So there was a non-linear jump at around 220ppm CO2, which is below pre-industrial levels. This means that there may be another such non-linearity but not that there is such a one. Underlines the need for exercising the precautionary principal, if we hit such an unknown non-linearity it may be game over pretty fast.
The paper mentions that the required data is not available for the Pliocene, which had 300-500ppm. As we are already at 500+ppm CO2e we may have already run that required real-time experiment. Using a GWP20 for methane we are already testing a forcing of over 600ppm CO2e. We are trying really hard to find any such non-linearities.
A shortcoming of the discussion part of the paper is the lack of acceptance of the true level of current GHG forcing taking into the non-CO2 gases.
I concatenated the following reposts related to likely increase of El Nino events with continued warming. In particular, N. J. Burls and A. V. Fedorov, (2014) notes that if we approach Pliocene conditions, there may be an abrupt/nonlinear change in the Equatorial Pacific into near continuous El Nino-like conditions:
The first linked reference uses CMIP5 projections to estimate that at least one source of currently increasing positive feedback for increases in atmospheric CO₂ concentrations accelerate is that during El Nino events lead to reductions in terrestrial productivity. So theoretically this natural source should decrease as our current El Nino event continues to degrade:
Jin-Soo Kim, Jong-Seong Kug, Jin-Ho Yoon and Su-Jong Jeong (2016), "Increased atmospheric CO2 growth rate during El Niño driven by reduced terrestrial productivity in the CMIP5 ESMs", Journal of Climate, doi:
http://dx.doi.org/10.1175/JCLI-D-14-00672.1 http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-14-00672.1Abstract: "Better understanding of factors that control the global carbon cycle could increase confidence in climate projections. Previous studies found good correlation between the growth rate of atmospheric CO2 concentration and the El Niño–Southern Oscillation (ENSO). Growth rate of atmospheric CO2 increases during El Niño but decreases during La Niña. In this study, long-term simulations of the Earth System Models (ESMs) in the Coupled Model Intercomparison Project Phase 5 archive were used to examine the interannual carbon flux variability associated with ENSO. The ESMs simulate the relationship reasonably well with a delay of several months between ENSO and the changes in atmospheric CO2. The increase in atmospheric CO2 associated with El Niño is mostly caused by decreasing Net Primary Production (NPP) in the ESMs. It is suggested that NPP anomalies over South Asia are at their maxima during boreal spring; therefore, the increase in CO2 concentration lags four to five months behind the peak phase of El Niño. The decrease in NPP during El Niño may be caused by decreased precipitation and increased temperature over tropical regions. Furthermore, systematic errors may exist in the ESM-simulated temperature responses to ENSO phases over tropical land areas, and these errors may lead to overestimation of ENSO-related NPP anomalies. In contrast, carbon fluxes from heterotrophic respiration and natural fires are likely underestimated in the ESMs compared with offline model results and observational estimates, respectively. These uncertainties should be considered in long-term projections that include climate–carbon feedbacks."
Also, many scientists are concerned that the Earth may be headed towards a Pliocene type of climate this century due to global warming. The linked reference indicates that changes in cloud cover/albedo for such conditions would rapidly induce the Equatorial Pacific Ocean into a permanent El Nino-like state. As cloud albedo is a rapid response feedback mechanism, such a change could happen in as little as a few decades from now (say 2040-2050). Permanent El Nino-like conditions would telecommunicate large amounts of heat from the Equatorial Pacific directly to West Antarctica.
N. J. Burls and A. V. Fedorov, (2014), "Simulating Pliocene warmth and a permanent El Niño-like state: the role of cloud albedo", Paleoceanography, DOI: 10.1002/2014PA00264
http://onlinelibrary.wiley.com/doi/10.1002/2014PA002644/abstractAbstract: "Available evidence suggests that during the early Pliocene (4-5 Ma) the mean east–west sea surface temperature (SST) gradient in the equatorial Pacific Ocean was significantly smaller than today, possibly reaching only 1-2°C. The meridional SST gradients were also substantially weaker, implying an expanded ocean warm pool in low latitudes. Subsequent global cooling led to the establishment of the stronger, modern temperature gradients. Given our understanding of the physical processes that maintain the present-day cold tongue in the east, warm pool in the west and hence sharp temperature contrasts, determining the key factors that maintained early Pliocene climate still presents a challenge for climate theories and models. This study demonstrates how different cloud properties could provide a solution. We show that a reduction in the meridional gradient in cloud albedo can sustain reduced meridional and zonal SST gradients, an expanded warm pool and warmer thermal stratification in the ocean, weaker Hadley and Walker circulations in the atmosphere. Having conducted a range of hypothetical modified cloud albedo experiments, we arrive at our Pliocene simulation, which shows good agreement with proxy SST data from major equatorial and coastal upwelling regions, the tropical warm pool, mid and high latitudes, and available subsurface temperature data. As suggested by the observations, the simulated Pliocene-like climate sustains a robust ENSO despite the reduced mean east–west SST gradient. Our results demonstrate that cloud albedo changes may be a critical element of Pliocene climate and that simulating the meridional SST gradient correctly is central to replicating the geographical patterns of Pliocene warmth.""
Also, Kim et al (2014) indicates that before 2040 CMIP5 models indicate that the amplitude of the ENSO phases will increase, indicating that when the El Nino events return for the next 25-years they are likely to be stronger than previously experienced leading to more abrupt climate change [see: Seon Tae Kim, Wenju Cai, Fei-Fei Jin, Agus Santoso, Lixin Wu, Eric Guilyardi & Soon-Il An, (2014), "Response of El Niño sea surface temperature variability to greenhouse warming", Nature Climate Change, doi:10.1038/nclimate2326].
Also, Praetorius & Mix (2014) provide paleo-evidence of the importance of the synchronization of the North Pacific, and the North Atlantic, Oceans on Artic amplification: Summer K. Praetorius, Alan C. Mix, (2014), "Synchronization of North Pacific and Greenland climates preceded abrupt deglacial warming", Science 25 July 2014: Vol. 345 no. 6195 pp. 444-448 DOI: 10.1126/science.1252000
Edit: For ease of reference, I provide the attached image which indicates the timeline for the most recent epochs (including the Pliocene)