Why We Have Already Overshot The Carbon Budget

There has been a lot of talk in the past few years about the remaining carbon budget. This is the amount of additional carbon dioxide that we can emit into the atmosphere that will give us a 66% chance of not triggering a “dangerous” increase in global temperatures of over 20C (450 ppm of atmospheric CO2). Yes, this is for only a “2 out of 3” chance of saving human society. At higher percentages, say 90%, there is no remaining budget.

Taking into account other climate-warming gases, such as methane, the remaining budget is estimated at 800 gigatonnes of CO2 from the start of 2017. At the current rate of global emissions, this budget will be used up within 20 years – the mid 2030’s[1]. The global climate policy body (the United Nations International Panel on Climate Change) already assumes that we will temporarily overshoot 450ppm. Speculative technologies will then be used to suck enough carbon out of the air to have net negative emissions[2], as I covered in another post[3].

The budget is calculated by using climate models to assess the responsiveness of the climate to increased levels of carbon dioxide. Unfortunately, recent scientific findings seem to be blowing gaping holes in the conservative assumptions used in the models. Yes, you read that correctly – the climate models are too conservative. Below are only some of the assumptions that have lead to the models underestimating the extent of future climate warming in response to increased levels of CO2.

Natural Emissions Will Increase

A new study points[4] to significant future increases in soil carbon emissions, equal to more than 10% of projected human-generated emissions. This amount would need to be removed from the carbon budget, knocking at least 2 years off the time left at current rates of emissions. The increased emissions are mainly from the Northern Hemisphere, the Arctic and sub-Arctic, as soils reach temperatures more conducive for microbes that break down the carbon in the soil. The results come from an analysis of 20 years worth of data from many different studies.

Increases In Artic Clouds Will Not Offset Reduced Reflectivity (Albedo) From Arctic Sea Ice Loss

The climate models tend to assume that the Arctic will not become ice-free for decades and that even when it does, increased levels of reflective summer clouds will help offset the reduced reflectivity of water versus ice. With this assumption, reduced Arctic sea ice will not result in increased levels of the sun’s energy from being captured (rather than reflected back out to space) by the increasing areas of dark waters. In the real world, Arctic sea ice is disappearing much faster than the models assume – especially when measured on a volume basis[5]. Research has shown that between 1979 and 2011 the Arctic albedo fell from 52% to 48%, adding as much warming as 25% of that from human climate change emissions[6] [7]. As more sea ice is lost around the peak of summer, when the Sun’s energy that reaches the Arctic is at its highest, the impact of the lost albedo will accelerate. By itself, this could have an effect greater than all of the incremental emissions assumed in the carbon budget.

It was thought that more open waters would lead to more evaporation and therefore increases in the amount of reflective clouds, but a recent analysis has shown that things are a bit more complicated than that[8] [9]. It seems that the Arctic summer is not that conducive to extra cloud formation, exactly the time when the sun is high in the Arctic sky. Even worse, the Fall may see a significant increase in cloudiness. In the darkness of the Arctic Fall, the extra clouds will reflect back energy emitted from the surface. They will act like a blanket to keep in the extra energy taken in during the summer, adding to the rate of warming.

Clouds Less Reflective Than Assumed

Clouds contain water in both liquid and solid-ice forms, and the amount of each greatly affects the reflectiveness of the clouds. Ice is much more reflective than water. Unfortunately, a new study seems to show that climate models have seriously under-estimated the amount of water in the clouds, and have therefore over-estimated their reflectivity[10]. In addition, less clouds may turn into a more reflective state than assumed as the climate warms up[11]. The result will be that more of the Sun’s energy will reach the Earth’s surface, producing a significantly faster rate of warming than given by the models.


[1] Global Carbon Project (2016), Carbon Budget, Global Carbon Project. Accessible at http://www.globalcarbonproject.org/carbonbudget/16/files/GCP_CarbonBudget_2016.pdf

[2] United Nations International Panel for Climate Change (2014), Climate Change 2014 Synthesis Report Summary for Policy Makers. Accessible at https://www.ipcc.ch/pdf/assessment-report/ar5/syr/AR5_SYR_FINAL_SPM.pdf

[3] Roger Boyd (2016), Climate Change Policy and The Super-Hero Syndrome, Humanity’s Test. Accessible at http://www.humanitystest.com/climate-change-policy-and-the-super-hero-syndrome/

[4] Thomas W. Crowther et. al. (2016), Quantifying global soil carbon losses in response to warming, Nature 540(7631):104-108 · November 2016 DOI: 10.1038/nature20150. Accessible at https://www.researchgate.net/publication/311163076_Quantifying_global_soil_carbon_losses_in_response_to_warming

[5] arctischepinguin (2017), PIOMAS Monthly Average Sea Ice Volume with exponential trend, arctischepinguin. Accessible at https://sites.google.com/site/arctischepinguin/home/piomas/grf/piomas-trnd2.png

[6] Laura Poppick (2014), Warming from Arctic Sea Ice Melting More Dramatic than Thought, Live Science. Accessible at http://www.livescience.com/43435-arctic-sea-ice-melt-causes-dramatic-warming.html

[7] Kristina Pistone et. al. (2014), Observational determination of albedo decrease caused by vanishing Arctic sea ice, Proceedings of the National Academy of Sciences doi: 10.1073/pnas.131820111. Accessible at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3948279/

[8] Roberto Molar Candanosa (2016), Clouds and Sea Ice: What Satellites Show About Arctic Climate Change, NASA. Accessible at https://www.nasa.gov/feature/langley/clouds-and-sea-ice-what-satellites-show-about-arctic-climate-change

[9] Patrick C. Taylor et. al. (2016), Covariance between Arctic sea ice and clouds within atmospheric state regimes at the satellite footprint level, Journal of Geophysical Research DOI: 10.1002/2015JD023520. Accessible at http://onlinelibrary.wiley.com/doi/10.1002/2015JD023520/full

[10] Oliver Milman (2016), Global warming may be far worse than thought, cloud analysis suggests, The Guardian. Accessible at https://www.theguardian.com/environment/2016/apr/07/clouds-climate-change-analysis-liquid-ice-global-warming

[11] Ivy Tran et. al. (2016), Observational constraints on mixed-phase clouds imply higher climate sensitivity, Science Vol. 352, Issue 6282, pp. 224-227
DOI: 10.1126/science.aad5300. Accessible at http://science.sciencemag.org/content/352/6282/224



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