Peer review: Atmospheric CO2 concentrations have little to no impact on Earth's surface temperature.

A paper by Marie-Michèle Ouellet-Bernier, Anne de Vernal, Claude HillaireMarcel and Matthias Moros published in the Journal The Holocene (link - pdf) contains the following:

Medieval warming (~1000–800 cal. yr BP) From ~1000 to ~800 cal. yrBP, summer SSTs increased to about 10°C, which is much higher than the present day summer SST of ~4.4°C ± 1.24°C at the coring site (NODC, 2001).

The Abstract: 

Map of the study area.
A star indicates the location of core MSM343300.
EGC: East Greenland Current; WGC: West Greenland Current;
IC: Irminger Current; LC: Labrador Current.

Micropaleontological analyses of a sediment core raised in Disko Bugt (West of Greenland) were undertaken in order to document paleoceanographical changes in the eastern Baffin Bay during the Holocene. The modern analogue technique (MAT) applied to dinocyst assemblages provided information on paleo-sea-surface conditions, whereas isotopic analyses of benthic foraminifers aimed at documenting the ‘deep’ water mass occupying the site. During the earlier interval recorded (~10 to ~7.3 cal. kyrBP), important discharge of ice and meltwater from the Greenland Ice Sheet (GIS) margin, notably through the Jakobshavn Isbrae, resulted in harsh conditions with a dense sea-ice cover and low temperatures, productivity, and foraminiferal abundances. Postglacial conditions settled at ~7.3 cal. kyrBP, with a sharp rise in dinocyst abundance and species diversity, which led to reconstruct increase in summer temperatures. We link this transition to the advection of West Greenland Current waters in the upper part of the water column after the reduction of meltwater inputs from GIS. Optimal temperature conditions reaching up to >10°C were finally achieved in surface waters at ~6cal. kyrBP. Slight cooling pulses were then recorded at ~4.2–4 and ~1.5–1cal. kyrBP, and the final optimum recorded in surface temperature from ~1 to 0.8cal. kyrBP is associated with the ‘Medieval Warm Period’. Throughout the postglacial interval, the data suggest an opposition between sea-surface temperatures and salinity, with warmer intervals being characterized by lower salinity waters, probably as a result of the higher freshwater discharge along the ice margin and notably the Jakobshavn Isbrae.

CO2 Science (link)

Can you guess how much warmer it was in west Greenland during the 200-year-long Medieval Warm Period than it is today? 

Providing the answer to this question was Ouellet-Bernier et al. (2014), who conducted several micro-paleontological analyses of a marine sediment core extracted from Disko Bugt (West of Greenland) in order to document paleo-oceanographical changes in the eastern part of Baffin Bay during the Holocene. Prominent among these analyses was the modern analogue technique, which they applied to dinocyst assemblages that provided information on paleo-sea-surface conditions, whereas isotopic analyses of benthic foraminifers were employed to assess the characteristics of the deep water mass occupying the site. 

These efforts revealed the existence of a number of periods of alternating cold and warm sea surface temperatures (SSTs) throughout the Holocene, the last of which warm periods was the Medieval Warm Period that occurred between approximately 1000 and 800 years BP. And in reporting their findings, Ouellet-Bernier et al. say that during this period the region's "summer SSTs increased to about 10°C," which they describe as being "much higher than the present day summer SST of about 4.4°C at the coring site," as recorded in the World Ocean Database of the NODC (2001). 

And a very special note should be made of this truly amazing level of warmth that occurred during a period of time when the atmosphere's CO2 concentration was only a mere 285 ppm compared to the 40%-greater 400 ppm of today, which facts clearly suggest that atmospheric CO2 concentrations have little to no impact on Earth's surface temperature.