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
- THE CARBON BUDGET CONUNDRUM https://tambonthongchai.com/2019/08/16/carbonbudgetconundrum/
- THE REMAINING CARBON BUDGET ANOMALY EXPLAINED https://tambonthongchai.com/?s=carbon+budget
- THE CARBON BUDGETS OF CLIMATE SCIENCE https://tambonthongchai.com/2019/09/21/boondoggle/
- CARBON BUDGETS AND THE TCRE https://tambonthongchai.com/2019/08/06/tcrebudget/
- ILLUSORY CARBON BUDGETS https://tambonthongchai.com/2019/08/02/illusorytcre/
- CARBON BUDGETS AND CLIMATE MITIGATION PATHWAYS https://tambonthongchai.com/2019/01/14/carbonbudget/
- TCRE: TRANSIENT CLIMATE RESPONSE TO CUMULATIVE EMISSIONS https://tambonthongchai.com/2018/05/06/tcre/
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
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- 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.