The following linked references cite research on forests, as an illustration of how sensitive such carbon sinks can be to future climate disruption (such as :wet-dry cycles, pests, fires, etc) especially as our current rate of increase of radiative forcing is much higher than at any time since the PETM; and thus vegetation (both on land & in the ocean) will not have adequate time to adapt to such rapidly changing climate conditions:
James Hansen, Pushker Kharecha, Makiko Sato (2013), "Doubling Down on Our Faustian Bargain", Environmental Research Letters.
http://iopscience.iop.org/article/10.1088/1748-9326/8/1/011006/meta&
http://www.columbia.edu/~jeh1/mailings/2013/20130329_FaustianBargain.pdfAbstract: "Rahmstorf et al 's (2012) conclusion that observed climate change is comparable to projections, and in some cases exceeds projections, allows further inferences if we can quantify changing climate forcings and compare those with projections. The largest climate forcing is caused by well-mixed long-lived greenhouse gases. Here we illustrate trends of these gases and their climate forcings, and we discuss implications. We focus on quantities that are accurately measured, and we include comparison with fixed scenarios, which helps reduce common misimpressions about how climate forcings are changing.
Annual fossil fuel CO2 emissions have shot up in the past decade at about 3% yr-1, double the rate of the prior three decades (figure 1). The growth rate falls above the range of the IPCC (2001) 'Marker' scenarios, although emissions are still within the entire range considered by the IPCC SRES (2000). The surge in emissions is due to increased coal use (blue curve in figure 1), which now accounts for more than 40% of fossil fuel CO2 emissions."
The second linked article is entitled: "Forests 'held their breath' during global warming hiatus, research shows". This illustrates Hansen's Faustian Bargain.
https://phys.org/news/2017-01-forests-held-global-hiatus.htmlExtract: "The study shows that, during extended period of slower warming, worldwide forests 'breathe in' carbon dioxide through photosynthesis, but reduced the rate at which they 'breathe out'—or release the gas back to the atmosphere."
The third linked reference indicates that forests play a more important role in keeping the planet cool than was previously appreciated. Thus, if one assumes that they are entitled to make self-serving assumptions one could assume that decision makers will not only preserve forests but will expand them in the future. However, the reality is that we are currently losing forests at a rate appropriate for a BAU scenario; and which could accelerate in the future. Thus, if we keep losing forest, the AR5 projections may err on the side of least drama:
Ryan M. Bright et al. Local temperature response to land cover and management change driven by non-radiative processes, Nature Climate Change (2017). DOI: 10.1038/nclimate3250
http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate3250.htmlAbstract: "Following a land cover and land management change (LCMC), local surface temperature responds to both a change in available energy and a change in the way energy is redistributed by various non-radiative mechanisms. However, the extent to which non-radiative mechanisms contribute to the local direct temperature response for different types of LCMC across the world remains uncertain. Here, we combine extensive records of remote sensing and in situ observation to show that non-radiative mechanisms dominate the local response in most regions for eight of nine common LCMC perturbations. We find that forest cover gains lead to an annual cooling in all regions south of the upper conterminous United States, northern Europe, and Siberia—reinforcing the attractiveness of re-/afforestation as a local mitigation and adaptation measure in these regions. Our results affirm the importance of accounting for non-radiative mechanisms when evaluating local land-based mitigation or adaptation policies."
The fourth reference (see also the attached image) indicates a two-fold increase of carbon cycle sensitivity to tropical temperature variations:
Wang, X., Piao, S., Ciais, P., Friedlingstein, P., Myneni, R.B., Cox, P., Heimann, M., Miller, J., Peng, S.P., Wang, T., Yang, H. and Chen, A., (2014), "A two-fold increase of carbon cycle sensitivity to tropical temperature variations", Nature, 2014; DOI: 10.1038/nature12915.
http://www.nature.com/nature/journal/v506/n7487/full/nature12915.html#extended-datahttp://sites.bu.edu/cliveg/files/2014/01/wang-nature-2014.pdfAbstract: "Earth system models project that the tropical land carbon sink will decrease in size in response to an increase in warming and drought during this century, probably causing a positive climate feedback. But available data are too limited at present to test the predicted changes in the tropical carbon balance in response to climate change. Long-term atmospheric carbon dioxide data provide a global record that integrates the interannual variability of the global carbon balance. Multiple lines of evidence demonstrate that most of this variability originates in the terrestrial biosphere. In particular, the year-to-year variations in the atmospheric carbon dioxide growth rate (CGR) are thought to be the result of fluctuations in the carbon fluxes of tropical land areas. Recently, the response of CGR to tropical climate interannual variability was used to put a constraint on the sensitivity of tropical land carbon to climate change. Here we use the long-term CGR record from Mauna Loa and the South Pole to show that the sensitivity of CGR to tropical temperature interannual variability has increased by a factor of 1.9 ± 0.3 in the past five decades. We find that this sensitivity was greater when tropical land regions experienced drier conditions. This suggests that the sensitivity of CGR to interannual temperature variations is regulated by moisture conditions, even though the direct correlation between CGR and tropical precipitation is weak. We also find that present terrestrial carbon cycle models do not capture the observed enhancement in CGR sensitivity in the past five decades. More realistic model predictions of future carbon cycle and climate feedbacks require a better understanding of the processes driving the response of tropical ecosystems to drought and warming."
Caption for the attached image: "Figure 1 | Change in detrended anomalies in CGR and tropical MAT, in dCGR/dMAT and in ªintCGR over the past five decades. a, Change in detrended CGR anomalies at Mauna Loa Observatory (black) and in detrended tropical MAT anomalies (red) derived from the CRU data set16. Tropical MAT is calculated as the spatial average over vegetated tropical lands (23uN to 23u S). The highest correlations between detrended CGR and detrended tropicalMAT are obtained when no time lags are applied (R50.53, P,0.01). b, Change in dCGR/dMAT during the past five decades. c, Change in cintCGR during the past five decades. In b and c, different colours showdCGR/dMATor cint CGR estimated with moving time windows of different lengths (20 yr and 25 yr). Years on the horizontal axis indicate the central year of the moving time window used to derive dCGR/dMAT or cintCGR (for example, 1970 represents period 1960–1979 in the 20-yr time window). The shaded areas show the confidence interval of dCGR/dMATand cintCGR, as appropriate, derived using 20-yr or 25-yr moving windows in 500 bootstrap estimates."