Not sure where you're getting "lowered" from. The range given by the IPCC for as long as I've been following the subject (about 15 years) has been 1.5C - 4.5C, with some estimates at both extremes but with a gradual convergence on the middle of that range.
The growing consensus, from much of the current material I've been looking at, is that 3.0C is the lower limit with most indicators pointing toward the higher ranges being better fits for the available data than the middle or lower end of this range.
I'm aware of a couple of recent papers giving estimates nearer 4C than 3C but nothing suggesting the range you quoted ...
... so I'd be interested in seeing the source for that.
Really? even the SRES report that was used in large part to help form, and were repeated in, the IPCC AR4 forecast estimates, gave a warming range for 2100 of 1.6–6.9°C above preindustrial (one of the A1 senarios - A1F1 I believe). These aren't a part of my current reading list, but here is a copy of a Metoffice report that repeats essentially the same information back in 2009.
"When could global warming reach 4°C?" -
http://www.metoffice.gov.uk/media/pdf/m/q/HCTN_80.pdf
"The IPCC Fourth Assessment Report (AR4) assessed a range of scenarios of future greenhouse gas emissions without policies to specifically reduce emissions, and concluded that these would lead to an increase in global mean temperatures of between 1.4°C and 6.9°C by the end of the 21st Century, relative to pre-industrial. While much political attention is focussed on the potential for global warming of 2°C relative to preindustrial, the AR4 projections clearly suggest that much greater levels of warming are possible by the end of the 21st Century in the absence of mitigation. The centre of the
range of AR4 projected global warming was approximately 4°C...Using these GCM projections along with simple climate model projections including uncertainties in carbon cycle feedbacks, and also comparing against other model projections from IPCC, our best estimate is that the A1FI emissions
scenario would lead to a warming of 4°C relative to pre-industrial during the 2070s. If carbon cycle feedbacks are stronger, which appears less likely but still credible, then 4°C warming could be reached by 2060..."
Here are a few of the more recent papers that I have been reading that lend support for increasing sensitivity assessments.
“High Climate Sensitivity Suggested by Multi-satellite Observations: the Role of Circulation and Cloud Feedback”
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http://adsabs.harvard.edu/abs/2013AGUFM.A43K..03S
“It has long been recognized that cloud feedback is a leading contributor to the spread of models' climate sensitivity. Here we show that simulated cloud feedbacks are strongly correlated with the changes of the Hadley Circulation. We further show the present-day simulations of circulation and clouds have imprints that foreshadow the strength of circulation and cloud feedback under global warming. Compared with multi-satellite observations and reanalysis data, the high sensitivity models perform better than the low sensitivity models. Using observational metrics that emphasize the coherent spatial variabilities of clouds and relative humidity in association with the Hadley Circulation, we find the best estimates of equilibrium climate sensitivity likely range from 3.6 to 4.7 K, with a mean of 4.1 K and a standard deviation of 0.4 K.”
“Weakening and strengthening structures in the Hadley Circulation change under global warming and implications for cloud response and climate sensitivity”
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http://yly-mac.gps.caltech.edu/reprintsyly/A_RecentPapers/Su_etal_ECS_JGR2014.pdf
“…Lastly, the new model performance metrics that emphasize the entire spatial variations of cloud fraction and relative humidity associated with the Hadley Circulation show that the models that perform better by these measures tend to have higher ECS. These models also produce stronger Hadley Circulation in general. Consistent with this, composites over the high-sensitivity models perform substantially better by these measures than those over the low-sensitivity models. Although better performance in current climate is not necessarily an indicator of a better projection of future climate, these new metrics are for climate variables closely associated with circulation and energy budget changes under global warming. Any reasonable weighting of the multimodel ensemble by these metrics would yield an estimate of ECS higher than the multimodel mean.”
“Super Interglacials and persistent warmth paced Arctic Climate Evolution over the Past 3.6 Myr: Lake El'gygytgyn, Western Beringia, a new polar lens focused on high latitude environmental change”
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http://adsabs.harvard.edu/abs/2013EGUGA..1511829B
“…In 2009, International Continental Deep drilling at Lake El'gygytgyn (67o30' N, 172 o 05' E) recovered lacustrine sediments dating back to 3.58 Ma that now provide the first time-continuous Pliocene-Pleistocene Arctic paleoclimate record of alternating glacial-interglacial change. The warmest/wettest Pliocene interval of the lake record occurs from ~3.58-3.34 Ma and is dominated by exceptional tree pollen implying July temperatures nearly 7-8o C warmer than today with nearly ~3 times the annual precipitation. Atmospheric CO2 levels are estimated to have been 360 to 400 ppm implying exceptionally high climate sensitivity and polar amplification. In fact, pollen spectra and modern analog analysis show an unbroken persistence of summers much warmer and wetter than the last interglacial, MIS 5e until nearly 2.2 Ma. Extreme warmth in the Mid Pliocene Arctic occurs at the same time ANDRILL results suggest the West Antarctic Ice Sheet was non-existent. Modeling sensitivity experiments using 300 and 400 ppm CO2 are consistent with sustained forests at Lake El'gygytgyn during this interval and restricted glacial ice over Greenland in both cold and warm boreal summer orbits especially for the PRISM interval…”
“Spread in model climate sensitivity traced to atmospheric convective mixing”
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http://www.lmd.jussieu.fr/~jldufres/publi/2014/Sherwood.Bony.ea-nature-2014-smi.pdf
“Climate sensitivity refers to the change in global mean temperature in response to a change in external forcing, usually a doubling of CO2. Despite decades of research attempting to narrow uncertainties, equilibrium climate sensitivity estimates from climate models still span roughly 2 to 5◦C, precluding accurate projections of future climate. The spread arises largely from differences in the feedback from low clouds, for reasons not yet understood. Here we show that differences in the simulated strength of convective mixing between the lower and middle tropical troposphere explain about half of the variance in climate sensitivity estimated by 43 climate models. The apparent mechanism is that such mixing dehydrates the low-cloud layer at a rate that increases as climate warms, and this rate of increase depends on the initial mixing strength, linking the mixing to cloud feedback. Mixing inferred from observations appears sufficiently strong to imply a climate sensitivity greater than 3◦C for a carbon dioxide doubling. This is significantly higher than the currently accepted lower bound of 1.5◦C, thereby constraining model projections toward relatively severe future warming.”
“Quantifying greenhouse warming, aerosol cooling and the transient climate sensitivity - A novel approach”
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http://adsabs.harvard.edu/abs/2013AGUFM.A21B0025S
“We present an interdisciplinary study combining climate observations and modeling with econometric methodology to identify the relative contributions to recent climate change from greenhouse warming and aerosol cooling. The cooling effect of aerosols has likely masked some of the warming due to increasing GHG concentrations to date. Several publications have recently pointed out the intimate relationship between the aerosol cooling effect and climate sensitivity. Generally, a GCM that has a large aerosol cooling effect will typically have a high climate sensitivity, and vice versa. Given this connection between the aerosol effect and the GCMs climate sensitivity, it is problematic that the aerosol effect is still very poorly constrained, even after decades of dedicated research on aerosol effects on climate. It is time to seek out other complimentary and parallel approaches to the traditional ones, and this is at the heart of the presented study. We apply econometric methods to determine the parameters of an energy balance model, using observed time series of surface air temperature, radiation and CO2 concentrations. This framework allows us to decompose of the observed temperature trend into a GHG component and an aerosol component for the time period 1960 to 2000. We find that aerosols have likely masked about 50% of the warming due to increasing GHG concentrations during this time period. This is compared to a much weaker aerosol masking simulated by the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM). The framework also allows for an estimate of the transient climate sensitivity, with the caveat that the estimate is only valid for the parts of the globe which have sufficient data coverage to be included in our analysis. This is the case for most land areas, for which our framework yields a transient climate sensitivity of 6.1K, with a 95% confidence interval of [3.6, 8.7]K.”
“New constraints on atmospheric CO2 concentration for the Phanerozoic”
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http://droyer.web.wesleyan.edu/Franks_et_al_2014_GRL_new_stomatal-CO2_proxy.pdf
“…These results provide critical new empirical support for the emerging view that large (~2000–3000 ppm), long-term swings in ca do not characterize the post-Devonian and that Earth’s long-term climate sensitivity to ca is greater than originally thought.”
“Climate sensitivity, sea level and atmospheric carbon dioxide”
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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3785813/
“…Cenozoic temperature, sea level and CO2 covariations provide insights into climate sensitivity to external forcings and sea-level sensitivity to climate change. Climate sensitivity depends on the initial climate state, but potentially can be accurately inferred from precise palaeoclimate data. Pleistocene climate oscillations yield a fast-feedback climate sensitivity of 3 ± 1◦C for a 4Wm−2 CO2 forcing if Holocene warming relative to the Last Glacial Maximum (LGM) is used as calibration, but the error (uncertainty) is substantial and partly subjective because of poorly defined LGM global temperature and possible human influences in the Holocene. Glacial-to-interglacial climate change leading to the prior (Eemian) interglacial is less ambiguous and implies a sensitivity in the upper part of the above range, i.e. 3–4◦C for a 4Wm−2 CO2 forcing. Slow feedbacks, especially change of ice sheet size and atmospheric CO2, amplify the total Earth system sensitivity by an amount that depends on the time scale considered…”
“High Arctic forests during the middle Eocene supported by moderate levels of atmospheric CO2”
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http://geology.gsapubs.org/content/early/2014/10/14/G36014.1.abstract
“An abundance of data from Napartulik suggests mean annual temperatures at least 30 °C warmer than today and atmospheric water loads 2× current levels...Individual reconstructions vary between 392 ppm and 474 ppm, with a site median of 424 ppm (351–523 ppm at 95% confidence). These estimates represent the first direct constraints on CO2 for a High Arctic forest and suggest that the temperate conditions present at Napartulik during the middle Eocene were maintained under CO2 concentrations ∼1.5× pre-industrial levels. Our results support the case that long-term climate sensitivity to CO2 in the past was sometimes high, even during largely ice-free periods, highlighting the need to better understand the climate forcings and feedbacks responsible for this amplification.”
These are just a handful of the papers on the top page of my reading list that relate to the focus shift to the upper part of the C
a climate sensitivity range. I'd be happy to provide a more extensive listing for any who are interested.
