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COP 21: A Pathway for 1.5°C

The case for limiting the rise in global temperatures to 2°C was made many years ago and finally agreed at COP16 in Cancun in 2010. But the text noted the importance of an even more aggressive target, notably 1.5°C, proposed by the small island states who were deeply concerned about future sea level rise. While 1.5°C doesn’t guarantee to limit sea level rise such that certain island nations remain safe, it does further shift the global risk profile in terms of possible major changes in the ice shelves.

The idea of a 1.5°C goal has remained largely in the background since 2010, but COP21 has brought the issue to the forefront of negotiators minds, with a reported group of some 100 countries now willing to support such an objective. At a reception early in the second week, the UK Climate Minister was very upbeat about the 1.5°C goal and the government’s role in working with AOSIS (Alliance of Small Island States). At the COP Plenary on Wednesday night (9th December), many groups and nations spoke about the need for a 1.5°C goal.  But while there is increasing enthusiasm for and talk about such a goal, there seems to be limited substantive discussion on the feasibility of achieving it.

As often discussed in my postings, the expected global temperature rise is closely linked with cumulative emissions over time, not the level of emissions in a certain year. This means that what might have seemed achievable in 2010, is all the more difficult in 2015 with higher emissions and continued upward pressure. In fact, between 2010 and 2015 another 60 billion tonnes of carbon has been released into the atmosphere. Total emissions since 1750 now stand at just under 600 billion tonnes carbon, with 1.5°C equivalent to some 750 billion tonnes carbon based on a climate sensitivity of 2°C per trillion tonnes. Even if emissions were to continue to plateau as we have seen over 2014-2015, the 1.5°C threshold would be reached as early as 2028.

There are always a variety of trajectories possible for any temperature goal, but 1.5°C offers little room for flexibility, given its stringency. One such pathway which adds up to ~750 billion tonnes carbon by 2100 is shown below (global CO2 emissions on the vertical scale). In this pathway, global net zero emissions must be reached in just 40 years (860 billion tonnes accumulation), followed by another half century of atmospheric carbon removal and storage (~100 billion tonnes removal). Some 10 billion tonnes of CO2 must be removed and stored each year by late in the century, either through bio-energy with carbon capture and storage (BECCS) or direct air capture of CO2 and subsequent storage (DACCS). Significant reforestation would also play a major role. With infrastructure in place, the 22nd century might even offer the possibility of drawing down on CO2 below a level that corresponds with 1.5°C.


Apart from massive reliance on CCS both on the way to net zero emissions and afterwards to correct the over accumulation, such a plan would require a complete rebuild of the energy system in just 40 years. This would include the entire industrial system, all transport and power generation. Alternatives would have to be found for many petroleum based products and a new large scale synthetic hydrocarbon industry would be needed for sectors such as aviation and shipping. While agriculture is largely a bio based emissions system, a solution to agricultural methane emissions would also nevertheless be needed.

A pathway that doesn’t involve future use of CCS would require net zero emissions in just 23 years – an option that isn’t even remotely feasible. Returning to the 40 year pathway, even this presents an immensely challenging task. While it might be feasible to have a zero emissions power sector in under 40 years, particularly given that all the necessary technologies to do so exist in one form or another, electricity still represents only 20% of final energy use. Solutions would have to be found for all other sectors, which in many instances involves electrification and therefore places a significant additional load on the redevelopment of the power generation system. Aviation would be particularly tricky.

Finally, there is CCS itself. The pathway above (and almost any other 1.5°C pathway) is completely dependent on it, yet the technology is hardly deployed today. It is certainly commercially ready, but the barriers to deployment are many, ranging from the lack of an economic case for project development to public concern about deep storage of carbon dioxide. The later that net zero emissions is reached, the greater the post net zero dependence on CCS becomes.

While the case for 1.5°C has certainly been made from a climate perspective, it has yet to be demonstrated from an implementation perspective.

David Hone's picture

Thank David for the Post!

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Jim Baird's picture
Jim Baird on Dec 10, 2015 4:36 pm GMT

Sea level rise has three components all of which are mitigated by producing energy moving surface ocean heat to the deep through heat engines. The first is thermal expansion as the oceans heat up. The coefficient of expansion of sea water at 1000 meters is half that of the tropical surface thus this component of sea level rise is also halved by moving surface heat to deeper water. The greatest long-term sea level threat is melting of the polar icecaps and here too heat moved to the deep is no longer available to melt ice. And the final component is the movement of terrestrial fossil water into the sea as a result of aquifer pumping. To get OTEC power to shore, in most cases, requires the conversion of electricity to an energy carrier like hydrogen by electrolysis. Utilized to its maximum potential OTEC would convert 15 cubic kilometers worth of sea water to gas each year and then back to water on land as the hydrogen is used to produce energy. This could provide 600 gallons annually for every individual on the planet and greatly reduce the need for aquifer pumping. And if you want CCS, the14 terawatts of primary energy  use to produce the energy/water carrier hydrogen using the “supergreen” electrolysis technique developed by a team from Lawrence Livermore Laboratories would sequester 79 billion metric tons of carbon dioxide annually. 

There are faux answers to climate change and then real ones based on the science.  

Mark Heslep's picture
Mark Heslep on Dec 13, 2015 2:50 am GMT

Even if emissions were to continue to plateau as we have seen over 2014-2015, the 1.5°C threshold would be reached as early as 2028.”

David – could you exlain that 2028/1.5°C possibility?   Current temperature anomaly is ~0.8°C above pre-industrial.  What conditions could drive the balance, 0.7°C in 13 yrs, or more than ten times the trend for the last 15 years?

IPCC Synythesis

…the rate of warming over the past 15 years (1998–2012; 0.05 [–0.05 to 0.15] °C per decade), which begins with a strong El Niño, is smaller than the rate calculated since 1951 (1951–2012; 0.12 [0.08 to 0.14] °C per decade). {1.1.1, Box 1.1}

David Hone's picture
David Hone on Dec 13, 2015 12:04 pm GMT


This is based entirely on cumulative emissions, not observed temperature. Although we don’t know the exact relationship, I have assumed a base climate sensitivity of 2C per trillion tonnes carbon emitted (Allen et. al, 2009), which implies we have locked in 1.2C already as emissions from 1750 are some 600 billion tonnes carbon ( Of course we haven’t actually observed 1.2C yet.

This relationship implies that 750 billion tonnes carbon is 1.5C, which we get to in about 2028, assuming a 600 billion tonne starting point and 11+ billion tonnes carbon per annum.

It may of course transpire that the relationship is not 2C per trillion.


Mark Heslep's picture
Mark Heslep on Dec 13, 2015 6:44 pm GMT

Thanks, I see then that the your point is 1.5°C of eventual temerature rise would be locked in by 2028, to be realized some decades hence.  A realized temperature anomaly of 1.5°C actually in 2028 would be impossible. 

David Hone's picture
David Hone on Dec 13, 2015 8:50 pm GMT

Yes, agree. I should have made that a bit clearer.

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