Climate change seems to polarise comment into denialists and believers. But good skeptics should be able to address specific issues with the data without falling into the anti-science denialist camp. Hence this question.

There is good historic evidence that the Medieval Warm Period was warmer than today (Greenland and much of Europe clearly were as the evidence from agriculture shows). But leading climate change scientists don't always agree (see the Wikipedia entry).

I'd say the world is getting warmer, but is it yet warmer than it ever has been in human history? What is the real evidence?

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    It's about heat transfer rates too. Sure, there was a warm period that was several hundred years long. This period is only decades. It takes time for quantities of ice to melt. Given all that I have seen we are indeed at the warmest time in recorded human history, but with out 6 second attention spans, and northern hemisphere winter coming on, most people will totally forget. – Larian LeQuella Sep 24 '11 at 17:07
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    I think the question would be better without the first paragraph, as it is the sort of thing which is likely to do more to polarise the discussion that bring it back to a more rational tone. It is possible to just discuss the science. The second two paragraphs pose the question very well (+1). – Dikran Marsupial Jun 19 '14 at 17:31

It is highly unlikely that the medieval warm period was, globally, warmer than current.

The IPCC 4th Assessment Report graphed a dozen different replications of the "hockey stick" graph, along with the instrumental record. This is for the Northern Hemisphere, which has both the best paleoclimate record and that's also where the various forms of historical evidence (such as the agricultural evidence and Viking settlements) are located. IPCC AR4 Figure 6-10

We can see from this that there are a few short patches in the 10th and 12th century which may possibly be warmer than today - it's just not very likely that they are. While most reconstructions do include a MWP that was slightly warmer than the temperatures around that period, it was probably not higher than the present-day average.

In addition, according to Mann et. al. (2008):

The reconstructed amplitude of change over past centuries is greater than hitherto reported, with somewhat greater Medieval warmth in the Northern Hemisphere, albeit still not reaching recent levels.

Mann et. al. (2008)

According to Surface Temperature Reconstructions for the Last 2,000 Years, a book written by the National Research Council:

Presently available proxy evidence indicates that temperatures at many, but not all, individual locations were higher during the past 25 years than during any period of comparable length since A.D. 900.

For interest, Ljungqvist (2009) plotted a number of paleoclimate proxies for a number of different regions. Remember, many of these will only include data up to about 1950 or so. In many of these records, current temperatures are lower than during their medieval warm peak. However, the exact time of that peak varies - which probably helps to explain why many locations contain a MWP and yet it is less pronounced in larger scale hemispheral or global records.

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    This is a very good summary of the conventional consensus. But the reason I asked the question in the first place was because some of the methods that built that consensus have been controversial (and are at the heart of the climate gate scandal). The historic evidence says medieval Europe was warmer (and the Ljunquist paper says "Late 20th century temperatures are in some of the records the highest for the last two millennia, although more records seem to show peak medieval temperatures exceeding the modern temperatures.") So do we trust the reliability of the consensus? – matt_black Sep 25 '11 at 16:34
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    One problem with these reconstructions is that they seem to severely understate uncertainty in the distant past. For example, in the third graph, uncertainty does not seem to widen as much further back in time as might be expected. Similarly in the fourth and fifth graphs, reconstructions which do show uncertainty often do not overlap: for example the CIV and EPS "land with uncertainties" bands are often highly distinct despite having been produced in the same paper. – Henry Sep 25 '11 at 17:29
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    @Sklivvz - with all due respect, scientific "consensus" can be invalidated by a single fact (would you like me to list the major examples?). The only thing I will trust is a reproducible research explaining the known facts by showing a controlled experiment producing the predicted outcome. You know, the whole "falsifyability" thing that some people like to have in real science. A backwards-looking model created by people who set out to prove a specific result doesn't qualify no matter how many people invested in status quo produce such models. None of the work explains MWP. – user5341 Sep 26 '11 at 21:05
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    @DVK Funny, I don't remember having contested the agricultural and historical evidence. Or having said anything about theory. I explicitly acknowledge the MWP. The data from Ljungqvist shows that such warm periods were not synchronous, and the data from Esper, Cook, Schweingruber, Oerlemans, D’Arrigo, Wilson, Jacoby and many others supports this. Better in short: "My large number of better-distributed data points adds context to this smaller number of data points, and improves our ability to compute accurate averages." – Joel Rein Sep 27 '11 at 0:05
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    @matt_black: M&M criticise the MBH98 paper, not cited above. Most of the above papers (where relevant) use different methods of statistical analysis to which such criticisms do not apply, and yet reproduce the results. – Joel Rein Oct 3 '11 at 4:03

From Mann (2009), using roughly 1000 different proxy datasets: enter image description here

Some recent temperatures via NOAA: enter image description here

So according to a comparison of these data sources, it all depends on the location and the scale you are looking at. Were temperatures during the MWP globally warmer than the modern warm period? Very likely not.

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    This would be totally convincing apart from two considerations. Mann's MWP world map doesn't look consistent with what we know of history (though written history is admittedly patchy). And Mann's proxy reconstruction methods are controversial (so using them as primary evidence here feels weak without some discussion of why his methods are unimpeachable). – matt_black Jun 21 '14 at 8:53

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