Pathways to 1.5 °C and 2 °C warming based on observational and geological constraints

  • Philip Goodwin, Anna Katavouta, Vassil M. Roussenov, Gavin L. Foster, Eelco J. Rohling, Richard G. Williams
  • Nature Geoscience, January 2018, Nature
  • DOI: 10.1038/s41561-017-0054-8

How much time is left before we reach global warming targets of 1.5°C or 2°C?

What is it about?

The Paris climate agreement aspires to restrict the rise in global mean surface temperature since the pre-industrial period to 2°C or less for this century by reducing global carbon emissions. However, there are large uncertainties in how much carbon may be emitted before reaching a warming target. We demonstrate a new approach to reduce the uncertainty of climate projections; using theory and geological constraints we generate a very large ensemble of several thousand projections that closely match historical records for nine key climate metrics, including surface warming and ocean heat content. Our breakthrough is by ensuring our projections are consistent with the historical record, our analysis significantly narrows the uncertainty in surface warming projections. We find that there is less than two decades before we reach 1.5°C warming and less than four decades before we reach 2°C warming. In more detail, we find that a warming target of 1.5°C above the preindustrial requires that the total emitted carbon from the start of year 2017 to be less than 195 to 205 PgC in over 66% of simulations (equivalently 715 to 750 Gt CO2). At current emission rates, this 1.5°C warming target is reached in 17 to 18 years. We find that a warming target of 2°C is only likely if the emitted carbon remains less than 395 to 455 PgC in over 66% of simulations (equivalently 1450 to 1670 Gt CO2). At current emission rates, this 2 °C warming is reached in 35 to 41 years. In comparison to our work, 13 Earth system models forced by a realistic forcing suggest that 2°C warming might occur with cumulative carbon emissions ranging from 84 to 581 PgC from year 2017 (or equivalently 308 to 2130 Gt CO2). This range in how much future carbon may be emitted to meet specific warming targets is much larger than our projections. Note about units: 1 Peta gram of carbon is denoted by 1 PgC=10 power 15 gC, which may equivalently be written as 1 Giga tonne C denoted by 1 GtC. 1 GtC of carbon is equivalent to 3.7 Gt of carbon dioxide.

Why is it important?

We need to know how much carbon we may emit and over what timescale before we meet warming targets. In these windows of 17 to 18 years before 1.5°C warming or 35 to 41 years before 2°C warming, we need either to develop a more carbon-efficient future or prepare to mitigate for the adverse effects of a warming climate. To develop a more carbon-efficient future, we need to develop and adopt new technologies, and plan and organise our societies, to use energy more efficiently and reduce carbon emissions. Ultimately, we need to explore ways of moving towards a carbon-neutral future and ways of capturing carbon to reduce how much carbon dioxide is in the atmosphere. We all face challenges in making this transition, but the earlier that we start moving towards a more carbon-efficient future the easier it will be to meet these warming targets.


Professor Richard G Williams
University of Liverpool Department of Earth Ocean and Ecological Sciences

In our view, the timescale of nearly two decades before the expected warming of 1.5°C is very short to deliver the necessary mitigation, but the timescale of three to four decades before the expected warming of 2°C is sufficiently long for there to be the possibility to develop new technologies and planning systems, and to implement them on a global scale. Given the technical and societal challenges to overcome in moving towards a carbon neutral future, it is important that as much effort is made as possible in moving forward. Without concerted effort, in one to two decades, we will instead be discussing how to cope with living with a warmer world and how to avoid exceeding higher temperature targets. The requirement that our model projections needed to be consistent with historical records led to a much narrower projection, than previously found, for how much carbon we may emit before reaching the warming targets. How much time is left before reaching these warming targets is then estimated by assuming that the carbon emission rate stays the same. If the carbon emission rate reduces, then we have more time left before exceeding the warming targets, while if the carbon emission rate increases, then we have even less time before exceeding the warming targets. In terms of how we worked on this study, the paper draws upon inputs from researchers with different skills. The underlying theory was published in Nature Geoscience in 2014, where for the first time Goodwin and Williams provided a single equation connecting global warming to cumulative carbon emissions. Goodwin then developed an efficient, Earth system model (WASP) to develop an ensemble of model projections. These projections drew on geological constraints for climate sensitivity from Foster and Rohling. The model projections were only retained if they satisfied consistency tests against historical data, including surface temperature records and ocean heat content. Katavouta and Roussenov helped provide the reanalyses of ocean heat content data. Our breakthrough is in how the historical data has constrained our ensemble of model projections and, thus, provided a narrower window of how much carbon we emit and how much time is left before reaching the Paris warming targets.

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