The Medieval Warm(ish) Period In Pictures Posted on 10 July 2011 by Rob Painting The Medieval Warm(ish) Period In Pictures The Medieval Warm Period (MWP) is a subject of "skeptic" focus, primarily because it was a time of natural warming. It took place from about 950-1250 AD, and, as opposed to today's warming which is global in extent and due to human activities, the MWP was mainly a northern hemisphere phenomenon and smaller in scale. Indeed, the advance of North American glaciers during the MWP is in stark contrast to what is happening in North America today. The MWP in global maps Mann (2009) was an analysis of a large set of climate/temperature proxies (ice cores, tree rings, cave mineral deposits, sediments, etc.) covering the MWP. See below: Figure 1: Reconstructed surface temperature anomaly for Medieval Warm Period (950 to 1250 A.D.), relative to the 19611990 reference period. Gray areas indicate regions where adequate temperature data are unavailable. It's clear from the map that only limited areas of the world were warmer during the MWP than the 19611990 reference period, Greenland in particular. Much of the rest of the planet, especially the oceans, were cooler. And take note of the cooler North American west coast, which is consistent with the glacier advance there during the MWP. Obviously, the Earth has continued to warm even further since the 19611990 period, so the difference in temperature between the MWP shown in the reconstruction and today is even greater. Climate Models and the MWP The climate proxies suggest a rather different distrubition of heat and rainfall than today. So the question arises: based on the current well-understood climate system, can ocean-atmosphere processes produce temperature patterns like the MWP? For answers we turn to climate models. The cool central and eastern Pacific suggested by Mann 2009 explain some of planet-wide climate features observed in the MWP, but they fail to explain other aspects, such as as a more positive North Atlantic Oscillation (NAO), and increased rainfall in parts of Asia. Graham (2010) used the NCAR climate model and, based on findings from other modelling studies and proxy evidence for a warmer Western Pacific Ocean, found that by forcing the simulation to have a slightly warmer Indian and Western Pacific Ocean relative to other ocean basins, the model results match the climate suggested by the proxies. Below are the comparisons between the climate model control runs (a pre-industrial baseline) and the simulations including a warmer Indian-Western Pacific Ocean, for the northern hemisphere (boreal) winter (figure 2 and 3) Figure 2: Differences in DecemberMarch temperature between the Indian Ocean Warm Pool (25) and control simulations (°C, color; values are Sea surface temperature over ocean and 2-m temperature over land).Lined contour interval is 0.5°C between 30°N and 30°S and 1°C elsewhere. From Graham 2010. Figure 3: Differences in DecemberMarch precipitation (expressed as fraction of Control simulation, color) and Sea level Pressure (difference, hPa) for the Indian Ocean Warm Pool simulation. SLP contour interval is 1.0 hPa except 0.25 hPa in the region 30°S40°N and 25°E100°E where the ﬁner interval highlights temperature-driven low pressure around continental periphery of northern Indian Ocean. From Graham 2010. The warm Indian-Western Pacific Ocean, combined with the cool Central and Eastern tropical Pacific reveals a good match with the paleoclimate proxies shown in the Mann 2009 reconstruction, namely: The NAO becomes more positive (A in figure 2 and 3), matching proxies of Atlantic sea surface temperature and sea ice distribution. This due to an atmospheric teleconnection (two connected atmospheric processes separated by large distances) between the NAO and warming over the Indian Ocean. Seasonal dryness in northeast Africa (B in Figure 3 ) And wetter conditions in India, Southeast Asia and China(C in figure 3) A mechanism for such warming of the Indian and western Pacific Oceans has not been found, nor do the climate model simulations resolve all the patterns suggested by the proxies (such as warming of the British Isles); however, these findings represent another small step forward in piecing together the puzzle of the MWP, and also support the interpretation of the proxy data in Mann 2009. Science marches on while skeptics don't The MWP was very unlike warming today; the growing North American glaciers during the MWP being somewhat of a giveaway. The MWP only affected warming in a handful of regions, with Greenland being especially warm (Figure 1), whereas much of the Earth was actually cooler than the late 20th century. By comparison; today virtually every glacier and ice sheet on the planet is in rapid retreat. Both the climate proxies and the climate models imply that the MWP was a re-organization of the Earth's climate, and that much of this re-organization can be explained by oceanic patterns of warming and cooling, although what started all this rolling in the first place is still unknown. So while some climate "skeptics" are stuck in a time loop, wilfully reliving their own version of Groundhog Day, science continues to move forward. .... Holy crap this shows that most of the world outside of europe/arcitc and some of the US was below the baseline for the period of 1961-1990 during the med evil warm period. They say that the glacial in north America were growing. I disagree because I believe the med evil was caused by the same thing that caused the little ice age. hehehe I believe most of the bigger short term 20-200 year movements within the climate are mostly caused by changes within the solar output. Of course not all as you know---another is the orbit around the sun that goes from more oval to circular in nature, which causes more solar energy to reach the earth when it is avged over the year with slightly more energy and another is the angle of the planet. Mostly longer term cycles. Of course if the different area's---were warmer at different times that would build support for a change within internal patterns like the nao, pdo, enso. We will see as more and more science is coming into support of such, but I will stick with a mixture of the solar output and maybe lean towards some internal patterns. Maybe the sun causes the internal changes?