What baseline should we use in our Paris target global warming analyses?

With the IPCC report on 1.5°C due to come out next year and a rush of publications on the impacts of 1.5°C and 2°C global warming likely before that, it’s important to consider what baseline we use for our analyses. This is the subject of an upcoming article in The Conversation that I wrote with Ben Henley and Ed Hawkins. Here, I briefly discuss some of the different options and their pros and cons. Of course for different studies some baselines might be more appropriate than others.

Humans have been releasing greenhouse gases into the atmosphere for a long time. This makes choosing a true pre-industrial baseline difficult. Source: FreeImages.com- Markus Biehal.

The main choices seem to be:

1. A late 19th century period, such as 1850-1900 (e.g. Henley and King 2017) or 1860-1880 (e.g. Mitchell et al. 2017).
2. An earlier period, such as 1720-1800 (e.g. Hawkins et al. 2017).
3. Climate model simulations without anthropogenic forcings (e.g. King et al. 2017).

1. Late 19th century baselines:
Pros:

• The major instrumental global temperature records start in this period (1850 for Hadley Centre, 1880 for GISS and NOAA). This provides a greater degree of confidence over global temperatures prior to the mid-19th century.
• We can directly calculate how much warmer the present period is relative to this baseline.

Cons:    

• This period is well after the Industrial Revolution started. As a result greenhouse gas concentrations are above natural levels. There were also several major volcanic eruptions, including Krakatoa in 1883, which may have countered any substantial anthropogenic warming effect.
• For a coupled model analysis, using a late 19th century baseline that is shorter than about 30 years, or assuming an historical warming to a more recent but similarly short baseline such as the HAPPI experiment (Mitchell et al. 2017), would cause difficulties. Decadal climate variability associated with accelerated warming and “hiatus” phases in the current climate, would mean that different climate model simulations would be in different phases during baseline periods. This would result in an inaccurate representation of the anthropogenic warming to the 1.5°C target as warming effects would be conflated with the effects of decadal variability.

2. Earlier baselines:
Pros:

• Using an earlier baseline, such as the 1720-1800 period proposed by Hawkins et al. (2017), provides a truer “pre-industrial” baseline.
• A period can also be selected to match relatively well with the present with respect to external forcings (i.e. solar variability and major volcanic eruptions).

Cons:    

• We have far less information available to us about the Earth’s climate at that time. This would make understanding the difference in climates between then and either the present or a 1.5°C world more difficult.
• It would also be more difficult to run climate model experiments over earlier windows as we have less information and less confidence in important factors used to force the models, such as aerosol concentrations.

3. Natural climate model simulation baselines:
Pros:

• Allows for easy comparison with CMIP5 model projections at 1.5°C and 2°C global warming.
• We have a large selection of available suitable simulations in the CMIP5 archive (either historicalNat or de-drifted piControl experiments) and can construct long baselines less influenced by decadal climate variability and with no anthropogenic forcings.

Cons:    

• We can’t make easy direct comparisons to the observed warming.

 
The best choice of baseline depends on the specific design of the experiment and purpose of analysis. This poses a problem for selecting and putting forward a specific baseline to be used across analyses. If a pre-determined baseline is used for all analysis conducted in preparation for the IPCC special report on 1.5°C then it will need to compromise between the competing advantages and disadvantages of each baseline.

References:

Hawkins, E., et al., 2017: Estimating changes in global temperature since the pre-industrial period. Bull. Amer. Meteorol. Soc., doi: 10.1175/BAMS-D-16-0007.1.

Henley, B. J. and A. D. King, 2017: Trajectories towards the 1.5°C Paris target: modulation by the Interdecadal Pacific Oscillation. Geophys. Res. Lett., doi: 10.1002/2017GL073480.

King, A. D., D. J. Karoly, and B. J. Henley, 2017: Australian climate extremes at 1.5 and 2 degrees of global warming. Nature Climate Change, doi:10.1038/nclimate3296.

Mitchell, D., et al., 2017: Half a degree additional warming, prognosis and projected impacts (HAPPI): background and experimental design. Geosci. Model Dev., 10, 571-583, doi:10.5194/gmd-10-571-2017, 2017.