Carbon cycle feedbacks and the worst-case greenhouse gas pathway

Originally published at Skeptical Science on July 7, 2015


The worst-case emissions pathway, RCP8.5, is a scenario that burns a huge amount of fossil fuels, especially coal. The model has sometimes been criticized as implausible because of its huge resource consumption and emissions of ~1700 billion tonnes of carbon (GtC) over the century. Those emissions are based in part on carbon cyclemodel assumptions, which recent work suggests may be too optimistic. New research shows that future plant growth may be restricted by nutrient availability, turning the land carbon sink into a source. Also, permafrost feedbacks (not considered in IPCC CMIP5 models) may also add significant emissions to the atmosphere under the RCP8.5 pathway. In addition, the latest research on the Amazon Basin reveals that the tropical forest carbon sinks may already be diminishing there. Together, these feedbacks suggest that the greenhouse gas concentrations in the RCP8.5 case could be achieved with ~400 GtC smaller human emissions, making the RCP8.5 worst-case scenario more plausible.

The climate models referred to  in the recent IPCC Fifth Assessment Report (AR5) are founded on one of four Representative Concentration Pathways or RCPs. The key word in RCP is concentration. In the RCPs, the concentration of greenhouse gases is fixed at different times in the future and the climate model (or general circulation model or GCM) uses those atmospheric concentrations to calculate future climate states. Underpinning the concentration pathways are socio-economic and emissions scenarios. There can be more than one underlying emissions scenario capable of producing the concentration pathway.

If you are unfamiliar with RCPs, check out the great guide that Graham Wayne wrote in August 2013 for Skeptical Science.

This way of modelling differs from previous approaches in which the starting point was a story or scenario about economic and social development that led to emissions. These emissions are run through a carbon-cycle model (which may be simple or complex) to produce atmospheric concentrations over time.

The schematic illustrates the differences in approach. The elements in red boxes are the prescribed inputs into the models, whereas the elements in blue ellipses are outputs. The advantage of the RCP prescribed-concentration approach is that the climate model outputs do not depend to the same degree on carbon-cycle models as they did in the emissions scenario method. The disadvantage is that there is no unique link between concentrations and emissions. The schematic is simplified in that there are feedbacks and loops in the processes that are not illustrated. Continue reading

Are we overestimating our global carbon budget?

Research on carbon-cycle feedbacks suggests we have less wiggle room to turn the climate ship around.

(First published at Corporate Knights on July 7, 2015.)

Illustration by Yarek Waszul

The latest research suggests that natural sinks of carbon on land may be slowing or even turning into sources, creating climate consequences potentially worse than first thought.

Nature has provided humans with a buffer against the worst effects of our carbon pollution. Since 1750, we have emitted about 580 billion tonnes of carbon into the atmosphere by burning fossil fuels, cutting down forests and making cement. If those emissions had simply accumulated in the air, the concentration of carbon dioxide would have increased from 280 parts per million (ppm), as it was before the Industrial Revolution, to about 550 ppm today. Instead, we currently measure around 400 ppm, which is still a whopping 40 per cent above the planet’s pre-industrial atmosphere, but much less than a doubling.

Some 60 per cent of our emissions have been taken up in natural sinks by, in roughly equal parts, dissolving into the ocean and by being taken up by plants growing faster on land. Were it not for these natural carbon sinks, we would by now be much deeper into dangerous climate change.

As we continue to burn fossil fuels, our climate troubles will become worse should those sinks start to falter. And the outlook will be worse still if those sinks turn into sources of carbon.

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Unsure about how to talk about uncertainty? Read this

2015-07-05_15-39-46Adam Corner, Stephan Lewandowsky, Mary Phillips, and Olga Roberts have today published The Uncertainty Handbook, which is a twenty-page practical guide for climate communicators. It is excellent, clearly written and I recommend that everyone read it all. Adam Corner has a blogpost outlining the Handbook at Shaping Tomorrow’s World.

As the authors say, everyone is already familiar with making decisions under uncertainty. To make plans means accepting that they may not unfold as expected. The natural world is chaotic, with many known-unknowns and unknown-unknowns. The human world is even more unpredictable. We all develop rules-of-thumb to deal with everyday uncertainty: whether to take an umbrella, when to take a vacation, whose advice to trust. But we humans do not always have good instincts when it comes to grasping the uncertainty of novel and unfamiliar situations. As Daniel Kahneman showed in his masterpiece Thinking, Fast and Slow we are often not very smart when confronting probability or logic problems in our lives, especially when they are expressed numerically.  The Economist reviewer wrote:

In one experiment described by Mr Kahneman, participants asked to imagine that they have been given £50 behave differently depending on whether they are then told they can “keep” £20 or must “lose” £30—though the outcomes are identical. He also shows that it is more threatening to say that a disease kills “1,286 in every 10,000 people”, than to say it kills “24.14% of the population”, even though the second mention is twice as deadly. Vivid language often overrides basic arithmetic.

Framing is everything.

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