So what's the evidence? This is all old stuff, but I'm going to go through it again.
Before I get to the evidence, it's necessary to point out that deniers don't understand the significance of this question. They think that aha, I got you, if was warmer back then before there were SUVs then you can't blame modern warming on fossil fuels! Which is just a bad logical error.
Yes, the climate changes, but not always for the same reason. There isn't only one reason why climate changes -- CO2 -- and nothing else can ever change it. There are many factors that cause climate change besides greater atmospheric CO2 -- orbital (Milankovitch) cycles, changes in solar output, significant volcanic aerosols that lead to cooling (with enough emitted aerosols the cooling can be enhanced by the ice-albedo feedback, which is probably what caused the Little Ice Age (also not global)), an asteroid or comet strike, meltwater from ice sheets, some reasons I've probably forgotten, and then that one damn butterfly in China who alone seems responsible for a great deal of the trouble and strife in this sordid, chaotic world.
Most "contrarians" (cough cough) seem to think the Sun caused a global Medieval Warm Period (MWP). What's the data?
Recently Judith Lean wrote a paper giving the reconstructed solar output from 850 C.E. onward; plot to the right. (Click to enlarge).
Looking at the top graph, for TSI, there doesn't seem to be much of a global MWP there, does there?
But let's play along with the deniers and assume a 1 W/m2 increase in solar irradiance, at the top of the atmosphere. How much temperature change might that cause?
The IPCC 5AR puts solar forcing at about 0.1°C per W/m2. So does this rough calculation: the zero-dimensional energy balance, no atmosphere model:
(For its derivation, see Pierrehumbert's textbook, equation 3.6, pp 114-115) where S is solar irradiance = 1365 W/m2. This gives T = 255°C, Earth's "brightness temperature" -- the global average surface temperature if Earth had no atmosphere, or as it is seen from outside the atmosphere. Then
So how is the sun going to cause a warm period? It's not. Deniers think the climate is much more sensitive to solar radiance than it really is.
If the climate was so sensitive to solar forcing -- say, 1°C per W/m2 -- then the forcing from CO2 doubling = α*ln(2) = (5.35 W/m2)*ln(2) = 3.7 degC, before feedbacks.
So then we'd have even more to worry about from anthropogenic GHGs.
None of this is new, as I wrote. But I wanted to make this point again.
So, was the MWP global, like Lars Larson suggested? The data say no.
First, here are the results from the PAGES 2k Consortium published a few years ago -- using proxies like tree rings and pollen grains to reconstruct the last 2000 years of regional temperatures. It's a huge study done worldwide by about six dozen scientists. This is from their abstract:
"There were no globally synchronous multi-decadal warm or cold intervals that define a worldwide Medieval Warm Period or Little Ice Age, but all reconstructions show generally cold conditions between AD 1580 and 1880, punctuated in some regions by warm decades during the eighteenth century."They produced a beautiful grid of temperatures by 30-year intervals by continent -- it's from their Supplementary Material:
So around 1000 CE is was quite warm in North America -- I think on the show I mentioned sand dunes in Kansas, but what I was thinking of was the Sandhills in Nebraska, which formed around then. It was warm in Antarctica and the Arctic. It wasn't warm in South America or Australasia or much in Asia, and only some bins in Europe show special warmth then.
So nothing global, but yes, certainly some reasons experienced warmth for a period of 30 years to, in North America, about 200 years.
What about the ocean surface? Nothing much of anything happened their either, according to the Ocean 2k study:
If the ocean surface wasn't warmer -- it covers 70% of the globe -- and only some land regions were warmer, then the MWP
1) wasn't global, and
2) would be bad news for us if it had been global with warming, and
3) wasn't global.