Increasing the estimated value of the (past) climate forcing doesn't necessarily increase the expected magnitude of future warming. In fact, it may decrease our expectation of future warming.
The rate of warming is determined by the forcing and the climate sensitivity. For the past, the rate of warming is fixed -- so if our estimated value of the forcing goes up, it suggests that climate sensitivity is actually lower than previously believed. That in turn suggests less warming in the future.
It does suggest it, but it is not necessarily the case, as climate sensitivity generally refers to equilibrium climate sensitivity, which is not actually a measured value at any given point in time. It could be that the ocean heat uptake is higher than previously thought, for example. In that case, we might expect greater warming in the future.
Additionally, to BenB's point:
First, both anthropogenic and natural aerosols are radiative forcings that are normally net negative. So for example, if the radiative forcing for methane is higher, then if modeler's underestimated the historical/paleo negative forcing from aerosol then climate sensitivity may be higher than AR5 assumes. This point is well illustrated by Shrivastava et al (2017) which states:
"Several SOA processes highlighted in this review are complex and interdependent, and have non-linear effects on the properties, formation and evolution of SOA. Current global models neglect this complexity and non-linearity, and thus are less likely to accurately predict the climate forcing of SOA, and project future climate sensitivity to greenhouse gases."
Shrivastava M, Kappa CD, Fan J, et al. (2017), "Recent Advances in Understanding Secondary Organic Aerosol: Implications for global climate forcing", Reviews of Geophysics, DOI: 10.1002/2016RG000540
http://onlinelibrary.wiley.com/doi/10.1002/2016RG000540/fullAbstract: "Anthropogenic emissions and land-use changes have modified atmospheric aerosol concentrations and size distributions over time. Understanding pre-industrial conditions and changes in organic aerosol due to anthropogenic activities is important because these features 1) influence estimates of aerosol radiative forcing and 2) can confound estimates of the historical response of climate to increases in greenhouse gases. Secondary organic aerosol (SOA), formed in the atmosphere by oxidation of organic gases, represents a major fraction of global submicron-sized atmospheric organic aerosol. Over the past decade, significant advances in understanding SOA properties and formation mechanisms have occurred through measurements, yet current climate models typically do not comprehensively include all important processes. This review summarizes some of the important developments during the past decade in understanding SOA formation. We highlight the importance of some processes that influence the growth of SOA particles to sizes relevant for clouds and radiative forcing, including: formation of extremely low-volatility organics in the gas phase; acid-catalyzed multi-phase chemistry of isoprene epoxydiols (IEPOX); particle-phase oligomerization; and physical properties such as volatility and viscosity. Several SOA processes highlighted in this review are complex and interdependent, and have non-linear effects on the properties, formation and evolution of SOA. Current global models neglect this complexity and non-linearity, and thus are less likely to accurately predict the climate forcing of SOA, and project future climate sensitivity to greenhouse gases. Efforts are also needed to rank the most influential processes and non-linear process-related interactions, so that these processes can be accurately represented in atmospheric chemistry-climate models."
Second, mounting evidence indicates a nonlinear climatic sensitivity to GHG, as indicated by Li Lo et. al. (2017); which indicates that with increasing levels of GHG concentration the ENSO climate attractor can abruptly and dramatically increase:
Li Lo et. al. (2017), "Nonlinear climatic sensitivity to greenhouse gases over past 4 glacial/interglacial cycles", Scientific Reports 7, Article number: 4626, doi:10.1038/s41598-017-04031-x
https://www.nature.com/articles/s41598-017-04031-x.Abstract: "The paleoclimatic sensitivity to atmospheric greenhouse gases (GHGs) has recently been suggested to be nonlinear, however a GHG threshold value associated with deglaciation remains uncertain. Here, we combine a new sea surface temperature record spanning the last 360,000 years from the southern Western Pacific Warm Pool with records from five previous studies in the equatorial Pacific to document the nonlinear relationship between climatic sensitivity and GHG levels over the past four glacial/interglacial cycles. The sensitivity of the responses to GHG concentrations rises dramatically by a factor of 2–4 at atmospheric CO2 levels of >220 ppm. Our results suggest that the equatorial Pacific acts as a nonlinear amplifier that allows global climate to transition from deglacial to full interglacial conditions once atmospheric CO2 levels reach threshold levels."
Also, 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:
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/abstractAlso, 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
There are numerous other reason that masking could be hiding both high current GHG radiative forcing (I note that if Etminan et al (2016) is correct that the GWP100 of methane is 25% higher than in AR5 then CO2e is was above
548ppm after 2016); and high current ECS; so that if these masking factors are reduced (say by deforestation reducing SOA concentrations) then temperatures could increase rapidly.