Jochem Marotzke, Director of Germany’s Max Planck Institute for Meteorology (MPIM), says we would have to wait 20 years before seeing any impact on climate from CO2 reductions, based on model simulations. Climate variability prevails…
Jochem Marotzke, director of Germany’s Max Planck Institute for Meteorology (MPIM), wondered whether CO2 savings could really have a direct influence on the temperature in the near future. In a new paper (Marotzke 2018), the Hamburg-based climate researcher simulates the temperature profile of the 2030s predicted by climate models and uses once again a conventional emission profile (Scenario RCP 4.5), and once a politically reduced emission scenario.
Conclusion: Most likely, there would probably be no difference as natural climate variability prevails over these time scales. The paper was published in WIRE’s Climate Change and can be downloaded free of charge as a pdf:
Quantifying the irreducible uncertainty in near‐term climate projections
If the Paris agreement at the Conference of Parties 21 is implemented very effectively, greenhouse‐gas emissions might decrease after year 2020. Whether this would lead to identifiable near‐term responses in “iconic” climate quantities of wide scientific and public interest is unclear, because the climate response would be obscured by quasi‐random internal variability. I define the climate response as an increase or decrease in a linear climate trend over the period 2021–2035, compared to 2006–2020, and establish the probability of such a trend change being caused by an assumed policy shift toward emissions reductions after 2020. I quantify the irreducible uncertainty in projecting such a trend change through very large (100‐member) ensembles of the state‐of‐the‐art climate model MPI‐ESM‐LR. Trends in global‐mean surface temperature (GMST) are higher over the period 2021–2035 than over 2006–2020 in one‐third of all realizations in the mitigation scenario RCP2.6, interpreted as implementing the Paris agreement, compared to around one‐half in the no‐mitigation scenario RCP4.5. Mitigation is sufficient to cause a GMST trend reduction with a probability of 0.40 and necessary with a probability of 0.33. Trend increases in Arctic September sea‐ice area and the Atlantic meridional overturning circulation are caused by the emissions reductions with a probability of only around 0.1. By contrast, emissions reductions are necessary for a trend decrease in upper‐ocean heat content with a probability of over one‐half. Some iconic climate quantities might thus by year 2035 exhibit an identifiable response to a successful Paris agreement but sometimes with low probability, creating a substantial communication challenge.”
In the conclusion, there are some even clearer statements. Marotzke warns that even painful efforts to reduce CO2 in the next two decades could have little impact on the climate:
My thought experiment demonstrates that it is crucial to have realistic expectations of the efficacy of climate policy in the near‐term: Even if greenhouse‐gas emissions begin to decline after year 2020, the probability is substantial that the response of iconic climate quantities to this decline will not have emerged by year 2035.”
Science communication challenge
With 90% probability, the Arctic Sea Ice (SIA) and the Gulf Stream (AMOC) will not respond to changes in CO2 emissions in the 2030s. Marotzke already sees a great communication challenge for the scientists, similar to the unexpected hiatus of recent years.
The major advance brought about by my analysis lies in the ability to quantify the degree of irreducible uncertainty about whether the assumed emissions reduction will cause the desired climate response over a given timescale. The probability of this response occurring depends on the quantity in question but also on the type of causation; for the time horizon out to 2035 the probability lies here in the range between a bit under 0.1 for causation both sufficient and necessary for SIA and AMOC and a bit above one‐half for necessary causation for ocean heat content.
Communicating these probabilities will be nontrivial but will be aided by the precise definitions and meanings underlying them (Hannart et al., 2016; Pearl, 2000). The communication challenge (Deser et al., 2012) furthermore supports the notion that the recent hiatus was not a distraction to the scientific community (Lewandowsky, Risbey, & Oreskes, 2016) but instead provided an opportunity to communicate the role of internal variability (Fyfe et al., 2016) to an audience that might otherwise be disinclined to engage in this discourse.”
The climate sciences continue to navigate in difficult waters. The natural variability causes them huge problems because it has been neglected in the models.
Marotzke still dreams that nature only produces noise (“quasi-random internal variability”). However, the day will surely come when he will also acknowledge the systematic effect of natural climate factors such as ocean cycles and solar activity fluctuations. Perhaps he should start to take an interest in paleoclimatology, which is leaving him in the dust…