The medieval warm period (MWP) refers to a period of warmer temperatures in medieval times, approximately 950-1300 AD. Until very recently, this was generally accepted by scientists. The expression was introduced by Hubert Lamb in his classic book, "Climate, history and the modern world", where he writes "By the late tenth to twelfth centuries most of the world for which we have evidence seems to have been enjoying a renewal of warmth, which at times during those centuries may have approached the level of the warmest millennia of post glacial times."

Recently however, a group of 'climate scientists' and activists have attempted to deny the existence of the MWP, or downplay its magnitude, or claim that it was only local to the North Atlantic region. The motivation for these people is to try to be able to claim that current warming is unprecedented and man-made. To support their false claims they have used flawed statistical techniques to construct the totally discredited 'hockey stick' picture. 

There are hundreds of scientific papers that confirm the MWP, and confirm that it was a global phenomenon. Here are a few of these:

• Evidence for a ‘Medieval Warm Period’ in a 1,100 year tree-ring reconstruction of past austral summer temperatures in New Zealand.  E.R. Cook et al, Geophys. Res. Lett., 29(14), 1667  (2002).  ("clear evidence for persistent above-average temperatures within the interval commonly assigned to the MWP";  "the New Zealand temperature reconstruction supports the global occurrence of the MWP").
  
• Climate, Environment and Society in the Pacific during the Last Millennium.  P.D. Nunn (2007).  Most of  this 300-page book can be found on Google books here. See also Nunn's press release here. Nunn, who is a Professor of Geography and IPCC author, finds clear evidence for the MWP and a rapid cooling event around 1300 AD. At the end of a 28-page chapter on the MWP, Nunn writes: "Key points:
  1. The climate of the MWP in the pacific basin was marked by warm dry conditions exhibiting a low degree of interannual variability.
  2. Available data suggest that sea level rose in many parts of the Pacific Basin during the MWP, reaching a maximum at its end that exceeded present sea level.
  3. Most Pacific Basin societies enjoyed times of plenty during the MWP to which the comparatively constant climate contributed. Many societies also show adaptation to increasing warm and dry conditions. Food crises arising from droughts affected parts of the eastern Pacific Rim."
Climate and hydrographic variability in the Indo-Pacific Warm Pool during the last millennium
Climate and hydrographic variability in the Indo-Pacific Warm Pool during the last millennium. A. Newton, R. Thunell, L. Stott, Geophys. Res. Lett., 33, L19710 (2006). ("The warmest temperatures and highest salinities occurred during the Medieval Warm Period (MWP), while the coolest temperatures and lowest salinities occurred during the Little Ice Age (LIA)").
1400 yr multiproxy record of climate variability from the northern Gulf of Mexico. JN Richey, RZ Poore, BP Flower, TM Quinn, Geology,35; 423-426 (2007) ("Two multidecadal intervals of sustained high Mg/Ca indicate that Gulf of Mexico sea surface temperatures (SSTs) were as warm or warmer than near modern conditions between 1000 and 1400 yr B.P.")
2,000-year-long temperature and hydrology reconstructions from the Indo-Pacific warm pool. DW Oppo, Y Rosenthal and BK Linsley, Nature, 460, 1113-1116 (2009). ("Reconstructed SST was, however, within error of modern values from about AD 1000 to AD 1250, towards the end of the Medieval Warm Period.")
The little ice age and medieval warm period in the Sargasso Sea. LD Keigwin, Science 274, 1504-1508 (1996). ("Results from a radiocarbondated box core show that SST was ~1°C cooler than today ~400 years ago (the Little Ice Age) and 1700 years ago, and ~1°C warmer than today 1000 years ago (the Medieval Warm Period).")

Glacial geological evidence for the medieval warm period. JM Grove and R Switsur, Climatic Change, 26, 143-169 (1994). ("The results suggest that it was a global event occurring between about 900 and 1250 A.D") 
Evidence for the existence of the medieval warm period in China. D Zhang, Climatic Change, 26, 289-297 (1994). ("It can be estimated that the annual mean temperature in south Henan Province in the thirteenth century was 0.9–1.0°C higher than at present.")

 See CO2Science for  hundreds of research papers providing evidence for the MWP.