway (timing of emissions or peak emission rate). Hence policy targets based on limiting cumulative emissions of carbon dioxide are likely to be more robust to scientific uncertainty than emission-rate or concentration targets. Total anthropogenic emissions of one trillion tonnes of carbon (3.67 trillion tonnes of CO₂), about half of which has already been emitted since industrialization began, results in a most likely peak carbon-dioxide-induced warming of 2 °C above pre-industrial temperatures, with a 5–95% confidence interval of 1.3–3.9 °C.
3. Mackey, Brendan, et al. “Untangling the confusion around land carbon science and climate change mitigation policy.” Nature climate change 3.6 (2013): 552.  Depletion of ecosystem carbon stocks is a significant source of atmospheric CO₂ and reducing land-based emissions and maintaining land carbon stocks contributes to climate change mitigation. We summarize current understanding about human perturbation of the global carbon cycle, examine three scientific issues and consider implications for the interpretation of international climate change policy decisions, concluding that considering carbon storage on land as a means to ‘offset’ CO₂ emissions from burning fossil fuels (an idea with wide currency) is scientifically flawed. The capacity of terrestrial ecosystems to store carbon is finite and the current sequestration potential primarily reflects depletion due to past land use. Avoiding emissions from land carbon stocks and refilling depleted stocks reduces atmospheric CO₂concentration, but the maximum amount of this reduction is equivalent to only a small fraction of potential fossil fuel emissions.
4. Gignac, Renaud, and H. Damon Matthews. “Allocating a 2 C cumulative carbon budget to countries.” Environmental Research Letters 10.7 (2015): 075004.  Recent estimates of the global carbon budget, or allowable cumulative CO₂ emissions consistent with a given level of climate warming, have the potential to inform climate mitigation policy discussions aimed at maintaining global temperatures below 2 °C. This raises difficult questions, however, about how best to share this carbon budget amongst nations in a way that both respects the need for a finite cap on total allowable emissions, and also addresses the fundamental disparities amongst nations with respect to their historical and potential future emissions. Here we show how the contraction and convergence (C&C) framework can be applied to the division of a global carbon budget among nations, in a manner that both maintains total emissions below a level consistent with 2 °C, and also adheres to the principle of attaining equal per capita CO₂emissions within the coming decades. We show further that historical differences in responsibility for climate warming can be quantified via a cumulative carbon debt (or credit), which represents the amount by which a given country’s historical emissions have exceeded (or fallen short of) the emissions that would have been consistent with their share of world population over time. This carbon debt/credit calculation enhances the potential utility of C&C, therefore providing a simple method to frame national climate mitigation targets in a way that both accounts for historical responsibility, and also respects the principle of international equity in determining future emissions allowances.
5. Rogelj, Joeri, et al. “Mitigation choices impact carbon budget size compatible with low temperature goals.” Environmental Research Letters 10.7 (2015): 075003.  Global-mean temperature increase is roughly proportional to cumulative emissions of carbon-dioxide (CO₂). Limiting global warming to any level thus implies a finite CO₂ budget. Due to geophysical uncertainties, the size of such budgets can only be expressed in probabilistic terms and is further influenced by non-CO₂ emissions. We here explore how societal choices related to energy demand and specific mitigation options influence the size of carbon budgets for meeting a given temperature objective. We find that choices that exclude specific CO₂mitigation technologies (like Carbon Capture and Storage) result in greater costs, smaller compatible CO₂ budgets until 2050, but larger CO₂ budgets until 2100. Vice versa, choices that lead to a larger CO₂ mitigation potential result in CO₂ budgets until 2100 that are smaller but can be met at lower costs. In most cases, these budget variations can be explained by the amount of non-CO₂ mitigation that is carried out in conjunction with CO_2, and associated global carbon prices that also drive mitigation of non-CO₂ gases. Budget variations are of the order of 10% around their central value. In all cases, limiting warming to below 2 °C thus still implies that CO₂ emissions need to be reduced rapidly in the coming decades.
6. Riahi, Keywan, et al. “Locked into Copenhagen pledges—implications of short-term emission targets for the cost and feasibility of long-term climate goals.” Technological Forecasting and Social Change 90 (2015): 8-23.  This paper provides an overview of the AMPERE modeling comparison project with focus on the implications of near-term policies for the costs and attainability of long-term climate objectives. Nine modeling teams participated in the project to explore the consequences of global emissions following the proposed policy stringency of the national pledges from the Copenhagen Accord and Cancún Agreements to 2030. Specific features compared to earlier assessments are the explicit consideration of near-term 2030 emission targets as well as the systematic sensitivity analysis for the availability and potential of mitigation technologies. Our estimates show that a 2030 mitigation effort comparable to the pledges would result in a further “lock-in” of the energy system into fossil fuels and thus impede the required energy transformation to reach low greenhouse-gas stabilization levels (450 ppm CO₂e). Major implications include significant increases in mitigation costs, increased risk that low stabilization targets become unattainable, and reduced chances of staying below the proposed temperature change target of 2 °C in case of overshoot. With respect to technologies, we find that following the pledge pathways to 2030 would narrow policy choices, and increases the risks that some currently optional technologies, such as carbon capture and storage (CCS) or the large-scale deployment of bioenergy, will become “a must” by 2030.
7. Rogelj, Joeri, et al. “Differences between carbon budget estimates unravelled.” Nature Climate Change 6.3 (2016): 245.  Several methods exist to estimate the cumulative carbon emissions that would keep global warming to below a given temperature limit. Here we review estimates reported by the IPCC and the recent literature, and discuss the reasons underlying their differences. The most scientifically robust number — the carbon budget for CO₂-induced warming only — is also the least relevant for real-world policy. Including all greenhouse gases and using methods based on scenarios that avoid instead of exceed a given temperature limit results in lower carbon budgets. For a >66% chance of limiting warming below the internationally agreed temperature limit of 2 °C relative to pre-industrial levels, the most appropriate carbon budget estimate is 590–1,240 GtCO₂ from 2015 onwards. Variations within this range depend on the probability of staying below 2 °C and on end-of-century non-CO₂ warming. Current CO₂ emissions are about 40 GtCO₂ yr⁻¹, and global CO₂ emissions thus have to be reduced urgently to keep within a 2 °C-compatible budget.
8. Rogelj, Joeri, et al. “Paris Agreement climate proposals need a boost to keep warming well below 2 C.” Nature 534.7609 (2016): 631.  The Paris climate agreement aims at holding global warming to well below 2 degrees Celsius and to “pursue efforts” to limit it to 1.5 degrees Celsius. To accomplish this, countries have submitted Intended Nationally Determined Contributions (INDCs) outlining their post-2020 climate action. Here we assess the effect of current INDCs on reducing aggregate greenhouse gas emissions, its implications for achieving the temperature objective of the Paris climate agreement, and potential options for overachievement. The INDCs collectively lower greenhouse gas emissions compared to where current policies stand, but still imply a median warming of 2.6–3.1 degrees Celsius by 2100. More can be achieved, because the agreement stipulates that targets for reducing greenhouse gas emissions are strengthened over time, both in ambition and scope. Substantial enhancement or over-delivery on current INDCs by additional national, sub-national and non-state actions is required to maintain a reasonable chance of meeting the target of keeping warming well below 2 degrees Celsius.
9. Anderson, Kevin, and Glen Peters. “The trouble with negative emissions.” Science 354.6309 (2016): 182-183.  In December 2015, member states of the United Nations Framework Convention on Climate Change (UNFCCC) adopted the Paris Agreement, which aims to hold the increase in the global average temperature to below 2°C and to pursue efforts to limit the temperature increase to 1.5°C. The Paris Agreement requires that anthropogenic greenhouse gas emission sources and sinks are balanced by the second half of this century. Because some nonzero sources are unavoidable, this leads to the abstract concept of “negative emissions,” the removal of carbon dioxide (CO₂) from the atmosphere through technical means. The Integrated Assessment Models (IAMs) informing policy-makers assume the large-scale use of negative-emission technologies. If we rely on these and they are not deployed or are unsuccessful at removing CO₂from the atmosphere at the levels assumed, society will be locked into a high-temperature pathway.
10. Pfeiffer, Alexander, et al. “The ‘2 C capital stock’for electricity generation: Committed cumulative carbon emissions from the electricity generation sector and the transition to a green economy.” Applied Energy 179 (2016): 1395-1408.  This paper defines the ‘2°C capital stock’ as the global stock of infrastructure which, if operated to the end of its normal economic life, implies global mean temperature increases of 2°C or more (with 50% probability). Using IPCC carbon budgets and the IPCC’s AR5 scenario database, and assuming future emissions from other sectors are compatible with a 2°C pathway, we calculate that the 2°C capital stock for electricity will be reached by 2017 based on current trends. In other words, even under the very optimistic assumption that other sectors reduce emissions in line with a 2°C target, no new emitting electricity infrastructure can be built after 2017 for this target to be met, unless other electricity infrastructure is retired early or retrofitted with carbon capture technologies. Policymakers and investors should question the economics of new long-lived energy infrastructure involving positive net emissions.
11. Peters, Glen P., et al. “Key indicators to track current progress and future ambition of the Paris Agreement.” Nature Climate Change 7.2 (2017): 118.  Current emission pledges to the Paris Agreement appear insufficient to hold the global average temperature increase to well below 2 °C above pre-industrial levels¹. Yet, details are missing on how to track progress towards the ‘Paris goal’, inform the five-yearly ‘global stocktake’, and increase the ambition of Nationally Determined Contributions (NDCs). We develop a nested structure of key indicators to track progress through time. Global emissions^2,3 track aggregated progress¹, country-level decompositions track emerging trends^4,5,6 that link directly to NDCs⁷, and technology diffusion^8,9,10 indicates future reductions. We find the recent slowdown in global emissions growth¹¹ is due to reduced growth in coal use since 2011, primarily in China and secondarily in the United States¹². The slowdown is projected to continue in 2016, with global CO₂ emissions from fossil fuels and industry similar to the 2015 level of 36 GtCO₂. Explosive and policy-driven growth in wind and solar has contributed to the global emissions slowdown, but has been less important than economic factors and energy efficiency. We show that many key indicators are currently broadly consistent with emission scenarios that keep temperatures below 2 °C, but the continued lack of large-scale carbon capture and storage¹³ threatens 2030 targets and the longer-term Paris ambition of net-zero emissions.
12. Millar, Richard J., et al. “Emission budgets and pathways consistent with limiting warming to 1.5 C.” Nature Geoscience10.10 (2017): 741.  The Paris Agreement has opened debate on whether limiting warming to 1.5 °C is compatible with current emission pledges and warming of about 0.9 °C from the mid-nineteenth century to the present decade. We show that limiting cumulative post-2015 CO₂ emissions to about 200 GtC would limit post-2015 warming to less than 0.6 °C in 66% of Earth system model members of the CMIP5 ensemble with no mitigation of other climate drivers. We combine a simple climate–carbon-cycle model with estimated ranges for key climate system properties from the IPCC Fifth Assessment Report. Assuming emissions peak and decline to below current levels by 2030, and continue thereafter on a much steeper decline, which would be historically unprecedented but consistent with a standard ambitious mitigation scenario (RCP2.6), results in a likely range of peak warming of 1.2–2.0 °C above the mid-nineteenth century. If CO₂emissions are continuously adjusted over time to limit 2100 warming to 1.5 °C, with ambitious non-CO₂ mitigation, net future cumulative CO₂emissions are unlikely to prove less than 250 GtC and unlikely greater than 540 GtC. Hence, limiting warming to 1.5 °C is not yet a geophysical impossibility, but is likely to require delivery on strengthened pledges for 2030 followed by challengingly deep and rapid mitigation. Strengthening near-term emissions reductions would hedge against a high climate response or subsequent reduction rates proving economically, technically or politically unfeasible.

Posted by: chaamjamal on: August 26, 2020

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