What should be baseline/reference period for climate change impact simulation ?

Well, This is look hot topic for current time. In climate change studies, baseline year period is more important and necessary as it considered as reference period to assess the impact of future climate change. But the baseline year period  is suggested 30 years time periods of 1960-1990 by WMO (WMO, 2007). According to IPCC definition A baseline period is needed to define the observed climate with which climate change information is usually combined to create a climate scenario. Initially, IPCC suggested to use time period 1931 to 1950 then 1951 to 1980 thereafter 1961 to 1990 for climate change studies. And the reason is later periods (1961 to 1990) are likely to have larger anthropogenic trends embedded in the climate data, especially the effects of sulfate aerosols over regions such as Europe and eastern USA.

After regularly update the recommended period is now 1981-2010 by WMO (Page 1) and in future, WMO will use time period 1991-2020. However, IPCC used 20 years of period 1986-2005 to compare the climate change (Table  3.1) and will use the years 1995–2014 in its Sixth Assessment Report. But what I remember GCM’s input data were 1850-2005 and afterwards year periods 2005 to 2100 are GCM’s output and most of studies considered time period 1970-2004 as baseline period.

Effect of different baseline period of US

According to NOAA blog "The influence of long-term global warming is obvious: the earliest map in the series has the most widespread and darkest blues, and the most recent map has the most widespread and darkest reds. Today, the normal annual temperatures across the country are warmer than the 20th-century average virtually everywhere. From 1901-1930, the annual average temperature was mostly colder than the 20th-century average"